Electric field-induced control of protein crystal morphology

In a previous study (D. Ray, <i>et al</i>., <i>J. Phys. Chem. Lett.</i>, 2024, <b>15</b>, 8108-8113), we found that an alternating electric field considerably affects the location of the crystallization boundary and the liquid-liquid phase separation line as well as crystallization kinetics in lysozyme solutions containing sodium thiocyanate (NaSCN). The present study extends this work by investigating the influence of the same electric field on the microscopic appearance of lysozyme crystals as they form from a supersaturated solution. We observe a variety of distinct crystal morphologies, which we classify as single- and multi-arm crystals, flower-like crystal structures, whiskers, and sea-urchin crystals. Crystal morphologies exhibit significant variations with changes in protein and salt concentrations, and the electric field strongly alters the morphology-state diagram in the protein-versus-salt concentration plane. This alteration is likely due to the field effect on protein-protein interactions. We believe the effect is mediated by the field-enhanced adsorption of SCN^- ions to the surface of lysozyme, ultimately driving the observed changes in crystallization behavior. These findings offer insights into how electric fields can be used to control crystal formation and morphology in protein systems.