Mechanisms of Singlet Oxygen Oxidation and

1

Mechanisms of Singlet Oxygen Oxidation and
Nutritional and Flavor Quality of Food
David B. Min Department of Food Science and
Technology The Ohio State University Columbus,
Ohio USA
2
Challenging Research Areas in Food Chemistry

• Soybean Oil Reversion Beany Flavor Formation
• Sunlight Flavor Formation in Milk
• Mechanisms of Riboflavin Degradation under Light

3
1772 Scheele Priestley Discovered
Oxygen 1811 Avogadro Oxygen is a Diatomic
Molecule 1848 Faraday Oxygen is a
Paramagnetic Molecule 1934 Herzberg
Observation of Singlet Oxygen 1964 Foote
Wexler Rediscovered Singlet Oxygen  
               
Oxygen History
4
Food Oxidation

• Triplet oxygen oxidation
• Singlet oxygen oxidation

5
Molecular Orbital of Triplet Oxygen
Molecular
?
Atomic
Atomic
?
?
?
?
2Px 2Py 2Pz
2Pz 2Py 2Px
?
Energy
?
2S
2S
?
6
Lipid Oxidation
14 13 12 11 10 9
C
H
C
H
C
H
C
H
C
H
C
H
C
H
R
(
C
H
)
C
H
2
2
2
2
3
3
Initiation
Metal Reactive oxygen species
H
Preventive Antioxidants
13 12 11 10 9
(
C
H
)
C
H
C
H
R
C
H
C
H
C
H
C
H
C
H
2
4
3
2
?
E0 600mV
3O2
K109 M-1sec-1
7
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
2
4
3
2
O
3O2
.
H from RH (Lipid)
R. ROO
O
Propagation
?
K 100 M-1sec-1
E01000mV
.
Radical Scavenging Antioxidant
O
O
H
.
C
(
C
H
)
(
C
H
)
C
H from
3
3
3
3
O
C
H
O
C
H
3
3
E0 400mV
K 107 M-1sec-1
8
C
H
C
H
C
H
C
H
C
H
C
H
R
(
C
H
)
C
H
2
2
4
3
O
OH
?
E02300 mV
O
H
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
3
2
2
4
O
E01600 mV
?
Termination
C
H
(
C
H
)
C
HO
3
2
4
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
3
2
2
4
O
O
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
3
2
2
4
9
Molecular Orbital of Singlet Oxygen
Molecular
?
Atomic
Atomic
?
?
?
?
2Px 2Py 2Pz
2Pz 2Py 2Px
?
Energy
?
2S
2S
?
?
1S
1S
?
10
Reaction of 3O2 and 1O2 with Linoleic Acid
.
OOH
3O
3O
2
2
R
R
2
R
1
R
R
2
R
1
2
1
OOH
1O
O
2
R
R
R
2
1
R
H
O
2
1
OOH
gtgtgt
1O
3O
2
2
R
R
2
1
11
Photosensitizers in Foods

• Chlorophyll
• Protoporphyrine
• Riboflavin

12
Chlorophyll
H
C
H
H
C
R
2
C
H
C
H
H
C
2
3
3
N
N
M
g
H
H
N
N
C
H
3
H
C
3
O
C
O
C
H
2
3
C
H
2

C
O
2
C
H
C
H
C
H
C
H
3
3
3
3
13
Riboflavin
C
O
H
H
2
C
H
O
H
C
H
O
H
C
H
O
H
C
H
2
C
H
O
N
N
3
H
N
N
C
H
3
O
14
Singlet Oxygen Formation by Photosensitizer
Excited State
1Sen
K1- 20?108/sec ISC
Fluorescence K 2?108/sec
3Sen
hv
K1- 3?109/sec
Phosphorescence
3O2
K10- 104/sec
1Sen
Singlet Oxygen
Ground State
15
Singlet Oxygen Determination by Trapping
O
O
H3C
CH3
1O2
H3C
CH3
N
N
CH3
H3C
H3C
CH3
H
O

