Heterofunctional carboxylic acids.

1

• Lecture ? 9
• Heterofunctional carboxylic acids.

Prepared by ass. Medvid I.I., ass. Burmas N.I.
2

• Outline
• Physical and chemical properties of oxoacids.
  Acetoacetic ester.
• Physical and chemical properties of halogenacids
• Physical and chemical properties of hydroxyacids.
• Physical and chemical properties of phenolacids.
• Physical and chemical properties of aminoacids.
• Chloranhydrides of carbonic acid
• a) Physical and chemical properties of a phosgene
• 7. Amides of carbonic acid
• Physical and chemical properties of an urea
• Physical and chemical properties of a guanidine
• 8. Sulfoacids
• aliphatic sulfoacids
• aromatic sulfoacids

3

• 9. Aminoacids
• 1). a-Aminoacids as structure components of
  proteins.
• 2). Classification and structure of aminoacids.
• 3). Chirality of aminoacids.
• 4). Acid - base properties of aminoacids.
• 5). Chemical properties of a-aminoacids.
• 6). Indentification of aminoacids.
• 7). Proteins. Structure of ? protein.
• 8). Globular and fibrous proteins
• 9). Simple and conjugated proteins.
• 10.Peptides
• a) acid-base properties
• b) optical properties of peptides
• c) chemical properties of peptides
• d) synthesis of peptides.

4

• The most important heterofunctional carboxylic
  acids are halogenocarboxylic acid (halogenoacid),
  hydroxycarboxylic acid (hydroxyacid)
  oxocarboxylic acid (aldehydo- and ketonoacid) and
  amino- carboxylic acids (aminoacids)

5

• 1. Oxoacids
• To oxoacids include aldehydo- and ketonoacids.
  These compounds include in the structure of the
  carboxyl group, aldehyde functional group or
  ketone functional group.

pyroracemic acid, 2-oxopropanoic acid
acetoacetic acid, 3-oxobutanoic
acid, ß-ketobutyric acid
glyoxylic acid, oxoethanoic acid
?-ketovaleric acid, 4-oxopentanoic
acid, levulinic acid
oxalacetic acid, oxobutanedioic
acid, ketosuccinic acid
6

• Methods of extraction of oxoacids
• Oxidation of hydroxyacids
• Hydrolysis dihalogenocarboxylic acids

lactic acid
pyroracemic acid
2,2-dichlorpropanoic acid pyroracemic
acid (pyruvic acid)
7

• Chemical properties of oxoacids
• Decarboxylation of a-oxoacids
• Decarboxylation of ß-oxoacids

8

• Acetoacetic ester
• Acetoacetic ester synthesis is a chemical
  reaction where ethyl acetoacetate is alkylated at
  the a-carbon to both carbonyl groups and then
  converted into a ketone, or more specifically an
  a-substituted acetone.
• Acetoacetic ester is a tautomeric substance. He
  characterized keto-enol tautomery.

9

• The enol form of "acetoacetic ester" stand by
  formation of hydrogen bond

10

• Chemical properties of acetoacetic ester
• Reactions of ketone form

11

• 2. Reactions to enol form
• interaction of acetoacetic ester with metallic
  sodium
• interaction of acetoacetic ester with NaOH
• c) interaction acetoacetic ester with PCL5

ethyl-3-chlorbutene-2-oate
12

• d) interaction of acetoacetic ester with
  bromine water.
• The discolouration of bromine water, that
  explained unsaturated of "acetoacetic ester.
• e) interaction of acetoacetic ester with FeCL3

13

• The characteristic feature of acetoacetic
  ester is the ability to ketone decomposition and
  acid decomposition .
• Ketone decomposition occurs when heated in the
  presence of the dilute solutions of acids or
  alkalis.
• Acid decomposition of acetoacetic ester

14

• An acetoacetic ester used in the organic
  synthesis for the extraction of difference
  ketones and carboxylic acids.

15

• 2. Halogenoacids
• Halogenoacids are the derivatives of carboxyl
  acids that contain halogen radical (1 or more).
• a-bromopropanoic acid
• 2-bromopropanoic acid

2-bromo-3-methylbutanoic acid, a -
bromoisovaleric acid
16

• Methods of extraction of halogenocarboxylic acid
  
• Halogenation of saturated carboxylic acids
• Hydrohalogenation of unsaturated carboxylic acids
• Halogenation of aromatic carboxylic acids

acrylic acid
ß-chloropropanoic acid

m-chlorobenzoic acid
17

• Physical and chemical properties of
  halogenocarboxylic acid
• For physical properties of halogencarboxylic
  acids are colorless liquids or crystalline
  substance, soluble in water.
• Chemical properties in the molecule of
  halogenoacids either carboxyl group or halogen
  radical can react.
• As the halogen atom separation of carboxyl
  group inductive effect decreases, and so the
  acidity decreases. In the transition from mono-
  to di- and polyhalogencarboxylic acids the
  acidity increases. The most powerful of
  carboxylic acid is trifluoroacetic acid
  CF3-COOH (pKa 0,23)

18

• I. Carboxyl group can react with formation of
• Salts

chloroacetat sodium
19

• b) complex ethers
• c) amides

methyl ether of ß-chloropropanoic acid
amide ß-chloropropanoic acid
20

• II. Halogen radical can react with
• ammonium
• b) NaOH (water solution)
• 1) for a-halogenoacids

ammonium salt of ß-aminopropanoic acid
lactic acid
21

• 2) for ß-halogenoacids
• 3) for ?,s-halogenoacids

ß-chloropropanoic acid ß-hydroxypropanoic
acid acrylic acid
?-butyrolactone
22

• Representatives of halogenocarboxylic acid

Monochloroacetic acid
Trichloroacetic acid
Dichloroacetic acid
These acids are used in organic synthesis
Ureide of a-bromisovaleric acid (bromisoval) used
in medical practice as a hypnotic.
23

• 3. Hydroxyacids
• Hydroxyacids are the derivatives of carboxyl
  acids that contain OH group (1 or more).

ß a
2-hydroxypropanoic acid a-hydroxypropanoic acid
24
glycolic acid, hydroxyacetic acid, hydroxyethanoic
acid
tartaric acid a,a-dihydroxysuccinic
acid, 2,3-dihydroxybutandioic acid,
lactic acid, a- hydroxypropanoic acid, 2-
hydroxypropanoic acid
malic acid, hydroxysuccinic acid hydroxybutanedioi
c acid
citric acid, 2-hydroxy-1,2,3-propantricarboxylic
acid
25

• In a row of hydroxyacids often found the optical
  isomery.

