The helix–coil transition in an aqueous solution of poly-L-glutamic acid has been observed using 13C nuclear magnetic resonance. The transition from coil to helix results in pronounced deshielding of the carbonyl and C-α carbons of the polymer and a less dramatic shielding effect on the C-β carbon. These results are contrasted with the titration behavior of L-glutamic acid to delineate those shifts directly attributed to the secondary-structure change. The biological significance of this observation is supported by the 13C chemical shift differences observed for the 13C-enriched phenylalanyl residue 8 in the free 1–15 amino-terminal peptide of ribonuclease (RNase) and the 1–15 peptide:RNase S-protein complex.