Quenching Mechanisms of Singlet Oxygen Oxidation

1
Quenching Mechanisms of Singlet Oxygen Oxidation
2
Effect of Chlorophyll on the Volatile Compounds
of Soybean Oil under Light at 10oC
Volatile Content (EU)
Storage in Hours
3
Carotenoids
C
H
O
Apo - 8'- carotenal
b -
b -
Carotene
O
Canthaxanthin
O
4
Effects of ?-apo-8-carotenal, ?-carotene or
canthaxanthin on the peroxides of soy oil
containing 3 ppm chlorophyll
Peroxide Value (meq/kg oil)
Storage Time (hr)
5
Effects of ?-apo-8-carotenal, ?-carotene or
canthaxanthin on the peroxides in soybean oil
containing 3 ppm chlorophyll
Peroxide Value (meq/kg oil)
Storage Time (hr)
6
Kinetic Equation for the Formation of Product AO2
n
h
O
A
Sen
1
AO
2
3
1
Sen
Sen
O
2
kisc
2
k
k
r
0
kQ
Q
Q
k
k
q
d
Sen
Sen
Q
k
ox
-Q
3
O
3
O
2
QO
2
2
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Definition of Steady State Approximation
Multistep reaction mechanisms usually involve one
or more intermediate species that do not appear
in the overall equation. Frequently, these
intermediate species are very reactive and
therefore do not accumulate to any significant
extent during the reaction. It is, therefore,
frequently a good approximation to take d1/dt
0, where 1 is intermediate. This is the steady
state (or Stationary State) approximation.
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Definition of Rate Constant
Rate Constant (k) is indication of reaction
rate. The unit for the k If the reaction is
first-order reaction, the unit is sec-1 If the
reaction is second-order reaction, the unit is
M-1sec-1.
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Schematic Diagrams of Formation of Product AO2
n
h
O
A
Sen
1
AO
2
3
1
Sen
Sen
O
2
kisc
2
k
k
r
0
kQ
Q
Q
k
k
q
d
Sen
Sen
Q
k
ox
-Q
3
O
3
O
2
QO
2
2
kisc 1Sen - kQ 3Sen Q - ko 3O23Sen
0 ----- (1) ko3O23Sen -
kr1O2A - kqQ1O2 - kox-Q 1O2Q -kd
1O2 0 ---- (2) kr A1O2
dAO2/dt ---------- (3)
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From equation (1) kisc1 Sen (kQQ
ko3O2) 3Sen 3Sen kisc 1Sen/
(kQQ ko 3O2) --------- (4) From
equation (2) ko3O23Sen 1O2(krA
kq Q kox-Q Q kd) 1O2
ko3O23Sen/(krA kqQ kox-QQ kd)
------(5) From equation (4) and (5) 1O2
ko 3O2 kisc 1Sen/(kQQ ko3O2)/ (krA
kq Q kox-Q Q kd) ------- (6)
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From equations (3) and (6) dAO2/dt kr A
1O2 dAO2/dt krA ko3O2 kisc1Sen
/ (kQQ k0 3O2) / (krA kqQ
kox-QQ kd) kisc1Sen
k03O2 / (kQQ k03O2 ) krA / (krA
kqQ kox QQ kd) When kisc1Sen
K When inverse the denomination and nomination
of the equation
dAO2/dt Kk03O2 / (kQQ k03O2 )
krA / (krA kqQ kox-QQ kd)
A (
k
k
k
)Q
k

k
3O
k
Q
r
q
d
ox-Q
0
Q
2
-1
-1

dAO
/
K
dt
2
k
A
k
3O

r
0
2
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Singlet Oxygen Quenching only
A (
k
k
k
)Q
k

k
3O
k
Q
r
q
d
ox-Q
0
Q
2
-1
-1

dAO
/
K
dt
2
k
A
k
3O

r
0
2
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Singlet oxygen quenching only
(k
k
)Q k
ox-Q
q
d
-1
-1
1
dAO
/dt
K
2
k
A
r
-1
-1
Plots of AO

versus A
at various Q
2
-1
Constant Intercept K
-1
Slope K
k
(k
k
)Q/k
d
ox-Q
q
r
Therefore, Slope/Intercept k
(k
k
) Q/k
d
ox-Q
q
r
Plots of Slope/Intercept vs. Q
Intercept k
/ k
d
r
Slope (k
k
) / k
r
q
ox-Q
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Singlet Oxygen Quenching

