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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Origin of the Transition State on the Free Energy Surface: Intramolecular Proton Transfer Reaction of Glycine in Aqueous Solution
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Origin of the Transition State on the Free Energy Surface: Intramolecular Proton Transfer Reaction of Glycine in Aqueous Solution

机译：自由能表面上过渡态的起源：甘氨酸在水溶液中的分子内质子转移反应

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摘要

Not only to elucidate the origin of the reaction barrier in liquid phase, i.e., the free energy of activation, but also to locate the proper transition state (TS) for a chemical reaction, the molecular dynamics method and the free energy perturbation theory have been applied to the intramolecular proton transfer reaction of glycine in aqueous solution, i.e., the zwitterion (ZW), to the neutral form (NF). The potential energy surface varies drastically as its environment changes from gas phase to aqueous solution, and experimentally, the existence of an entropy barrier is also suggested due to the solvent molecules. In this study, it is reported that the TS on the free energy surface (FES) corresponds approximately to the geometry at s ≈ 0.66 amu~(1/2) bohr, where s denotes the intrinsic reaction coordinate (IRC) for the gas-phase reaction, and therefore, the TS geometry is completely different from that for the gas phase. The free energy difference between ZW and NW is 8.46 ± 1.45 kcal/mol, and then the free energy of activation of ZW IS 16.85 ± 1.36 kcal/mol at the temperature 300 K, both of which are in very good agreement with the experimental values. Further, the entropy contribution to the free energy change increases almost monotonously along the IRC, while the enthalpy contribution has a maximum at s ≈ 0.6 amu~(1/2) bohr, being understood as the origin of the free energy of activation. By the interaction energy distribution and the radial distribution functions, it is shown that solvent water molecules interact with ZW more strongly than both TS and NF, especially at both the positive amino and negative carboxyl groups. Therefore, from a microscopic point of view, the reaction barrier in aqueous solution is clearly explained by the fact that as the forward reaction (ZW → NF) proceeds, the Coulomb interaction between the charged groups of ZW and solvent water molecules becomes weaker while the intramolecular potential energy is stabilized compensatorily to form a free energy maximum.
机译：分子动力学方法和自由能微扰理论不仅阐明了液相反应势垒的起源，即活化的自由能，而且还为化学反应确定了合适的过渡态（TS），应用于水溶液中甘氨酸的分子内质子转移反应，即两性离子（ZWitterion）（ZW）到中性形式（NF）。势能表面随着其环境从气相到水溶液的变化而急剧变化，并且在实验中，由于溶剂分子的存在，也表明存在熵屏障。在这项研究中，据报道，自由能表面（FES）上的TS大致对应于s≈0.66 amu〜（1/2）bohr的几何形状，其中s表示气体的固有反应坐标（IRC）。相反应，因此，TS几何与气相完全不同。 ZW和NW之间的自由能差为8.46±1.45 kcal / mol，然后在300 K的温度下ZW活化的自由能为16.85±1.36 kcal / mol，这两个值与实验值非常吻合。此外，熵对自由能变化的贡献沿IRC几乎单调增加，而焓贡献在s≈0.6 amu〜（1/2）bohr处具有最大值，这被理解为激活自由能的起源。通过相互作用能分布和径向分布函数，表明溶剂水分子与ZW的相互作用比TS和NF更为强烈，特别是在正氨基和负羧基上。因此，从微观的角度来看，水溶液的反应障碍可以通过以下事实清楚地解释：随着正向反应（ZW→NF）的进行，ZW带电基团与溶剂水分子之间的库仑相互作用变弱，而分子内势能被补偿稳定以形成最大自由能。

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