Dynamical Structure of Peptide Molecules: Fourier Transform Microwave Spectroscopy of N-Methylpropionamide

Published

January 1, 2003

Author(s)

Y Kawashima, R D. Suenram, E Hirota

Abstract

To clarify the dynamical aspects of the peptide structure, N-methylpropionamide (NMPA) was investigated as an example of peptide molecules: XCONHY (X=CH₃CH₂ and Y = CH₃ for NMPA), paying special attention to the internal rotation of the two methyl groups. NMPA was found to have an almost planar skeleton with an extended syn/trans conformation, as indicated by the observed value of I_aa + I_bb -I_cc, and its rotational spectra were interpreted in terms of group G₁₈ consisting of six symmetry species: A₁, A₂, E₁, E₂, E₃, and E₄. The A₁ and E₂ spectra were observed to be split for most of the b-type transitions, yielding the internal-rotation potential barrier V₃ of 796 (21) cm^-1 for CH₃ in the ethyl group referred to as C-CH₃. The spectra of the three E species: E₁, E₃, and E₄ are separated several tens to thousands of MHz from the corresponding A₁ spectrum, suggesting a low internal-rotation potential barrier of the CH₃ bonded to the nitrogen, called N-CH₃. In sharp contrast with the A₁ spectra, which were well fitted to the ordinary asymmetric-rotor spectral pattern, a few higher-order terms were required to reproduce the E₁ spectrum, presumably because of the low N-CH₃ barrier. The spectral analysis thus performed led to a V₃ of 80.06487 (14) cm^-1, an order of magnitude lower than that of C-CH₃. The E₃ and E₄ spectra are found to form triplets with the corresponding E₁ lines at the center, and the E-3-E₁ and E₄-E₁ splittings are explained essentially by the contributions of the C-CH₃ internal rotation combined with the kinetic-energy coupling between the two ethyl groups. The torsion around the C - C bond between the ethyl and carbonyl groups is suggested by an ab initio calculation to be of double minimum nature, but the observed A₁ spectrum did not show any indication of such a double-minimum potential for the C-C torsion, although the possibility of a small hump being present at a planar conformation can not be entirely eliminated.

Citation

Journal of Molecular Spectroscopy

Pub Type

Journals

Keywords

Fourier transform microwave spectrum, internal rotation, molecular beam spectrum, N-methyl propionamide, peptide mimetics, rotational spectrum

Citation

Kawashima, Y. , Suenram, R. and Hirota, E. (2003), Dynamical Structure of Peptide Molecules: Fourier Transform Microwave Spectroscopy of N-Methylpropionamide, Journal of Molecular Spectroscopy (Accessed April 25, 2024)