Liu Jianbo
Department of Chemistry and Biochemistry, Queens College and the Graduate Center of the City University of New York, 65-30 Kissena Blvd., Queens, NY 11367, USA.
Phys Chem Chem Phys. 2017 Nov 22;19(45):30616-30626. doi: 10.1039/c7cp06124b.
Direct dynamics trajectories were calculated at the B3LYP/6-31G* level of theory to examine the intra-base pair proton transfer and dissociation of the deprotonated guanine (G)·cytosine (C) base pair under different excitation conditions, and to explore the origin of the nonstatistical product branching reported in a collision-induced dissociation (CID) experiment (Phys. Chem. Chem. Phys. 2016, 18, 32222). Trajectories for thermal excitation were initiated at two major conformers G·[C-H] (hydrogen-bonded guanine and N1-deprotonated cytosine) and G·[C-H]_PT (formed by proton transfer from the N1 of guanine to the N3 of deprotonated cytosine), and at their transition state (TS). Thermal excitation was realized by sampling molecular vibrational levels and TS's reaction coordinate energy with Boltzmann distributions at temperatures of 960 and 1330 K, which correspond to classical energies of 3.0 and 5.0 eV, respectively. Thermally excited trajectories undergo intra-base pair proton transfer extensively. The resulting conformation scrambling leads to nearly equal branching between the dissociation channels of [G-H] + C and G + [C-H]. Collisions of G·[C-H] and G·[C-H]_PT with Ar were each simulated at collision energies of 3.0 and 5.0 eV, respectively. The probability for intra-base pair proton transfer decreases substantially in collision trajectories. The CID product branching calculated on the basis of the population-weighted trajectory results of G·[C-H] and G·[C-H]_PT reveals a strong preference for [G-H] + C, consistent with the experiment. Trajectory analysis corroborates that nonstatistical CID is attributed to inadequate conformation interconversion during collisional activation, and to the faster dissociation of the G·[C-H]_PT conformer albeit G·[C-H]_PT has nearly the same translational-to-vibrational energy transfer as G·[C-H].
在B3LYP/6 - 31G*理论水平下计算直接动力学轨迹，以研究不同激发条件下脱质子鸟嘌呤（G）·胞嘧啶（C）碱基对的碱基对内质子转移和解离，并探究碰撞诱导解离（CID）实验（《物理化学化学物理》2016年，18卷，32222页）中报道的非统计产物分支的起源。热激发轨迹从两个主要构象体G·[C - H]（氢键结合的鸟嘌呤和N1脱质子的胞嘧啶）和G·[C - H]_PT（由鸟嘌呤的N1向脱质子胞嘧啶的N3质子转移形成）及其过渡态（TS）开始。通过在960和1330 K温度下用玻尔兹曼分布对分子振动能级和TS的反应坐标能量进行采样来实现热激发，这分别对应于3.0和5.0 eV的经典能量。热激发轨迹广泛经历碱基对内质子转移。由此产生的构象混乱导致[G - H] + C和G + [C - H]解离通道之间的分支几乎相等。分别在3.0和5.0 eV的碰撞能量下模拟了G·[C - H]和G·[C - H]_PT与Ar的碰撞。在碰撞轨迹中，碱基对内质子转移的概率大幅降低。基于G·[C - H]和G·[C - H]_PT的总体加权轨迹结果计算的CID产物分支显示出对[G - H] + C的强烈偏好，这与实验结果一致。轨迹分析证实，非统计CID归因于碰撞激活过程中构象互变不足，以及G·[C - H]_PT构象体尽管与G·[C - H]具有几乎相同的平动到振动能量转移，但解离速度更快。
