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

T1 relaxation in the rotating frame (T1ρ) is a sensitive magnetic resonance imaging (MRI) contrast for acute brain insults. Biophysical mechanisms affecting T1ρ relaxation rate (R1ρ) and R1ρ dispersion (dependency of R1ρ 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 R1ρ in heat-denatured phantoms displaying proton resonances from a number of side-chain chemical groups in high-resolution 1H NMR spectra. At pH of 5.5, R1ρ 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 R1ρ. The present data point to a crucial role of proton exchange on R1ρ and R1ρ dispersion in immobilized protein solution mimicking tissue relaxation properties. © 2001 Elsevier Science.