Thermodynamics of the uptake of proteins by reverse micelles: a first approximation model

Interfacial thermodynamics of protein adsorption, ion co-adsorption and ion binding in solution. II. Model interpretation of ion exchange in lysozyme chromatography

In this paper we present a model for the ion exchange effects in protein adsorption. The model is applied to chromatography of lysozyme on strong cation exchanger 'mono S'. The experimental and general thermodynamic aspects have been discussed in Part 1, the preceding paper. The main modelling assumptions are (i) the charge regulation is confined to the small layer of contact between adsorbed protein and exchanger surface, (ii) the contact layer as a whole is electroneutral and (iii) the number of protein acid/base groups and exchanger surface acid groups which participate in the ion exchange is proportional to the area of the contact layer. The model is fitted to the experimental data by adjustment of only two or three parameters. The experimental co-adsorption numbers are very well reproduced. A few conspicuous features emerge: (i) the number of protein acid/base groups and exchanger surface acid groups in the contact layer varies with the medium conditions, such that the number is higher when the interaction between protein and exchanger surface is stronger. (ii) There is indirect evidence for structural alterations in the upper layers of the exchanger surface: the adsorbed protein is probably partly 'buried' in the surface.

Competitive adsorption of amphiphilic molecules and the stability of water-swollen micelles in oil

Motivated by recent attempts to confine biochemical processes inside water-in-oil microemulsions, we studied the composition and stability of mixed-amphiphile water-swollen micelles in oil from a theoretical point of view. A novel adsorption model demonstrates how the micellar contents (DNA, proteins, etc.) can dramatically affect the composition of the amphiphilic film and the resulting distribution of micelles. Special attention is given to the effect of electrostatic interactions within the micelles as well as between different ones. Since in a low dielectric medium charge fluctuations can lead to long-range intermicellar attractions, we suggest that the presence of amphiphilic polymers in the surfactant film may be needed to stabilize such microemulsions.

Electrostatic interactions of charged dipolar proteins in reverse micelles

The electrostatic interactions in a reverse micelle containing a small-ionized protein are studied by Monte Carlo simulation. The electrostatic contribution to the potential of mean force of the protein in the reverse micelle is determined for a neutral protein, a uniformly charged protein, and a uniformly charged protein with a dipole moment. The effect of addition of a simple electrolyte is studied. While symmetrically distributed micellar charge exerts no force on enclosed ionic species, the protein is driven to the micellar wall due to interactions with simple ions. Protein binding to the inner wall of the micelle can be regulated by added salt. The presence of a dipole drives the protein further to the wall. These effects are studied for several proteins characterized by different charges and dipole moments. For a weakly charged protein with a strong dipole moment the contribution of dipolar interaction to the free energy can represent a major driving force for protein solubilization in the microemulsion.

Cutinase-AOT interactions in reverse micelles: the effect of 1-hexanol

Cutinase encapsulated in dioctyl sulfosuccinate reverse micelles displays very low stability, undergoing fast denaturation due to an anchoring at the micellar interface. The denaturation process and the structure of the reverse micelle were characterized using biophysical techniques. The kinetics of denaturation observed from fluorescence match the increase of the hydrodynamic radius of reverse micelles. Denaturation in reverse micelles is mainly the unfolding of the three-dimensional structure since the decrease in the circular dichroism ellipticity in the far-UV range is very small. The process is accompanied by an increase in the steady-state anisotropy, as opposed to what happens for denaturation in aqueous solution. Since 1-hexanol used as co-surfactant in dioctyl sulfosuccinate reverse micelles slows or even prevents cutinase denaturation, its effect on cutinase conformation and on the size of reverse micelles was analyzed. When 1-hexanol is present, cutinase is encapsulated in a large reverse micelle, as deduced from dynamic light scattering. The large reverse micelle filled with cutinase was built from the fusion of reverse micelles according to a pseudo-unimolecular process ranging in time from a few minutes to 2h depending on the reverse micellar concentration. This slow equilibrium driven by the encapsulated cutinase has not been reported previously. The encapsulation of cutinase in dioctyl sulfosuccinate reverse micelles establishes a completely new equilibrium characterized by a bimodal population of empty and filled reverse micelles, whose characteristics depend greatly on the interfacial characteristics, that is, on the absence or presence of 1-hexanol.

