pH Dependence of the 15N and 17O nuclear magnetic resonance chemical shifts of glycylglycine

The pH chemical shift titration curves of the glycylglycine amide and ammonium nitrogens and the amide and carboxylate oxygens, measured by 15N and 17O Fourier transform n.m.r. spectroscopy, indicated that on going from the cationic to zwitterionic state the amide C–N double bond character increases and is accompanied by a transfer of electron density from the amide nitrogen to oxygen.