This work reports a promising approach to the development of novel self-buffering and biocompatible ionic liquids for biological research in which the anions are derived from CAPS Buffer (Good’s buffers, GB). Five Good’s buffers (Tricine, TES, CHES, HEPES, and MES) were neutralized with four suitable hydroxide bases (1-ethyl-3-methylimidazolium, tetramethylammonium, tetraethylammonium, and tetrabutylammonium) producing 20 Good’s buffer ionic liquids (GB-ILs). The presence of the buffering action of the synthesized GB-ILs was ascertained by measuring their pH-profiles in water. Moreover, a series of mixed GB-ILs with wide buffering ranges were formulated as universal buffers. The impact of GB-ILs on bovine serum albumin (BSA), here used as a model protein, is discussed and compared with more conventional ILs using spectroscopic techniques, such as infrared and dynamic light scattering. They appear to display, in general, a greater stabilizing effect on the protein secondary structure than conventional ILs. A molecular docking study was also carried out to investigate on the binding sites of GB-IL ions to BSA. We further used the QSAR-human serum albumin binding model, log K(HSA), to calculate the binding affinity of some conventional ILs/GB-ILs to HSA. The toxicity of the GB and GB-ILs was additionally evaluated revealing that they are non-toxic against Vitro fischeri. Finally, the GB-ILs were also shown to be able to form aqueous biphasic systems when combined with aqueous solutions of inorganic or organic salts, and we tested their extraction capability for BSA. These systems were able to extract BSA with an outstanding extraction efficiency of 100% in a single step for the GB-IL-rich phase, and, as a result, the use of GB-IL-based ABS for the separation and extraction of other added-value biomolecules is highly encouraging and worthy of further investigation.
Room temperature ionic liquids (RTILs) have been considered as a new type of non-aqueous solvent for chemical synthesis, biocatalysis, electro-chemical devices, polymerization, engineering fluids, and other purposes. This wide variety of applications is a major result of their unusual and tunable physicochemical properties.1-5 ILs are salts that remain in the liquid state below the boiling point of water (100 °C). They are characterized by a high ionic conductivity, high chemical/thermal stability, non-flammability, and high solubility for a large range of materials. Several studies have shown that some ILs, either pure or in aqueous solution, can increase the stability of biomolecules like proteins, enzymes and DNA, which is expressed in the vast number of manuscripts published in this field.5-9 The cations and anions of biocompatible ILs are usually more complex than common salts, such as NaCl. The IL cations are often nitrogen-based, namely alkylammonium, dialkylimidazolium, alkylpyridinium and alkylpyrrolidinium, or phosphorous-containing compounds, such as the widely employed tetralkylphosphoniums. IL anions can be halides, nitrates, sulfates, alkylsulfates, alkylsulfonates, [BF4]−, [PF6]−, [CH3CO2]−, [CF3CO2]−, [Tf2N]−, and [R2PO4]−, among others.
Proteins remain in their native (folded) state under physiological conditions, whereas their denatured (unfolded) state is induced by thermal or chemical unfolding. The effects of ions on protein folding, enzyme activity, and protein crystallization are typically described by the Hofmeister series.10 Although it has been accepted that salt ions exert their effects indirectly by changing the water structure, recent results have questioned this model and shown that in most cases a direct interaction of the salt ions with the protein is involved.11 A particularly useful aspect of ILs results from the combination between chaotropic cations and kosmotropic anions that were shown previously to stabilize proteins.12 Another important aspect of ILs is that their polarity and hydrophobicity can be tuned by varying the alkyl side-chain length of the cations and by an appropriate selection of the cation core or anion nature. There are several reports showing that the enzyme activity increases with the IL hydrophobicity up to a maximum, and then decreases with a further increase in the IL hydrophobicity.12 In contrast, there are also some conflicting studies reporting a relatively high enzyme stability and activity in hydrophilic ILs.12