Homayoon Zahra, Pratihar Subha, Dratz Edward, Snider Ross, Spezia Riccardo, Barnes George L, Macaluso Veronica, Martin Somer Ana, Hase William L
Department of Chemistry and Biochemistry, Texas Tech University , Lubbock, Texas 79409-1061, United States.
Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, Université d'Evry Val d'Essonne UMR 8587 CNRS-CEA-UEVE , Bd. F. Mitterrand, 91025 Evry Cedex, France.
J Phys Chem A. 2016 Oct 27;120(42):8211-8227. doi: 10.1021/acs.jpca.6b05884. Epub 2016 Oct 13.
Direct dynamics simulations, utilizing the RM1 semiempirical electronic structure theory, were performed to study the thermal dissociation of the doubly protonated tripeptide threonine-isoleucine-lysine ion, TIK(H), for temperatures of 1250-2500 K, corresponding to classical energies of 1778-3556 kJ/mol. The number of different fragmentation pathways increases with increase in temperature. At 1250 K there are only three fragmentation pathways, with one contributing 85% of the fragmentation. In contrast, at 2500 K, there are 61 pathways, and not one dominates. The same ion is often formed via different pathways, and at 2500 K there are only 14 m/z values for the product ions. The backbone and side-chain fragmentations occur by concerted reactions, with simultaneous proton transfer and bond rupture, and also by homolytic bond ruptures without proton transfer. For each temperature the TIK(H) fragmentation probability versus time is exponential, in accord with the Rice-Ramsperger-Kassel-Marcus and transition state theories. Rate constants versus temperature were determined for two proton transfer and two bond rupture pathways. From Arrhenius plots activation energies E and A-factors were determined for these pathways. They are 62-78 kJ/mol and (2-3) × 10 s for the proton transfer pathways and 153-168 kJ/mol and (2-4) × 10 s for the bond rupture pathways. For the bond rupture pathways, the product cation radicals undergo significant structural changes during the bond rupture as a result of hydrogen bonding, which lowers their entropies and also their E and A parameters relative to those for C-C bond rupture pathways in hydrocarbon molecules. The E values determined from the simulation Arrhenius plots are in very good agreement with the reaction barriers for the RM1 method used in the simulations. A preliminary simulation of TIK(H) collision-induced dissociation (CID), at a collision energy of 13 eV (1255 kJ/mol), was also performed to compare with the thermal dissociation simulations. Though the energy transferred to TIK(H) in the collisions is substantially less than the energy for the thermal excitations, there is substantial fragmentation as a result of the localized, nonrandom excitation by the collisions. CID results in different fragmentation pathways with a significant amount of short time nonstatistical fragmentation. Backbone fragmentation is less important, and side-chain fragmentation is more important for the CID simulations as compared to the thermal simulations. The thermal simulations provide information regarding the long-time statistical fragmentation.
利用RM1半经验电子结构理论进行直接动力学模拟，以研究双质子化三肽苏氨酸 - 异亮氨酸 - 赖氨酸离子TIK(H)在1250 - 2500 K温度下的热解离情况，该温度范围对应的经典能量为1778 - 3556 kJ/mol。不同碎裂途径的数量随温度升高而增加。在1250 K时只有三种碎裂途径，其中一种途径贡献了85%的碎裂。相比之下，在2500 K时有61种途径，且没有一种占主导。相同的离子常常通过不同途径形成，在2500 K时产物离子只有14个m/z值。主链和侧链碎裂通过协同反应发生，伴随着质子转移和键断裂同时进行，也通过无质子转移的均裂键断裂发生。对于每个温度，TIK(H)的碎裂概率随时间呈指数变化，这与赖斯 - 拉姆齐 - 卡斯尔 - 马库斯理论和过渡态理论一致。确定了两条质子转移途径和两条键断裂途径的速率常数与温度的关系。从阿伦尼乌斯图中确定了这些途径的活化能E和指前因子A。质子转移途径的活化能E为62 - 78 kJ/mol，指前因子A为(2 - 3)×10 s；键断裂途径的活化能E为153 - 168 kJ/mol，指前因子A为(2 - 4)×10 s。对于键断裂途径，产物阳离子自由基在键断裂过程中由于氢键作用发生显著的结构变化，这降低了它们的熵以及相对于烃分子中C - C键断裂途径的E和A参数。从模拟阿伦尼乌斯图确定的E值与模拟中使用的RM1方法的反应势垒非常吻合。还进行了TIK(H)在13 eV（1255 kJ/mol）碰撞能量下的碰撞诱导解离（CID）的初步模拟，以与热解离模拟进行比较。尽管碰撞中传递给TIK(H)的能量远小于热激发能量，但由于碰撞引起的局部非随机激发，仍有大量碎裂发生。CID导致不同的碎裂途径，有大量短时间的非统计性碎裂。与热模拟相比，主链碎裂在CID模拟中不太重要，侧链碎裂更重要。热模拟提供了关于长时间统计性碎裂的信息。