D-, or (R,R)-tartaric acid
L-, or (S,S)-tartaric acid
mezo-, or (R,S)-tartaric acid
26

• Methods of extraction of hydroxyacids
• Hydrolysis of a-halogenoacids
• Oxidations of diols and hydroxyaldehydes
• Hydration of a,ß-unsaturated carboxylic acids

lactic acid
ß-hydroxypropanoic acid
27

• 4. Hydrolysis of hydroxynitriles (cyanohydrins)

28

• Physical and chemical properties of
  hydroxycarboxylic acid
• For physical properties of hydroxycarboxylic
  acids are colorless liquids or crystalline
  substance, soluble in water.
• Chemical properties in the molecule of
  hydroxyacids ether OH group or carboxyl group
  can react.
• Carboxyl group can react forming
• a) salts

sodium ß-hydroxypropanoic acid
29

• b) complex ethers

methyl ether of ß-hydroxypropanoic acid
30

• c) amides
• II. OH group can react with
• hydrohalogens (HCl, HBr, HI, HF)
• b) can oxidize

amide of ß-hydroxypropanoic acid
ß-oxopropanoic acid
31
Related to heat of
1. a-hydroxyacids
lactic acid
lactide
2. ß-hydroxyacids
heating
3-hydroxybutanoic acid
butene-2-onic (crotonic) acid
32

• 3. ?-hydroxyacids

heating
4-hydroxybutanic acid
?-butyrolacton
33
Decomposition of a-hydroxyacids
acetic acid
formic acid
34

• Representatives of hydroxyacids
• Milk acid . Milk acid is
  a trivial name because at
  first it was extracted from milk. It is present
  in kefir yogurt, sour milk and
  other milk products. It can form in muscles
  during hard and prolonged work. That is why
  peoples can feel ache in their muscles after
  physical training. Salts of milk acid are used in
  medicine.
• Apple acid . It is present in green apples
  and some berries. It takes part in biological
  processes in human organisms and organisms of
  other alive creatures. In industry it is used for
  manufacturing of wine, fruit waters and sweets.
  It is used in medicine for synthesis of some
  medical preparations.
• Tartaric acid . It is present in grape. It is
  used in medicine for synthesis of some
  medical preparations.

35

• Citric acid . It is present in orange,
  lemon and other citric fruits. It takes part
  in biological processes in human
  organism.

36

• 4. Phenolacids.

Phenolacids are the derivatives of aromatic
carboxyl acids that contain OH group (1 or more).
salicylic acid, 2-hydroxybenzoic acid
o-hydroxycinnamic acid
4-hydroxybenzoic acid
3,4,5-trihydroxybenzoic acid, gallic acid
37

• Methods of phenolacids extraction
• Carboxylation of phenols by carbon oxide (IV)
• In the Kolbe synthesis, also known as the
  KolbeSchmitt reaction, sodium phenoxide is
  heated with carbon dioxide under pressure, and
  the reaction mixture is subsequently acidified to
  yield salicylic acid
• 2. Hydroxylation of arencarboxylic acids

38
3. Alloying of sulphobenzoic acid with alkalis
m-sulphobenzoic acid potassuim
salt of
3-hydroxybenzoic acid
39
Chemical properties of phenolacids Chemical
properties of phenolacids due to the presence in
their structure of carboxyl group, phenolic
hydroxyl and the aromatic nucleus.
Decarboxylation
40
(No Transcript)
41
(No Transcript)
42

• The best known aryl ester is O-acetylsalicylic
  acid, better known as aspirin. It is prepared by
  acetylation of the phenolic hydroxyl group of
  salicylic acid
• Aspirin possesses a number of properties that
  make it an often-recommended drug. It is an
  analgesic, effective in relieving headache pain.
  It is also an antiinflammatory agent, providing
  some relief from the swelling associated with
  arthritis and minor injuries. Aspirin is an
  antipyretic compound that is, it reduces fever.
  Each year, more than 40 million lb of aspirin is
  produced in the United States, a rate equal to
  300 tablets per year for every man, woman, and
  child.

43
5. Aminoacids

• An aminoacid is an organic compound that contains
  both a amino (N?2) group and a carboxyl (-????)
  group. The amino acids found in proteins are
  always a-amino acids.

44
(No Transcript)
45

• Methods of aminoacids extraction
• Effects of ammonia on halogencarboxylic acids
• Effects of ammonia and HCN on aldehydes

a-chlorpropanoic acid
a-aminopropanoic acid
a-aminopropanonitrile
acetalaldehyde
aldimine
a-aminopropanoic acid
46
3. Accession of ammonia to the a, ßunsatured
acids
acrylic acid
ß-aminopropanoic acid
4. Reduce of nitrobenzoic acid
n-nitrobenzoic acid n-aminobenzoic acid
47
Optical properties
48
Physical and chemical properties of aminoacids

• Both an acidic group (-????) and ? basic group
  (-N?2) are present on the same carbon in an
  a-amino acid.
• The net result is that in neutral solution, amino
  acid molecules have the structure
• ? zwitter-ion is ? molecule that has ? positive
  charge on one atom and ? negative charge on
  another atom.

49

• Reactions on a amino-group

50

• Reactions to a carboxylic group

51

• Heating of
• a-aminoacids
• ß-aminoacids

a-aminopropanoic acid
3,6-dimethyl-2,5-diketopiperazine
ß-aminooil acid
crotonic acid
52
3. ?-aminoacids
?-aminooil acid ?-lactam
53

• React a-aminoacids with ninhydrin

54

• Carbonic acid (ancient name acid of air or
  aerial acid) has the formula H2CO3. It is also a
  name sometimes given to solutions of carbon
  dioxide in water, which contain small amounts of
  H2CO3. The salts of carbonic acids are called
  bicarbonates (or hydrogen carbonates) and
  carbonates. It is a weak acid.
• Carbonic acid is diprotic it has two hydrogen
  atoms which may dissociate from the parent
  molecule. Thus there are two dissociation
  constants, the first one for the dissociation
  into the bicarbonate (also called hydrogen
  carbonate) ion HCO3
• H2CO3 ? HCO3 H
• Ka1 2.5104  pKa1 3.45 /- 0,15 at 25
• The second for the dissociation of the
  bicarbonate ion into the carbonate ion CO3²
• HCO3 ? CO3 H
• Ka2 5.6110-11  pKa2 10.25 at 25 C.

