Observation of individual carboxyl groups in hen egg-white lysozyme by use of high field 13C-nuclear magnetic resonance.

Several of the carboxyl carbon atom resonances of hen egg-white lysozyme (mucopeptide N-acetylmuramoyl hydrolase, EC 3.2.1.17) have been resolved by 13C-nuclear magnetic resonance (NMR) at 68 MHz. The change in chemical shift of the carboxyl carbon atom resonances, as a function of pH, has enabled the distinction of these resonances against the background of many nontitrating carbonyl group resonances. Several apparent microscopic ionization constants have been determined from the carboxyl group NMR titration curves, and possible assignments are discussed. Preliminary experiments were carried out in the presence of cobaltous ion, and selective shifts of several resonances were observed. Our results indicate the possibility of the direct observation of a wide range of single functional groups of proteins in solution by NMR techniques.