Amorati Riccardo, Cavalli Andrea, Fumo Maria Grazia, Masetti Matteo, Menichetti Stefano, Pagliuca Chiara, Pedulli Gian Franco, Viglianisi Caterina
Dipartimento di Chimica Organica A. Mangini Università di Bologna, Via San Giacomo 11, 40126 Bologna, Italy.
Chemistry. 2007;13(29):8223-30. doi: 10.1002/chem.200700309.
Sulfur-containing analogues of vitamin E (thiachromanols), either linked or not to a catechol moiety, were synthesized and their hydrogen-atom donating ability evaluated. The determination of the O--H bond dissociation enthalpy (BDE) of the alpha-tocopherol analogue 4 by the electron paramagnetic resonance (EPR) equilibration technique provided a value of 78.9 kcal mol(-1), that is, approximately 1.8 kcal mol(-1) higher than that of alpha-tocopherol. The kinetic rate constants for the reaction with peroxyl radicals (kinh), measured by inhibited autoxidation studies, showed that thiachromanols react 2.5 times slower than the corresponding tocopherols, in agreement with the higher BDE value. This behavior was explained, on the basis of crystallographic analyses and DFT calculations, in terms of a change in the molecular geometry caused by insertion of a sulfur atom into the framework of vitamin E. This behavior implies a greater deviation of the condensed ring from coplanarity with the aromatic ring, thus giving rise to a decrease in the conjugative stabilization of the phenoxyl radical and consequently to an increase in the O--H bond strength. Although less reactive than tocopherols, thiachromanols may, however, act as bimodal antioxidants as a result of the hydroperoxide decomposing ability of the sulfur atom.
合成了与儿茶酚部分相连或未相连的维生素E含硫类似物（硫杂色满醇），并评估了它们的氢原子供体能力。通过电子顺磁共振（EPR）平衡技术测定α-生育酚类似物4的O-H键解离焓（BDE），得到的值为78.9 kcal mol⁻¹，即比α-生育酚的该值高约1.8 kcal mol⁻¹。通过抑制自氧化研究测量的与过氧自由基反应的动力学速率常数（kinh）表明，硫杂色满醇的反应速度比相应的生育酚慢2.5倍，这与更高的BDE值一致。根据晶体学分析和密度泛函理论（DFT）计算，这种行为是由于硫原子插入维生素E骨架导致分子几何形状发生变化而解释的。这种行为意味着稠环与芳环的共平面性偏差更大，从而导致苯氧基自由基的共轭稳定性降低，进而导致O-H键强度增加。尽管硫杂色满醇的反应性比生育酚低，但由于硫原子的氢过氧化物分解能力，它们可能作为双功能抗氧化剂起作用。
