The image to the right shows insulin crystals grown with the use of an ionic liquid, choline dihydrogen phosphate. 

Ionic liquids have been found effective as additives in protein crystallization, with different ionic liquids used to increase crystallization rates and crystal size.1-4 The inclusion of ionic liquids in crystallization experiments has been reported to lead to less crystal polymorphism as well as less precipitation at higher precipitant concentrations.2 Ionic liquids have been used as additives to produce crystals in reagents that had previously not resulted in crystallization and results suggest ionic liquids may be applicable for the solubilization and crystallization of membrane proteins.2

Ionic liquids are organic salts with melting points below 100°Celsius. They are thermally stable, nonflammable and demonstrate very low vapor pressure. Ionic liquids are soluble in a variety of organic and inorganic reagents and can be highly water soluble. Ionic liquids can demonstrate a degree of localized structuring about each ion compared to materials composed of disassociated ions, setting them apart from salt solutions.5-6 Ionic liquids can participate in ionic, hydrophobic and hydrogen bond interactions. Ionic liquids are often chaotropic, composed of low symmetry ions with charge delocalization and weak intermolecular interactions.1 These organic salts generally consist of combinations of organic cations and either an organic or inorganic anion. Ionic liquids have been demonstrated to suppress protein aggregation and significantly increase protein folding yields.7-8 Ionic liquids have been reported to stabilize protein activity and structure.9-11 The inclusion of the ionic liquid 1-n-Butyl-3-methylimidazolium tetrafluoroborate improved the thermal stability and solubility of integral membrane proteins for membrane proteomic studys.12

Some ionic liquids, such as ethylammonium nitrate have water-like characteristics, including the capacity for hydrogen bonding and the promotion of micelle formation by some surfactants.13 Many ionic liquids are also organic acids and have ionic character in addition to the hydrophobic behavior, which makes them unique and useful solvents in protein chemistry.

Hampton Research Ionic Liquid Solutions for Crystal Growth

HR2-214 Ionic Liquid Screen 0.5 mL, tube format
HR2-214-** Individual Ionic Liquid Reagents 9 mL, tube format
HR2-078 PEG/Ionic Liquid 1 10 mL, tube format
HR2-079 PEG/Ionic Liquid 2 10 mL, tube format
HR2-462 PEG/Ionic Liquid HT 1 mL, Deep Well block format
HR2-078-** and HR2-079-** Individual PEG/Ionic Liquid Reagents 185 mL, bottle


References and Readings

1. Pusey, M.L., Paley, M.S., Turner, M.B., and Rogers, R.D. 2007. Protein crystallization using room temperature ionic liquids. Crystal Growth & Design. 74:787-793.

2. Hekmat, D., Hebel, D., Sebastian, J. Schmidt, M., and Weuster-Botz, D. 2007. Advanced protein crystallization using water-soluble ionic liquids as crystallization additives. Biotech. Lett., 29:703-1711.

3. Garlitz, J.A., Summers, C.A., Flowers, R.A. and Borgstahl, G.E.O. 1999. Ethylammonium nitrate: A protein crystallization reagent. Acta Cryst. D53:2037-2038.

4. Judge, R.A., Takahashi, S., Longnecker, K.L. Fry, E.H., Abad-Zapatero, C., and Chi, M.L. 2009. The effects of ionic liquids on protein crystallization and X-ray diffraction resolution. Crystal Growth & Design 9:3463-3469.

5. Bowran, D.T., Hardacre, C. Holbrey, J.D., McMath, J.E., and Soper, A.K. 2003. J. Chem. Phys. 118:173-178.

6. Cadena, C., Zhao, Q., Snurr, and R.Q., Maginn, E.J. 2006. J. Chem. Phys. B 110:2821-2832.

7. Summers, C.A. and Flowers, R.A. 2000. Protein renaturation by the liquid organic salt ethylammonium nitrate. Protein Science 9:2001-2008.

8. Lange, C., Patil, G., and Rudolph, R. 2005. Ionic liquids as refolding additives: N′-alkyl and N′-(w-hydroxyalkyl) N-methylimidazolium chloride. Protein Science 14:2693-2701.

9. Lozano, P. , de Diego, T. Guegan, J.P., Vaultier, M., and Ibora, J.L. 2001. Stabilization of a-chymotrypsin by ionic liquids in transesterification reactions. Biotehnol. Bioeng. 75:363-369.

10. Baker, S.N., McCleskey, T.M., Pandy, S., and Baker, G.A. 2004. Fluorescence study of protein thermostability in ionic liquids. Chem. Commun. 2004:940-941.

11. De Diego, T., Lozano, P., Gmouth, S. Vaultier, M., and Iborra, J.L. 2004. Fluorescence and CD spectroscopic analysis of the a-chymotrypsin stabilization by the ionic liquid 1-ethyl-3-methyimidazolium bis[(trifluormethyl)sulfonyl]amide. Biotechnol. Bioeng. 88:916-924.

12. Sun, L., Tao, D., Han, B., Ma, J., Zhu, G., Liang, Z., Shan, Y., Zhang, L., and Zhang, Y. 2010. Ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate for shotgun membrane proteomics. Anal. Bioanal. Chem. DOI 10.1007/s0216-010-4381-5.

13. Evans, D.F., Yamauchi, A., Roman, R., and Casassa, E.Z. 1982. J. Colloids Interface Sci. 88:89-96.

14. Proteins in Ionic Liquids: Current Status of Experiments and Simulations. Christian Schrode, Top Curr Chem (J). 2017; 375(2): 25.

15. Ionic Liquids as Stabilization and Refolding Additives and Solvents for Proteins. Fujita K. Adv Biochem Eng Biotechnol. 2018 Jul 13. doi: 10.1007/10_2018_65.

16. Fujita K. (2018) Ionic Liquids as Stabilization and Refolding Additives and Solvents for Proteins. In: . Advances in Biochemical Engineering/Biotechnology. Springer, Berlin, Heidelberg.