Enhancement and suppression of protein crystal nucleation due to electrically driven convection

We investigated the effects of the constant electric fields from 2.0 to 6.0 kV cm−1 on the nucleation of ferritin, apoferritin and lysozyme crystals. For this, supersaturated solutions of the three proteins were held between electrodes separated by 1.0 cm in batch and sitting drop geometries without contact between electrodes and solutions. The nucleation rate was characterized by the number of crystals appearing after a certain time (1–3 days). We show that in sitting drop arrangements, weak electric fields (<4 kV cm−1) either suppress or have no effect on the nucleation rate of ferritin and apoferritin, while electric fields of 5 or 6 kV cm−1 reproducibly enhance crystal nucleation of both proteins. Electric fields of all tested strengths consistently enhance lysozyme crystal nucleation. All batch experiments showed no effect of the electric field on the nucleation rates. Since the solutions contain high electrolyte concentrations and are conductive, the electric field strengths within them are negligible. We show that the electric field causes solution stirring with rates of up to 100 μm s−1, depending of the field strength. Thus, our observations indicate that at slow solution flow rates, the rates of nucleation of ferritin and apoferritin crystal are suppressed, while faster stirring enhances crystal nucleation of these proteins. All solution flow rates enhance lysozyme crystal nucleation. Our results suggest that solution convection may strongly affect nucleation, and that for some systems, an optimal convection velocity, leading to fastest nucleation, exists.