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

Abstract

Abstract 15 N n.m.r. spectra of [ 13 C‐2, 3‐ 15 N 2 ‐ guanidino ]arginine and [ 13 C, 15 N 2 ] urea were obtained in D 2 O and H 2 O 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 15 N spectra. When the guanidino group is deprotonated (p K = 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 1 J (CN) using a Blizzard–Santry type calculation. The ramifications of the proton exchange on 15 N n.m.r. spectra of amino acids and peptides are discussed.