Precrystallization structures in supersaturated lysozyme solutions studied by dynamic light scattering and scanning force microscopy

A comparitive study of the nanostructure evolving during aggregation of hen-egg white lysozyme in supersaturated solution was carried out by dynamic light scattering (DLS) and scanning force microscopy (SFM). Lysozyme aggregate (cluster) formation was observed in solution in the presence of NaCl, ${\mathrm{(NH}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}\mathrm{,}$ and ${\mathrm{NaNO}}_{\mathrm{3}}$ as precipitating agents. The growth kinetics were examined by DLS and revealed fractal growth of the clusters with a fractal dimension of 1.8 obtained independently of the type of inert salt. Such behavior is typical for diffusion-limited cluster–cluster (DLCA) aggregation. Initial lysozyme cluster sizes were in the range of 12–35 nm. SFM images of individual lysozyme clusters at the liquid–solid interface were obtained in the presence of NaCl and ${\mathrm{NaNO}}_{\mathrm{3}}$ under crystallization conditions, and revealed cluster sizes in agreement with those determined by DLS. Extended domains of smaller sized clusters appeared on the mica surface after subjecting supersaturated lysozyme solutions to a dialysis step. The feasibility of DLS and SFM for monitoring the nano- and mesoscopic morphology of lysozyme aggregates in supersatured solutions and at the solid–liquid interface is discussed.