Cyclic Constraints on Conformational Flexibility in 纬-Peptides: Conformation Specific IR and UV Spectroscopy

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• 作者：Patrick S. Walsh ; Ryoji Kusaka ; Evan G. Buchanan ; William H. James III ; Brian F. Fisher ; Samuel H. Gellman ; Timothy S. Zwier
• 刊名：Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：November 27, 2013
• 年：2013
• 卷：117
• 期：47
• 页码：12350-12362
• 全文大小：588K
• 年卷期：v.117,no.47(November 27, 2013)
• ISSN：1520-5215

文摘

Single-conformation spectroscopy has been used to study two cyclically constrained and capped 纬-peptides: Ac-纬_ACHC-NHBn (hereafter 纬_ACHC, Figure 1a), and Ac-纬_ACHC-纬_ACHC-NHBn (纬纬_ACHC, Figure 1b), under jet-cooled conditions in the gas phase. The 纬-peptide backbone in both molecules contains a cyclohexane ring incorporated across each C尾-C纬 bond and an ethyl group at each C伪. This substitution pattern was designed to stabilize a (g+, g+) torsion angle sequence across the C伪鈥揅尾鈥揅纬 segment of each 纬-amino acid residue. Resonant two-photon ionization (R2PI), infrared鈥搖ltraviolet hole-burning (IR鈥揢V HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been used to probe the single-conformation spectroscopy of these molecules. In both 纬_ACHC and 纬纬_ACHC, all population is funneled into a single conformation. With RIDIR spectra in the NH stretch (3200鈥?500 cm^鈥?) and amide I/II regions (1400鈥?800 cm^鈥?), in conjunction with theoretical predictions, assignments have been made for the conformations observed in the molecular beam. 纬_ACHC forms a single nearest-neighbor C9 hydrogen-bonded ring whereas 纬纬_ACHC takes up a next-nearest-neighbor C14 hydrogen-bonded structure. The gas-phase C14 conformation represents the beginning of a 2.6₁₄-helix, suggesting that the constraints imposed on the 纬-peptide backbone by the ACHC and ethyl groups already impose this preference in the gas-phase di-纬-peptide, in which only a single C14 H-bond is possible, constituting one full turn of the helix. A similar conformational preference was previously documented in crystal structures and NMR analysis of longer 纬-peptide oligomers containing the 纬_ACHC subunit [Guo, L., et al. Angew. Chem. Int. Ed. 2011, m>50m>, 5843鈭?846]. In the gas phase, the 纬_ACHC-H₂O complex was also observed and spectroscopically interrogated in the molecular beam. Here, the monosolvated 纬_ACHC retains the C9 hydrogen bond observed in the bare molecule, with the water acting as a bridge between the C-terminal carbonyl and the 蟺-cloud of the UV chromophore. This is in contrast to the unconstrained 纬-peptide-H₂O complex, which incorporates H₂O into both C9 and amide-stacked conformations.