55
Functional derivates of carbonic acid.
56
6. Chloranhydrides of carbonic acid
Produces phosgene by interaction of carbon oxide
(II) with chlorine on the light.
57
Physical and chemical properties of a
phosgene Phosgene is the chemical compound with
the formula COCl2. This colorless gas gained
infamy as a chemical weapon during World War I,
and is also a valued industrial reagent and
building block in organic synthesis. In low
concentrations, its odor resembles freshly cut
hay or grass. Some soldiers during the First
World War stated that it smelled faintly of May
Blossom. In addition to its industrial
production, small amounts occur naturally by the
breakdown of chlorinated compounds and the
combustion of chlorine-containing organic
compounds
1. Hydrolysis of phosgene
58

• 2. Interaction of phosgene with alcohols
• 3. Interaction of phosgene with ammoium

59
7. Amides of carbonic acid
Esters of carbamic acid are named urethanes
60
Meprothan used in a medicine as a medicament,
which has tranquilization and hypnotic effects.
61
Urea or carbamide is an organic compound with the
chemical formula (NH2)2CO. The molecule has two
amine (-NH2) residues joined by a carbonyl (-CO-)
functional group. Urea was first discovered from
urine in 1773 by the French chemist Hilaire
Rouelle. In 1828, the German chemist Friedrich
Wöhler obtained urea by treating of silver
isocyanate with ammonium chloride in a failed
attempt to prepare ammonium cyanate
AgNCO NH4Cl ? (NH2)2CO AgCl
In the industry urea produces by interaction of
an ammonia with carbon oxide (IV)
62
Physical and chemical properties of an urea The
urea molecule is planar. Each carbonyl oxygen
atom accepts four N-H-O hydrogen bonds. This
dense and energetically favourable hydrogen-bond
network is probably established at the cost of
efficient molecular packing. The structure is
quite open, the ribbons forming tunnels with
square cross-section. The carbon in urea is
described as sp² hybridized, the C-N bonds have
significant double bond character, and the
carbonyl oxygen is basic compared to
formaldehyde. Its high solubility is due to
extensive hydrogen bonding with water up to
eight hydrogen bonds may form - two from the
oxygen atom, one from each hydrogen atom and one
from each nitrogen atom.
1. Interaction of an urea with strong acids
2. Hydrolysis of an urea during to heating
63
3. Interaction of an urea with halohenalkanes
(alkylation)
4. Interaction of an urea with halohenanhydrides
of carboxylic acids (acylation)
64
Dicarboxylic acids can form with an urea cycle
ureides. For example, barbituric acid or
malonylurea or 6-hydroxyuracil is an organic
compound based on a pyrimidine heterocyclic
skeleton. It is an odorless powder soluble in hot
water. Barbituric acid is the parent compound of
barbiturate medicine, although barbituric acid
itself is not pharmacologically active.
65
5. Interaction of an urea with HNO2
6. Interaction of an urea with water solution of
hypobromides. This reaction as the previous can
be used to quantitative determination of an urea.
66
5. Biuret reaction. Used for qualitative
determination of an urea and proteins, as
containing in its structure of a group?O-NH-.
67
By-product of a biuret reaction is the
isocyanuric acid, which forms as a result of
trimerazation of cyanuric acid.
68
Physical and chemical properties of a guanidine
1. Interaction a guanidine with acids
2. Interaction a guanidine with bifunctional
compounds (diesters, diketones)
69
The remain of guanidine is the structural
components of many compounds. For example

• Arginine plays an important
  role in cell division, the healing of
  wounds, removing
  ammonia from the body,
  immune function, and the release
  of hormones.

Guanine is one of the five main nucleobases
found in the nucleic acids DNA and RNA.
70

• 
  Streptomycin is an antibiotic
  drug, the
  first of a class of drugs
  called aminoglycosides
  to be
  discovered, and was the first
  antibiotic remedy for
  tuberculosis.

Streptomycin - (IUPAC) name 5-(2,4-diguanidino-3,
5,6-trihydroxy-cyclohexoxy)- 4-4,5-dihydroxy-6-(h
ydroxymethyl)-3-methylamino-tetrahydropyran-2-yl
oxy-3-hydroxy-2-methyl-tetrahydrofuran-3-carbald
ehyde
71

• 8. Sulfoacids called the derivatives of
  organic compounds in which an atom of hydrogen
  replaced by the residue of sulfuric acid
  sulfogroup SO3H.
• Aliphatic sulfoacids
• ?H3-SO2OH
  ?H3-?H2-SO2OH
• methanesulfonic acid
  ethanesulfonic acid
• (methanesulfoacid)
  (ethanesulfoacid)

72

• Functional derivatives of sulfoacids
• CH3-SO2Cl - choranhydride of methanesulfoacid
  (methane
  sulfonylchloride)
• CH3-SO2ONa sodium salt of methanesulfoacid
  (methanesulfate sodium)
• CH3-SO2NH2 amide of methanesulfoacid
  (methanesulfonamide
  )
• CH3-SO2-OC2H5 ethyl ester of methanesulfoacid
  (ethylmetanesulfonat
  e)

73

• Sulfonic acids are typically much stronger acids
  than their carboxylic equivalents, and have the
  unique tendency to bind proteins and
  carbohydrates tightly most "washable" dyes are
  sulfonic acids (or have the functional sulfonyl
  group in them) for this reason. They are also
  used as catalysts and intermediates for a number
  of different products. Sulfonic acids and their
  salts (sulfonates) are used extensively in
  obtaining such diverse products like detergents,
  antibacterial drugs, medicine with derivatives of
  sulfonic acid, anion exchange resins (water
  purification), and dyes. The simplest example is
  methanesulfonic acid, CH3SO2OH, which is a
  reagent regularly used in organic chemistry.
  p-toluenesulfonic acid is also an important
  reagent.

74

• Extraction of aliphatic sulfoacids
• Sulfochlorination
• Sulfooxidation
• 2R-H 2SO2 O2 2R-SO2OH
• 
  alkanesulfonic acid
• 3. Oxidation of thiols

75

• 4. Sulfonation of alkanes by conc. H2SO4
• 5. Accession of hydrosulfites to alkenes

76

• Chemical properties of aliphatic sulfoacids
• 1. Formation salts of sulfoacids
• C2H5-SO2-OH NaOH C2H5-SO2-ONa H2O
• 2 C2H5-SO2-OH 2 Na 2 C2H5-SO2-ONa H2
• 2. Formation of sulfonylchlorides
• R-SO2-OH PCl5 R-SO2Cl POCl3 HCl
• 3. Formation of sulfonamides
• R-SO2-Cl 2 NH3 R-SO2-NH2 NH4Cl
• 4. Formation esters of sulfoacids
• R-SO2-Cl 2 NaO-R' R-SO2-O-R' NaCl

