Fischer Kaitlyn C, Voss Jonathan M, Zhou Jia, Garand Etienne
Department of Chemistry , University of Wisconsin-Madison , 1101 University Ave. , Madison , Wisconsin 53706 , United States.
J Phys Chem A. 2018 Oct 18;122(41):8213-8221. doi: 10.1021/acs.jpca.8b07546. Epub 2018 Oct 4.
IR predissociation spectroscopy of the GlyH(HO) complex formed inside of a cryogenic ion trap reveals how the flexible model peptide structurally responds to solvation by a single water molecule. The resulting one-laser spectrum is quite congested, and the spectral analyses were assisted by both HO/DO substitution and IR-IR double resonance spectroscopy, revealing the presence of two contributing isomers and extensive anharmonic features. Comparisons to structures found via a systematic computational search identified the geometries of these two isomers. The major isomer, with all trans amide bonds and protonation on the terminal amine, represents ∼90% of the overall population. It noticeably differs from the unsolvated GlyH, which exists in two isomeric forms: one with a cis amide bond and the other with protonation on an amide C═O. These results indicate that interactions with just one water molecule can induce significant structural changes, i.e., cis- trans amide bond rotation and proton migration, even as the clustering occurs within an 80 K cryogenic ion trap. Calculations of the isomerization pathways further reveal that the binding energy of the water molecule provides sufficient internal energy to overcome the barriers for the observed structural changes, and the minor solvation isomer results from a small fraction of the ions being kinetically trapped along one of the pathways.
低温离子阱内形成的GlyH(HO)络合物的红外预解离光谱揭示了柔性模型肽如何在结构上响应单个水分子的溶剂化作用。由此产生的单激光光谱相当拥挤，光谱分析借助了HO/DO取代和红外-红外双共振光谱，揭示了两种有贡献的异构体的存在以及广泛的非谐特征。与通过系统计算搜索找到的结构进行比较，确定了这两种异构体的几何结构。主要异构体具有全反式酰胺键且末端胺基质子化，占总体的约90%。它与未溶剂化的GlyH明显不同，未溶剂化的GlyH存在两种异构体形式：一种具有顺式酰胺键，另一种酰胺C═O质子化。这些结果表明，即使聚集发生在80K的低温离子阱内，与单个水分子的相互作用也能引发显著的结构变化，即顺反酰胺键旋转和质子迁移。异构体化途径的计算进一步表明，水分子的结合能提供了足够的内能来克服观察到的结构变化的障碍，次要的溶剂化异构体是一小部分离子沿其中一条途径被动力学捕获的结果。
