CONFORMATION DEPENDENCE OF THE SH AND CS STRETCH FREQUENCIES OF THE CYSTEINE RESIDUE

In order to relate the observed SH and CS stretch frequencies of the cysteine residue in proteins more closely to its conformation, we have done normal mode calculations on a model for this structure, viz., (CCONH) (CNHCO) CHCH2SH. A range of chi1 and chi2 were studied, combined with the phi,psi of the alpha-helix, beta-sheet, glutathione, and extended-helix conformations. The force field was a combination of a scaled ab initio force field of the -CH2SH group, obtained from ethanethiol and tested on 1-propanethiol and 3-thiol-N-methylpropionamide, and our empirical force field for the peptide group. The results provide more detailed structure-spectra correlations than are possible from experimental studies of model compounds.