T 1ρ 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 ρ, the rotating frame relaxation rate, was investigated for several proteins using the on‐resonance spin‐lock technique, for locking fields B 1 < 200 μT. The measured values of 1/ T 1 ρ, were fitted to a simple theoretical model to obtain the dispersion curves 1/ T 1 ρ(ω 1 ) and the relaxation rate at zero B 1 field, 1/ T 1 ρ,(O). 1/T 1 ρ, 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 ρ. In all samples, 1/ T 1 ρ(O) was equal to 1/T 2 ρ measured at the main magnetic field B o = 0.1 T, but at higher locking fields the dispersion curves mono‐tonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for 1/ T 1 ρ relaxation in protein solutions is not valid at very low B 1 fields. The underlying mechanism for the relaxation rate 1/ T 1 ρ at B 1 fields below 200 μT is discussed.