Synthesis and conformational study in solution and in solid state of oligopeptides containing L-leucine and glycine

The synthesis and conformational studies of the oligopeptide N-tert.-butyloxycarbonyl-L-Leu-(L-Leu-Gly)n-OBzl (n = 1, 3, 5) and N-tert.-butyloxycarbonyl-(L-Leu-Gly)2-OBzl are described. The peptides were synthesized by stepwise and fragment condensation techniques using dicyclohexylcarbodiimide as the condensing agent in solution. The conformational study of the oligopeptides was carried out using CD, u.v. and i.r. spectra. The conformation in solution was examined in trifluoroethanol, hexafluoroisopropanol, hexafluoroacetone trihydrate, and methanol. CD spectra in trifluoroethanol exhibited a gradual variation with increasing peptide chain length. This can be interpreted as a formation of an ordered structure which is already present in the heptapeptide and, to a greater extent, in the undecapeptide. The results obtained from the CD profiles and i.r. spectra showed the presence of beta structure with antiparallel chains in the heptapeptide and undecapeptide. Finally, CD spectra revealed in trifluoroethanol-water solution the binding of Ca2+ to heptapeptide and undecapeptide together with a contemporaneous conformational change. This change is probably due to the formation of beta-turns. No change in the CD profiles was obtained by using Mg2+, K+, Na+, and Li+ ions instead of Ca2+.