14N NMR relaxation times of several protein amino acids in aqueous solution -: comparison with 17O NMR data and estimation of the relative hydration numbers in the cationic and zwitterionic forms

The N-14 nuclear magnetic resonance (NMR) linewidths of the alpha-amino groups of several protein amino acids were measured in aqueous solution, with and without composite proton decoupling, to estimate the effect of proton exchange and molecular weight on the linewidths. It is shown that, contrary to earlier claims, the increase in the linewidth at low pH is not exclusively due to the effect of proton exchange broadening. The N-14 linewidths, under composite proton decoupling, increase with the bulk of the amino acid, and increase at low pH. Statistical treatment of the experimental N-14 and literature O-17 NMR data was performed assuming two models: (i) an isotropic molecular reorientation of a rigid sphere in a medium of viscosity eta, (ii) a stochastic diffusion of the amino and carboxyl groups comprising contributions from internal (tau(int)) and overall (tau(mol)) motions. Assuming a single correlation time from overall molecular reorientation (tau(mol)), then, a linear correlation was found between the linewidths and the molecular weights of the protein amino acids at the pH values 0.5 and 6.0, which are characteristic of the cationic and zwitterionic forms, respectively. The slopes of the straight-lines were found to be dependent of pH for N-14, contrary to the O-17 linear correlations whose slopes were found to be independent of pH. Assuming effective correlation times of the amino and carboxyl groups, which comprise contributions from the internal (tau(int)) and overall (tau(mol)) motions, then, a significant improvement of the statistics of the regression analysis was observed. The N-14 relaxation data, in conjunction with O-17 NMR linewidths, can be interpreted by assuming that the N-14 quadrupole coupling constants (NQCCs) are influenced by the protonation state of the carboxyl group, the O-17 NQCCs remain constant, and the cationic form of the amino acids is hydrated by an excess of 1-3 molecules of water relative to the zwitterionic state. (C) 2003 Elsevier Inc. All rights reserved.