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be increased compared to symmetrical carotenoids. This enhanced solvent interaction will lower
the energy of the triplet state, making energy transfer from
1
O
2
to the carotenoid faster.
As noted earlier, environments such as water/methanol mixtures are useful models of membrane
environments. These mixed solvents lead to a reduced efi ciency of
1
O
2
quenching and the quench-
ing becomes negligible at high water concentrations. Figure 14.2 shows an example of this behavior
for zeaxanthin (ZEA), as the aggregation of ZEA is increased.
At 70% methanol (30% D
2
O), very little quenching is observed and this correlates with the
formation of a new band in the ground state spectrum in methanol/water mixtures as shown in
Figure 14.3.
In general, when water is added to homogeneous organic solutions containing carotenoids, spec-
tral changes indicate that carotenoid aggregation occurs. The absorption band attributed to the
monomer decreases with the addition of water (>15%) with the concomitant increase in a new
absorption band at lower wavelength attributed to a carotenoid dimer/aggregate. The spectral shift
of the carotenoid dimer/aggregate to shorter wavelength is attributed to exciton coupling interac-
tions. This splitting leads to a forbidden lower energy transition and an allowed higher energy
transition leading to a blue shift. Overall, the stacking of carotenoids occurs in order to reduce the
exposure of the hydrophobic system to the polar aqueous environment.
Cantrell et al. (2003) studied the quenching of
1
O
2
by several dietary carotenoids in dipalmitoyl
phosphatidylcholine (DPPC) unilamellar liposomes. These workers used water soluble and lipid
soluble
1
O
2
sensitizers so that a comparison of the efi ciencies of quenching
1
O
2
generated within
and outside the membrane model could be made. Perhaps surprisingly there was little difference
in the efi ciency of quenching in either situation. Typical results are presented in Table 14.3 (taken
from Cantrell et al. (2003 and 2006)).
This implies that the rate-determining step is the migration of the
1
O
2
through the membrane
rather than through the water to the membrane surface. However, as can be seen, there was a marked
difference in the behavior of the different dietary carotenoids with all-
trans
-
-CAR)
and lycopene (LYC) being the most efi cient and the XANs, especially lutein (LUT), being rather
inefi cient. For ZEA, a pivotal XAN in the protection of the macular, a particularly unexpected
result was reported. It is instructive to compare
β
-carotene (
β
β
-cryptoxanthin (
β
-CRYP), with only one terminal
hydroxyl group, and ZEA, with two such groups.
3.0
1
O
2
decay in the absence of ZEA
1
O
2
decay in the presence of 10 μM ZEA
2.5
2.0
1.5
1.0
0.5
0.0
0
10
20
30
40
50
D
2
O (%)
FIGURE 14.2
The effect of increasing D
2
O (inducing zeaxanthin aggregation) on the singlet oxygen deac-
tivation efi ciency of zeaxanthin.
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