Growth mechanism of crystals of glycine/S-methyl-L-cysteine copolymer in the course of polymerization

In order to examine the growth mechanism of copolymer crystals formed during polymerization, the heterogeneous copolymerization of glycine N-carboxy anhydride (2,5-dioxo-1-oxa-3-azacyclopentane) and S-methyl-L-cysteine N-carboxy anhydride (4-methylthiomethyl-2,5-dioxo-1-oxa-3-azacyclopentane) was studied in acetonitrile. In all the polymerization systems, the crystal growth occurs through formation of the cross-β-type structure. The high conversion in the glycine-rich systems is accounted for by the widening of the cross-section of the β-chain in the backbone crystal, due to the introduction of S-methyl-L-cysteine residues into the polyglycine chain. On the other hand, the narrowing of the cross-sectional area per chain in the skeleton crystals of the oligomer gave rise to the lowering of the conversion in the S-methyl-L-cysteine-rich systems. This shows the importance of the effect of the cross-sectional area of the oligomer chain (of the β structure) in the crystal formed in the begining relative to that of the growing (α-helical) chain. The glycine residue seems to be incorporated into the crystalline lattice of the S-methyl-L-cysteine and vice versa.