The complete microspeciation of arginine and citrulline.

(1)H NMR-pH titrations of arginine, the most basic natural amino acid and citrulline, its neutral counterpart were carried out. Two other closely related auxiliary compounds were also studied. The 8 macroscopic protonation constants were determined. Combining the four datasets in a deductive method, all the 12 microconstants of arginine, the 4 microconstants of citrulline and arginine amide were calculated. An error-propagation analysis and the pH-dependent distribution of the 8 arginine microspecies are provided.