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

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.