Calculation of the CD Spectrum of a Peptide from Its Conformational Phase Space: The Case of Met-enkephalin and Its Unnatural Analogue.

ABSTRACT

We have investigated the conformational phase spaces of both Met-enkephalin and Ada-enkephalin in 2,2,2-trifluoroethanol in order to connect them to their respective CD spectra. To this end, we have characterized the conformational preferences of the zwitterionic and neutral forms of Met-enkephalin and of both the R- and the S-epimers of Ada-enkephalin, as obtained by classical molecular dynamics. The CD spectrum for each peptide was subsequently obtained with a procedure of successive averaging, which accounts for the behavior of the solvent, the side chains, and the backbone variations of the peptides. To make an appropriate comparison with experimental results, we have produced composite spectra that account for the appropriate contributions of the zwitterionic and neutral forms of the peptides as well as the expected epimeric ratio. Such a procedure results in theoretically obtained CD spectra that show significant promise in terms of reproducing their experimentally measured counterparts.