77
Aromatic sulfoacids
78
Extraction of aromaric sulfoacids
1. Sulfonation of aromatic ring
79
(No Transcript)
80

• Chemical properties of aromatic sulfoacids
• Reactions of the sulfogroup
• a) formation salts of sulfoacids
• C6H5SO2OH NaOH C6H5SO2Na H2O
• b) formation of sulfonylchlorides
• C6H5SO2OH PCl5 C6H5-SO2-Cl POCl3 HCl
• C6H5 2 HO-SO2Cl C6H5-SO2Cl H2SO4 HCl
• c) formation of sulfonamides
• C6H5SO2Cl 2 NH3 C6H5-SO2-NH2 NH4Cl
• d) formation esters
• C6H5SO2Cl HO-C2H5 C6H5-SO2-O-C2H5 HCl
• e) reduced of the sulfogroup

81
f) synthesis of a saccharin
82
II. Reactions SE, SN of sulfogroup
a) desulfonation
b) a reaction of alkalic floating
83
III. Substitution reaction of atom H in the
benzene nucleus
84
7.Sulphanylamidic preparations

• Sulfanilamide is a molecule containing the
  sulfonamide functional group attached to an
  aniline. Sulfanilamide is a sulfonamide
  antibiotic. The sulfonamides are synthetic
  bacteriostatic antibiotics with a wide spectrum
  against most gram-positive and many gram-negative
  organisms. However, many strains of an individual
  species may be resistant. Sulfonamides inhibit
  multiplication of bacteria by acting as
  competitive inhibitors of p-aminobenzoic acid in
  the folic acid metabolism cycle. Bacterial
  sensitivity is the same for the various
  sulfonamides, and resistance to one sulfonamide
  indicates resistance to all. Most sulfonamides
  are readily absorbed orally. However, parenteral
  administration is difficult, since the soluble
  sulfonamide salts are highly alkaline and
  irritating to the tissues. The sulfonamides are
  widely distributed throughout all tissues. High
  levels are achieved in pleural, peritoneal,
  synovial, and ocular fluids. Although these drugs
  are no longer used to treat meningitis, CSF
  levels are high in meningeal infections. Their
  antibacterial action is inhibited by pus.
  Mechanism of action Sulfanilamide is a
  competitive inhibitor of bacterial
  para-aminobenzoic acid (PABA), a substrate of the
  enzyme dihydropteroate synthetase. The inhibited
  reaction is necessary in these organisms for the
  synthesis of folic acid. Indication For the
  treatment of vulvovaginitis caused by Candida
  albicans

85

• Sulphanylamidic preparations. All
  sulphanylamidic medicines contain the next
  fragment
• Albucyde (sulphacyl) is an antibacterial mean,
  is a part of eye-drops.
• Urosulphane is an antibacterial mean by
  infection of urinal canals.
• Norsulphazol is used by pneumonia, meningitis,
  staphylococcal and streptococcal sepsis,
  infectious diseases.
• Bucarbane is a hypoglycemic mean.

Albucyde Urosulphane Norsulphazol
Bucarbane (sulphacyl)
86
9. Aminoacids.1). a-Aminoacids as structure
components of proteins.

• Next to water, proteins are the most abundant
  substances in most cells - from 10 to 20 of the
  cells mass. All proteins contain the elements
  carbon, hydrogen, oxygen, and nitrogen most also
  contain sulfur. The presence of nitrogen in
  proteins sets them apart from carbohydrates and
  lipids, which generally do not contain nitrogen.
  The average nitrogen content of proteins is 15.4
  by mass. Other elements, such as phosphorus and
  iron, are essential constituents of certain
  specialized proteins. Casein, the main protein of
  milk, contains phosphorus, an element very
  important in the diet of infants and children.
  Hemoglobin, the oxygen-transporting protein of
  blood, contains iron.

87

• The word protein comes from the Greek proteios,
  which means "of first importance." This
  derivation alludes to the key role that proteins
  play in life processes.
• ? protein is in polymer in which the monomer
  units are aminoacids. Thus the starting point for
  ? discussion of proteins is an understanding of
  the structures and chemical properties of
  aminoacids.
• An aminoacid is an organic compound that contains
  both an amino (N?3) group and a carboxyl (-????)
  group. The aminoacids found in proteins are
  always a-aminoacids - that is, aminoacids in
  which the aminogroup is attached to the a-carbon
  atom of the carboxylic acid carbon chain.

88

• The general structural formula for an a-aminoacid
  is
• The R group present in an a-aminoacid is called
  the aminoacid side chain. The nature of this side
  chain distinguishes ?-aminoacids from each other.
  Side chains vary in size, shape, charge, acidity,
  functional groups present, hydrogen-bonding
  ability, and chemical reactivity.

89

• 2). Classification and structure of aminoacids.
  
• Over 700 different naturally occurring
  aminoacids are known, but only 20 of them, called
  standard aminoacids, are normally present in
  proteins. ? standard aminoacid is one of the 20
  a-aminoacids normally found in proteins.
  Aminoacids are grouped according to side-chain
  polarity. In this system there are four
  categories (1) nonpolar aminoacids, (2) polar
  neutral aminoacids, (3) polar acidicamino acids,
  and (4) polar basic aminoacids. This
  classification system gives insights into how
  various types of aminoacid side chains help
  determine the properties of proteins.

90

• Nonpolar aminoacids contain one amino group, one
  carboxyl group, and a nonpolar side chain. When
  incorporated into ? protein, such aminoacids are
  hydrophobic (water fearing) that is, they are
  not attracted to water molecules. They are
  generally found in the interior of proteins,
  where there is limited contact with water. There
  are eight nonpolar aminoacids. The three types of
  polar aminoacids have varying degrees of affinity
  for water. Within ? protein, such aminoacids are
  said to be hydrophilic ("water-loving").
  Hydrophilic aminoacids are often found on the
  surfaces of proteins.

91

• Polar neutral aminoacids contain one amino
  group, one carboxyl group, and ? side chain that
  is polar but neutral. The side chain is neutral
  in that it is neither acidic nor basic in
  solution at physiological pH. There are seven
  polar neutral aminoacids. Polar acidic aminoacids
  contain one amino group and two carboxyl groups,
  the second carboxyl group being part of the side
  chain. In solution at physiological ??, the side
  chain of ? polar acidic aminoacid bears ?
  negative charge the side-chain carboxyl group
  has lost its acidic hydrogen atom. There are two
  polar acidic aminoacids aspartic acid and
  glutamic acid. Polar basic aminoacids contain one
  amino groups and one carboxyl group, the second
  amino group being part of the side chain. In
  solution at physiological ??, the side chain of ?
  polar basic aminoacid bears ? positive charge
  the nitrogen atom of the amino group has accepted
  ? proton. There are three polar basic aminoacids
  lysine, arginine, and histidine.

