Chemistry Reference
In-Depth Information
radical cations (Car
•+
), dications (Car
2+
), and the loss of H
+
to form the carotenoid neutral radical
(
#
Car
•
) (Gao et al. 1996, Jeevarajan et al. 1996a) according to the following equations:
0
1
E
•
+
−
Car
Car
+
e
(9.1)
0
2
E
•
+
2
+
−
Car
Car
+
e
(9.2)
K
com
2
+
•+
Car
+
Car
2Car
(9.3)
K
dp
K
dp
2
+
#
+
+
Car
Car
+
H
(9.4)
'
dp
K
•+
#
•
+
Car
Car
+
H
(9.5)
0
3
E
#
+
−
#
•
Car
+
e
Car
(9.6)
It has been demonstrated (Mairanovski et al. 1975, Park 1978, Grant et al. 1988, Chen 1991, Khaled
1992, Jeevarajan et al. 1994a-c, Jeevarajan 1995, Jeevarajan and Kispert 1996, Jeevarajan et al.
1996a, Gao et al. 1997, Deng 1999, Liu and Kispert 1999, Hapiot et al. 2001, Konovalov et al. 2002)
that great care must be taken to eliminate any traces of water or oxygen during the electrochemi-
cal studies, in order to obtain reproducible results. Accurate oxidation potentials could be deduced
(Hapiot et al. 2001) only if i ts were made to cyclovoltammograms (CV) recorded over six orders
of magnitude of sweep times. The more traditional way of recording CV (Mairanovskii et al. 1975,
Park 1978, Grant et al. 1988, Chen 1991, Khaled 1992, Jeevarajan et al. 1994a-c, Jeevarajan 1995,
Jeevarajan and Kispert 1996, Jeevarajan et al. 1996a, Gao et al. 1997, Deng 1999, Liu and Kispert
1999, Hapiot et al. 2001, Konovalov et al. 2002) gave oxidation potentials some 50-100 mV lower.
Radical cations and neutral radicals of carotenoids can be measured and detected by electron
paramagnetic resonance spectroscopy (EPR). Such techniques have been used to detect and char-
acterize their properties. Unfortunately, the large number of different proton hyperi ne couplings
(
18) results in approximately 300,000 EPR lines for symmetrical carotenoids, if all couplings were
resolved, and even a greater number for asymmetrical carotenoids. There would be an even larger
number of EPR lines, if it was not for the rapid rotation of the methyl groups, even at 5 K, which
causes the methyl proton couplings to be averaged out, so each methyl group exhibits one set of
proton couplings. The large number of proton couplings results in a single, unresolved, inhomoge-
neously broadened powder EPR line of 14 Gauss peak-to-peak linewidth. To resolve the hyperi ne
couplings, continuous wave (CW) electron-nuclear double resonance (ENDOR) measurements have
been carried out (Piekara-Sady et al. 1991, 1995, Wu et al. 1991, Jeevarajan et al. 1993b). For each
set of equivalent protons, only two ENDOR lines separated by the hyperi ne coupling constant,
A
,
occur instead of multiple EPR lines. The ENDOR spectrum is recorded as a function of swept radio
frequency (rf) centered at the free proton frequency, while the observing magnetic i eld is set to the
center of the EPR line. To achieve the greatest spectral resolution, ENDOR measurements should
be carried out in solution where the proton dipolar anisotropy is averaged out, and at a steady-state
carotenoid concentration of
∼
1 mM. However, because the carotenoid radical cations are short-lived
in chlorinated (electron acceptor) solvents (Khaled et al. 1990, Deng et al. 2000) (lifetimes on
the order of 100-200 s and even shorter in other solvents), a steady-state mM concentration is not
achieved for ENDOR measurement in a closed sample tube (over 30 min to 1 h needed). A possible
<
Search WWH ::
Custom Search