Abstract
Despite the progress achieved by aqueous biphasic systems (ABSs) comprising ionic liquids (ILs) in extracting valuable proteins, the quest for bio-based and protein-friendly ILs continues. To address this need, this work uses natural organic acids as precursors in the synthesis of four ILs, namely tetrabutylammonium formate ([N4444][HCOO]), tetrabutylammonium acetate ([N4444][CH3COO]), tetrabutylphosphonium formate ([P4444][HCOO]), and tetrabutylphosphonium acetate ([P4444][CH3COO]). It is shown that ABSs can be prepared using all four organic acid-derived ILs paired with the salts potassium phosphate dibasic (K2HPO4) and tripotassium citrate (C6H5K3O7). According to the ABSs phase diagrams, [P4444]-based ILs outperform their ammonium congeners in their ability to undergo liquid–liquid demixing in the presence of salts due to their lower hydrogen-bond acidity. However, deviations to the Hofmeister series were detected in the salts’ effect, which may be related to the high charge density of the studied IL anions. As a proof of concept for their extraction potential, these ABSs were evaluated in extracting human transferrin, allowing extraction efficiencies of 100% and recovery yields ranging between 86 and 100%. To further disclose the molecular-level mechanisms behind the extraction of human transferrin, molecular docking studies were performed. Overall, the salting-out exerted by the salt is the main mechanism responsible for the complete extraction of human transferrin toward the IL-rich phase, whereas the recovery yield and protein-friendly nature of these systems depend on specific “IL-transferrin” interactions.
Highlights
Aqueous biphasic systems (ABSs) are liquid–liquid systems used as extraction techniques, comprising water as the main constituent and two-phase splitting promoters such as two polymers or a polymer and a salt [1]
The potential for natural organic acid-derived ionic liquids (ILs) to create ABSs was initially evaluated by determining their phase diagrams with C6 H5 K3 O7 and K2 HPO4 as the salting-out agents
[N4444 ][CH3 COO]) and the respective binodal curves were determined at 25 ◦ C and atmospheric pressure, and further correlated using the equation proposed by Merchuk et al [47]
Summary
Aqueous biphasic systems (ABSs) are liquid–liquid systems used as extraction techniques, comprising water as the main constituent and (at least) two-phase splitting promoters such as two polymers or a polymer and a salt [1]. These systems may be formed free of volatile organic solvents. If their components are wisely chosen, these systems may be used as low-cost, simple, bio- and eco-friendly extraction platforms [2,3]. Imidazolium-based ILs, mostly those carrying fluorinated anions, are among the most widely adopted to formulate
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