92

• According to the chemical origin of the residue
  connected with a-aminoacid fragment
  CH(NH2)COOH, a-aminoacids divided on aliphatic,
  aromatic and heterocyclic.
• In heterocyclic a-aminoacids proline and
  oxyproline a-aminoacids fragment presents in
  hetecyclic structure.

According to the quantity of NH2 and COOH
groups in molecule a-aminoacids divided on
monoaminocarbonic, monoaminodicarbonic and
diaminomonocarbonic.
93

• Classification and structure of amino acids

94

• The names of the standard aminoacids are often
  abbreviated using three-letter codes. Except in
  four cases, these abbreviations are the first
  three letters of the aminoacids name. In
  addition, ? new one-letter code for aminoacid
  names is currently gaining popularity
  (particularly in computer applications). These
  abbreviations are used extensively when
  describing peptides and proteins, which contain
  tens and hundreds of aminoacid units.

95

• The essential aminoacids. All of the 20
  aminoacids are necessary constituents of human
  protein. Adequate amounts of 11 of the 20
  aminoacids can be synthesized from carbohydrates
  and lipids in the body if ? source of nitrogen is
  also available. Because the human body is
  incapable of producing 9 of these 20 acids fast
  enough or in sufficient quantities to sustain
  normal growth, these 9 aminoacids, called
  essential aminoacids, must be obtained from food.
  Essential aminoacids are aminoacids that must be
  obtained from food. An adequate human diet must
  include foods that contain these essential
  aminoacids. The human body can synthesize small
  amounts of some of the essential aminoacids, but
  not enough to meet its needs, especially in the
  case of growing children. The 9 essential
  aminoacids for adults are histidine, isoleucine,
  leucine, lysine, methionine, phenylalanine,
  threonine, tryptophan, and valine. (In addition,
  arginine is essential for children).

96

• ? complete dietary protein contains all the
  essential aminoacids in the same relative amounts
  in which human being require them. ? complete
  dietary protein may or may not contain all the
  nonessential aminoacids. Most animal proteins,
  including casein from milk and proteins found in
  meat, fish, and eggs, are complete proteins,
  although gelatin is an exception (it lacks
  tryptophan). Proteins from plants (vegetables,
  grains, and legumes) have quite diverse aminoacid
  patterns, and some tend to be limiting in one or
  more essential aminoacids. Some plant proteins
  (for example, corn protein) are far from
  complete. Others (for example, soy protein) are
  complete. Thus vegetarians must eat ? variety of
  plant foods to obtain all of the essential
  aminoacids in appropriate quantities.

97
3). Chirality of aminoacids.

• Four different groups are attached to the
  a-carbon atom in all of the standard aminoacids
  except glycine, where the R group is ? hydrogen
  atom. This means that the structures of 19 of the
  20 standard aminoacids possess ? chiral center at
  this location, so enantiomeric forms (left- and
  right-handed forms) exist for each of these
  aminoacids. With few exceptions (in some
  bacteria), the aminoacids found in nature and in
  proteins are isomers. Thus, as is the case with
  monosaccharides, nature favors one mirror-image
  form over the other. Interestingly, for
  aminoacids the L isomer is the preferred form,
  whereas for monosaccharides the n isomer is
  preferred.

98
(No Transcript)
99

• The rules for drawing Fischer projections for
  aminoacid structures follow
• 1. The - ???? group is put at the top of the
  projection, the R group at the bottom. This
  positions the carbon chain vertically.
• 2. The N?2 group is in ? horizontal position.
  Positioning it on the left denotes the L isomer,
  and positioning it on the right denotes the D
  isomer.

100
4). Acid - base properties of aminoacids.

• In pure form, aminoacids are white crystalline
  solids with relatively high decomposition points.
  (Most amino acids decompose before they melt.)
  Also most aminoacids are not very soluble in
  water because of strong intermolecular forces
  within their crystal structures. Such properties
  are those often exhibited by compounds in which
  charged species are present. Studies of
  aminoacids confirm that they are charged species
  both in the solid state and in solution. Both an
  acidic group (-????) and ? basic group (-N?2) are
  present on the same carbon in an a-aminoacid.

101

• We learned that in neutral solution, carboxyl
  groups have ? tendency to lose protons (?),
  producing ? negatively charged species
• ???? ???- ?
• We learned that in neutral solution, amino groups
  have ? tendency to accept protons (?), producing
  ? positively charged species
• NH2 H NH3
• As is consistent with the behavior of these
  groups, in neutral solution, the ???? group of
  an aminoacid donates ? proton to the NH2 of the
  same aminoacid. We can characterize this behavior
  as an internal acid base reaction. The net
  result is that in neutral solution, aminoacid
  molecules have the structure.

102

• Such ? molecule is known as ? zwitterion, from
  the German term meaning double ion. ?
  zwitterion is ? molecule that has ? positive
  charge on one atom and ? negative charge on
  another atom. Note that the net charge on ?
  zwitterion is zero even though parts of the
  molecule carry charges. In solution and also in
  the solid state, a-aminoacids are zwitterions.
  Zwitterion structure changes when the pH of ?
  solution containing an aminoacid is changed from
  neutral either to acidic (low pH) by adding an
  acid such as ??1 or to basic (high pH) by adding
  ? base such as NaOH. In an acidic solution, the
  zwitterion accepts ? proton (?) to form ?
  positively charged ion.

103

• In basic solution, the NH3 of the zwitterion
  loses ? proton, and ? negatively charged species
  is formed.
• Thus, in solution, three different aminoacid
  forms can exist (zwitterion, negative ion, and
  positive ion). The three species are actually in
  equilibrium with each other, and the equilibrium
  shifts with pH change. The overall equilibrium
  process can be represented as follows
• In acidic solution, the positively charged
  species on the left predominates nearly neutral
  solutions have the middle species (the
  zwitterion) as the dominant species in basic
  solution, the negatively charged species on the
  right predominates.

104

• The previous discussion assumed that the side
  chain (R group) of an aminoacid remains unchanged
  in solution as the pH is varied. This is the case
  for neutral aminoacids but not for acidic or
  basic ones. For these latter compounds, the side
  chain can also acquire ? charge, because it
  contains an amino or ? carboxyl group that can,
  respectively, gain or lose ? proton. Because of
  the extra site that can be protonated or
  deprotonated, acidic and basic aminoacids have
  four charged forms in solution. The existence of
  two low-pH forms for aspartic acid results from
  the two carboxyl groups being deprotonated at
  different pH values. For basic aminoacids, two
  high-pH forms exist because deprotonation of the
  amino groups does not occur simultaneously. The
  side-chain amino group deprotonates before the
  a-amino group.

