Multiexponential proton relaxation processes of compartmentalized water in gels

The proton relaxation times, T1 and T2, of water in Sephadex gels, exhibiting pores of varying size (i.e., with exclusion limits of molecular weight between 10(3) and 10(5)) and water contents in the range 30 to 70% (w/w, weight of water to total weight), were measured at 20 MHz in the temperature range 5 to 50 degrees C. Multiexponential analysis of the relaxation curves revealed the existence of two relaxation components in all gel systems. A component with long T1 and T2 (T1,1 and T2,1) is associated with a large water fraction alpha 1,1 and alpha 2,1 and a component with short T1 and T2 (T1,2 and T2,2) with a small water fraction alpha 1,2 and alpha 2,2. An analysis of the temperature behavior of the relaxation components gives insight into the relaxation mechanisms. The relaxation process in water, compartmentalized in the gel matrix, is mainly controlled by dipole-dipole interactions. In addition, proton exchange processes between hydration water and hydroxyl groups of the matrix chain contribute under specific conditions and lead to a dramatic enhancement of the relaxation rate. In particular, for gels with small pores and with low water content proton exchange is observed. Compartments of water in gels could be models for compartments of water in biological tissues.