High-resolution nuclear magnetic resonance studies at 270 MHZ of alternating and random poly(benzyl D,L-glutamates)

The solution conformations of several D,L copoly benzyl glutamates both random and alternating are studied by comparing their NMR spectra in chloroform and also in dioxane and dimethylformamide. The alpha CH chemical shifts characteristic of the alpha helix of strictly alternating DL copolymers in chloroform/0.5% TFA are established (3.65 and 3.82 ppm) and differ from that of the regular alpha helix (3.92 ppm). It is concluded that alternating copolymers prepared by an essentially racemization-free method are completely in the alpha-helical conformation which is characteristic of strictly alternating D-L copolymers, whereas random copolymers are largely regular alpha. The alpha leads to piDL helix/helix transition of an alternating copolymer in dioxane has been monitored and the alpha CH resonance characteristic of the piDL helix is found to be at the unusually low chemical shift of 4.45 ppm.