CONFORMATIONAL STUDIES OF THE N-ACETYL-S-METHYL-L-CYSTEINYL-L-HISTIDYLGLYCINE METHYL AMIDE CATION USING THE AM1 METHOD

Conformational studies of N-acetyl-S-methyl-L-cysteine (AcCysMe), the N-acetyl-L-histidylglycine cation([AcHis-Gly](+)) and the N-acetyl-S-m ethyl-L-cysteinyl-L-histidylglycine methyl amide cation ([AcCysMe-His- Gly-NHMe](+)) have been carried out using the AM1 semiempirical method . Three-dimensional energy surfaces of AcCysMe and [AcHis-Gly](+) were obtained. The conformation of [AcCysMe-His-Gly-NHMe] + was computed u sing the combined optimum structures of AcCysMe and [AcHis-Gly](+)(the acetyl group was deleted). From our calculations, the most stable con former of the tripeptide exhibits a hydrogen bond between the carbonyl group of the glycine and the ring-NH+ of the histidine moiety. This c onformation is in accordance with that of a tripeptide chelate of pall adium(II) proposed in one of our previous articles. These results prov ide direct evidence for the importance of the His18 residue of cytochr ome c concerning the selective cleavage of cytochrome c promoted by pa lladium(II) complexes.