Secondary structure of peptides. 4: 13 C‐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 13 C‐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) NH 2 prevent the precipitation of oligoalanine and, thus, the formation of a β‐sheet structure. 13 C‐nmr CP/MAS measurements also show that tri‐ and tetra‐ L ‐alanines form insoluble β‐sheet structures.