SEQUENTIAL HYDROPHOBIC POLYPEPTIDES CONSISTING OF LEUCINE AND ISOLEUCINE - SYNTHESIS, CONFORMATIONAL CHARACTERIZATION IN THE SOLID-STATE, AND GOVERNING FACTORS FOR THE SPECIFICATION IN THE BETA-STRUCTURE ALPHA-HELIX DECISION

Six new sequential hydrophobic polypeptides consisting of leucyl and i soleucyl residues were synthesized by polycondensation of peptide acti ve esters. Solid samples of these polypeptides were obtained by fast r eprecipitation from a solution in 1,1,1,3,3,3-hexafluoropropan-2-ol (H FIP) or HFIP/dichloroacetic acid with diethyl ether. The conformation of the solid sample was determined by IR spectroscopy as follows: the beta-structure for (Leu-Ile-Ile-Leu)(n), a mixture of the beta-structu re and alpha-helix for (Leu-Ile-Leu)(n), and the alpha-helix for (Ala- Ile-Ile-Leu)(n), (D-Leu-Ile-Ile-Leu)(n), (Leu-Ile-Ile-Gly)(n), and (Le u-Ile-Ile-Aib)(n). The factor governing the specification of the beta- structure/alpha-helix was deduced to be the consecutiveness of the tid ily aligned hydrophobic side chains of the amino acid residues to affo rd sterically well-regulated intermolecular aggregation of the polypep tides prior to the conformational preference of the individual amino a cid residue.