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Solvation of the Glycyl Radical
The Journal of Physical Chemistry A ( IF 2.9 ) Pub Date : 2018-08-30 00:00:00 , DOI: 10.1021/acs.jpca.8b06833
Bun Chan 1, 2 , Jamie Rintelman 3 , Geoffrey P. F. Wood 2 , Leo Radom 2 , Mark S. Gordon 3
Affiliation  

The effect of adding explicit water molecules to the neutral (N) and zwitterionic (Z) forms of the glycyl radical has been examined. The results show that a minimum of three water molecules is required to stabilize the Z radical as a local minimum, with an energy gap of 123 kJ mol–1 between the N and Z forms at this point, in favor of the N form. Increasing the number of water molecules to ∼20 leads to a converged Z–N energy difference of ∼50 kJ mol–1 still in favor of the N form, even though the radical is not considered fully solvated from a structural point of view. Thus, energetic convergence is determined mainly by solvation of the polar functional groups, and a complete coverage of the entire molecule is not necessary. Because aqueous closed-shell glycine exists as a zwitterion while aqueous glycyl radical prefers the neutral form, the conversion between the two necessitates a change along the hydrogen-abstraction reaction pathway. In this regard, the transition structure for α-hydrogen abstraction by the ·OH radical has greater resemblance to glycine than to the glycyl radical. Overall, the barrier for hydrogen abstraction from Z glycine is larger than that from the N isomer, and this might act to provide some protection against radical damage to the free amino acid in the (aqueous) biological environment.

中文翻译:

糖基自由基的溶剂化

已经研究了将明显的水分子添加到甘氨酸自由基的中性(N)和两性离子(Z)形式中的效果。结果表明,至少需要三个水分子才能将Z自由基稳定在局部最小值,此时N和Z形式之间的能隙为123 kJ mol –1,有利于N形式。将水分子的数量增加到约20会导致Z-N能量差收敛到约50 kJ mol –1即使从结构的观点来看,该基团并没有被完全溶剂化,但仍然支持N的形式。因此,高能收敛主要由极性官能团的溶剂化决定,而整个分子的完全覆盖不是必需的。由于闭壳甘氨酸水溶液以两性离子的形式存在,而甘氨酰基自由基的水溶液更倾向于中性形式,因此两者之间的转化必然导致沿吸氢反应路径的变化。在这方面,通过·OH基团夺取α-氢的过渡结构与甘氨酸的相似性大于与糖基的相似性。总体而言,从Z甘氨酸提取氢的壁垒比从N异构体提取氢的壁垒大,
更新日期:2018-08-30
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