Formation dynamics of protein precrystallization fractal clusters

The enzyme lysozyme served as a prototype to study the early stages of protein crystallization or precipitation. The crystallization and precipitation aggregates reactions were initiated by supersaturating the lysozyme solutions through addition of NaCl and (NH4)2SO4 respectively and monitored via photon correlation spectroscopy. Laplace inversion of the spectra revealed in general bimodal particle distributions consisting of monomeric and high molecular weight protein species. The time evolution of the formation of large aggregates obeys the dynamic scaling characteristics of fractal structures in the diffusion, crossover or reaction limited aggregation regimes; it is a function of the protein concentration, and of the concentration and type of the employed precipitant. The derived scaling exponents provide estimates which correlate with the various parameters inducing crystallization in the very early stages of the process. Rules quite similar to those governing the aggregation of inorganic colloids seem to be applicable and provide assistance in dynamically controlling the crystallization process.