Measuring Ti(III)−Carotenoid Radical Interspin Distances in TiMCM-41 by Pulsed EPR Relaxation Enhancement Method

Abstract

Interspin distances between the Ti(3+) ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7'-apo-7'-(4-carboxyphenyl)-beta-carotene (coordinated to Ti(3+)), canthaxanthin (formed as a random distribution of isomers), and beta-ionone (model for a short-chain polyene) to Ti(3+) framework sites were determined using the pulsed EPR relaxation enhancement method. To estimate the electron transfer distances, the temperature dependence of relaxation rates was analyzed in both siliceous and metal-substituted siliceous materials. The phase memory times, T(M), of the carotenoid radicals were determined from the best fits of two-pulse ESEEM curves. The spin-lattice relaxation times, T(1), of the Ti(3+) ion were obtained from the inversion recovery experiment with echo detection on a logarithmic time scale in the temperature range of 10-150 K. The relaxation enhancement for the carotenoid radicals in TiMCM-41 as compared to that in MCM-41 is consistent with an interaction between the radical and the fast relaxing Ti(3+) ion. For canthaxanthin and beta-ionone, a dramatic effect on the carotenoid relaxation rate, 1/T(M), occurs at 125 and 40 K, respectively, whereas for carboxy-beta-carotene 1/T(M) increases monotonically with increasing temperature. The interspin distances for canthaxanthin and beta-ionone were estimated from the 1/T(M) - 1/T(M0) difference, which corresponds to the Ti(3+) contribution at the temperature where the maximum enhancement in the relaxation rate occurs. Determination of the interspin distances is based on calculations of the dipolar interaction, taking into consideration the unpaired spin density distribution along the 20-carbon polyene chain, which makes it possible to obtain a fit over a wider temperature interval. A distribution of the interspin distances between the carotenoid radical and the Ti(3+) ion was obtained with the best fit at approximately 10 A for canthaxanthin and beta-ionone and approximately 9 A for 7'-apo-7'-(4-carboxyphenyl)-beta-carotene with an estimated error of +/-3 A. The interspin distances do not depend on 1/T(M) - 1/T(M0) for carboxy-beta-carotene which shows no prominent peak in the relaxation rate over the temperature range measured.

Carotenoid Radical Formation: Dependence on Conjugation Length

“…Similar experimental Mims ENDOR spectra have been reported for the lutein radicals 13 , astaxanthin radicals 25 , canthaxanthin 26 and the 9'- cis neoxanthin radicals 11 . These spectra can be compared to Figures 9, 10 and 11 and used as a guide for detecting similar carotenoid radicals by Mims ENDOR measurements.…”

“…11,12,13,25,26 DFT-calculated isotropic and anisotropic proton hyperfine coupling constants for different carotenoid radicals are accurate enough 19 to be used in simulation and interpretation of overlapping powder ENDOR spectra composed of a mixture of carotenoid radicals. 11,13 Simulated spectra for xanthophylls found signatures for the presence of the radical cation Car •+ , as well as the presence of carotenoid neutral radicals, #Car • (n).…”

“…DFT calculations of carotenoid radical cations containing cyclohexene rings such as β-carotene 14,19 and the xanthophylls zeaxanthin 12 , violaxanthin 12 , lutein 13 , astaxanthin 25 , canthaxanthin 26 and 9'- cis neoxanthin 11 predicted isotropic β-methyl proton couplings less than 10 MHz. The largest couplings correspond to C9 or C9' methyl groups (see Table 2).…”

Titanium Ions Dispersed into the ZrO2 Matrix: Spectroscopic Properties and Photoinduced Electron Transfer

“…This signal was never reported before for ZrO 2 matrix and, on the basis of its similarity with the signals usually observed in reduced TiO 2 , is assigned to reduced Ti 3+ centers in ZrO 2 matrix. The existence of Ti 3+ centers in oxides different from TiO 2 , however, is well documented in the literature. − …”

Chemistry of carotenoid neutral radicals

“…63,64 EPR techniques that been used to characterize carotenoid radicals to resolve g-anisotropy (HF-EPR, located at the National High Magnets Field Lab at Florida State University), anisotropic coupling constants due to α-protons (CW, pulsed ENDOR, HYSCORE), isotropic coupling constants due to β-methyl protons (CW, pulsed ENDOR), to determine distances between carotenoid radicals and electron acceptor site (ESEEM, relaxation enhancement), etc., are listed with references in Table 1. 5,6,72,73,74,75,76,77 Measuring distances between carotenoid radicals and formed upon photo-irradiation of carotenoids adsorbed on silica-alumina 24,63,64,65,88 or on the molecular sieves MCM-41, 34,88 Ti-MCM-41 89 or Cu-MCM-41. 33,86 Davies and Mims ENDOR complement each other because hyperfine coupling are shown using one method but not in the other due to different pulse delay times.…”