Q5


Q4

Q3



1/AO
2
Q2

K-1

Q1

1/A


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Plot of Slope/Intercept(S/I) vs. Concentration of
QuenchersQ
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Triplet Sensitizer Quenching only
A (
k
k
k
)Q
k

k
3O
k
Q
r
q
d
ox-Q
0
Q
2
-1
-1

dAO
/
K
dt
2
k
A
k
3O

r
0
2
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Triplet sensitizer quenching only, equation
becomes
k
Q
k
Q
d
-1
-1
1
1
dAO
/dt
K
2
k
A
k
O

r
0
2
-1
-1
Plots of AO

vs. A
at different Q.
2
Slope/Intercept k
/k
d
r
The ratio of slope to the intercept of this plot
is independent of Q.
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Triplet Sensitizer Quenching
1/A
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Quenching Mechanism Determination of b-Carotenene
1/ Peroxide Value ( 1/ M )
Intercept K-1 Singlet Oxygen Formation (K)
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Determination of singlet oxygen oxidation rate
with soybean oil and b-carotenal singlet oxygen
quenching rate
0.115 Kd / kr Kd 1.1x104sec-1 in methylene
chloride
(kq kox-Q)/kr 0.296 x 105
kq kox-Q 0.296 x 105 x 1.0 x105 M-1 Sec-1
Kr1.0x105 M-1 Sec-1
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QUENCHING EFFECT OF ?-CAROTENE
1O2 1?-CAROTENE 3O2
3?-CAROTENE 3 ?-CAROTENE
1 ?-CAROTENE
RADIATIONLESS
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Quenching Mechanism Determination of b-Carotenene
1/ Peroxide Value ( 1/ M )
Intercept Singlet oxygen Formation (K-1)
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Determination of singlet oxygen oxidation rate
with soybean oil and b-carotenal singlet oxygen
quenching rate
0.115 Kd / kr Kd 1.1x104sec-1 in methylene
chloride
(kq kox-Q)/kr 0.296 x 105
kq kox-Q 0.296 x 105 x 1.0 x105 M-1 Sec-1
Kr1.0x105 M-1 Sec-1
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Homework

• Figure 1. Effects of different concentrations of
  a
• tocopherol on the 1/headspace oxygen of 0.03,
  0.06, 0.10
• and 0.16 M soybean oil in methylene chloride
  containing
• 3.3 x 10-9 M chlorophyll during light storage.
• Figure 2. Relation of slope/intercept of the
  plots of
• 1/headspace oxygen vs 1/soybean oil (Figure 1) to
  the
• concentraiton of a-tocopherol.

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Homework

• 1. What is the antioxidant mechanism of
  a-tocopherol in the chlorophyll photosensitized
  oxidation of soybean oil? Justify your answer.
• 2. What is the rate of singlet oxygen formation
  (K) in the sample bottle per hour if the average
  value of y-intercept of Figure 1 is 0.12?

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Homework

• 3. Kd of the singlet oxygen in methylene chloride
  is 1.1 x 104 sec-1. What is the rate constant
  (Kr) of singlet oxygen oxidation of soybean oil?
• 4. What is the total singlet oxygen quenchuing
  rate constant (Kq Kox-Q) of a-tocopherol?

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Singelt Oxygen Formation,and Tocopherol
Quenching Mechanism and Rate The effect of 0,
1.0, 2.0, or 4.0 (x10-3M) a-tocopherol on the 3.3
x 10-9 M chlorophyll sensitized photooxidation
of 0.03, 0.06, 0.10 and 0.16 M soybean oil in
methylene chloride was studied. Ten mL of oil
sample was transferred into 30 mL serum bottle.
The bottle was sealed with Teflon coated septa
and aluminum caps and placed in the light box.
The samples were held 8 hrs and evaluated by
measuring headspace oxygen content every hour.
The depleted oxygen in the headspace is singlet
oxygen according to previous studies.
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Homework

• What is the rate of singlet oxygen formation (K)
  in the sample bottle per hour if the average
  value of y-intercept of Figure 1 is 0.12?