Chemistry Reference
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no such peaks are observed. This difference between Zea and Vio in the location of proton loss has
been shown to be critical to the quenching of chlorophyll in light harvesting center (LHCII) in the
presence of excess light (Focsan et al. 2008).
The relative DFT energies
E for the neutral radicals formed by the loss of a proton from C4-C
α
H
2
,
C5-C
β
H
3
, C9-C
β
H
3
, or C13-C
β
H
3
for 7
Δ
′
-apo-7
′
,7
′
-dicyano-
β
-carotene, lycopene,
β
-carotene, and
8
0.5 kcal/mol) but higher
than zeaxanthin by approximately 2.5 kcal/mol and higher than canthaxanthin by 7.0 kcal/mol (Focsan
et al. 2008).
As a consequence of deprotonation, a change in the unpaired electron spin distribution of the
neutral radicals produces larger methyl proton hyperi ne constants (on the order of 10-16 MHz) than
for the radical cation. Examples of the unpaired spin distribution of the radical cations and neutral
radicals are depicted in Figure 9.4. It was observed that the unpaired spin density for the carotenoid
neutral radicals increases at carbons along the chain distant from the position from where the proton
was lost upon light irradiation. The
#
Vio
•
(4 or 4
′
-apo-
β
-caroten-8
′
-al are all about the same (5.0
±
0.5, 11.0
±
0.5, and 12.5
±
) radicals do not show this distri-
bution due to the presence of the epoxy group that localizes the unpaired spin density on the C4(4
′
) and
#
Vio
•
(5 or 5
′
′
)
atom and C5(5
′
), C6(6
′
) atoms, respectively (see Figure 9.4). Proton loss from the C4(4
′
)-C
α
H
2
and
)-C
β
H
3
groups of violaxanthin radical cation forms structures higher in energy than the most
stable neutral radical,
#
Vio
•
(9 or 9
C5(5
′
), and generates large couplings which are not experimentally
observed in the EPR spectrum. However, loss of a proton from the methyl group at the C9(9
′
′
)
or C13(13
) position exhibits similar hyperi ne couplings to those of zeaxanthin neutral radicals
formed by proton loss from these two positions.
′
OH
OH
O
O
HO
HO
Zeaxanthin
Violaxanthin
FIGURE 9.4
Unpaired spin distribution for zeaxanthin (left) and violaxanthin (right) radicals. From up to
down: Zea
•+
,
#
Zea
•
(4),
#
Zea
•
(5) and Vio
•+
,
#
Vio
•
(4),
#
Vio
•
(5), respectively. The black represents excess α and
the dark gray excess β unpaired spin density. (Adapted from Focsan, A.L. et al.,
J. Phys. Chem. B
, 112, 1806,
2008. With permission.)
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