Crystallization of polypeptides during polymerization

Abstract

In order to pursue the possibilities of formation of extended chain crystals in the course of polymerization, kinetics of the heterogeneous polymerization of L- and DL-alanine N-carboxy anhydride (NCA) initiated by butylamine in acetonitrile and the morphologies of the resultant polymers were studied. The fibers formed in the beginning of polymerization thickened as the polymerization proceeded. The growing chain molecules, lost solubility in the system when they attained a degree of polymerization of 3 to 4 and crystallized in the form of ribbon-like fibers in which molecules with the β-conformation were aligned perpendicularly to the fiber axis. As the growing chains attained a critical length by further polymerization, they began to take the α-helical conformation. The crystals grew successively thereafter, taking the α-helical conformation through addition of the monomer molecules onto the reactive chain ends. Possible mechanisms are proposed for the nucleation and the subsequent growth of the crystals in the course of polymerization. The polymerization of NCA's of glycine and β-alanine has also been studied. The polymer precipitated in the form of spherulites consisting of ribbon-like crystalline units. The molecules in the ribbon-like crystals were arranged antiparallel, forming a hydrogen-bonded sheet. Homopolymerizations of other NCA's and copolymerizations were also examined in the same heterogeneous system.