Determination of stereochemistry stability coefficients of amino acid side-chains in an amphipathic alpha-helix.

We describe here a systematic study to determine the effect on secondary structure of d-amino acid substitutions in the nonpolar face of an amphipathic alpha-helical peptide. The helix-destabilizing ability of 19 d-amino acid residues in an amphipathic alpha-helical model peptide was evaluated by reversed-phase HPLC and CD spectroscopy. l-Amino acid and d-amino acid residues show a wide range of helix-destabilizing effects relative to Gly, as evidenced in melting temperatures (DeltaTm) ranging from -8.5 degrees C to 30.5 degrees C for the l-amino acids and -9.5 degrees C to 9.0 degrees C for the d-amino acids. Helix stereochemistry stability coefficients defined as the difference in Tm values for the l- and d-amino acid substitutions [(DeltaTm' = TmL and TmD)] ranging from 1 degrees C to 34.5 degrees C. HPLC retention times [DeltatR(XL-XD)] also had values ranging from -0.52 to 7.31 min at pH 7.0. The helix-destabilizing ability of a specific d-amino acid is highly dependent on its side-chain, with no clear relationship to the helical propensity of its corresponding l-enantiomers. In both CD and reversed-phase HPLC studies, d-amino acids with beta-branched side-chains destabilize alpha-helical structure to the greatest extent. A series of helix stability coefficients was subsequently determined, which should prove valuable both for protein structure-activity studies and de novo design of novel biologically active peptides.