Effects of Hydrodynamic Convection and Interionic Electrostatic Forces on Protein Crystallization

The biological function of a protein is intimately related to its three-dimensional molecular structure. Although X-ray diffraction from single crystals can be employed to solve for the molecular structure, use of this method is often impeded by the slow rate of precipitation of crystals in the pH-buffered, water-based, electrolyte solutions which ordinarily serve as growth media. By taking into account the interionic electrostatic forces that affect protein solubility, nucleation, growth, and Ostwald ripening, we find that the following sequence of growth solution procedures should be effective in producing crystals of any water-soluble protein, which dissolves endothermically. The protein should be dissolved at room temperature in a growth solution, and then the temperature should be lowered to the cold room temperature at 4 °C to establish the supersaturation. To control nucleation, establish a measurable crystallization rate, and limit the number of crystals competing for the dissolved protein, the salt concentration should be minimal, and the pH should be different from the pI. As the rate of decay of the supersaturation approaches zero, Ostwald ripening will commence. If the salt concentration and temperature are maintained as above, and the value of the pH is chosen to be intermediate between the two most widely spaced but numerically adjacent pK a values of any of the ionizable amino acid residues along the protein chain, the number of crystals will decrease and the average crystal size will increase. By taking into account hydrodynamic convection in a growth solution in a gravitational field, we construct a figure of merit, M, that when evaluated using terrestrial measurements, can be used to discriminate between proteins that should benefit from crystallization in microgravity and those that should receive no benefit. The threshold value for the onset of benefits appears to be M ≥ 0.004. Finally, we discriminate between the magnetic field requirements appropriate for the complete levitation of a crystal growth solution in a gravitational field and those appropriate for the suppression of natural convection alone.