High-quality single crystals of proteins are required for studies related to structure-guided drug design and controlled drug delivery, because the crystal quality governs the refinement of the three-dimensional structure of protein molecules obtained from X-ray structure analysis. However, it is quite difficult to obtain high-quality single crystals of proteins. In addition, the crystallization of proteins is also difficult, and therefore the establishment of a crystallization technique that can obtain high-quality single crystals of proteins and control the nucleation of proteins is required.

Previously, we have successfully achieved both an increase and a decrease in the nucleation rate of protein crystals by applying an external alternating current (AC) electric field, by focusing on the magnitude of the electrostatic energy added to the chemical potentials of the liquid and solid phases ^{1), 2)}. It is considered that such effect of the electrostatic energy is added to not only the chemical potential but also entropy. Thus, we demonstrate the improvement of the crystal quality of protein crystals by applying an external AC electric field.

The FWHMs obtained from crystals prepared without an external electric field increased for diffraction peaks with order higher than the 440 reflection. In contrast, for crystal growth with an external electric field at 1 MHz, the FWHMs were smaller than those for crystals grown without. This suggests that the crystal quality be improved by applying an external AC electric field. In this presentation, the improvement of the crystal quality is discussed in the light of the change in entropy of the solid.

Reference

1) H. Koizumi, K. Fujiwara, and S. Uda, Cryst. Growth Des. 9, 2420-2424 (2009).
2) H. Koizumi, Y. Tomita, S. Uda, K. Fujiwara and J. Nozawa, J. Crystal Growth 352, 155-157 (2012).

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