ANALYSIS OF THE HELICAL CONFORMATIONS IN POLY(BETA-L-ASPARTATE)S - POLY(ALPHA-N-BUTYL BETA-L-ASPARTATE) AND POLY[ALPHA-(2-METHOXYETHYL) BETA-L-ASPARTATE]

The conformation of the two title poly(beta-L-aspartate)s was analyzed by both molecular mechanics calculations and the linked-atom least-squares (LALS) methodology using X-ray diffraction data previously published by us. A right-handed 13/4 helix composed of 14-member hydrogen-bonded rings was found to be the most favored conformation for the so-called ''hexagonal form'' in which both compounds are known to crystallize. The preferred conformation for the tetragonal form, which is the second crystal modification adopted by the n-butyl derivative, consisted of a right-handed 4/1 helix. Different modes of packing compatible with experimental data were examined for each crystal form, the most favored being those corresponding to space groups P2(1) and P4(1) for hexagonal and tetragonal, respectively. These results appear to be substantially identical to those earlier described for poly(alpha-isobutyl beta-L-aspartate) which is the only poly(beta-L-aspartate) investigated so far by these methods. Comparison between the relative stabilities calculated for the two polymorphs of each polymer allowed one to account for the different tendency to crystallize in the tetragonal form that is experimentally observed among poly(beta-L-aspartate)s possessing side chains of intermediate size.