Carbon-13 and proton NMR studies of helix-coil transition of poly(gamma-benzyl L-glutamate)

Results of carbon-13 NMR of the solvent (CDCl3/TFA) induced and thermally induced helix-coil transition of poly (γ-benzyl L-glutamate) (PBLG) are presented in this paper. Sample of PBLG had the DP of ca.820. Proton-NMR spectra were obtained with a JEOL PS-100 spectrometer, and carbon-13 NMR spectra with the same one equipped with PFT-100 Fourier Transform System. Internal TMS was used as a reference. Trifluoroacetic acid (TFA) was purchased from Tokyo Kasei Co., Ltd., and dentrated chloroform of isotopic purity 99.7% was from Merck. Polypeptide solution was of concentration 15 (w/v) %. The solvent induced helix-coil transition midpoint and the shift difference Δ C/H for each carbon atom were obtained. From the results, carbon atoms can be divided into two classes, such as backbone carbons and side chain carbons, according to their chemical shift and helix-coil transition midpoint. The helix-coil transition midpoint of the side chain carbons is found at 17.0 - 18.0 (v/v) % TFA, which is larger than that of the backbone carbons. This means that side chain takes helical conformation before backbone does. The thermally induced helix-coil transition midpoint and the shift difference Δ C/H were also obtained. The shift difference of the backbone carbons is the same as that of the solvent induced helix-coil transition. That of the side chain carbons is smaller than that found in the solvent induced helix-coil transition. The helix-coil transition midpoint of Csub(α)(46.00C) is different from that of C = Osub(Amide) (50.50C). This difference shows existence of the intermediate state which is between α-helix and random coil, and is free from intramolecular hydrogen-bonding. The carbon-13 helix-coil transition midpoint, in particular Csub(α), is different from proton one (α-CH 60.00C). (Kato, T.)