Proteins are highly labile molecules, thus requiring the presence of appropriate solvents and excipients in their liquid milieu to keep their stability and biological activity. In this field, ionic liquids (ILs) have gained momentum in the past years, with a relevant number of works reporting their successful use to dissolve, stabilize, extract, and purify proteins. Different approaches in protein-IL systems have been reported, namely, proteins dissolved in (i) neat ILs, (ii) ILs as co-solvents, (iii) ILs as adjuvants, (iv) ILs as surfactants, (v) ILs as phase-forming components of aqueous biphasic systems, and (vi) IL-polymer-protein/peptide conjugates. Herein, we critically analyze the works published to date and provide a comprehensive understanding of the IL-protein interactions affecting the stability, conformational alteration, unfolding, misfolding, and refolding of proteins while providing directions for future studies in view of imminent applications. Overall, it has been found that the stability or purification of proteins by ILs is bispecific and depends on the structure of both the IL and the protein. The most promising IL-protein systems are identified, which is valuable when foreseeing market applications of ILs, e.g., in "protein packaging" and "detergent applications". Future directions and other possibilities of IL-protein systems in light-harvesting and biotechnology/biomedical applications are discussed.

This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 (DOI 10.54499/UIDB/50011/2020), UIDP/50011/2020 (DOI 10.54499/UIDP/50011/2020), and LA/P/0006/2020 (DOI 10.54499/LA/P/0006/2020), financed by national funds through the FCT/MCTES (PIDDAC). This work was funded by the PRR - Recovery and Resilience Plan and by the NextGenerationEU funds at Universidade de Aveiro, through the scope of the Agenda for Business Innovation “InsectERA” (Project no. 20 with the application C644917393-00000032). P.B. acknowledges La-Caixa junior research leadership-post doctoral program (ID: 100010434, fellowship code: LCF/BQ/P122/11910023); A.A.T. acknowledges the Knut and Alice Wallenberg Foundation via the Wallenberg Centre for Molecular and Translational Medicine (AT), Swedish Research Council (Project no. 2020-04299) and Cancerfonden, Sweden (Project no. 222409Pj); D.M. acknowledges DST-SERB, India (EEQ/2021/000059), and NANOPLANT project, which received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 856961. T.S.K. acknowledges DST-SERB, India (CRG/2021/005897), and A.K. acknowledges CSIR-CSMCRI under in-house Project MLP0029 for financial supports. Authors acknowledge Ana Sofia Marques from CICECO-Aveiro Institute of Materials for assistance during drawing of some figures.

With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Peer reviewed