Molecular basis of water proton relaxation in gels and tissue

An extensive set of water-H magnetic relaxation dispersion (MRD) data are presented for aqueous agarose and gelatin gels. It is demonstrated that the EMOR model, which was developed in a companion paper to this study (see Halle, this issue), accounts for the dependence of the water-H spin-lattice relaxation rate on resonance frequency over more than four decades and on pH. The parameter values deduced from analysis of the H-1 MRD data are consistent with values derived from H-2 MRD profiles from the same gels and with small-molecule reference data. This agreement indicates that the water-H-1 relaxation dispersion in aqueous biopolymer gels is produced directly by exchange-mediated orientational randomization of internal water molecules or labile biopolymer protons, with little or no role played by collective biopolymer vibrations or coherent spin diffusion. This ubiquitous mechanism is proposed to be the principal source of water-H-1 spin-lattice relaxation at low magnetic fields in all aqueous systems with rotationally immobile biopolymers, including biological tissue. The same mechanism also contributes to transverse and rotating-frame relaxation and magnetization transfer at high fields.