Rotational spectra of the ground torsional state of the N-acetyl alanine methyl ester molecule, a derivative of the alanine dipeptide biomimetic molecule, have been measured using a mini Fourier transform spectrometer between 10 and 20 GHz as part of a project undertaken to determine the conformational structures of various peptide mimetics from the torsion-rotation parameters of low-barrier methyl tops. Torsional rotation splittings from two of the three methyl tops (the CH3 group capping the ?NH-C(=O)- group with a low potential barrier and the CH3 group capping the ?C(=O)-O- group with an intermediate potential barrier), account for most of the observed lines. In addition to the AA-state, two E-states have been assigned and include an AE-state having a V3 barrier of 396.29(8) cm-1 and an EA-state having a lower torsional barrier of 64.98(4) cm-1. (The two letters indicate the symmetry species of the wave function with respect to the nuclear-permutation-inversion group of the two tops; AA, AE, EA, and EE correspond to A1, E2, E1, and E3/E4 respectively in the permutation-inversion group G18). A third set of unassigned splittings, involving the EA-state, are likely associated with the two remaining EE-states. We have fit the observed torsional splittings corresponding to the AA, AE, and EA states using two different theoretical fitting procedures. In the first method, rotational transitions within the AA torsional state are fit separately from transitions within the AE and EA torsional states.
Citation: Journal of Molecular Spectroscopy
Pub Type: Journals
Alanine dipeptide, biomolecular structure, methyl torsional barriers, peptide bonds, torsional-rotation models