105

• The isoelectric point for an aminoacid is the
  pH at which the total charge on the aminoacid is
  zero. Every aminoacid has ? different isoelectric
  point. Fifteen of the 20 amino acids, those with
  nonpolar or polar neutral side chains, have
  isoelectric points in the range of 4.8 - 6.3. The
  three basic aminoacids have higher isoelectric
  points (His 7.59, Lys 9.74, Arg 10.76), and
  the two acidic aminoacids have lower ones (Asp
  2.77, Glu 3.22).
• ? ?? below the isoelectric point favors the
  positively charged form of the aminoacid.
  Conversely, ? ?? above the isoelectric point
  favors the negatively charged form of the
  aminoacid.
• When two electrodes (one positively charged and
  one negatively charged) are immersed in ?
  solution containing an aminoacid, molecules with
  ? net positive charge are attracted to the
  negatively charged electrode, and negatively
  charged aminoacid molecules migrate toward the
  positively charged electrode. The zwitterion form
  exhibits no net migration toward either
  electrode. This behavior is the basis for the
  measurement of isoelectric points. The pH of the
  solution is adjusted until no net migration
  occurs.
• Mixtures of aminoacids in solution can be
  separated by using their different migration
  patterns at various pH values. This type of
  analytical separation is called electrophoresis.
  Electrophoresis is the process of separating
  charged molecules on the basis of their migration
  toward charged electrodes.

106
Methods of aminoscids obtaining

• Proteins hydrolysis. It can be alkaline, acidic
  or fermentative hydrolysis. Widely use
  fermentative hydrolysis, for separation of
  a-aminoacids ionchange chromatography is used.
• Microbiological synthesis. In these method use
  special microorganisms that produce a-aminoacids.

107
5). Chemical properties of a-aminoacids

• Reaction on amino-group
• 1) Formation of N-acylderivatives. This reaction
  use for blocking (protection) of aminogroup at
  the synthesis of peptides. As acylation agents
  use benzoxycarbonylchloride (a) or
  tret-butoxycarboxazide (b)

Blocked carbobenzoxygroup removed by catalytic
hydrogenolysis or by action of HBr in acetic acid
in cold.
108

• Tret-butoxycarbonyl group destroyed by action of
  triftoracetic acid

109

• 2) Deamination
• oxidation deamination important pathway for the
  biodegradation of a-aminoacids
• hydrolytic deamination reaction with nitrous
  acid. Aminoacids react with nitrous acid to give
  hydroxyacid along with the evolution of nitrogen.
• The nitrogen can be collected and measured.
  Thus this reaction constitutes one of the methods
  for the estimation of amino acids.

110

• c) intramolecular deamination - unsaturated acids
  are formed
• d) redaction deamination saturated carboxylic
  acid formation

111

• 3) Tranceamination. Reaction goes under the
  present of enzymes tranceaminases and coenzyme
  pyridoxalphosphate

4) Interaction with carbonyl compounds
112

• 5) Reaction with phenylisothiocyanate (Edmane
  reaction). Form derivatives of 3-phenyl-2-thiohyda
  ntoine (derivatives of phenylthiohydantoine)

6) Interaction with 2,4-dinitroftorbenzol
(Senhers reagent)
113
B. Reaction on carboxyl group

• 1) Formation of helate compounds ( complex salts
  with ions of heard metals)

• 2) Reaction with alcohols difficult esters
  formation

114

• 3) Reaction with ammonia amides formation. The
  amides of aspartic and glutamic acid acids,
  asparagine and glutamine, play important role in
  the transport of ammonia in the body.

4) Formation of halogenanhydrides and anhydrides
( like carbonyl acids). Before these reaction
blocked aminogroup by formation of
N-acylderivatives.
115

• 5) Decarboxylation. Aminoacids may be
  decarboxylated by heat, acids, bases or specific
  enzymes to the primary amines
• Some of the decarboxylation reaction are of great
  importance in the body, decarboxylation of
  histidine to histamine
• In the presence of foreign protein introduced
  into the body, very large quantities of histamine
  are produced in the body and allergic reactions
  become evident. In extreme cases shock may
  result. The physiological effects of histamine
  may be neutralized or minimized by the use of
  chemical compounds known as antihistamines.

116

• C. Formation of salts. All aminoacids can react
  with some inorganic acids and bases and form two
  kind of sold

117

• D. Peptide formation. Two aminoacids can react in
  ? similar way - the carboxyl group of one
  aminoacid reacts with the amino group of the
  other aminoacid. The products are ? molecule of
  water and ? molecule containing the two
  aminoacids linked by an amide bond. Removal of
  the elements of water from the reacting carboxyl
  and amino groups and the ensuing formation of the
  amide bond are better visualized when expanded
  structural formulas for the reacting groups are
  used.

118
In aminoacid chemistry, amide bonds that link
aminoacids together are given the specific name
of peptide bond. ? peptide bond is ? bond between
the carboxyl group of one aminoacid and the amino
group of another aminoacid. Under proper
conditions, many aminoacids can bond together to
give chains of aminoacids containing numerous
peptide bonds. For example, four peptide bonds
are present in ? chain of five aminoacids.
119

• Short to medium-sized chains of aminoacids are
  known as peptides. ? peptide is ? sequence of
  aminoacids, of up to 50 units, in which the
  aminoacids are joined together through amide
  (peptide) bonds. ? compound containing two amino
  acids joined by ? peptide bond is specifically
  called ? dipeptide three aminoacids in ? chain
  constitute ? tripeptide and so on. The name
  oligopeptide is loosely used to refer to peptides
  with 10 to 20 aminoacid residues and polypeptide
  to larger peptides. In all peptides, the
  aminoacid at one end of the aminoacid sequence
  has ? free H3N group, and the aminoacid at the
  other end of the sequence has ? free ???- group.
  The end with the free H3N group is called the
  N-terminal end, and the end with the free ???-
  group is called the ?-terminal end. By
  convention, the sequence of aminoacids in ?
  peptide is written with the N-terminal end
  aminoacid at the left. The individual aminoacids
  within ? peptide chain are called aminoacid
  residues.

120

• The structural formula for ? polypeptide may be
  written out in full, or the sequence of
  aminoacids present may be indicated by using the
  standard three-letter aminoacid abbreviations.
  The abbreviated formula for the tripeptide
• which contains the aminoacids glycine, alanine,
  and serine, is Gly Ala Ser. When we use
  this abbreviated notation, by convention, the
  aminoacid at the N-terminal end of the peptide is
  always written on the left.

121

• The repeating chain of peptide bonds and
  a-carbon atoms in ? peptide is referred to as the
  backbone of the peptide. The R group side chains
  are substituents on the backbone. Peptides that
  contain the same aminoacids but in different
  order are different molecules (structural
  isomers) with different properties. For example,
  two different dipeptides can be formed from one
  molecule of alanine and one molecule of glycine.
• In the first dipeptide, the alanine is the
  N-terminal residue, and in the second molecule,
  it is the ?-terminal residue. These two compounds
  are isomers with different chemical and physical
  properties.

