Weak Interactions in Aqueous Systems

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Weak Interactions in Aqueous Systems
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Learning Objectives
Describe the properties of a hydrogen bond
including strength and directionality.
Identify the most common hydrogen bonds in
biological systems.
Define and describe hydrophobic interactions.
Explain with chemical equations the bicarbonate
buffer system in the blood.
Define amphipathic and identify the molecule
types that it applies to.
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Learning Objectives
Identify and describe the four weak interaction
forces hydrogen bonding, ionic, hydrophobic, and
van der Waals.
Describe the process of osmosis, and the effect
of hypertonic and hypotonic solutions on cell
structure.
Describe the function of aquaporin-1.
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Water
Water is the most abundant substance found in
living systems, making up 70 or more of the
weight of most organisms.
Most of the earths surface is covered with water.
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The dipolar nature of the water molecule
Tetrahedral arrangement of 2 hydrogens and 2 sets
of unshared electron pairs
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Hydrogen-bonding in water
Acceptor atom
The length of hydrogen bonds are typically
measured (in angstroms) from the donor atom to
the acceptor atom.
The shorter the distance, the stronger the
hydrogen bond.
Donor atom
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Directionality of the hydrogen bond
Hydrogen bonds are strongest when the bonded
molecules are orientated to maximize
electrostatic interactions. This occurs when the
hydrogen atom and the two atoms that share it are
in a straight line. Hydrogen bonds are thus
highly directional and capable of holding two
hydrogen-bonded molecules or groups in a specific
geometric arrangement.
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Water forms hydrogen bonds with polar solutes.
Uncharged but polar biomolecules such as sugars
dissolve readily in water because of the
stabilizing effect of hydrogen bonds between the
hydroxyl groups and carbonyl oxygen of the sugar
and the polar water molecules. Alcohols,
aldehydes, ketones, and compounds containing NH
bonds all form hydrogen bonds with water
molecules and tend to be soluble in water.
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Common hydrogen bonds in biological systems
The acceptor atom is usually oxygen or nitrogen.
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Some biologically important hydrogen bonds
thymine
Between complimentary bases in DNA
Between the peptide backbone in proteins
adenine
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Water interacts electrostatically with charged
solutes.
Water dissolves salts such as NaCl by hydrating
and stabilizing the Na and Cl- ions, and
counteracting their tendency to associate in a
crystalline lattice.
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Nonpolar gases are poorly soluble in water.
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Carbon dioxide, which is produced during
oxidation of fuels, forms carbonic acid in
aqueous solution and is transported in the blood
as the bicarbonate ion.
CO2
H2O

H2CO3
HCO3-
H

Carbonic acid/bicarbonate is one of the main
buffers that occurs in the blood. The other
involves the protein hemoglobin.
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Amphipathic compounds contain regions that are
polar (or charged) and regions that are nonpolar.
phenylalanine
phosphatidylcholine
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When an amphipathic compound is mixed with water,
the polar, hydrophilic region interacts favorably
with water and tends to dissolve, but the polar,
hydrophobic region tends to avoid contact with
water.
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Clusters of lipid molecules fewer water
molecules are ordered and entropy increases.
Dispersion of lipids in water water molecules
are forced to become highly ordered.
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Micelles all hydrophobic groups are sequestered
from water the ordered shell of water molecules
is minimized, and entropy is further increased.
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The forces that hold the nonpolar regions of the
molecules together are called hydrophobic
interactions. The strength of hydrophobic
interactions is not due to any intrinsic
attraction between nonpolar molecules. Rather,
it results from the systems achieving greatest
thermodynamic stability by minimizing the number
of ordered water molecules required to surround
hydrophobic portions of the solute molecules.
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Hydrogen bonds
Ionic interactions
Hydrophobic interactions
Van der Waals interactions
Any two atoms in close proximity
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Ionic Interactions
Interactions between charged functional groups
q1q2
k
U
r
D
U is the energy of association of two electric
charges q1 and q2, separated by a distance, r.
(from Coulombs Law)
Most charged residues in proteins are located on
the surface of the molecule where they are
solvated by water molecules and/or small
counterions.
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Van der Waals Forces
Average separation of two non-bonded atoms is
known as their van der Waals separation. There
is a very high repulsion if atoms get closer than
this, and there is a rapid fall off in attractive
force if the distance becomes larger.
At a point where the van der Waals attractive
forces exactly balance the repulsive forces, the
nuclei of the atoms involved are said to be in
van der Waals contact.
The basis for this phenomena is proposed to be
the result of induced dipoles.
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Movement of Water Molecules
Water molecules tend to move from a region of
high concentration to one of lower concentration.
When two different aqueous solutions are
separated by a semi-permeable membrane (dialysis
tubing, plasma membrane), water molecules
diffusing from the region of high concentration
to the region of low concentration produce
osmotic pressure.
Osmosis the bulk flow of water through a
semi-permeable membrane to another aqueous
compartment containing solute at a higher
concentration.
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The effect of extracellular osmolarity on water
movement across the plasma membrane.
Extracellular solutes
Intracellular solutes
Cell in isotonic solution no net water movement
across the plasma membrane.
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Cell in hypertonic solution
Water flows out and cell shrinks.
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Cell in hypotonic solution
Water flows in creating outward pressure. The
cell swells and may eventually burst.
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Dissociation of Water Molecules
H2O
OH-
H

Free protons do not exist in solution. Hydrogen
ions formed in water are immediately hydrated to
hydronium ions. Hydrogen bonding between water
molecules makes hydration of dissociated protons
virtually instantaneous.
H3O
Hydronium ion
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Transfer of protons in a proton wire
Proton hopping is much faster than true diffusion
and explains the remarkable high ionic mobility
of hydrogen ions compared with other monovalent
cations such as Na or K.
Short hops of protons between a series of
hydrogen-bonded water molecules effects an
extremely rapid net movement of a proton over a
long distance.
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Aquaporins
Aquaporins are transmembrane water channels. In
the kidney, at least seven aquaporins are
expressed at distinct sites. AQP1 is extremely
abundant in the proximal tubule and descending
thin limb and is essential for urine
concentration. AQP1 is a multisubunit oligomer
that is organized as a tetrameric assembly of
four identical polypeptide subunits with a large
glycan attached to only one of the subunits. Not
all aquaporins assemble as tetramers. Some
aquaporins selectively pass specific ions or
small molecules (glycerol), and some are gated
presumably as a regulatory mechanism for
vassopressin (ADH, antidiuretic hormone, secreted
by the posterior pituitary).
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Aquaporin-1 water channel
PDB File 1J4N