Mimicking the First Turn of an 伪-Helix with an Unnatural Backbone: Conformation-Specific IR and UV Spectroscopy of Cyclically Constrained 尾/纬-Peptides

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• 作者：Joseph R. Gord ; Patrick S. Walsh ; Brian F. Fisher ; Samuel H. Gellman ; Timothy S. Zwier
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：July 17, 2014
• 年：2014
• 卷：118
• 期：28
• 页码：8246-8256
• 全文大小：482K
• ISSN：1520-5207

文摘

The folding preferences of two capped, constrained 尾/纬-dipeptide isomers, Ac-尾_ACPC-纬_ACHC-NHBn and Ac-纬_ACHC-尾_ACPC-NHBn, (designated 尾纬 and 纬尾, respectively), have been investigated using single- and double-resonance ultraviolet and infrared spectroscopy in the gas phase. These capped 尾/纬-dipeptides have the same number of backbone atoms between their N- and C-termini as a capped 伪-tripeptide and thus serve as a minimal structural unit on which to test their ability to mimic the formation of the first turn of an 伪-helix. Resonant two-photon ionization and UV鈥揢V hole-burning spectroscopy were performed in the S₀鈥揝₁ region, revealing the presence of three unique conformations of 尾纬 and a single conformation of 纬尾. Resonant ion-dip infrared spectra were obtained in the NH stretch region from 3300 to 3500 cm^鈥? and in both the amide I and amide II regions from 1400 to 1800 cm^鈥?. These infrared spectra were compared to computational predictions from density functional theory calculations at the M05-2X/6-31+G(d) level, leading to assignments for the observed conformations. Two unique bifurcated C8/C13 H-bonded ring structures for 尾纬 and a single bifurcated C9/C13 H-bonded ring structure for 纬尾 were observed. In all cases, the H-bonding patterns faithfully mimic the first full turn of an 伪-helix, most notably by containing a 13-membered H-bonded cycle but also by orienting the interior amide group so that it is poised to engage in a second C13 H-bond as the 尾/纬-peptide lengthens in size. The structural characteristics of the 尾/纬-peptide version of the 13-helix turn are compared with the 伪-helix counterpart and with a reported crystal structure for a longer 尾/纬-peptide oligomer.