The coupled effect of ionic strength (50-400 mM) and pH (2-8) on ionization and conformation equilibria of lysozyme was studied using NMR spectroscopy. Observed changes in pK(a) values of the ionizable groups were found to originate from perturbations in the geometry of hydrogen bonds rather than screening of electric fields. Moreover, at the ionic strengths used here, salt-induced local conformational changes had a dominant effect on chemical shifts measured on H-1(N) and N-15 amide nuclei. Accurate modeling of these localized perturbations in structure-based energy calculations is a necessary prerequisite on the way to complete understanding of any salt-induced processes in proteins. (C) 2013 Elsevier B.V. All rights reserved.