Chemistry of Lipid Oxidation

1
Chemistry of Lipid Oxidation
2
Molecular Orbital of Triplet Oxygen
Molecular
?
Atomic
Atomic
?
?
?
?
2Px 2Py 2Pz
2Pz 2Py 2Px
?
?
Energy
2S
2S
?
3
Mechanism of Autooxidation
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
Metal Energy Reactive oxygen species
- ? H
Initiation
13 12 11 10 9
(
C
)
C
H
C
H
R
H
C
H
C
H
C
H
C
H
C
H
4
3
2
?
2
K109/sec
3O2
E0 600mv
13 12 11 10 9
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
2
4
3
2
(K 10o M-1sec-1)
O
Propagation
E01000mv
R.
? H from RH
O
?
4
13 12 11 10 9
C
H
C
H
C
H
C
H
C
H
C
H
R
(
C
H
)
C
H
2
2
4
3
O
Hydroperoxide Decompositin
O
- ? OH
E02300 mv
H
13 12 11 10 9
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
3
2
2
4
O
?
E01600 mv
C
H
(
C
H
)
C
HO
3
2
4
Termination
C
H
(
C
H
)
C
H
3
3
2
3
5
Mechanisms of Oxidation
Initiation
RH

R
H


R
O


ROO

2
Propagation
ROO

R
H
ROOH

R



1
1
Termination

R
R


RR
O




R
O
O
R
O
O
R
O
O
R
2

R
O
R


R
O
R
R
O
O


R

R
O
O
R
O

2RO
R
O
O

2
R
O
O
R

2

2
6
Oxidation of Mono-Olefines
Oleic acid - 4 Hydroperoxides
Double bond shifts to
7
Hydroperoxides from Linolenate
8
Peroxide Decomposition
9
Effects of Metal on Peroxide Decomposition



-
C
u
u
R
O
O
H
R
O
O
H
C






C
u





H
R
O
O
C
u
R
O
O
H


-
H
O
H


R
O
R
O
O


R
O
O
H
2

H
O
2
10
Decomposition of Hydroperoxide
O
H
H
H
H
H
H
C
C
C
C
R
C
C
C
H
H
O
B
O
A
H
O
O
C
C
C
H
2
O
H
H
C
C
H
3
H
C
H
C
H
C
H
C
H
R
O
H
H
H
H
2
2
C
C
C
R
C
C
H
11
Lactones in butter flavor
Important lactones in butter are d-decalactone d-d
odecalactone d-tetradecalactone 5-20 ppm The
lactones have coconut-like flavor which is
desirable in molten butter, undesirable in fresh
butter and dry whole milk. fresh butter
content of lactones is low heated
butter lactone increases
12
Lactones come from d-hydroxy acids in milk
13
Lactone Formation
C
O
O
H
(
C
H
)
C
H
C
O
C
H
R
2
2
O
g-(d)-
a-
lactone
carboxyl-
odorless, well crystalized
C, decarboxylated to lactone
at 80120
14
Molecular Orbital of Singlet Oxygen
Molecular
?
Atomic
Atomic
?
?
?
?
2Px 2Py 2Pz
2Pz 2Py 2Px
?
Energy
?
2S
2S
?
?
1S
1S
?
15
Excitation and Deactivation of Photosensitizer
1
Excited state
Sen
x
8
k 25
10
/sec
ISC
3
Sen
x
8
n
k 2
10
/sec
h

3

O
2
4
k 10 - 10
/sec
x
9
k 3
10
/sec
1
Sen
Ground state
Singlet oxygen formation
16
Excitation and Deactivation of Photosensitizer
Excited State
1Sensitizer
K1- 20?108/sec
ISC
Fluorescence K 2?108/sec
3Sensitizer
hv
Energy
K1- 3?109/sec
Phosphorescence
K10-104/sec
3O2
1Sensitizer
Singlet Oxygen
Ground State
17
Type 1 and 2 Mechanisms
n
h
ISC
3Sen
Sen
1Sen


