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/ T 1 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 “ 14 N quadrupolar peaks.” 14 N 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 14 N 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 14 N quadrupolar components were largest in concentrated samples, but with different concentration dependencies. The dispersive components of samples above ≈ 19% protein concentration had a fixed functional form, the amplitude of which vaned 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 14 N peaks are not resolved below ≈ 19% protein and increase linearly with incremental volume fraction at protein concentrations above 19%. In addition, the 14 N peaks in nuclear homogenates are 50–100% larger than those ofcortical homogenates at the same concentrations. Partial substitution of solvent D 2 O for H 2 O decreases the peak heights, indicating that an exchangeable proton mediates the interaction between solvent protons and protein 14 N nuclei. © 1988 Academic Press, Inc.