Strong and weak binding of water to proteins studied by NMR triple- quantum filtered relaxation spectroscopy of ¹⁷O-water

The triple-quantum filtered (TQF) spin-echo signal of ¹⁷O-water, in the presence of proteins, was analysed to yield estimates of the number of weakly, and strongly bound water molecules. The analysis used a constrained direct iterative regression procedure with a three-state model of fast- exchange. Thus, the population size of free, weakly, and strongly bound water were determined simultaneously. The two fractions of the bound water were estimated by using correlation time(s) estimated in other studies. Bovine serum albumin (BSA), basic pancreatic trypsin inhibitor (BPTI), lysozyme and oxyhaemoglobin were studied. Of the four proteins, BSA contained the largest number of strongly and weakly bound water molecules, there being ~ 30 of the former and ~ 3000 of the latter under conditions of high protein concentration. The correlation time of the proteins increases with their concentration in solution, and when this was taken into account for BSA the estimated number of strongly bound water molecules did not change significantly. This NMR technique, and data analysis, will probably also be useful in studies of water binding and mobility in various systems including hydrogels, protein networks, membranes, cells and tissues.