HYDROGEN-BONDING AND ISOMERIZATION IN THIOAMIDE PEPTIDE DERIVATIVES

The conformations of fourteen thiopeptides in three solvents (acetonit rile, methylene chloride, DMSO) are discussed as inferred by interpret ation of the amide infrared absorptions. The spectra indicate that the enhanced acidity of the thioamide N-H proton (and hence the stronger hydrogen-bonding donor potential) promotes the formation of C-5 ring s tructures that are not often found in unsubstituted peptides, and stro ng C(S)-N-H --> O=C hydrogen bonds render both C-7 (gamma-turn) and C- 10 (beta-turn) structures more stable than their oxoamide counterparts . The ability of infrared spectra to discriminate among C-5, C-7, and C-10 structures is evaluated and discussed.