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

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• 作者：Lauren P. DeFlores ; Ziad Ganim ; Sarah F. Ackley ; Hoi Sung Chung ; Andrei Tokmakoff
• 刊名：Journal of Physical Chemistry B
• 出版年：2006
• 出版时间：September 28, 2006
• 年：2006
• 卷：110
• 期：38
• 页码：18973 - 18980
• 全文大小：487K
• 年卷期：v.110,no.38(September 28, 2006)
• ISSN：1520-5207

文摘

We investigate the influence of isotopic substitution and solvation of N-methylacetamide (NMA) on anharmonicvibrational coupling and vibrational relaxation of the amide I and amide II modes. Differences in the anharmonicpotential of isotopic derivatives of NMA in D₂O and DMSO-d₆ are quantified by extraction of the anharmonicparameters 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 anharmonicpotential, density functional theory and potential energy distribution calculations are performed. It is shownthat the origin of anharmonic variation arises from differing local mode contributions to the normal modesof the NMA isotopologues, particularly in amide II. The time domain manifestation of the coupling is thecoherent 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 beunderstood by the rapid exchange of strongly coupled modes within the peptide backbone, followed bypicosecond dissipation into weakly coupled modes of the bath.