Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy of N-methylpropionamide

Abstract

In order to clarify the dynamical aspects of the peptide structure, N-methylpropionamide (NMPA) was investigated as an example of peptide molecules: XCONHY (X=CH 3CH 2 and Y=CH 3 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 G 18 consisting of six symmetry species: A 1, A 2, E 1, E 2, E 3, and E 4. The A 1 and E 2 spectra were observed split in most of b-type transitions, yielding the internal-rotation potential barrier V 3 of 796 (21) cm −1 for CH 3 in the ethyl group referred to as C–CH 3. The spectra of the three E species: E 1, E 3, and E 4 appeared several tens to thousands MHz apart from the corresponding A 1 spectra, suggesting the internal-rotation potential barrier of CH 3 bonded to the nitrogen, called N–CH 3, to be quite low. In sharp contrast with the A 1 spectra, which were well fitted to the ordinary asymmetric-rotor spectral pattern, a few higher-order terms were required to reproduce the E 1 spectra, presumably because of the low N–CH 3 barrier. The spectral analysis thus performed, in fact, led to the V 3 of 80.06487 (14) cm −1, an order of magnitude lower than that of C–CH 3. The E 3 and E 4 spectra were found to form triplets with the corresponding E 1 lines at the center, and the E 3– E 1 and E 4– E 1 splittings were explained essentially by the contributions of the C–CH 3 internal rotation combined with the kinetic-energy coupling between the two methyl groups. The torsion around the C–C bond between the ethyl and carbonyl groups was suggested by an ab initio calculation to be of double minimum nature, but the observed A 1 spectra 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 could not be entirely eliminated. The present results on NMPA along with those obtained on other peptide molecules will be of some significance in clarifying important problems of structural biology such as protein folding and signal transfer through biological systems.