RH
K1
3O2


R

Sen H
K2
3O2
1O2
RH
3O2
ROOH
O2- Sen
ROOH
Photooxidation process (RH Substrate K1 1
- 3 ? 109 /M ?sec. K2 lt 107 /M ?sec)
18
Production of 1O2 by Photochemical, Chemical,
and Biological Systems
19
Reactions of Singlet Oxygen with Various Types of
Double Bonds
1,4- Cycloaddition
ENE Reaction
1,2 Cycloaddition
20
Reaction of Singlet Oxygen with Double Bond
R
R
R
'
O
R'
O
O
H
H
O
I
II
21
Conjugated and Nonconjugated Hydroperoxides
Arising via the 6-Centered Transition state
22
Reversion Flavor
15 14 13 12 11 10
9 8
1
O
2

15 14 13 12 11 10
9 8
O

15 14 13 12 11 10
9 8
C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
O
H
(
C
H
)
C
H
C
H
2
2
2
2
6
O


23
4-Keto-6-nonenal Formation from Linolenic Acid
24
Mechanism for 4-Keto-5-nonenal Formation from
4-Keto-6-nonenal
25
Formation of 2-(1-Pentenyl)furan from
4-Keto-5-nonenal and 2-(2-Pentenyl)furan from
4-Keto-5-nonenal
26
Formation of 2- Pentyl furan
27
Formation of 2- Pentyl furan
28
Enzymatic lipid oxidation (Lipoxygenase)
Detrimental effects. a. Destruction of the
essential fatty acids. b. The free radicals
produced damage other compounds including
vitamins and proteins. c. Development of
off-flavor and odor in beans and peas ? a
hay-like flavor.
29
Specificity of Lipoxygenase

• a cis, cis-Penta-1,4-Diene Unit
  (-CHCH-CH2-CHCH-)
• - Methylene group of the Penta-1,4-Diene Unit to
  be in the ?-8 position.

30
Mechanism of Action

• The enzyme forms a stereospecific complex with
  the unsaturated fatty acid.
• 2. The enzyme abstracts either an electron or a
  hydrogen atom stereospecifically from the ?-8
  position producing a free radical at ?-8 of the
  fatty acid.
• 3. While still attached to the enzyme, the fatty
  acid free radical isomerizes to place the
  unshared electron at ?-8 causing conjugation and
  isomerization of the double bond.
• 4. O2 reacts with the free radical at ?-6 to give
  a peroxy free radical.
• 5. A hydrogen from the medium forms the
  hydroperoxide which then dissociates
  from the enzyme.

31
Mechanism of Lipoxygenase Oxidation
m
32
Hydroperoxide Formation
m
33
Aldehyde and Alcohol Formation in Tomato from
Linolenic and Linoleic Acid.
mm
Linolenic Acid C
14
O
Lipoxygenase
2
O

H
C
C
C
C
C
C
H
cis-3-hexenal
AOR
AOR
O
C
C
C
C
C
C
H
n-
hexanal
H
AOR
C
C
C
C
C
O
H
H
cis - 3 - hexenol

34
Aldehyde and Alcohol Formation in Tomato from
Linolenic and Linoleic Acid
m
Linoleic Acid C14
35
Ethyl vinyl ketone
Isolated and identified in raw soybean
O
H
C
C
H
C
C
H
C
H
( raw beany, grasoy )
3
2
2
36
Ethyl vinyl ketone
H
C
C
H
C
H
C
H
C
H
C
H
C
H
C
O
O
H
CH
3
2
2
O
O
H
H
C
C
H
C
H
C
H
C
H
3
2
2
C
H
C
H
H
C
C
H
C
H
2
3
2
O
, RH
2
C
H
C
H
C
H
C
H
H
C
2
2
3
O
O
H
C
H
C
H
C
H
C
H
C
2
2
3
O
37
(No Transcript)