Abstract Decomposition of the protonated tripeptide, GlyProAla (H+GPA), through collision-induced dissociation with Xe in a guided ion beam tandem mass spectrometer (GIBMS) is examined. Cross sections for the formation of [b2]+, [y2 + 2H]+, [a1]+, [b3]+, [a3]+, [a2]+, and H+(1-pyrroline) fragment ions were collected as a function of kinetic energy. Analysis of these cross sections include consideration of the effects of multiple collisions, dissociation lifetimes, reactant internal and kinetic energy distributions, competition among channels, and evolution into sequential channels. Structures and molecular parameters of reactants, transition states, intermediates, and products were identified using the B3LYP/6-311+G(d,p) level of theory, with single-point energy calculations performed at the B3LYP, B3P86, and MP2(full) levels of theory using a 6-311+G(2d,2p) basis set. Good agreement between experimental threshold energies and those calculated for rate-limiting transition states allow key reaction pathways for the formation of each product to be identified. The MP2(full) level of theory is found to agree best with the experimental results. Notably, even though H+GPA starts with a cis-peptide bond, it forms an oxazolone as its [b2]+ fragment and the absence of a diketopiperazine [b2]+ fragment is verified by other key observations.

中文翻译：

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顺反异构化不是 b2 离子结构的速率决定因素：引导离子束和 H+ (GlyProAla) 分解的计算研究

摘要 研究了质子化三肽 GlyProAla (H+GPA) 在引导离子束串联质谱仪 (GBBSS) 中通过与氙碰撞诱导解离的分解。[b2]+、[y2 + 2H]+、[a1]+、[b3]+、[a3]+、[a2]+ 和 H+（1-吡咯啉）碎片离子形成的横截面被收集为动能的函数。对这些横截面的分析包括考虑多次碰撞的影响、解离寿命、反应物内部和动能分布、通道之间的竞争以及向连续通道的演变。使用 B3LYP/6-311+G(d,p) 理论水平确定反应物、过渡态、中间体和产物的结构和分子参数，并在 B3LYP、B3P86、和 MP2(full) 理论水平，使用 6-311+G(2d,2p) 基组。实验阈值能量与为限速过渡态计算的阈值能量之间的良好一致性允许确定形成每种产物的关键反应途径。发现 MP2(full) 理论水平与实验结果最吻合。值得注意的是，即使 H+GPA 以顺式肽键开始，它也会形成恶唑酮作为其 [b2]+ 片段，并且其他关键观察结果证实了二酮哌嗪 [b2]+ 片段的缺失。