Several ionic liquids (ILs) are known to revert aggregation processes and improve the in vitro refolding of denatured proteins. In this paper the capacity of a particular class of ammonium based ILs to act as refolding enhancers was tested using lysozyme as a model protein. Raman spectra of ILs treated fibrillar lysozyme as well as lysozyme in its native and fibrillar conformations were collected and carefully analyzed to characterize the refolding extent under the effect of the IL interaction. Results obtained confirm and largely extend the earlier knowledge on this class of protic ILs and indicate Ethyl Ammonium Nitrate (EAN) as the most promising additive for protein refolding. The experiment provides also the demonstration of the high potentiality of Raman spectroscopy as a comprehensive diagnostic tool in this field.
IntroductionProtein aggregation is one of the most important problems in the production and storage industrial processes and therefore among the causes of the major economic loss in biotechnology and pharmaceutical factories. Indeed in manufacturing commercial products, the goal is to obtain a stable and correct protein folding for allowing the full functionality. 1 The problems of protein aggregation and structural stability are not limited to the manufacturing processes. Protein misfolding diseases are a well-known class of ailments including Alzheimer, Parkinson and Huntington diseases. They all involve protein aggregation and share common features such as the presence of insoluble fibrous protein aggregation in a specific structural motif characterized by a cross-β sheet structure. 2 Key issues on aggregation are not yet fully addressed such as the detailed microscopic mechanism leading to aggregation, the structure of aggregates, how the environmental conditions can affect the rate and the amount of aggregation and how aggregation can be prevented and/or removed. Additives may promote the stabilization of the native state of the protein accelerating the kinetics of the correct folding and removing/inhibiting the aggregation of denatured polypeptides and intermediates of the folding pathways.
3In recent years ionic liquids (ILs) have been used to stabilize the protein activity, to inhibit or reduce aggregation, and to improve the in vitro refolding of denatured proteins. 4,5 ILs have numerous attractive characteristics including their non-volatility, good solvating properties, thermal stability, and recyclability, that render these compounds "environmentally green". 6,7,8,9,10 One of the most important qualities of these solvents is the high tunability of their chemical structure. Indeed, they can be designed to have specific physical and chemical qualities by acting on either the alkyl chains (i.e. modifying the length, the presence of hydrophobic groups, etc.) or the anion (i.e. varying the degree of the charge delocalization, its hydrogen bonding ability, etc). 11 ILs having coordinating anions which are strong hydrogen bond acceptors (e.g. Cl − , NO 3 − , CH 3 COO − and (MeO) 2 PO 2 − )...