Chemistry Reference
In-Depth Information
0.1
1.2 μs
2 μs
5 μs
40 μs
0.09
(a)
0.06
0.08
(a) 570 nm
(b) 720 nm
0.03
(b)
0.06
0
0
5
10
15
20
25
t/ μs
0.04
0.02
0
400
500
600
700
800
900
1000
Wavelength (nm)
FIGURE 14.7
Transient absorption spectra observed following pulse radiolysis of CAN and formate in
argon-saturated aqueous 2% TX-100 (pH = 7.1). Inset: Kinetic traces of CANH
•
at 570 nm and CAN
•−
at
720 nm, showing the decay of the radical anion and concomitant formation of the neutral radical.
14.4
REACTIVITY OF CAROTENOID RADICALS
14.4.1 I
NTERACTION
WITH
O
XYGEN
Carotenoid radical anions contrast with radical cations in that they have been shown to react with
oxygen at diffusion-controlled rates (Conn et al. 1992) whereas the radical cations do not react with
oxygen (Dawe and Land 1975) at all.
For the neutral addition radicals of carotenoids, with acylperoxyl radicals, it was shown
(El-Agamey and McGarvey 2003) that no reaction could be observed with up to 0.01M oxygen,
giving an upper limit of ≈10
5
M
−1
s
−1
for the rate constant. However, more recently, the same authors
(El-Agamey and McGarvey 2005) have reported a reversible oxygen addition to a neutral carbon-
centered carotenoid addition radical from the reaction of carotenoids with phenylthiyl radicals. In
the absence of oxygen, these radicals decay over hundreds of milliseconds, and the decay was
shown to increase with the addition of oxygen. For PhS-77DH
•
, the rate of oxygen addition was
shown to be 4.3 × 10
4
M
−1
s
−1
, that is, below their previously suggested limit. This work has been
recently extended (El-Agamey and McGarvey 2007) to a wide range of carotenoid-phenylthiyl
addition radicals leading to the rate constants of 0.32-4.3 × 10
4
M
−1
s
−1
.
14.4.2 I
NTERACTION
WITH
O
THER
C
AROTENOIDS
14.4.2.1 Radical Anions
In hexane,
-carotene (DECA) were
studied and Table 14.7 gives the electron transfer second-order rate constants for various pairs, with
β
-CAR, LYC, septareno-
β
-carotene (SEPTA), and decapreno-
β
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