Thirumoorthy Krishnan, Nandi Nilashis
Chemistry Department, Birla Institute of Technology and Science, Pilani, Rajasthan 333031, India.
J Phys Chem B. 2006 May 4;110(17):8840-9. doi: 10.1021/jp055295n.
In the present work, we compare the intermolecular energy surfaces of the alanine molecule in its neutral and zwitterionic state using ab initio theory (HF/6-311++G) as a function of mutual orientation. Starting from the optimized structures of the nonbonded homochiral (l-l) and heterochiral (d-l) pairs of molecules, the energy surfaces are studied with rigid geometry by varying the distance and orientation. The potential energy surfaces of the l-l and d-l pairs are found to be dissimilar and reflect the underlying chirality of the homochiral pair and racemic nature of the heterochiral pair. The intermolecular energy surface of the l-l pair is more favorable than the corresponding energy surface of the d-l pair. The study, for the first time, reveals clear homochiral preference without use of parameters, which was unobserved in previous detailed simulations but predicted by theory. The electrostatic interaction further augments the chiral discrimination. The basis set superposition error (BSSE) corrected results show enhanced discrimination. Use of higher-level Møller-Plesset perturbation theory (MP2) and further BSSE correction do not change the conclusions made at the Hartree-Fock (HF) level. The major conclusions based on HF and MP2 level calculations remain unaltered when the calculations of the potential energy surfaces for the neutral and zwitterionic pairs are repeated using the density functional theory (DFT) (B3LYP/6-311++G). The observed orientation dependence has significance in the biological chiral recognition as well as peptide synthesis at the peptidyl transferase center where the amino terminal and peptidyl terminal undergo mutual rotatory motion.
在本工作中，我们使用从头算理论（HF/6 - 311++G），将丙氨酸分子处于中性和两性离子状态时的分子间能量表面作为相互取向的函数进行比较。从非键合的同手性（l - l）和异手性（d - l）分子对的优化结构出发，通过改变距离和取向，以刚性几何结构研究能量表面。发现l - l和d - l分子对的势能表面不同，反映了同手性对的潜在手性和异手性对的外消旋性质。l - l分子对的分子间能量表面比d - l分子对的相应能量表面更有利。该研究首次揭示了无需使用参数的明显同手性偏好，这在先前的详细模拟中未被观察到，但理论上有预测。静电相互作用进一步增强了手性识别。基组叠加误差（BSSE）校正后的结果显示出增强的识别能力。使用更高水平的莫勒 - 普莱塞微扰理论（MP2）以及进一步的BSSE校正不会改变在哈特里 - 福克（HF）水平得出的结论。当中性和两性离子对的势能表面计算使用密度泛函理论（DFT）（B3LYP/6 - 311++G）重复进行时，基于HF和MP2水平计算得出的主要结论保持不变。观察到的取向依赖性在生物手性识别以及肽基转移酶中心的肽合成中具有重要意义，在该中心氨基末端和肽基末端会进行相互旋转运动。
