Relaxometry of calf lens homogenates, including cross-relaxation by crystallin NH groups

We studied the magnetic field dependence of the longitudinal relaxation rates of water protons (1/T1 nuclear magnetic relaxation dispersion (NMRD) profiles) in transparent homogenates of calf lens. The samples included nuclear homogenates with total (heterogeneous) crystallin contents between 34% (v/v) (native) and 14% (diluted) as well as cortical homogenate, 21% (native) and 34% (concentrated). The NMRD profiles had two components: a monotonic dispersive component (analogous to that of both globular protein solutions and diamagnetic tissue) and "14N quadrupolar peaks." 14N peaks have never been reported for protein solutions, only for tissues and dehydrated proteins. These peaks occur between 0.5 and 5 MHz proton Larmor frequency and arise from interactions of solvent water protons with NH moieties of proteins. The 14N peaks in lens cytoplasm are very large and may correlate with the crystallin structure and interactions required to maintain short-range order and lens transparency. The monotonic and 14N quadrupolar components were largest in concentrated samples, but with different concentration dependencies. The dispersive components of samples above approximately 19% protein concentration had a fixed functional form, the amplitude of which varied with protein volume fraction, f, by the multiplicative factor f/(1 - f), suggesting spatial organization and dynamics of the solute proteins that are relatively independent of water content. In contrast, at concentrations less than 19%, the NMRD profiles are concentration dependent, indicating a dependence of the orientational relaxation time of the proteins on protein-protein interactions seen previously in other globular proteins at these concentrations. The 14N peaks are not resolved below approximately 19% protein and increase linearly with incremental volume fraction at protein concentrations above 19%. In addition, the 14N peaks in nuclear homogenates are 50-100% larger than those of cortical homogenates at the same concentrations. Partial substitution of solvent D2O for H2O decreases the peak heights, indicating that an exchangeable proton mediates the interaction between solvent protons and protein 14N nuclei.