Chemistry Reference
In-Depth Information
TABLE 14.11
Second-Order Rate Constants for the
Quenching of Carotenoid Radical
Cations by Tyrosine and Cysteine in
Detergent Micelles
k
(×10
5
M
−1
s
−1
)
CAR
•+
Tyrosine
Cysteine
0.22
16.9
β
-CAR in TX-100
CAN in TX-100
0.41
10.9
ZEA in TX-100
0.81
7.8
ASTA in TX-100
0.94
10.1
There is evidence that carotenoid radical cations can persist for up to one second in some micel-
lar environments (Burke et al. 2001a) so that, in the absence of a “repair” process, the radical may
survive until it reacts with another biomolecule.
For tryptophan, there seems to be an equilibrium, which could also lead to some tryptophan radi-
cal formation and subsequent protein damage:
•
+
•+
TrpH
+
CAR
TrpH
+
CAR
(14.14)
The studies of this equilibrium as a function of pH enabled the estimation of the absolute one-
electron reduction potentials of CAR
•+
in an aqueous micellar environment (Edge et al. 2000, Burke
et al. 2001b) (see Table 14.12 for typical results). As can be seen, the potentials of all the dietary car-
otenoid radical cations are very similar but LYC
•+
has the lowest potential implying that it is the best
carotenoid antioxidant against free radicals (of course, this is an oversimplii cation, see above).
14.5 BIOMEDICAL CONSEQUENCES
A number of speculations can arise from the photo-physical data presented earlier. The major caro-
tenoids that protect the macular are the XANs, ZEA, and LUT, yet, at least in the model membranes
studies, these are rather poor
1
O
2
quenchers (compared, e.g., with
β
-CAR and LYC). However, free
TABLE 14.12
One-Electron Reduction Potentials for CAR
•+
CAR
•+
E
0
(mV) ± 25 mV
1060
β -CAR in TX-100
CAN in TX-100
1041
ZEA in TX-100
1031
ASTA in TX-100
1030
1028
β -CAR in TX-405/TX-100
LYC in TX-405/TX-100
980
Note:
Solubility problems require mixed micelles for LYC.
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