Chemistry Reference
In-Depth Information
ENDOR spectrum exhibited chlorine and nitrogen splittings indicating a carotenoid-quinone radi-
cal adduct formation.
9.6 SPIN TRAPPING EPR METHOD
Spin trapping EPR technique and UV-Vis spectroscopy have been used (Polyakov et al. 2001b) to
determine the relative rates of reaction of carotenoids with
•
OOH radicals formed by the Fenton
reaction in organic solvents. The Fe
3+
species generated via the Fenton reaction
2
+
3
+
•
−
Fe
+
H O
→+
Fe
OH
+
OH
(9.23)
22
can oxidize (Polyakov et al. 2001c) the carotenoid to Car
•+
. At low concentrations of H
2
O
2
(1 m M),
the generated
•
OH radical reacts with the solvent DMSO to produce
•
CH
3
(Figure 9.1). At increas-
ing H
2
O
2
concentration (1-10 mM), the
N-tert
-butyl-
-phenylnitrone (PBN) spin adducts of both
•
OH and
•
CH
3
radicals appear in the EPR spectrum (Figure 9.1) (Polyakov et al. 2001b). At high
concentration of H
2
O
2
(50 0 m M), only
•
OOH radicals were detected. Use of EPR along with optical
absorption spectroscopy has demonstrated that the scavenging ability of carotenoid toward
•
OOH
increases with its oxidation potential (Figure 9.2) (Polyakov et al. 2001b). In Scheme 9.3 are listed
the carotenoids for which the PBN-
•
OOH adduct was formed.
α
(2)
500 mM
(1)
(3)
10 mM
(3)
1 mM
3360
3380
3400
3420
Magnetic field (gauss)
FIGURE 9.1
EPR spectra of spin adducts recorded during the Fenton reaction in DMSO at different H
2
O
2
concentrations ([FeCl
2
] = 1 mM), (1), (2), and (3) are
•
OH,
•
OOH, and
•
CH
3
radicals, respectively.
25
IV
20
15
V
10
VI
5
I
I
I
II
I
0
0.50
0.55
0.60
0.65
0.70
0.75
Potential (V) vs. SCE
FIGURE 9.2
Dependence of scavenging ability of carotenoids in Scheme 9.3 (I-VI) with oxidation potential.
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