Determination of stereochemistry stability coefficients of amino acid side‐chains in an amphipathic α‐helix

Abstract: 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 α‐helical peptide. The helix‐destabilizing ability of 19 d ‐amino acid residues in an amphipathic α‐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 (ΔT_m) ranging from ?8.5°C to 30.5°C for the l ‐amino acids and ?9.5°C to 9.0°C 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 (ΔT_m′ = T_mL and T_mD)] ranging from 1°C to 34.5°C. HPLC retention times Δt_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 CD and reversed‐phase HPLC studies, d ‐amino acids with β‐branched side‐chains destabilize α‐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.