Molecular structure of 5-methyl thiophene acryloyl ethyl thiolester: A vibrational spectroscopic and density functional theory study

Enzyme-substrate intermediates involving the acyl group 5-methyl thiophene acryloyl (5-MTA) bound to the active site of an enzyme via a sulfur or selenium atom have been characterized by Raman spectroscopy (e.g., J. D. Doran and P. R. Carey, Biochemistry 1996, 35, 12495–12502, and M. J. O'Connor et al., J Amer Chem Soc 1996, 118, 239–240). Raman difference spectra reveal the Raman spectrum of the acyl group in the active site and, in turn, these can be used to probe acyl group conformation and active site forces and interactions. In order to improve the understanding of the relationship between conformational states and vibrational spectra of 5-MTA thiolesters, calculations based on a density functional theory analysis are undertaken for 5-methyl thiophene acryloyl ethyl ester. The calculations provide the precise geometries and energies of rotomers of 5-MTA ethyl thiolester involving rotational isomerism about the CC single bonds flanking the ethylenic linkage and the SC bond linking the ethyl group to the sulfur atom. The calculations also provide the vibrational spectrum for each conformer and these predictions are compared with the experimental Raman an IR data for the thiolester in carbon tetrachloride. Modes are identified that can act as conformational markers for isomerism about the CC and SC₂H₅ single bonds. These findings are used to identify the two conformational states giving rise to the Raman spectrum of the 5-MTA-S-enzyme formed by the viral cysteine protease HAV-3C. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 201–218, 1999