Lysozyme aggregation and solution properties studied using PGSE NMR diffusion measurements

The solution behavior of lysozyme was studied as a function of protein conc entration, NaCl concentration, pH, and temperature using pulsed-gradient sp in-echo NMR diffusion measurements. The lysozyme solutions clearly exhibite d nonideal behavior which was sensitive to both the salt concentration and pH. Lysozyme has an isoelectric point of pH 11, and it is often overlooked that at normal pH it has a net positive charge. Since lysozyme is a charged species, the changes in the diffusion coefficients were interpreted, consi dering the competing effects of salt-mediated changes in protein interactio ns (e.g., electrostatic repulsion) and aggregation. The behavior is in agre ement with Derjaguin-Landau-Verwey-Overbeek (DLVO)-type modeling, accountin g for the attractive and repulsive forces present. The diffusion data was c ompared with various self-association models, including corrections for the effects of self-obstruction. The diffusion coefficients of the higher olig omers were calculated, assuming that the monomers aggregated as hard sphere s: Using an isodesmic association model, the equilibrium constant for the s elf-association of lysozyme at pH 4.6 and 298 K in the presence of 0.5 M Na Cl was estimated to be 118 +/- 12 M-1.