Tetramethyl-4-Piperidone-N-Oxyl
Tetramethyl-4-Piperidone
(TAN)
(TMPD)
16
Electron Spin Resonance Spectrum of
Tetramethyl-4-Piperidone-N-Oxyl
3370 G 3390 G
3410 G
17
Singlet Oxygen Oxidation of Soybean Oil
18
Soybean Oil Reversion Beany Flavor Compounds
C
H
C
H
C
H
C
H
C
H
3
2
2
O
2-(1-pentenyl)-furan
C
H
C
H
C
H
C
H
C
H
3
2
2
O
2-(2-pentenyl)-furan
19
Soybean Oil Purification by Chromatography
Soybean oil
Silicic acid
Sugar
Charcoal-Celite
Silicic acid
Vacuum
Purified soybean oil
20
Identified Compounds in Soybean Oil

• Phospholipids
• Tocopherols
• Monoglycerides
• Diglycerides
• Oxidized triglycerides
• Free fatty acids
• Chlorophylls

21
Purified Soybean Oils
22
Light Box for Soybean Oil Flavor Study
23
Effect of 0 and 5 ppm Chlorophyll on
2-Pentenylfurans in Soybean Oil
0 ppm
             
No beany flavor
5 ppm
2-pentenylfuran
trans-2-heptenal

Strong beany flavor
24
Mass Spectrum of 2-Pentenylfuran
25
2-Pentenylfuran from Linolenic Acid by 1O2
26
2-(2-pentenyl)-furan
C
H
C
H
C
H
C
H
C
H
3
2
2
O
27
Chlorophyll Effect on the Volatile Compounds of
Soybean Oil under Light at 10oC
Headspace Volatile Content (electronic
unit)
Storage in Hours
28
Carotenoids
CHO
Canthaxanthin
29
Effects of ?-Apo-8-Carotenal, ?-Carotene or
Canthaxanthin on Soybean Oil Oxidation
Peroxide Value (meq/kg oil)
Storage Time (hr)
30
Effects of ?-Apo-8-Carotenal, ?-Carotene or
Canthaxanthin on Soybean Oil Oxidation
Peroxide Value (meq/kg oil)
Storage Time (hr)
31
Diagram for Singlet Oxygen Soybean Oil Oxidation
Q Carotenoids
A
hv
3O2
ISC
Sen
1O2
1Sen
AO2
3Sen
kr
(Chlorophyll)
Q
Q
kQ
Q
kd
kQ
Sen
kox-Q
Sen
3O2
3O2
QO2
dAO2
Ko3O2
k rA
K

Ko3O2kQQ
k rA (kox-Q kQ)Q kd
dt

32
Steady State Kinetic Equation for Singlet Oxygen
Soybean Oil Oxidation
If carotenoids quenched singlet oxygen , the
equation is
dAO2
Ko3O2
k rA
K

Ko3O2kQQ
k rA (kox-Q kQ)Q kd
dt
kQQ0
dAO2

Ko3O2
k rA
K

Ko3O2
k rA (kox-Q kQ)Q kd
dt
Plot of AO2 -1 vs. A -1 at various Q
33
Singlet Oxygen Quenching
-1
K
Slope
Q4
?
1/AO2
?
?
Q3
Intercept K-1
?
?
Q2
?
?
?
1 K
?
Q1
?
?
?
?
1/A
34
Steady State Kinetic Equation for Singlet Oxygen
Soybean Oil Oxidation
If carotenoids quenched triplet chlorophyll ,
steady state kinetic equation is
dAO2
Ko3O2
k rA
K

Ko3O2kQQ
k rA (kox-Q kQ)Q kd
dt

(kox-Q kQ)Q 0
dAO2
Ko3O2
k rA
K

Ko3O2kQQ
k rA kd
dt
Plot of AO2 -1 vs. A -1 at various Q
35
Triplet Sensitizer Quenching
Q3
?
Q2

?

1/ AO2
?
Q1
?

?
?
?

Slope K-1kd(ko3O2 kQQ) /
ko3O2kr Intercept K-1(ko3O2 kQQ) /
ko3O2
?
1/A
36
Quenching Mechanism of b-Carotene
1/ Peroxide Value ( 1/ M )
37
Total Quenching Rate Constant (kqkox-Q) of
Singlet Oxygen by Carotenoids
Carotenoids Number of Conjugated
Quenching Rate Double Bonds
Constants ( M-1 sec-1) ? -
apo-8- Carotenal 10
2.86 ? 109 ? -
Carotene 11
4.60 ?
109 Canthaxanthin 13
1.12 ?
1010

