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(×10
3
)
ν
H
30
25
20
ν
A1
15
10
5
0
(a)
0
5
10
15
20
25
F2 (MHz)
22.5
20.0
17.5
ν
H
15.0
12.5
10.0
7.5
5.0
2.5
ν
A1
0.0
0.0
2.5
5.0
7.5
10.0
12.5
ν
1
(MHz)
15.0
17.5
20.5
22.5
(b)
FIGURE 9.3
Spectra of the mixture of canthaxanthin (2 mM) and AlCl
3
(2 mM) in CH
2
Cl
2
measured at 60 K
at the i eld
B
0
= 3349 G and microwave frequency 9.3757 GHz: (a) superimposed plot of a set of three-pulse
ESEEM spectra as the modulus Fourier transform and (b) HYSCORE spectrum measured with a τ = 152 ns.
(From Konovalova, T.A.,
J. Phys. Chem. B
, 105, 8361, 2001. With permission.)
1991, Konovalova and Kispert 1998, Konovalova et al. 1999, 2001a, Gao et al. 2002, 2003). It has
now been shown that the carotenoid neutral radical is not formed in the absence of UV photolysis,
and thus no resolvable methyl proton coupling of 13-16 MHz would have been observed.
The DFT calculations (Focsan et al. 2008) showed that the most energetically favorable neutral
radical produced by deprotonation of zeaxanthin radical cation, Zea
•+
, is the one resulting from the
loss of a proton from C4(4
′
)-C
α
H
2
group. In the case of violaxanthin radical cation, Vio
•+
, loss of
a C9(9
)-methyl proton produces the most energetically favorable neutral radical. The calculations
show that loss of a proton from a methyl group of position C5(5
′
′
) of the Vio
•+
, which contains an
epoxy group at the C5(5
) position, requires higher energy (~20 kcal/mol) and results in a
nonconjugated neutral allyl radical. Such a radical would exhibit large couplings (Fessenden and
Schuler 1963, Carrington and McLachlan 1967) which are easy to detect in the EPR spectrum, but
′
)-C6(6
′
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