Determination of the bound water fraction in cells and protein solutions using¹⁷O-water multiple-quantum filtered relaxation analysis

Multiple-quantum filtering NMR sequences were used to study the multiexponential relaxation behavior of H₂¹⁷O in the presence of macromolecules. By this means, the fraction and the correlation time of water in slow motion, or "bound" water, were determined, as well as the relaxation time of bulk water in the extreme-narrowing limit. Aqueous solutions of bovine serum albumin and intact human red blood cells were studied. The small fraction of bound water of less than 1% appeared to correspond to "strongly bound" water, whereas the behavior of bulk water was different from pure water and was interpreted as being due to weak (or transient) interactions with macromolecules. The experiments and the data analysis appeared to be reproducible, which suggests that diverse samples might be studied this way and thus help define the properties of water in these systems.