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Article

The Anharmonic Vibrational Potential and Relaxation Pathways of the Amide I and II Modes of N-Methylacetamide^†

Lauren P. DeFlores, Ziad Ganim, Sarah F. Ackley, Hoi Sung Chung, and Andrei Tokmakoff*

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

J. Phys. Chem. B, 2006, 110 (38), pp 18973–18980

DOI: 10.1021/jp0603334

Publication Date (Web): June 9, 2006

†

Part of the special issue “Robert J. Silbey Festschrift”.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

Abstract

We investigate the influence of isotopic substitution and solvation of N-methylacetamide (NMA) on anharmonic vibrational coupling and vibrational relaxation of the amide I and amide II modes. Differences in the anharmonic potential of isotopic derivatives of NMA in D₂O and DMSO-d₆ are quantified by extraction of the anharmonic parameters and the transition dipole moment angles from cross-peaks in the two-dimensional infrared (2D-IR) spectra. To interpret the effects of isotopic substitution and solvent interaction on the anharmonic potential, density functional theory and potential energy distribution calculations are performed. It is shown that the origin of anharmonic variation arises from differing local mode contributions to the normal modes of the NMA isotopologues, particularly in amide II. The time domain manifestation of the coupling is the coherent exchange of excitation between amide modes seen as the quantum beats in femtosecond pump−probes. The biphasic behavior of population relaxation of the pump−probe and 2D-IR experiments can be understood by the rapid exchange of strongly coupled modes within the peptide backbone, followed by picosecond dissipation into weakly coupled modes of the bath.

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