Potentialities and limitations of the use of H-1 NMRD technique for the characterization of the hydration properties of unfolded or partially folded states of proteins are discussed. The copper(I) form of monomeric Cu,Zn superoxide dismutase in its folded state and in the presence of 4 M guanidinium chloride is taken as case system. The dispersion profile, analyzed with an extended relaxation matrix analysis, indicates the presence of long-lived water molecules in the folded state. The observed increase in relaxation at high field upon addition of guanidinium chloride indicates an increase in the number of solvation. protons interacting with the protein and exchanging with a time shorter than the protein reorientational time. The observed effect is consistent with an exposed protein surface of SOD in the presence of 4 M guanidinium chloride smaller than what could be expected for a random coil. (C) 2005 Elsevier Inc. All rights reserved.