Carbon-13 NMR spectra of alpha single-helical and pipiDL double-helical conformations of alternating poly (gamma-benzyl D-L-glutamate) are presented. In both cases, the existence of doublet for the backbone carbon atoms is consistent with D and L residues being in different conformational states when engaged in the same helix. The resonances of the Calpha atoms in both helices, which are found at identical chemical shifts, and comparison with the 1H NMR observation, favor the hypotheses that solvent molecules are located inside the helical core of the double helices.