Conformational states of N-acylalanine dithio esters: correlation of resonance Raman spectra with structures

The conformational states of N-acylalanine dithio esters, involving rotational isomers about the RC(=O)NH--CH(CH3) and NHCH(CH3)--C(=S) bonds, are defined and compared to those of N-acylglycine dithio esters. The structure of N-(p-nitrobenzoyl)-DL-alanine ethyl dithio ester has been determined by X-ray crystallographic analysis; it is a B-type conformer with the amide N atom cis to the thiol sulfur. Raman and resonance Raman (RR) measurements on this compound and for the B conformers of solid N-benzoyl-DL-alanine ethyl dithio ester and N-(beta-phenylpropionyl)-DL-alanine ethyl dithio ester and its NHCH(CD3)C(=S) and NHCH(CH3)13C(=S) analogues are used to set up a library of RR data for alanine-based dithio esters in a B-conformer state. (Methyloxycarbonyl)-L-phenylalanyl-L-alanine ethyl dithio ester crystallizes in an A-like conformational state wherein the alanine N atom is nearly cis to the thiono S atom (C=S) [Varughese, K.I., Angus, R.H., Carey, P.R., Lee, H., & Storer, A.C. (1986) Can. J. Chem. 64, 1668-1673]. RR data for this solid material in its isotopically unsubstituted and CH(C-D3)C(=S) and CH(CH3)13C(=S) forms provide information on the RR signatures of alanine dithio esters in A-like conformations. RR spectra are compared for the solid compounds, for N-(p-nitrobenzoyl)-DL-alanine, N-(beta-phenylpropionyl)-DL-alanine, and (methyloxycarbonyl)-L-phenylalanyl-DL-alanine ethyl dithio esters, and for several 13C=S- and CD3-substituted analogues in CCl4 or aqueous solutions. The RR data demonstrate that the alanine-based dithio esters take up A, B, and C5 conformations in solution.(ABSTRACT TRUNCATED AT 250 WORDS)