Conformation and crystal structure of poly(alpha-cycloalkyl-beta-L-aspartate)s

The conformation and crystal structure of two nylon 3 derivatives bearing a cycloalkoxycarbonyl group on the beta-carbon of the main chain, namely, poly(alpha-cyclopentyl-beta-L-aspartate) and poly(alpha-cyclohexyl-beta-L-aspartate), were examined. X-ray diffraction data revealed that both compounds adopt a 13/4 helical conformation of the type usually found for the hexagonal form of poly(beta-L-aspartate)s bearing acyclic alkyl side chains. Whereas poly(alpha-cyclopentyl-beta-L-aspartate) crystallized in a monoclinic structure (pseudohexagonal, space group P2(1)), the cyclohexyl derivative deviated from the general pattern, crystallizing in a slightly distorted hexagonal lattice in the space group P1. AMBER energy optimization algorithms and linked-atom least-squares refinement applied to X-ray data were used to analyze the structure 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 sterically incompatible with the existence of mixed populations of axial and equatorial conformers within the same molecule. The analysis of the mimicked crystal favored the antiparallel arrangement of chains, but it was unable to discriminate between the different crystal models compatible with diffraction data. H-1 NMR measurements in solution proved that the helical conformation adopted in the solid state is still retained in the liquid state. C-13 CP-MAS NMR experiments furnished further evidence on the structural conclusions derived from theoretical methods.