Role of Hydrogen Bond in Polypeptides - ^2HNMR Studies in the Lyotropic Liquid Crystalline State

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1. Monodisperse PBLG samples having DP of 25 〜 50 have been prepared by using a solid-state peptide synthesizer. The helix-coil transitions in a mixed solvent comprising CHCl_3 and DCA were followed by ^1H- and ^<13>C-NMR. These samples exhibited two separate chemical shifts over the entire biphasic region : one from the helical and the other from the coil part. These observations are in contraditoin to those previously reported by Nagayame et al.2. The helix-sense inversion phenomena peculiar to poly(aspartic acid ester)s has been studies for systems such as poly (β-p-chlorobenzyl L-aspartate) and poly(β-pentafluorophenethyl L-aspartate). Chemical modifications at the sidechain terminal were found to cause appreciable changes in the solubility of polymers, and eventually the helix-helix transition temperatures were shifted. These results indicate that the stability of hydrogen bonds involved in the α-helix conformation sensitively alters with the sidechain structure as well as the character of the surrounding solvents. Further investigations are underway to elucidate these unique characteristics of the internal hydrogen-bonds.