Measuring Ti(III)-Carotenoid Radical Interspin Distances in TiMCM-41 by Pulsed EPR Relaxation Enhancement Method
Measuring Ti(III)-Carotenoid Radical Interspin Distances in TiMCM-41 by Pulsed EPR Relaxation Enhancement Method
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DOI：
10.1021/jp811369h
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发表时间：
2009-06-25
影响因子：
3.3
通讯作者：
Kispert, Lowell D.
中科院分区：
文献类型：
--
作者：
Konovalova, Tatyana A.;Li, Shenggang;Kispert, Lowell D.
使用脉冲EPR弛豫增强方法测定了Ti 3+离子和类胡萝卜素自由基之间的自旋间距，所述类胡萝卜素自由基通过光诱导电子转移从7 '-apo-7'-（4-羧基苯基）-β-胡萝卜素（与Ti 3+配位）、黄质（形成为随机分布的异构体）和β-紫罗酮（短链多烯的模型）到Ti 3+骨架位点而在TiMCM-41孔内产生。为了估计电子转移距离，分析了硅质和金属取代的硅质材料中弛豫速率的温度依赖性。从两脉冲ESEEM曲线的最佳拟合确定类胡萝卜素自由基的相记忆时间T-M。在10-150 K温度范围内，利用回波探测的反转恢复实验，得到了Ti ~（3+）离子的自旋-晶格弛豫时间T ~（-1）。类胡萝卜素自由基在TiMCM-41中的弛豫增强与MCM-41中的弛豫增强相比，与自由基和快速弛豫的Ti 3+离子之间的相互作用一致。对于黄质和β-紫罗兰酮，类胡萝卜素的松弛速率，1/T-M，发生在125和40 K，分别一个显着的效果，而羧基-β-胡萝卜素1/T-M随着温度的升高单调增加。从1/T-M - 1/T-M0差估计了黄质和β-紫罗兰酮的自旋间距离，其对应于在弛豫速率发生最大增强的温度下的Ti 3+贡献。自旋间距离的确定是基于偶极相互作用的计算，考虑到未成对的自旋密度分布沿着的20-碳多烯链，这使得有可能获得一个较宽的温度区间内的拟合。获得类胡萝卜素自由基和Ti 3+离子之间的自旋间距分布，其中对于黄质和β-紫罗兰酮，最佳拟合近似于10埃，对于7 ′-脱辅基-7 ′-（4-羧基苯基）-β-胡萝卜素，最佳拟合近似于9埃，估计误差为+/-3埃。对于羧基-β-胡萝卜素，自旋间距离不依赖于1/T-M - 1/T-M0，其在所测量的温度范围内的弛豫速率没有显著的峰。
Interspin distances between the Ti3+ ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7'-apo-7'-(4-carboxyphenyl)-beta-carotene (coordinated to Ti3+), canthaxanthin (formed as a random distribution of isomers), and beta-ionone (model for a short-chain polyene) to Ti3+ 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 Ti3+ 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 Ti3+ 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 Ti3+ 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 Ti3+ ion was obtained with the best fit at similar to 10 angstrom for canthaxanthin and beta-ionone and similar to 9 angstrom for 7'-apo-7'-(4-carboxyphenyl)-beta-carotene with an estimated error of +/- 3 angstrom. 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.