Detection of anisotropic hyperfine components of chemically prepared carotenoid radical cations: 1D and 2D ESEEM and pulsed ENDOR study

Canthaxanthin and 8 ' -apo-beta -caroten-8 ' -al radical cations chemically prepared on activated silica-alumina and in CH2Cl2 solution containing AlC l3 were studied by pulsed EPR and ENDOR spectroscopies. Both the 1D three-p ulse ESEEM and the 2D HYSCORE spectra of the carotenoid-AlCl3 mixtures exhi bited the Al-27 nuclei peak at 3.75 MHz. This indicates electron-transfer i nteractions between carotenoids and Al-III ions resulting in the formation and stabilization of carotenoid radical cations. Davies ENDOR measurements of the canthaxanthin radical cation on silica-alumina determined the hyperf ine couplings of protons belonging to three different methyl groups with a( H1) = 2.6 MHz, a(H2) = 8.6 MHz, and a(H3) ca. 13 MHz. The principal compone nts of the proton hyperfine tensors were obtained from HYSCORE spectra in A lCl3 solutions and on the solid support. Identification of the protons was made on the basis of isotropic hyperfine couplings determined by RHF-INDO/S P molecular orbital calculations. In frozen AlCl3 solution, the C(7,7 ')-H alpha and C(14,14 ')-H alpha alpha protons were observed for canthaxanthin and the C(8 or 14 ')-H alpha and C(15 ')-H alpha were observed for 8 ' -apo -beta -caroten-8 ' -al. On the silica-alumina support, the C(10,10 ')-H alp ha, C(11,11 ')-H alpha, and C(15,15 ')-H alpha alpha protons were measured for canthaxanthin and the C(12)-H alpha and C(15 ')-H alpha were measured f or 8 ' -apo- beta -caroten-8'-al. Some protons with large isotropic couplin gs (> 10 MHz) determined from HYSCORE analysis could be assigned to beta pr otons, but the principal components of their hyperfine tensors are much mor e anisotropic than those reported previously for protons. We suggest that c is/trans isomerization of carotenoids on silica-alumina results in stabiliz ation of di-cis isomers with large isotropic couplings for some alpha proto ns which are comparable to those of beta protons. HYSCORE is a promising te chnique to increase spectral resolution for proper assignment of protons wi th large hyperfine anisotropy, which cannot be resolved by ENDOR spectrosco py.