CONFORMATION AND CRYSTAL-STRUCTURE OF POLY(ALPHA-CYCLOALKYL-BETA-L-ASPARTATE)S

The conformation and crystal structure of two nylon 3 derivatives bear ing a cycloalkoxycarbonyl group on the beta-carbon of the main chain, namely, poly(alpha-cyclopentyl-beta-L-aspartate) and poly(alpha-cycloh exyl-beta-L-aspartate), were examined. X-ray diffraction data revealed that both compounds adopt a 13/4 helical conformation of the type usu ally found for the hexagonal form of poly(beta-L-aspartate)s bearing a cyclic alkyl side chains. Whereas poly(alpha-cyclopentyl-beta-L-aspart ate) crystallized in a monoclinic structure (pseudohexagonal, space gr oup P2(1)), the cyclohexyl derivative deviated from the general patter n, crystallizing in a slightly distorted hexagonal lattice in the spac e group P1. AMBER energy optimization algorithms and linked-atom least -squares refinement applied to X-ray data were used to analyze the str ucture of poly(alpha-cyclohexyl-beta-L-aspartate). Energy calculations made on an isolated chain revealed that the cyclohexyl side group is in the equatorial chair conformation and that the 13/4 helix will be s terically incompatible with the existence of mixed populations of axia l and equatorial conformers within the same molecule. The analysis of the mimicked crystal favored the antiparallel arrangement of chains, b ut it was unable to discriminate between the different crystal models compatible with diffraction data. H-1 NMR measurements in solution pro ved that the helical conformation adopted in the solid state is still retained in the liquid state. C-13 CP-MAS NMR experiments furnished fu rther evidence on the structural conclusions derived from theoretical methods.