EFFECTS OF DEHYDROALANINE ON PEPTIDE CONFORMATIONS

The conformational preferences of dehydroalanine were examined through a combined approach of X-ray diffraction, NMR spectroscopy, and molecular modeling. Monte Carlo simulated annealing was used in conjunction with X-ray diffraction data and semiempirical quantum mechanical calculations to determine appropriate parameters for modeling dehydroalanine with the DISCOVER consistent valence force field. The two molecules in the asymmetric unit of N-acetyldehydroalanine N'-methylamide were simulated in the crystalline environment using these optimized parameters. The rms deviations between simulation and experimental data for heavy atom bond lengths, bond angles, and torsions were 0.021 angstrom, 1.9-degrees, and 8.7-degrees, respectively. The dehydroalanine-containing ring A fragment of nisin and two analogs with either L- or D-alanine substituted for dehydroalanine were synthesized and examined by NMR spectroscopy. Using distance geometry followed by conformational energy minimization with our optimized parameters, families of conformations were determined for each molecule which satisfied the observed backbone NOE, J(alpha-N) coupling constants, and temperature coefficients. Dehydroalanine adopted a roughly planar conformation, with trans orientations for the phi and psi-torsions, and induced an inverse gamma-turn in the preceding residue. Similar effects have been observed for linear, dehydroalanine-containing peptides in solution and as crystals, suggesting that dehydroalanine exerts a powerful conformational influence independent of other constraints. The conformational preferences of the L- and D-Ala ring A analogs differed substantially from each other and from the ring A fragment.