122

• The number of isomeric peptides possible
  increases rapidly as the length of the peptide
  chain increases. Let us consider the tripeptide
  Ala Ser Cys as another example. In addition
  to this sequence, five other arrangements of
  these three components are possible, each
  representing another isomeric tripeptide Ala
  Cys Ser, Ser Ala Cys, Ser Cys Ala, Cys
  Ala Ser, and Cys Ser Ala. For ?
  pentapeptide containing 5 different aminoacids,
  120 isomers are possible.
• More than two hundred peptides have been
  isolated and identified as essential to the
  proper functioning of the human body. In general,
  these substances serve as hormones or
  neurotransmitters. Their functions range from
  controlling pain to controlling muscle
  contraction or kidney fluid excretion.

123

• Two important hormones produced by the
  pituitary gland are oxytocin and vasopressin,
  Each hormone is ? nonapeptide (nine amino acid
  residues) with six of the residues hells in the
  form of ? loop by ? disulfide bond formed from
  the interaction of two cysteine residues.
• Oxytocin regulates uterine contractions and
  lactation. Vasopressin regulates the excretion of
  water by the kidneys it also affects blood
  pressure. The structure of vasopressin differs
  from that of oxytocin at only two aminoacid
  positions the third and eighth aminoacid
  residues. The result of these variations is ?
  significant difference in physiological action.

124

• 6). Indentification of aminoacids.
• Biuret test. The protein is warmed gently with
  10 solution of sodium hydroxide and then ? drop
  of very dilute copper sulphate solution is added,
  the formation of reddish - violet colour
  indicates the presence of peptide link, ?? NH
  . The test is given by all proteins, peptones
  and peptides. Its name is derived from the fact
  that the test is also positive for the compound
  biuret, ?2? CONH CONH2 obtained from urea by
  heating. It should be noted that dipeptides do
  not give the biuret test, while all other
  polypeptides do so. Hence biuret test is
  important to know whether hydrolysis of proteins
  is complete or not. If the biuret test is
  negative, hydrolysis is complete, at least to the
  dipeptide stage.

125

• Xanthoprotein test. On treatment with
  concentrated nitric acid, certain proteins give
  yellow color. This yellow color is the same that
  is formed on the skin when the latter comes in
  contact with the concentrated nitric acid. The
  test is given only by the proteins having at
  least one mole of aromatic aminoacid, such as
  tryptophan, phenylalanine, and tyrosine which are
  actually nitrated during treatment with
  concentrated nitric acid. When you add after
  conc. HNO3 conc. NaOH forms light orange color
  (hynoid structure).

126

• Millon's test. Protein on adding Millon's
  reagent (? solution of mercuric and mercurous
  nitrates in nitric acid containing ? little
  nitrous acid) followed by heating the solution
  give ? red precipitate or colour. The test is
  responded by the proteins having tyrosine. The
  hydroxyphenyl group of tyrosine is the structure
  responsible for this test. Moreover, the
  non-proteinous material having phenolic group
  also responds the test.
• Foll reaction. This reaction reveals the sulfur
  containing aminoacids (cysteine, cystine).
  Treatment of the sulfur containing aminoacids
  with salt of lead and alkali yields a black
  sediment.
• Adamkevich reaction. This reaction detects the
  amino acid tryptophan containing indol ring. The
  addition of the concentrated acetic and sulfuric
  acids to the solution of tryptophan results in
  the formation of red-violet ring appearing on the
  boundary of different liquids.

127

• Ninhydrin test. The ninhydrin colour reaction
  is the most commonly test used for the detection
  of aminoacids. This is an extremely delicate
  test, to which proteins, their hydrolytic
  products, and a-aminoacids react. Although the
  test is positive for all free amino groups in
  aminoacids, peptides, or proteins, the test is
  much weaker for peptides or proteins because not
  as many free groups are available as in
  aminoacids. For certain aminoacids the test is
  positive in dilutions as high as 1 part in
  100,000 parts of water. When ninhydrin is added
  to ? protein solution and the mixture is heated
  to boil, blue or violet color appears on cooling.
  The colour is due to the formation of ? complex
  compound.

128
(No Transcript)
129

• The test is also given by ammonia, ammonium
  salts, and certain amines. Ninhydrin is also used
  as ? reagent for the quantitative determination
  of free carboxyl groups in solutions of
  aminoacids.
• Nitroprusside test. Proteins containing free -SH
  groups (of cysteine) give ? reddish colour with
  sodium nitroprusside in ammonical solution.

130
7). Proteins. Structure of the proteins.

• Proteins are polypeptides that contain more
  than 50 aminoacid units. The dividing line
  between ? polypeptide and ? protein is arbitrary.
  The important point is that proteins are polymers
  containing ? large number of aminoacid units
  linked by peptide bonds. Polypeptides are shorter
  chains of aminoacids. Some proteins have
  molecular masses in the millions. Some proteins
  also contain more than one polypeptide chain. To
  aid us in describing protein structure, we will
  consider four levels of substructure primary,
  secondary, tertiary, and quaternary. Even though
  we consider these structure levels one by one,
  remember that it is the combination of all four
  levels of structure that controls protein
  function.

131

• The primary structure of ? protein is the
  sequence of aminoacids present in its peptide
  chain or chains. Knowledge of primary structure
  tells us which aminoacids are present, the number
  of each, their sequence, and the length and
  number of polypeptide chains.
• The first protein whose primary structure was
  determined was insulin, the hormone that
  regulates blood-glucose level ? deficiency of
  insulin leads to diabetes. The sequencing of
  insulin, which took over 8 years, was completed
  in 1953. Today, thousands of proteins have been
  sequenced that is, researchers have determined
  the order of amino acids within the polypeptide
  chain or chains.

132

• The primary structure of ? specific protein is
  always the same, regardless of where the protein
  is found within an organism. The structures of
  certain proteins are even similar among different
  species of animals. For example, the primary
  structures of insulin in cows, pigs, sheep, and
  horses are very similar both to each other and to
  human insulin. Until recently, this similarity
  was particularly important for diabetics who
  required supplemental injections of insulin.
• An analogy is often drawn between the primary
  structure of proteins and words. Words, which
  convey information, are formed when the 26
  letters of the English alphabet are properly
  sequenced. Proteins, which function biologically,
  are formed from the proper sequence of 20
  aminoacids. The proper sequence of letters in ?
  word is necessary for it to make sense, just as
  the proper sequence of aminoacids is necessary to
  make biologically active protein.

