A MOLECULAR THEORY OF RELAXATION AND MAGNETIZATION-TRANSFER - APPLICATION TO CROSS-LINKED BSA, A MODEL FOR TISSUE

Homogeneous soft tissue, as regards its magnetic relaxation properties, is well-modeled by solutions of cross-linked protein (see Koenig and Brown, Prog. NMR Spectr. 22,487(1991)). Interactions at the solute-solvent interface after the hydrodynamics of solvent water, and also couple the solute and solvent proton Zeeman energy reservoirs, giving hydrodynamic and cross-relaxation contributions to water proton relaxation that respond differently to deuteration of solvent. We report messurements of the magnetic field dependence of 1/T1 of water protons in cross-linked bovine serum albumin (BSA), for partially deuterated solvent and, in order to separate these two contributions, of 1/T1 of deuterons. The major experimental finding is that, in addition to recently identified water-binding sites on protein (covering approximately 0.20% of the surface) with water lifetimes of about 1 mus, there is another group of sites with lifetimes of about 23 ns, covering approximately 2% of the surface, which are evident in both proton and deuteron data. In addition, we have formulated a theory of interfacial proton-proton magnetic interactions which-with these four parameters, plus two that quantify the protein-water coupling at each site-can account for all the proton and deuteron data, in both native and cross-linked BSA.