¹H magnetic cross‐relaxation between multiple solvent components and rotationally immobilized protein

Abstract

Magnetic cross-relaxation spectra or Z-spectra are presented for water, acetone, methanol, dimethylsulfoxide, and acetonitrile in cross-linked bovine serum albumin gels. Each solvent studied, reports the same Z-spectrum linewidth and shape for the solid component that follows from solutions of the coupled relaxation equations. The Z-spectra demonstrate competition among solvents for specific protein binding sites. The rate of magnetization transfer in the rotationally immobilized protein environment is approximated by 1/T2 for the solid component, which is shown to account for the observed magnetization transfer rates in the systems studied. The temperature dependence of the Z-spectra are different for water compared with the organic solvents. The cross-relaxation efficiency in the organic solvents decreases with increasing temperature because molecules bind less well at high temperature. For water, the hydrogen exchange path becomes increasingly important relative to the whole molecule path with increasing temperature, which improves the net cross-relaxation efficiency.

Modeling Magnetization Transfer Using a Three-Pool Model and Physically Meaningful Constraints on the Fitting Parameters

“…Convincing evidence that both dipolar coupling and physical exchange play a significant role in magnetization transfer has been demonstrated in hydrated systems of lipid bilayers, proteins, and gels (5,7,8,16,17,19,20,28,(33)(34)(35)(36). Work with hydrated lipids suggests that magnetic coupling occurs at specific dynamically restricted sites on the hydrophilic bilayer surface which are also characterized by the apparent necessity for the presence of a hydrogen-donor functionality such as a hydroxyl, amine, or carboxyl group (17,19,27,36,37).…”

“…The overall process of dipolar transfer and physical exchange leading to a decrease in the observable bulk-water NMR signal in the presence of saturation of the macromolecular magnetization is what will be referred to in the present work as magnetization transfer (MT). The degree of magnetization transfer has been found to be specific to the type of hydrated system (5,8,17,28,29). In human tissue, a significant magnetization transfer effect has been observed in muscle, the lens and cornea of the eye, cartilage, and gray and white matter of the brain (30).…”

“…It may serve as a site for hydrogen exchange of labile solvent and macromolecular hydrogens, with dipolar coupling between these labile hydrogens and the bulk of the macromolecular hydrogen population. Another possibility is that the hydrogen donor acts as a hydrogen bonding site to orient the solvent molecules at the macromolecular surface, allowing for dipolar coupling directly between the solvent and macromolecular hydrogens (17,19,28,35,36). Magnetization transfer has been observed using solvents without exchangeable hydrogens, suggesting that hydrogen exchange at these hydrogen-donor groups is not a necessary requirement for magnetization transfer (28).…”

“…The simplest models identify only the bulk solvent and macromolecular hydrogen populations with coupling described by a single coupling constant (5,16,17,28,38,39,40). These models are inherently incomplete where both physical exchange and dipolar coupling lead to magnetization transfer.…”

“…Another possibility is that the hydrogen donor acts as a hydrogen bonding site to orient the solvent molecules at the macromolecular surface, allowing for dipolar coupling directly between the solvent and macromolecular hydrogens (17,19,28,35,36). Magnetization transfer has been observed using solvents without exchangeable hydrogens, suggesting that hydrogen exchange at these hydrogen-donor groups is not a necessary requirement for magnetization transfer (28). Initial work studying the pH dependence of magnetization transfer suggested that hydrogen exchange played some role in coupling the water and macromolecular populations (27,36).…”