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ENDOR lines predicted by improved DFT calculations for methyl protons at C13(13
) located at
16.5 MHz were broad and not observable due to incomplete averaging of the methyl proton cou-
plings due to a hindering environment. Thus, the methyl groups at the C5(5
′
′
) and C9(9
′
) are located
away from the surface of the pore and rapidly rotate, while those at C13(13
) interact with the sur-
face. Steric hindrance by the terminal bulky trimethyl cyclohexene rings preclude attainment of the
requisite distance between Cu
2+
and the C7
′
=
C8 and C8
′ =
C7
′
bonds.
-carotene imbedded in Cu-MCM-41 exhibits
an echo decay with an echo modulation due to deuterons. The three-pulse ESEEM is plotted as a
function of time, and curves are drawn through the maximum and minima. From ratio analysis of
these curves, a best nonlinear least-squares i t determines the number of interacting deuterons, the
distance (3.3
Three-pulse ESEEM spectrum of perdeuterated
β
0.2 MHz). This analysis made it possible to
explain the observed reversible forward and backward electron transfer between the carotenoid and
Cu
2+
as the temperature was cycled (77-300 K).
±
0.2 Å), and the isotopic coupling (0.06
±
9.9 EPR ON ACTIVATED SILICA-ALUMINA
Adsorption of carotenoids on activated silica-alumina results in their chemical oxidation and
carotenoid radical formation. Tumbling of carotenoid molecules adsorbed on solid support is
restricted, but the methyl groups can rotate. This rotation is the only type of dynamic processes
which is evident in the CW ENDOR spectrum.
The Davies pulsed ENDOR spectrum of canthaxanthin oxidized on silica-alumina measured in
the temperature range of 3.3-80 K showed no lineshape changes, which is in agreement with previ-
ous 330 GHz EPR studies of canthaxanthin radical cations (Konovalova et al. 1999). This implies
very rapid rotation of the methyl groups down to 3.3 K.
Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the elec-
tron transfer between carotenoid molecule and the Al
3+
electron acceptor site. Both the three-pulse
ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/
AlCl
3
sample contain a peak at the
27
Al Larmor frequency (3.75 MHz). The existence of elec-
tron transfer interactions between Al
3+
ions and carotenoids in AlCl
3
solution can serve as a good
model for similar interactions between adsorbed carotenoids and Al
3+
Lewis acid sites on silica-
alumina.
9.10 DFT CALCULATIONS TO INTERPRET EPR SPECTRA
The Davies ENDOR spectrum of canthaxanthin radical photo-generated on silica-alumina con-
sists of two well-resolved doublets placed around the
1
H Larmor frequency with hyperi ne cou-
pling constants (hfc) of 2.6 and 8.6 MHz and a broad low-intensity line with hfc approximately
13 MHz (Konovalova et al. 2001a). The couplings were assigned to
β
-protons of three different
)-C
β
H
3
, respectively. The origi-
nal assignment of the large 13 MHz proton coupling to the C13(13
methyl groups, namely, C5(5
′
)-C
β
H
3
, C9(9
′
)-C
β
H
3
, and C13(13
′
)-C
β
H
3
group of the radical
cation was based on RHF-INDO/SP calculations, the best available at the time, but shown to be
in error by the more accurate DFT calculations (Gao et al. 2006, Focsan et al. 2008, Lawrence
et al. 2008).
Using different DFT functionals and basis sets (Focsan et al. 2008, Lawrence et al. 2008) it
was coni rmed that the isotropic
′
-methyl proton hyperi ne couplings do not exceed 9 MHz for the
carotenoid radical cation, Car
•+
. DFT calculations of neutral carotenoid radicals,
#
Car
•
, formed by
proton loss (indicated by
#
) from the radical cation, predicted isotropic
β
-methyl proton couplings
up to 16 MHz, a fact that explained the large isotropic couplings observed by ENDOR measure-
ments for methyl protons in UV irradiated carotenoids supported on silica gel, Nai on i lms, silica-
alumina matrices, or incorporated in molecular sieves (Piekara-Sady et al. 1991, 1995, Wu et al.
β
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