Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment and General Theory 2008-08-21

Elementary peptide motifs in the gas phase: FTIR aggregation study of formamide, acetamide, N-methylformamide, and N-methylacetamide.

Merwe Albrecht, Corey A Rice, Martin A Suhm

Index: J. Phys. Chem. A 112(33) , 7530-42, (2008)

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Abstract

Cold, isolated peptide model compounds and their aggregates are generated in pulsed supersonic jet expansions and detected by FTIR spectroscopy in the amide-A region, complemented by amide-I spectra. The most stable, symmetric dimer of formamide is unambiguously assigned in the gas phase for the first time, also by comparison to the analogous acetamide dimer. Efficient quenching of a hot-state Fermi resonance by cooling of the dimers is invoked. As the preferred relative orientation of the C=O and N-H groups in N-methylated formamide and acetamide is trans, these compounds show a fundamentally different dimerization pattern. Their most stable dimers, which would be analogous to those of formamide and acetamide, remain undetected as a consequence of kinetic control in the jet. Accurate benchmark quantities for multidimensional vibrational treatments of these peptide models are derived, and the influence of methyl groups on the N-H stretching dynamics is discussed.