Reverse micellar extraction for downstream processing of proteins/enzymes

New developments in the area of downstream processing are, hopefully, to fulfill the promises of modern biotechnology. The traditional separation processes such as chromatography or electrophoresis can become prohibitively expensive unless the product is of high value. Hence, there is a need to develop efficient and cost-effective downstream processing methods. Reverse micellar extraction is one such potential and a promising liquid-liquid extraction technique, which has received immense attention for isolation and purification of proteins/enzymes in the recent times. This technique is easy to scale-up and offers continuous operation. This review, besides briefly considering important physico-chemical and biological aspects, highlights the engineering aspects including mass transfer, mathematical modeling, and technology development. It also discusses recent developments in reverse micellar extraction such as affinity based separations, enzymatic reactions in reverse micelles coupled with membrane processes, reverse micellar extraction in hollow fibers, etc. Special emphasis has been given to some recent applications of this technique.

Reverse micelles as a water-property-control system to investigate the hydration/activity relationship of alpha-chymotrypsin

alpha-Chymotrypsin, solubilized in hydrated reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in n-octane, was used as a model system for studying the involvement of different water structures (strongly bound water, disordered water, water clusters and bulk water) in the development of the catalytically active conformation of the enzyme. Results presented in this study indicate a characteristic dependence of the stability/activity profile on the water content of the reverse-micellar system for values of wo of approximately 5 (wo is defined as [H2O]/[AOT]). The results are consistent with heat-capacity measurements for proteins. At very low wo values, the conformation of alpha-chymotrypsin changes to a very rigid structure in comparison to the structure observed in water. This is demonstrated by the overall center of gravity of the tryptophan fluorescence spectrum of the enzyme at wo = 0.65, which is blue shifted in comparison to the spectrum in bulk water indicating that the enzyme is in an apolar environment. In the absence of a hydration shell, the protein is to a great extent frozen and inactive. A small increase in the level of enzyme hydration (up to wo = 2.3) causes an increase in the amount of strongly bound water associated with the enzyme and the enzyme displays a high catalytic activity. Upon further addition of water, a new unstable water structure with unfavourable enthalpy is developed and the enzyme activity declines, reaching a minimum at wo = 5.1. A new increase of water content within a relatively small range, wo = 5-8, causes a dramatic increase in enzymic activity, reminiscent of a cooperative hydration dependence. In the range wo = 10-29, the effect of hydration on the activity is complete which shows that the enzyme activity depends on the amount of water in contact with the enzyme and not on the total amount of bulk water in the system. The experimental results on enzyme incubation at different wo values followed by dilution to constant high wo, are indicative of inactive conformational substates of alpha-chymotrypsin. It is demonstrated that highly active enzyme conformations at very low, wo values occur via an induced fit mechanism of substrate binding.

Comparison of the dynamic structure of alpha-chymotrypsin in aqueous solution and in reversed micelles by fluorescent active-site probing

A highly fluorescent anthraniloyl (Ant) group was covalently attached to the active site of alpha-chymotrypsin (CT), probably at Ser195. Ant-CT is stable at neutral pH for months, allowing a detailed fluorescence study of Ant-CT as a model protein to investigate its physical properties in 0.1 M Tris/HCl, pH 8.2, and in reversed micelles of n-octane, 0.1 M Tris/HCl, pH 8.2, and sodium bis(2-ethylhexyl)sulfosuccinate (AOT). Steady-state fluorescence measurements of the progressive red-shift of the center of gravity of the emission band as function of degree of hydration, wo, defined as [H2O]/[AOT], indicate that the average polarity in the vicinity of the probe is approaching that of bulk water at wo > 12. Time-resolved fluorescence measurements of Ant-CT in water and in reversed micelles showed that the active site has different properties in reversed micelles compared to those in water. Some specific changes at very low water content (0.6 < wo < 5) can be observed, which correlate with enzyme activity measurements in the same wo region (unpublished results). These effects are, for instance, significant changes in the average fluorescence lifetime and the internal flexibility of the probe. The overall rotational-correlation time of the enzyme in AOT reversed micelles seems to be independent on wo (5 < wo < 29), which suggests that the enzyme creates its own micelle.

Product inhibition of alpha-chymotrypsin in reverse micelles

The alpha-chymotrypsin-catalyzed hydrolysis of succinyl-L-alanyl-L-alanyl- L-prolyl-L-phenylalanyl p-nitroanilide has been studied in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane. It has been found that alpha-chymotrypsin is strongly inhibited competitively by the acidic peptide product which is formed during the course of the reaction. It has also been shown that the application of the integrated form of the Michaelis-Menten equation can be useful to detect possible inhibition effects and abnormal kinetic behavior of enzymes in reverse micelles. Furthermore, it has been shown that the turnover number (kcat) at low water content is lower than in water and increases as the water content in the system is lower than in water and increases as the water content in the system (wo = [H2O]/[AOT]) increases, kcat reaching the value in water at high wo. If however, initial velocity data, as obtained under conditions where the enzyme is not saturated with substrate, are plotted against wo, the curves are bell-shaped, with a maximum around wo = 15.