A Bioelectrochemical Method
for the Quantitative Description
of the Hofmeister Effect of Ionic Liquids in Aqueous Solution
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Abstract
It is imperative to establish a simple, efficient, and
practical
method to investigate the Hofmeister effect of ionic liquids (ILs)
on the behavior of proteins (enzymes). In this study, the effects
of the cations and anions of different ILs in aqueous media on the
structural stability of horseradish peroxidase (HRP), a model oxidoreductase,
were systematically investigated using electrochemical methods. It
is found that without ILs no direct electron transfer current signals
of HRP appear at bare glassy carbon electrode (GCE) in phosphate buffer
(pH 7.0) even after incubation and accumulation at a negative potential.
In the presence of ILs, however, a current signal occurs at GCE, depending
on the structure of the IL and its concentration. A linear relationship
between the peak currents and the scan rates demonstrates that the
direct electron transfer is a surface-confined thin-layer electrochemical
process. The redox signal at GCE is from the heme of HRP. An IL has
a perturbing effect on the HRP structure. The anodic peak current
of HRP at GCE, the catalytic activity of HRP, and the secondary structure
of HRP are well correlated. Different cations or anions at varied
concentrations have different effects on the structural stability
of HRP, resulting in different current signals at GCE. Thus, the anodic
peak current of HRP at GCE can be used as an indicator to quantitatively
characterize the effect of ILs on the structural stability of HRP.
The present Hofmeister series for cations and anions is in good agreement
with that reported elsewhere. To our knowledge, this is a first attempt
to establish a simple and practical electrochemical method to correlate
Hofmeister effects with characteristics of ions and solvents. The
present investigation not only deepens our understanding of the complex
electrochemical behavior of proteins in ILs media but also offers
a practical guidance to designing “green” and biocompatible
ILs for protein (enzyme) separation, purification, and enzymatic catalysis
and conversion
