Physical and Theoretical Chemistry Laboratory, Chemistry Department, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.
J Phys Chem A. 2008 May 22;112(20):4608-16. doi: 10.1021/jp800069n.
The conformational structures of protonated polyalanine peptides, Ala(n)H(+), have been investigated in the gas phase for n = 3, 4, 5, and 7 using a combination of resonant-infrared multiphoton dissociation (R-IRMPD) spectroscopy in the NH and OH stretch regions and quantum chemical calculations. Agreement between theoretical IR and experimental R-IRMPD spectral features has enabled the assignment of specific hydrogen-bonded conformational motifs in the short protonated peptides and revealed their conformational evolution under elevated-temperature conditions, as a function of increasing chain length. The shortest peptide, Ala(3)H(+), adopts a mixture of extended and cyclic chain conformations, protonated, respectively, at a backbone carbonyl or the N-terminus. The longer peptides adopt folded, cyclic, and globular charge-solvated conformations protonated at the N-terminus, consistent with previous ion-mobility studies. The longest peptide, Ala(7)H(+), adopts a globular conformation with the N-terminus completely charge-solvated, demonstrating the emergence of "physiologically relevant" intramolecular interactions in the peptide backbone. The computed conformational relative free energies highlight the importance of entropic contributions in these peptides.
采用 NH 和 OH 伸缩区域的共振红外多光子解离（R-IRMPD）光谱结合量子化学计算，研究了气相中质子化多丙氨酸肽，Ala(n)H(+)，中 n = 3、4、5 和 7 的构象结构。理论 IR 和实验 R-IRMPD 光谱特征之间的一致性，使得能够对短质子化肽中的特定氢键构象基序进行分配，并揭示了它们在高温条件下的构象演变，作为链长增加的函数。最短的肽，Ala(3)H(+)，采用伸展和环状链构象的混合物，分别在骨架羰基或 N 末端质子化。较长的肽采用折叠、环状和球形电荷溶剂化构象，在 N 末端质子化，与先前的离子迁移研究一致。最长的肽，Ala(7)H(+)，采用完全电荷溶剂化的球形构象，N 末端完全电荷溶剂化，证明了肽骨架中出现“生理相关”的分子内相互作用。计算的构象相对自由能突出了这些肽中熵贡献的重要性。
