6533b7d1fe1ef96bd125d4ec

RESEARCH PRODUCT

Secondary structure of peptides. 4:13C-Nmr CP/MAS investigation of solid oligo- and poly(L-alanines)

subject

description

Primary and tertiary amine-initiated polymerizations of L-alanine-N-carboxyanhydride (L-Ala-NCA) were conducted at 20 or 100°C in a variety of solvents. The 75.5-MHz 13C-nmr CP/MAS spectra of the resulting poly(L-alanines) revealed that all samples contain both α-helix and pleated-sheet structures. Depending on the reaction conditions the α-helix content varied between ca. 1 and 99%. Reprecipitation from aprotic nonsolvents does not change the α-helix/β-sheet ratio, indicating that this ratio is thermodynamically controlled. Since relatively large amounts of oligopeptides of degree of polymerization (DP) 4–6 can be extracted by means of acetic acid, it is concluded that (a) most poly(L-alanines) possess a bimodal molecular weight distribution, (b) the oligopeptide fraction with DP ⩽ 11 is responsible for the β-sheet fraction of all samples, and (c) the two-stage crystal growth proposed by Komoto and Kawai is not correct. Solubilizing initiators such as poly(ethylene oxide) NH2 prevent the precipitation of oligoalanine and, thus, the formation of a β-sheet structure. 13C-nmr CP/MAS measurements also show that tri- and tetra-L-alanines form insoluble β-sheet structures.