T-1 rho of protein solutions at very low fields: Dependence on molecular weight, concentration, and structure

The effect of molecular weight, concentration, and structure on 1/T-1 rho, the rotating frame relaxation rate, was investigated for several proteins using the on-resonance spin-lock technique, for locking fields B-1 < 200 mu T. The measured values of 1/T-1 rho were fitted to a simple theoretical model to obtain the dispersion curves 1/T-1 rho(omega(1)) and the relaxation rate at zero B-1 field, 1/T-1 rho(0), 1/T-1 rho was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra- and intermolecular hydrogen and disulfide bonds especially contributed to 1/T-1 rho. In all samples, 1/T-1 rho(0) was equal to 1/T-2 measured at the main magnetic field B-o = 0.1 T, but at higher locking fields the dispersion curves monotonically decreased, The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for T-1 rho relaxation in protein solutions is not valid at very low B-1 fields, The underlying mechanism for the relaxation rate 1/T-1 rho at B-1 fields below 200 mu T is discussed.