38
Singlet Oxygen Quenching Mechanism of Carotenoid
Energy Transfer
1O2 1?-Carotene 3O2 3?-Carotene
3 ?-Carotene
1?-Carotene
Radiationless Transfer
39
Singlet Oxygen Oxidation of Milk
40
Sunlight Flavor Formation in Milk
C
H
S
S
C
H
3
3
41
Milk Sample Bottle in Light Box
42
Light Box for Milk Flavor Study
43
Electron Spin Resonance Spectrum of Milk during
Light Exposure
0 Minutes
10 Minutes
3370G
3390G
3410G
44
Storage Effect on Volatiles of Milk under Light
D
D
D
45
Mass spectra of GC Peak D and Dimethyl Disulfide
46
Effect of Ascorbic acid on Dimethyl Disulfide
Formation in Milk
47
Mechanism of Dimethyl Disulfide Formation from
Methionine by Singlet Oxygen
C
O
H
O

O
O
C
O
O
H

1
O
2
N
H
C
H
C
H
C
H
S
C
H
N
H
C
H
C
H
C
H
S
C
H
2
2
3
2
2
2
3
2
O
C
O
O
H
O
H
O
C
O
O
H

N
H
C
H
C
H
C
H
S
C
H
O
H
N
H
C
H
C
H
C
H
S
C
H
2
2
3
2
2
3
C
O
O
H
O
C
O
O
H
N
H
C
H
C
H
C
H
O

S
C
H
N
H
C
H
C
H
C
H
S
C
H
2
2
3
2
2
3
C
H
S
S
C
H
S
C
H
2
3
3
3
48
Riboflavin Free Milk by Chromatography
Fluorosil
Vacuum
Riboflavin Free Milk
49
Electron Spin Resonance Spectrum of Riboflavin
Free Milk after Light Exposure
0 Minutes
20 Minutes
50
Mechanisms and Kinetics of Riboflavin
Photodegradation by Singlet Oxygen
51
Effects of Light on the Riboflavin ()
52
HPLC Chromatograms of Riboflavin at pH 6.5 Under
Light
Riboflavin
Riboflavin
53
Mass Spectrum of Riboflavin Photodegradation
Product
54
HPLC Chromatogram of Riboflavin at pH 8.5 Under
Light
2 hours at pH 8.5 under light
Lumichrome
New peak at pH 8.5
Riboflavin
Lumiflavin
55
Diagram for Riboflavin Degradation by
Photosensitization Reaction
KISC
3O2
RF
hv
3RF
1O2
RF
1RF
RFO2
kr
ko
kq
Q
Q
kQ
kd
Q
Kox-Q
QO2
RF
3O2
3O2
56
Lumichrome and Lumiflavin from Riboflavin
R
H
O
H
H
H
C
2
O
H
H
C
N
N
O
3
H
C
N
O
N
3

C
H
R
C
2
N
H
N
H
C
N
H
3
N
H
C
3
O
O
L
C
pH is acidic or neutral
R
R
H
O
H
O
H
H
C
H
H
C
2
2
H
C
N
N
O
H
C
N
N
O
3
h
v
3
I
S
C
1
R
F

N
H
N
H
N
H
C
N
H
C
3
3
O
O
3
R
F
pH is basic
1
R
F
H
O
R
C
H
3
H
H
C
H
C
N
N
O
H
2
3
H
C
N
N
O
O

C
3
R
N
H
N
H
C
3
N
H
N
H
C
O
3
L
F
O
57
Diagram for Riboflavin Degradation by
Photosensitization Reaction
KISC
3O2
RF
hv
3RF
1O2
RF
1RF
RFO2
kr
ko
kq
Q
Q
kQ
kd
Q
Kox-Q
QO2
RF
3O2
3O2
58
Formation of 2, 3-Butanedione from Riboflavin by
Singlet Oxygen
R
R
H
H
C
H3C
C
H3C
N
N
N
N
O
O
1O2
10
1
O
9
2
8
O
7
3
H3C
N
H
N
H
5
6
H3C
4
C
C
N
N
H
H
O
O
Riboflavin
R
R
H
H
H3C
C
H3C
C
N
N
O
N
N
O
O
O
1O2
O
O
O
O
O
O
N
H
N
H
C
H3C
H3C
C
N
N
H
H
O
O
R
O
CH3
C
H
N
N
O
C
Scission
O

O
C
N
H
C
H
CH3
N
O
O
2, 3-Butanedione
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