133

• The secondary structure of ? protein is the
  arrangement in space of the atoms in the backbone
  of the protein. Three major types of protein
  secondary structure are known the alpha helix,
  the beta pleated sheet, and the triple helix. The
  major force responsible for all three types of
  secondary structure is hydrogen bonding between ?
  carbonyl oxygen atom of ? peptide linkage and the
  hydrogen atom of an amino group (-NH) of another
  peptide linkage farther along the backbone. This
  hydrogen-bonding interaction may be diagrammed as
  follows

134

• The Alpha Helix The alpha helix (a-helix)
  structure resembles ? coiled helical spring, with
  the coil configuration maintained by hydrogen
  bonds between N ? and ? ? groups of every
  fourth aminoacid, as is shown diagrammatically in
  Figure.2.

135

• Figure. Three representations of (?) the ?
  helix protein structure. Hydrogen bonds between
  amide groups (peptide linkages) are shown in (b)
  and (?). (d) The top view of an ? helix shows
  that amino acid side chains (R groups) point away
  from the long axis of the helix.
• Figure. Two representations of the p pleated
  sheet protein structure. (?) ? representation
  emphasizing the hydrogen bonds between protein
  chains. (b) ? representation emphasizing the
  pleats and the location of the R groups. Proteins
  have varying amounts of a-helical secondary
  structure, ranging from ? few percent to nearly
  100 . In an a-helix, all of the aminoacid side
  chains (R groups) lie outside the helix there is
  not enough room for them in the interior.
  Figure.3d illustrates this situation. This
  structural feature of the a-helix is the basis
  for protein tertiary structure.

136

• The beta pleated sheet (ß-pleated sheet)
  secondary structure involves aminoacid chains
  that are almost completely extended. Hydrogen
  bonds form between two different side-by-side
  protein chains (interchain bonds) as shown in
  Figure.3, or between different parts of ? single
  chain that folds back on itself (intrachain
  bonds). The term pleated sheet arises from the
  repeated zigzag pattern in the structure
  (Figure.3b). Aminoacid side chains are located
  above and below the plane of the sheet. Very few
  proteins have entirely n helix or p pleated sheet
  structures. Instead, most proteins have only
  certain portions of their molecules in these
  conformations. The rest of the molecule assumes ?
  "random structure." It is possible to have both ?
  helix and p pleated sheet structures within the
  same protein.

137
Secondary ß-structure of proteins contains
parallel (a) and antiparallel (b) fragment
138

• Collagen, the structural protein of connective
  tissue (cartilage, tendon, and skin), has ?
  triple-helix structure. Collagen molecules are
  very long, thin, and rigid. Many such molecules,
  lined up alongside each other, combine to make
  collagen fibers. Cross-linking gives the fibers
  extra strength.

139

• The tertiary structure of ? protein is the
  overall three-dimensional shape that results from
  the attractive forces between aminoacid side
  chains (R groups) that are widely separated from
  each other within the chain. ? good analogy for
  the relationships among the primary, secondary,
  and tertiary structures of ? protein is that of ?
  telephone cord. The primary structure is the
  long, straight cord. The coiling of the cord into
  ? helical arrangement gives the secondary
  structure. The supercoiling arrangement the cord
  adopts after you hang up the receiver is the
  tertiary structure.

140

• Interactions responsible for the tertiary
  structure.
• Four types of attractive interactions contribute
  to the tertiary structure of ? protein
• 1) covalent disulfide bonds,
• 2) electrostatic attractions (salt bridges),

141

• 3) hydrogen bonds,

4) hydrophobic attractions.
142

• All four of these interactions are interactions
  between aminoacid R groups. This is ? major
  distinction between tertiary-structure
  interactions and secondary-structure
  interactions. Tertiary-structure interactions
  involve the R groups of aminoacids
  secondary-structure interactions involve the
  peptide linkages between aminoacid units.
  Disulfide bonds, the strongest of the
  tertiary-structure interactions, result from the
  SH groups of two cysteine molecules reacting
  with each other to form ? covalent disulfide.
  This type of interaction is the only one of the
  four tertiary-structure interactions that
  involves ? covalent bond. That SH groups are
  readily oxidized to give ? disulfide bond, S
  S . Disulfide bonds may involve two cysteine
  units in the same chain or in different chains.

143

• Figure. Four types of interactions between
  aminoacid R groups produce thetertiary structure
  of ? protein. (?) Disulfide bonds. (b)
  Electrostatic interactions (salt bridges). (?)
  Hydrogen bonds. (d) Hydrophobic interactions.
  Electrostatic interactions, also called salt
  bridges, always involve aminoacids with charged
  side chains. These aminoacids are the acidic and
  basic aminoacids. The two R groups, one acidic
  and one basic, interact through ion ion
  attractions. Figure.b shows an electrostatic
  interaction.

144

• Hydrogen bonds can occur between aminoacids
  with polar R groups. ? variety of polar side
  chains can be involved, especially those that
  possess the following functional groups
• Hydrogen bonds are relatively weak and are
  easily disrupted by changes in pH and
  temperature. Hydrophobic interactions result when
  two nonpolar side chains are close to each other,
  In aqueous solution, many proteins have their
  polar R groups outward, toward the aqueous
  solvent (which is also polar), and their nonpolar
  R groups inward (away from the polar water
  molecules). The nonpolar R groups then interact
  with each other. Hydrophobic interactions are
  common between phenyl rings and alkyl side
  chains. Although hydrophobic interactions are
  weaker than hydrogen bonds or electrostatic
  interactions, they are a significant force in
  some proteins because there are so many of them
  their cumulative effect can be greater in
  magnitude than the effects of hydrogen bonding.

145

• An example of ? protein with quaternary
  structure is hemoglobin, the oxygen-carrying
  protein in blood. It is ? tetramer in which there
  are two identical a chains and two identical ß
  chains. Each chain enfolds ? heme group, the site
  where oxygen binds to the protein.
• Figure. ? schematic diagram showing the
  quaternary structure of the oxygen-carrying
  protein hemoglobin.

146

• 8). Globular and fibrous proteins.
• On the basis of structural shape, proteins can
  be classified into two major types fibrous
  proteins and globular proteins. ? fibrous protein
  is ? protein that has ? long, thin, fibrous
  shape. Such proteins are made up of long
  rod-shaped or string-like molecules that can
  intertwine with one another and form strong
  fibers. They are water-insoluble and generally
  have structural functions within the human body.
  ? globular protein is ? protein whose overall
  shape is roughly spherical or globular. Globular
  proteins either dissolve in water or form stable
  suspensions in water, which allows them to travel
  through the blood and other body fluids to