Lysozyme crystallization rates controlled by anomalous fluctuations

Nucleation of protein aggregates and crystals is a process activated by statistical fluctuations of concentration. Nucleation rates may change by several orders of magnitude upon apparently minor changes in the multidimensional space of parameters (temperature, pH, protein concentration, salt type and concentrations, additives). We use available data on hen egg lysozyme crystal induction times in different solution conditions. We measure by static and dynamic light scattering the amplitudes and lifetimes of anomalously ample and long-lived fluctuations occurring in proximity of the liquid-liquid demixing region of the given lysozyme solutions. This allows determining the related spinodal temperatures TS and ε=(T-TS)/TS. Experimental induction times appear to depend solely upon ε over many orders of magnitude. This is quantitatively accounted for in terms of an extended two-stage nucleation model, which jointly takes into consideration amplitudes, lifetimes and scaling properties of anomalous fluctuations. One and the same relation describes quantitatively and equally well the present case of lysozyme crystallization (the best studied case of protein crystallization) and that of sickle hemoglobin fiber formation (the best studied case of protein fiber formation). Comparison with other recent models shows that taking into account lifetimes of anomalous fluctuations allows capturing the essence of the observed behavior.