Crossover from a Linear to a Branched Growth Regime in the Crystallization of Lysozyme

Using lysozyme as a crystallization model, existing intermediate clusters and aggregates have been previously identified as fractal systems using light scattering techniques. However, this has not been confirmed with neutron or X-ray scattering directly. In this work, we attempt to deepen our knowledge of the role of the fractal clusters during the crystallization process by following the evolution of the fractal dimension d(f) from the early stage of the nucleation process. Indeed, three different scattering techniques have been used simultaneously on the same sample: dynamic light scattering, small-angle neutron scattering, and static light scattering. We focused on the optimal batch crystallization condition in order to obtain large crystals (30 mg/mL lysozyme concentration and 3 wt % sodium chloride at pD 4.75 at 298 K). The selected temperature reduces the nucleation speed allowing us to investigate in detail the very early stage of the crystallization process. A direct temporal change of the fractal dimension d(f) during the initial growth phase of lysozyme was observed with d(f) rising from 1.0 to 1.7 in the first 90 min after initiating the crystallization process. The early phase of crystallization shows remarkable similarities to simulations on colloid aggregation. Long-term dynamic light scattering measurements allowed us to gain some insight into how fractal clusters may contribute during the crystal growth process. These findings help to improve theoretical models of crystal growth and may lead to the growth of larger crystals through a better understanding of the initial nucleation phase.