CONFORMATIONAL-ANALYSIS OF ALPHA-HELICAL POLYPEPTIDES IN 2 OPPOSITE SCREW FORMS - A COMBINED USE OF H-2 NMR AND MD-SIMULATION

In this work, we wish to report the results of conformational analysis of poly(beta-phenethyl L-aspartate) (PPLA) and poly(gamma-benzyl L-glutamate) (PBLG) in two opposite alpha-helical screw sense. Variously deuterated PPLA samples were prepared, and H-2 NMR measurements were carried out in tetrachloroethane at two different temperatures: one for the right-handed and the other for the left-handed alpha-helical state. The rotational isomeric state (RIS) analysis of the H-2 NMR quadrupolar splitting data was carried out, and the sidechain conformations were estimated. The results of the analysis for the right-handed alpha-helical PBLG have been reported previously. In the right-handed alpha-helix regime, i.e., r-PPLA and r-PBLG, the preferred form of the first two bonds was found to be (chi 1 chi 2)=tt, leading to an extended sidechain conformation in the direction perpendicular to the alpha-helical axis. The corresponding arrangements in the 1-form (I-PPLA) are more or less evenly distributed among three states, g(-)t, tt, and g(-)g(+). MD simulations were performed for the side chain flanking the right- and left-handed alpha-helical backbone by using Biosym's Discover program. An alpha-helix fragment consisting of 18 aminoacid residues, Ac-X(18)-NHMe with X=Asp(OPhe) or Glu(OBzl), was adopted as a model for the polymers in the simulation, The preferred sidechain conformations were elucidated as an average over an interval 100-200 ps. The simulations were performed in vacuo as well as in the presence of solvent molecules (chloroform). The agreement was found to be quite reasonable in all three polymer systems for which relevant experimental data are available.