Nitrogen‐15 n.m.r. studies of 13C, 15N labeled arginine

Abstract 15N n.m.r. spectra of [13C‐2, 3‐15N2‐guanidino]arginine and [13C, 15N2] urea were obtained in D2O and H2O at a variety of pH values both with and without proton decoupling. The effects of the proton exchange rate are readily observable in the proton coupled 15N spectra. When the guanidino group is deprotonated (pK = 12.5), the terminal nitrogens give a single resonance 6.6 ppm downfield of the protonated species, indicating a rapid tautomeric exchange. The observed NH and CN couplings are compared with calculated values, and good agreement is found for 1J(CN) using a Blizzard–Santry type calculation. The ramifications of the proton exchange on 15N n.m.r. spectra of amino acids and peptides are discussed.