Proton exchange as a relaxation mechanism for T1 in the rotating frame in native and immobilized protein solutions

T(1) relaxation in the rotating frame (T(1rho)) is a sensitive magnetic resonance imaging (MRI) contrast for acute brain insults. Biophysical mechanisms affecting T(1rho) relaxation rate (R(1rho)) and R(1rho) dispersion (dependency of R(1rho) on the spin-lock field) were studied in protein solutions by varying their chemical environment and pH in native, heat-denatured, and glutaraldehyde (GA) cross-linked samples. Low pH strongly reduced R(1rho) in heat-denatured phantoms displaying proton resonances from a number of side-chain chemical groups in high-resolution (1)H NMR, spectra. At pH of 5.5, R(1rho) dispersion was completely absent. In contrast, in the GA-treated phantoms with very few NMR visible side chain groups, acidic pH showed virtually no effect on R(1rho), The present data point to a crucial role of proton exchange on R(1rho) and R(1rho) dispersion in immobilized protein solution mimicking tissue relaxation properties. (C) 2001 Elsevier Science.