| Abstract
| - The conformation and crystal structure of two nylon 3derivatives bearing a cycloalkoxycarbonyl group onthe β-carbon of the main chain, namely,poly(α-cyclopentyl-β-l-aspartate) andpoly(α-cyclohexyl-β-l-aspartate),were examined. X-ray diffraction data revealed that both compoundsadopt a 13/4 helical conformation ofthe type usually found for the hexagonal form ofpoly(β-l-aspartate)s bearing acyclic alkyl sidechains. Whereaspoly(α-cyclopentyl-β-l-aspartate) crystallized ina monoclinic structure (pseudohexagonal, space groupP21),the cyclohexyl derivative deviated from the general pattern,crystallizing in a slightly distorted hexagonallattice in the space group P1. AMBER energyoptimization algorithms and linked-atom least-squaresrefinementapplied to X-ray data were used to analyze the structure ofpoly(α-cyclohexyl-β-l-aspartate).Energycalculations made on an isolated chain revealed that the cyclohexylside group is in the equatorial chairconformation and that the 13/4 helix will be sterically incompatiblewith the existence of mixed populationsof axial and equatorial conformers within the same molecule. Theanalysis of the mimicked crystal favoredthe antiparallel arrangement of chains, but it was unable todiscriminate between the different crystal modelscompatible with diffraction data. 1H NMR measurementsin solution proved that the helical conformationadopted in the solid state is still retained in the liquid state.13C CP−MAS NMR experiments furnishedfurther evidence on the structural conclusions derived from theoreticalmethods.
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