Single-conformation infrared spectra in the amide I and amide II regions have been recorded for a total of 34 conformations of three α-peptides, three β-peptides, four αβ-peptides, and one γ-peptide using resonant ion-dip infrared spectroscopy of the jet-cooled, isolated molecules. Assignments based on the amide NH stretch region were in hand, with the amide III data providing additional evidence in favor of the assignments. A set of 21 conformations that represent the full range of H-bonded structures were chosen to characterize the conformational dependence of the vibrational frequencies and infrared intensities of the local amide I and amide II modes and their amide II and amide IIII coupling constants. Scaled, harmonic calculations at the DFT M05-2X6-31G(d) level of theory accurately reproduce the experimental frequencies and infrared intensities in both the amide I and amide II regions. In the amide I region, Hessian reconstruction was used to extract local mode frequencies and amide II coupling constants for each conformation. These local amide I frequencies are in excellent agreement with those predicted by DFT calculations on the corresponding 13C 18O isotopologues. In the amide II region, potential energy distribution analysis was combined with the Hessian reconstruction scheme to extract local amide II frequencies and amide IIII coupling constants. The agreement between these local amide II frequencies and those obtained from DFT calculations on the N-D isotopologues is slightly worse than for the corresponding comparison in the amide I region. The local mode frequencies in both regions are dictated by a combination of the direct H-bonding environment and indirect, backside H-bonds to the same amide group. More importantly, the sign and magnitude of the inter-amide coupling constants in both the amide I and amide II regions is shown to be characteristic of the size of the H-bonded ring linking the two amide groups. These amide II and amide IIII coupling constants remain similar in size for α- β- and γ-peptides despite the increasing number of C-C bonds separating the amide groups. These findings provide a simple, unifying picture for future attempts to base the calculation of both nearest-neighbor and next-nearest-neighbor coupling constants on a joint footing.
Bibliographical noteFunding Information:
W.H.J., E.G.B., C.W.M., and T.S.Z. acknowledge support for this work from NSF CHE-0909619. CWM also acknowledges the “Nationale Akademie der Wissenschaften Leopoldina” for a postdoctoral scholarship (Grant No. BMBF-LPD 9901/8-159 of the “Bundesministerium für Bildung und Forschung”). L.G., S.H.C., and S.H.G. acknowledge support from NSF (CHE-0848847). Pat Bishop synthesized initial batches of Ac-Phe-NHMe and Ac-Phe-Ala-NHMe used in the present study. We would like to thank Michał H. Jamróz for providing us with the latest version of his potential energy distribution analysis program, VEDA 4.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry