The structure and resonance Raman spectra – structure correlations for methyloxycarbonyl-L-phenylalanyl-L-alanine ethyl dithioester

Abstract

The structure of methyloxycarbonyl-L-phenylalanyl-L-alanine ethyl dithioester has been determined by X-ray crystallographic analysis. The N—C—C—S(thiol) and C—N—C—C(S) torsional angles are 141(1)° and −69(2)°, respectively. Consequently, the N and thiono (C=S) sulfur atoms are in close contact and the [Formula: see text](thiono) distance is 3.078(5) Å. The structure is a perturbed A conformer, which we designate A′, by analogy to a previously characterised A conformer (Huber etal., Biochemistry, 21 3109 (1982)). The [Formula: see text] angles for the Phe residue are −87(2) and 151(2)°, respectively. A Raman spectroscopic study of a single crystal of the LL form and a resonance Raman (RR) spectroscopic study of powdered methyloxycarbonyl-L-phenylalanyl-DL-alanine ethyl dithioester establish the RR signature of the dithio moiety for the compound in the A′ form. The RR spectrum of the dithioester in CCl4 defines the RR signature for the C5 form in solution, while the RR spectrum from a solution in H2O/CH3CN provides information on the signatures of the A and B conformers. As for the case of N-benzoyl-DL-alanine dithioester, the A and C5 signatures of the title compound closely resemble those for glycine-based dithioesters but the key marker bands for the B conformer are shifted by ≈30 cm−1 to lower frequency compared to those for a glycine dithioester.