Characterisation by triple-quantum filtered 17O-NMR of water molecules buried in lysozyme and trapped in a lysozyme-inhibitor complex

Triple-quantum filtering NMR sequences were used to study the multiexponential relaxation behaviour of H-2 O-17 in the presence of hen egg white lysozyme. By this means, the fraction and the correlation time of water were determined in slow motion, as well as the relaxation time of water in the extreme narrowing limit. The small number of water molecules in slow motion, which is between four and five per lysozyme, seems to correspond to the 'integral' water, buried or in the cleft inside the protein, whereas water in fast motion corresponds to all other water molecules, interacting or not with the macromolecules. The same experiment was performed after addition of the inhibitor tri-N-acetylglucosamine (NAG)(3). For solutions of sufficient viscosity, there were approximately three supplementary water molecules in slow motion per lysozyme, probably trapped between the protein and the inhibitor. The correlation time of these water molecules was estimated at 2 ns, which should correspond to their residence time in the complex. (C) 1999 Elsevier Science B.V. All rights reserved.