Submitted to: Journal of the Bioelectrochemistry Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2005
Publication Date: 5/1/2006
Citation: Dicarlo, C.M., Compton, D.L., Evans, K.O., Laszlo, J.A. 2006. Bioelectrocatalysis in ionic liquids. Examining specific cation and anion effects on electrode-immobilized cytochrome c. Bioelectrochemistry. 68:134-143. Interpretive Summary: A study was conducted to investigate the behavior of electron transfer proteins immobilized on metal surfaces in a new type of solvent called ionic liquids. The objective was to determine if these proteins, which will be used to convert a portion of the 800,000 lbs of excess soybean oil produced in the U.S. to nutritional and cosmetic ingredients, remain active in ionic liquids. The proteins cannot be used in their natural environment to conduct these conversions since soybean oil is not soluble in water. Moreover, the use of traditional organic solvents should be avoided because of harmful health and environmental considerations. Ionic liquids are a new class of solvents that have negligible vapor pressure, making them less hazardous to use than traditional, volatile organic solvents. The results of this study show that the immobilized electron transfer proteins do not remain electrochemically active when exposed to the ionic liquids, but do become catalytically active again when reintroduced to water. The adverse effects of both the cationic and anionic components of the ionic liquids were determined. These fundamental results will be used by us and other scientists as a foundation to develop methods which will allow for the preservation of the immobilized electron transfer protein activity in ionic liquids.
Technical Abstract: Cytochrome c immobilized on mercaptothiol self-assembled monolayers exhibit a characteristic Fe(III)/Fe(II) redox signal that is lost when exposed to ionic liquids composed of a butylimidazolium cation combined with either hexafluorophosphate or bis(trifluoromethylsulfonyl)imide anion. In this study it was shown that exposure to the aqueous solubilized ionic liquid components, butyl-, hexyl-, and octyl- imidazolium cations and bis(trifluoromethylsulfonyl)imide anion, resulted in partial electrochemical signal loss. Absorbance and fluorescence measurements showed that signal loss due to the cationic ionic liquid component followed a different mechanism than that of the anionic component. Although a portion of the signal was recoverable, irreversible signal loss also occurred in both cases. The source of the irreversible component is suggested to be the loss of protein secondary structure through complexation between the ionic liquid components and the protein surface residues. The reversible electrochemical signal loss is likely due to interfacial interactions imposed between the electrode and the cytochrome heme group. The influence of the amount of exposed surface residues was explored with a simplified model protein, microperoxidase-11.