Characterization of copolyaspartates containing a long alkyl side chain

Abstract

Copoly(β-stearyl l-aspartate-β-benzyl l-aspartate) with various degrees of stearylation was synthesized and secondary and higher ordered structure in solid state was examined from the standpoint of composition and temperature of the copolyaspartates. Copolyaspartate with degree of stearylation < 30% transformed in solid state from left-handed α helix to right-handed α helix with increasing temperature. Copolyaspartate with degree of stearylation > 30% caused the reverse transformation, from right-handed α helix to left-handed α helix. The transition from α helix to β form was observed at higher temperature for all copolyaspartates studied. The transitions of helix sense and α helix to β form were irreversible. Copolyaspartate with degree of stearylation > 20% could furthermore cause an intramolecular side chain crystallization of a long alkyl chain. The side chain crystal of such a copolyaspartate melted but a helix backbone was still stable. The helix surrounded by a molten long side chain which plays a role as a solvent could be considered for construction of a thermotropic liquid crystalline phase. Copolyaspartate with a long alkyl side chain was compared with copolyglutamate and the difference of helix stability and side chain crystalline part was interpreted in terms of the competitive hydrogen bonding between NH groups in the backbone and CO groups in the side chain formed in the copolyaspartate.