The influence of macromolecular polymerization on spin-lattice relaxation of aqueous solutions

Abstract

The docking or polymerization of globular proteins is demonstrated to cause changes in proton NMR spin-lattice ( T 1) relaxation times. Studies on solutions of lysozyme, bovine serum albumin, actin, and tubulin are used to demonstrate that two mechanisms account for the observed changes in T 1. Polymerization displaces the hydration water sheath surrounding globular proteins in solution that causes an increase in T 1. Polymerization also slows the average tumbling rate of the proteins, which typically causes a contrary decrease in T 1. The crystallization reaction of lysozyme in sodium chloride solution further demonstrates that the “effective” molecular weight can either decrease or increase T 1 depending on how much the protein is slowed. The displacement of hydration water increases T 1 because it speeds up the mean motional state of water in the solution. Macromolecular docking typically decreases T 1 because it slows the mean motional state of the solute molecules. Cross-relaxation between the proteins and bound water provides the mechanism that allows macromolecular motion to influence the relaxation rate of the solvent. Fast chemical exchange between bound, structured, and bulk water accounts for monoexponential spin-lattice relaxation. Thus the spin-lattice relaxation rate of water in protein solutions is a complex reflection of the motional properties of all the molecules present containing proton magnetic dipoles. It is expected, as a result, that the characteristic relaxation times of tissues will reflect the influence of polymerization changes related to cellular activities.