Protein Crystallization in Very Weak DC Electric Field

How electric field (EF) assists in crystal growth has been difficult to describe uniformly. Many factors concerning the amplitude, frequency, and waveform of the field, gating and duration of field application, the requirement of auxiliary physical agents such as high magnetic field and pulsed radiation, and geometric confinement of the protein solution have made the process seemingly complex. It appears as though choosing conditions for EF-assisted protein crystallization is a bit hit-and-miss such that only a few proteins have been crystallized. This report describes EF-aided crystallization of a batch of five commercially available globular proteins from their respective 7 μM (∼0.02 mg/mL) solutions in a 1.3 V cm–1 DC field applied uniformly for ∼90 min. The crystals have been imaged and shown to diffract. The field mechanically unfolds the protein facilitating amorphous aggregation, which is a 'dense' state of molecules containing no tertiary structure, reduced α-helices, and excess β-structures than what the native state contains. The reduced surface tension of the molecules in the dense states lowers the Gibbs free energy barrier for nucleation. Nucleation from heterogeneous patches of dense states produces size distribution in the crystal population. It is shown that EF-assisted crystallization is an exceedingly easy and enjoyable experiment, requiring nothing special about apparatus and accessories, solution conditions, protein concentration, and experimental methods. Crystals can be grown in very weak DC field. The use of strong field, oscillating field in particular, in conjunction with auxiliary physical agents is redundant.