Pepin Robert, Laszlo Kenneth J, Peng Bo, Marek Aleš, Bush Matthew F, Tureček František
Department of Chemistry, Bagley Hall, Box 351700, University of Washington , Seattle, Washington 98195-1700, United States.
J Phys Chem A. 2014 Jan 9;118(1):308-24. doi: 10.1021/jp411100c. Epub 2013 Dec 18.
Experimental data from ion mobility measurements and electron transfer dissociation were combined with extensive computational analysis of ion structures and dissociation energetics for Gly-Leu-Gly-Gly-Lys cations and cation radicals. Experimental and computational collision cross sections of (GLGGK + 2H)(2+) ions pointed to a dominant folding motif that is represented in all low free-energy structures. The local folding motifs were preserved in several fragment ions produced by electron transfer dissociation. Gradient optimizations of (GLGGK + 2H)(+•) cation-radicals revealed local energy minima corresponding to distonic zwitterionic structures as well as aminoketyl radicals. Both of these structural types can isomerize to low-energy tautomers that are protonated at the radical-containing amide group forming a new type of intermediates, -C(•)O(-)NH2(+)- and -C(•)(OH)NH2(+)-, respectively. Extensive mapping with B3LYP, M06-2X, and MP2(frozen core) calculations of the potential energy surface of the ground doublet electronic state of (GLGGK + 2H)(+•) provided transition-state and dissociation energies for backbone cleavages of the N-Cα and amide C-N bonds leading to ion-molecule complexes. The complexes can undergo facile prototropic migrations that are catalyzed by the Lys ammonium group and isomerize enolimine c-type fragments to the more stable amide tautomers. In contrast, interfragment hydrogen atom migrations in the complexes were found to have relatively high transition energies and did not compete with fragment separation. The extensive analysis of the intermediate and transition-state energies led to the conclusion that the observed dissociations cannot proceed competitively on the same potential energy surface. The reactive intermediates for the dissociations originate from distinct electronic states that are accessed by electron transfer.
来自离子淌度测量和电子转移解离的实验数据，与对甘氨酰-亮氨酰-甘氨酰-甘氨酰-赖氨酸阳离子和阳离子自由基的离子结构及解离能的广泛计算分析相结合。(GLGGK + 2H)(2+)离子的实验和计算碰撞截面表明，在所有低自由能结构中都存在一种占主导的折叠基序。局部折叠基序在电子转移解离产生的几个碎片离子中得以保留。(GLGGK + 2H)(+•)阳离子自由基的梯度优化显示出对应于双离子两性离子结构以及氨基酮基自由基的局部能量最小值。这两种结构类型都可以异构化为在含自由基的酰胺基团处质子化的低能互变异构体，分别形成新型中间体-C(•)O(-)NH2(+)-和-C(•)(OH)NH2(+)-。使用B3LYP、M06-2X和MP2(冻结核心)对(GLGGK + 2H)(+•)基态双重电子态的势能面进行广泛映射，提供了N-Cα主链断裂和酰胺C-N键断裂导致离子-分子复合物的过渡态和解离能。这些复合物可以发生由赖氨酸铵基团催化的质子转移迁移，并使烯醇亚胺c型片段异构化为更稳定的酰胺互变异构体。相比之下，发现复合物中的片段间氢原子迁移具有相对较高的过渡能，并且不与片段分离竞争。对中间体和过渡态能量的广泛分析得出结论，观察到的解离不能在同一势能面上竞争性地进行。解离的反应中间体源自通过电子转移进入的不同电子态。
