Properties of Carotenoid Radicals and Excited States and Their Potential Role in Biological Systems - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 300

transfer second-order rate constants to be determined (see Table 14.9). These pulse radiolysis kinetic
studies show LYC efi ciently quenches the radical cations of all the XANs studied, whereas -CAR

reduces only ASTA •+ , CAN •+ , and APO •+ (XANs containing carbonyl groups) and give an order for
the ease of electron transfer as shown in Figure 14.11.
It is interesting that CAR •+ arising from the three carotenoids present in the human macular
(LUT, ZEA, and MZEA [mesozeaxanthin]) are all repaired efi ciently by LYC but not by

β

-CAR.
The retina is the only organ in the human body, which is continually exposed to the high levels of
focused radiation and is in a highly oxygenated environment and this combination means there is
a high likelihood of oxy-radical and 1 O 2 generation. LUT, ZEA, and MZEA all contain terminal
hydroxyl groups, and, as discussed in Section 14.2, this allows them to span membranes. If this is
the case, then those XAN containing hydroxyl groups will probably be more accessible to species
in the extracellular environment, such as vitamin C, which may be able to regenerate these XAN
from their radical cations.
The retina does not contain high concentrations of hydrocarbon carotenoids but Mares-Perlman
et al. (1995) have shown a correlation between age-related macular degeneration and low levels of
serum LYC and this apparent contradiction is discussed in Section 14.5.

β

TABLE 14.9
Bimolecular Rate Constants
for Electron Transfer between
Carotenoid Pairs (CAR 1 •− + CAR 2
Æ CAR 1 + CAR 2 •− )

Rate Constant (±10%)
(×10 9 M −1 s −1 ) for Reaction
with CAR 2

CAR 1 •+ LYC b -CAR ZEA

ASTA •+ 9.2 8.0 4.6

APO •+ 11.2 6.3 7.7

CAN •+ 7.9 4.8 <1.0

LUT •+ 5.2 <1.0 <1.0

MZEA •+ 7.8 <1.0 —

ZEA •+ 6.9 <1.0 —

Note: All other pairs and back reactions have
rate constants <1 × 10 9 M −1 s −1 .

ASTA •+ /APO •+ β-CAR •+

LUT •+ CAN MZEA/ZEA LYC

MZEA •+ /ZEA •+

CAN •+

LYC •+ ASTA/APO LUT β-CAR

Decreasing E (CAR •+ /CAR)

FIGURE 14.11 Relative ordering of the one-electron reduction potentials (E(CAR •+ /CAR)) of several caro-
tenoid radical cations in benzene.