Determination of stereochemistry stability coefficients of amino acid side-chains in an amphipathic α-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 a-helical peptide. The helix-destabilizing ability of 19 D-amino acid residues in an amphipathic a-helical model peptide was evaluated by reversed-phase HPLC and CID 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 (DeltaT(m)) ranging from -8.5degreesC to 30.5degreesC for the L-amino acids and -9.5degreesC to 9.0degreesC for the D-amino acids. Helix stereochemistry stability coefficients defined as the difference in T-m values for the L- and D-amino acid substitutions [DeltaT'(m) =T-mL and T-MD)] ranging from 1 C to 34.5degreesC. HPLC retention times [Deltat(R)(X-L-X-D)] 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 CID and reversed-phase HPLC studies, D-amino acids with P-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.