SynopsisPrimary and tertiary amine-initiated polymerizations of L-alanine-N-carboxyanhydride (L-Ala-NCA) were conducted a t 20 or 100°C in a variety of solvents. The 75.5-MHz '3C-nmr CP/MAS spectra of the resulting poly(L-alanines) revealed that all samples contain both &-helix and pleated-sheet structures. Depending on the reaction conditions the a-helix content varied between ca. 1 and 99%. Reprecipitation from aprotic nonsolvents does not change the a-helix/@-sheet ratio, indicating that this ratio is thermodynamiccy 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 5 11 is responsible for the @-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(ethy1ene oxide) NH;I prevent the precipitation of oligoalanine and, thus, the formation of a @-sheet structure. 13C-nmr CP/MAS measurements also show that tri-and tetra-L-alanines form insoluble @-sheet structures.

SynopsisA newly designed host-guest approach is introduced as a experimental tool to explore the relationship between the sequence of peptides and their secondary structure. From the CD spectra of the host-guest peptides studied, a tentative scale for the a-helix potential in 2,Z.Z-trifluoroethanol of guest amino acids is delineated. The conformational preferences are also examined in &structure supporting media (solid state, CH,Cl,, CH,OH, H20) using ir-absorption and CD techniques. Scales for the &forming tendency of guest amino acid residues in the different media are delineated. It is shown that the preferred conformation of the host-guest peptides is a function of the medium, the chain length, and the protecting groups. Given the fact that conformational effects are important in peptide synthesis, the tentative scales may serve as a guideline to predict secondary structures of side-chain-protected or -deprotected peptides in a given solvent, complementing the well-known empirical conformational prediction parameters.

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The disruption of the sparingly soluble, self-associated species (p-structure) of the polymer-bound, N-protected peptides Boc-~-Ala-Gly-~-lle-( L -A I ~) ~-N HPEG and Boc-(~-Ala),-Gly-~-lle-(L-AI~),-N H PEG [PEG = poly(ethy1ene glycol)] in methylene chloride by increasing amounts of N,N-dimethylformamide or dimethyl sulphoxide has been monitored by following the disappearance of the intense amide I or A i.r. bands of the strongly intermolecularly hydrogen-bonded molecules; this suggests the use of this method for the quantitative titration of the extent of self-association in PEG -bound peptides.