Rani Namrata
Quantum Chemistry Group, Department of Chemistry & Centre of Advanced Studies in Chemistry , Panjab University , Chandigarh 160014 , India.
J Phys Chem A. 2018 Sep 27;122(38):7572-7586. doi: 10.1021/acs.jpca.8b06659. Epub 2018 Sep 18.
Quantum-mechanical computations are performed to trace the mechanistic pathways for the gas-phase stereoinversion in proteinogenic l-threonine, an amino acid with two stereocenters. The pathways are explored employing density functional and coupled cluster theories along with a global reaction route mapping strategy to locate various intermediates and transition states along the stereoinversion pathways on the complex potential energy surface of l-threonine. A simultaneous intramolecular proton and hydrogen atom transfer is observed to drive the stereoinversion in threonine. The kinetics analysis of the stereoinversion pathways is also carried out using transition state theory while accounting for the quantum mechanical tunnelling under conditions akin to various temperature regions of interstellar medium (ISM). The key step leading to stereoinversion through an achiral intermediate or transition state is predicted to involve a low energy barrier with high stereoinversion rates. The temperature region of 500-1000 K corresponding to protoplanetary disks was found to be an optimum region for stereoinversion to occur in l-threonine with quite significant reaction rates. However, in the cold molecular clouds of ISM the stereoinversion is predicted to be a less likely event despite involving significant proton tunnelling. The stereoinversion pathways proposed in this work pay gainful insights, particularly, to the researchers looking for the complex organic molecules in outer space.
进行量子力学计算以追踪蛋白质原性L-苏氨酸（一种具有两个立体中心的氨基酸）气相立体反转的机理途径。利用密度泛函理论和耦合簇理论以及全局反应路线映射策略来探索这些途径，以在L-苏氨酸的复杂势能面上沿着立体反转途径定位各种中间体和过渡态。观察到分子内质子和氢原子同时转移驱动苏氨酸的立体反转。还使用过渡态理论对立体反转途径进行动力学分析，同时考虑在类似于星际介质（ISM）不同温度区域的条件下的量子力学隧穿。预计通过非手性中间体或过渡态导致立体反转的关键步骤涉及低能垒和高立体反转速率。发现对应于原行星盘的500 - 1000 K温度区域是L-苏氨酸发生立体反转的最佳区域，反应速率相当可观。然而，在ISM的冷分子云中，尽管涉及显著的质子隧穿，但预计立体反转不太可能发生。这项工作中提出的立体反转途径为特别是在外层空间寻找复杂有机分子的研究人员提供了有益的见解。
