The influence of asymmetric carbon atoms of the side chains on the conformational properties of polypeptides: Comparison of diastereomeric homooligopeptides of L-isoleucine and D-alloisoleucine

The conventionally protected oligopeptides of the two homologous series Boc‐(L‐Ile)n‐OMe and Boc‐(D‐aIle)n‐OMe (n = 2–6) were synthesized in a standard stepwise fashion and their uv and CD spectra in 2,2,2‐trifluoroethanol, and solid‐state ir spectra were investigated. In addition, two oligomeric products derived from the NCAs of L‐isoleucine and of D‐allo‐isoleucine and having a DP of 20 and 12, respectively, were studied in the solid state by x‐ray and ir. No substantial differences between the properties of the diastereomeric oligomers in the solid state were noticed, a β‐structure being very likely at least for the Boc‐protected hexapeptides and the higher oligomers. In contrast, differences were observed between the spectroscopic properties of the diastereomeric oligopeptides, and especially of the hexapeptides, in trifluoroethanol solution. The different properties of the hexapeptides in solution were related to the existence, in the case of Boc‐(L‐Ile)6‐OMe, of soluble molecular aggregates in which the peptide chains assume the β‐conformation. These results provide an additional example of the influence of the configuration of asymmetric carbon atoms of the side chains on the conformational properties of peptide molecules in solution.