High-resolution NMR studies at 270 MHz of alternating poly(gamma-benzyl D-L-glutamate) in double-stranded helical conformations.

NMR spectra (270MHz) are presented of strictly alternating poly(gamma-benzyl D-L-glutamate) in dioxane, chloroform, and methylene chloride solutions from which cast films have been demonstrated to have a molecular conformation in the pi pi DL 9.0 and pi pi DL 7.2 double-helical forms. Unusually low chemical shift values are observed for the alpha-CH (approximately 5.5 ppm) and NH (approximately 9.0 ppm) resonances, well separated from those of the alpha and pi DL single helices. The aromatic protons and benzyl-CH2 resonances are found slightly upfield of those of the single helices. It is proposed that these shift values are characteristic of the double helices and that the conformation of the PBD-LG sample in these solvents is the same as in the film obtained therefrom. This conclusion is supported by NMR spectra of a solvent-induced pi pi DL 7.2 leads to pi pi DL 9.0 double helical transconformation and a similar TFA-induced double helix leads to single helix conformational transition.