Applications of EPR Spectroscopy to Understanding Carotenoid Radicals - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 178

Ni(I)

g = 2.0027

g = 2.09

2800 3000 3200 3400 3600

Magnetic field (gauss)

FIGURE 9.11 9 GHz EPR spectrum at 77 K of β-carotene in Ni-MCM-41 after 350 nm irradiation. (From
Konovalova, T.A., J. Phys. Chem. B , 105, 7549, 2001. With permission.)

(Hartmann et al. 1996). The g || component is often too weak to observe. The Ni(I) EPR signals were
not detected upon 350 nm irradiation of Ni-MCM-41 samples before adsorption of carotenoids.
Detected at 9 GHz, EPR signals of an isolated Ni(I) species with g = 2.09 provide direct evidence for

the reduction of Ni(II) ions by carotenoids.

9.14.2 C AROTENOIDS IN F E -MCM-41
Multifrequency EPR spectroscopy was applied to study Fe(III)-MCM-41 mesoporous molecular
sieves with incorporated carotenoids (Konovalova et al. 2003).
EPR spectroscopy is a useful technique for characterizing the iron sites in both the low-spin

5/2) electronic coni gurations. The spin Hamiltonian for high-spin iron
is given by the following equation (Dowsing and Gibson 1969, Sweeney et al. 1973):

( S = 1/2) and high-spin ( S =

1

⎛ ⎞

2 2 2 2

Hg =β BS + DS − S + E ( SS − ) (9.24)

⎜ ⎟

S Z X Y

⎝ ⎠

3

In this case the g -tensor exhibits extremely small anisotropy, and the spectral characteristics are
determined by the ZFS parameters D (axial) and E (rhombic). When the symmetry is axial, D ≠ 0

and E 1/3. Most high-spin d 5 systems do not belong to

one of these special cases. Several different symmetries of Fe 3+ contribute to multicomponent EPR
spectra with overlapping signals. Such complex spectra arising from more than one center can be
analyzed at different microwave frequencies. For high-spin Fe 3+ in proteins, zeolites, and MCM-41
molecular sieves, the electron Zeeman interaction ( gbB 0 S ) is much smaller at the X-band frequency
than the ZFS interaction. This makes interpretation of the 9 GHz EPR spectra difi cult due to inho-
mogeneous broadening arising from the ZFS and overlapping signals. Use of higher microwave
frequency is particularly advantageous in this case.
Studies with 9-287 GHz EPR (Konovalova et al. 2003) were carried out to characterize the Fe 3+
sites in Fe-MCM-41 molecular sieves. Multifrequency EPR measurements were also performed to
elucidate the types of iron sites which are responsible for carotenoid oxidation, their stability, and
accessibility. The X-band EPR spectrum of Fe-MCM-41 activated at 260°C and recorded at 77 K
consists of a strong sharp peak at g

= 0. In the case of rhombic symmetry, E/D =

9.0 (Figure 9.12a). The presence of
these signals originating from the middle Kramers doublet and the lowest Kramers doublet, respec-
tively, is characteristic of high-spin Fe 3+ when E/D

= 4.3 with a shoulder at g =

= 1/3 (Abragam and Bleaney 1970, Pilbrow

1990). The observation of a g 4.3 signal in zeolites and aluminophosphate molecular sieves is usu-

ally considered as evidence for the presence of framework Fe 3+ ions (Goldfarb et al. 1994, Kosslick

=