Scattering approaches to unravel protein solution behaviors in ionic liquids and deep eutectic solvents: From basic principles to recent developments

Proteins in ionic liquids (ILs) and deep eutectic solvents (DESs) have gained significant attention due to their potential applications in various fields, including biocatalysis, bioseparation, biomolecular delivery, and structural biology. Scattering approaches including dynamic light scattering (DLS) and small-angle X-ray and neutron scattering (SAXS and SANS) have been used to understand the solution behavior of proteins at the nanoscale and microscale. This review provides a thorough exploration of the application of these scattering techniques to elucidate protein properties in ILs and DESs. Specifically, the review begins with the theoretical foundations of the relevant scattering approaches and describes the essential solvent properties of ILs and DESs linked to scattering such as refractive index, scattering length density, ion-pairs, liquid nanostructure, solvent aggregation, and specific ion effects. Next, a detailed introduction is provided on protein properties such as type, concentration, size, flexibility and structure as observed through scattering methodologies. This is followed by a review of the literature on the use of scattering for proteins in ILs and DESs. It is highlighted that enhanced data analysis and modeling tools are necessary for assessing protein flexibility and structure, and for understanding protein hydration, aggregation and specific ion effects. It is also noted that complementary approaches are recommended for comprehensively understanding the behavior of proteins in solution due to the complex interplay of factors, including ion-binding, dynamic hydration, intermolecular interactions, and specific ion effects. Finally, the challenges and potential research directions for this field are proposed, including experimental design, data analysis approaches, and supporting methods to obtain fundamental understandings of complex protein behavior and protein systems in solution. We envisage that this review will support further studies of protein interface science, and in particular studies on solvent and ion effects on proteins.

The attenuating ability of deep eutectic solvents towards the carboxylated multiwalled carbon nanotubes induced denatured β-lactoglobulin structure

The stabilization of proteins has been a major challenge for their practical utilization in industrial applications. Proteins can easily lose their native conformation in the presence of denaturants, which unfolds the protein structure. Since the introduction of deep eutectic solvents (DESs), there are numerous studies in which DESs act as promising co-solvents that are biocompatible with biomolecules. DESs have emerged as sustainable biocatalytic media and an alternative to conventional organic solvents and ionic liquids (ILs). However, the superiority of DESs over the deleterious influence of denaturants on proteins is often neglected. To address this, we present the counteracting ability of biocompatible DESs, namely, choline chloride-glycerol (DES-1) and choline chloride-urea (DES-2), against the structural changes induced in β-lactoglobulin (Blg) by carboxylated multiwalled carbon nanotubes (CA-MWCNTs). The work is substantiated with various spectroscopic and thermal studies. The spectroscopic results revealed that the fluorescence emission intensity enhances for the protein in DESs. Contrary to this, the emission intensity extremely quenches in the presence of CA-MWCNTs. However, in the mixture of DESs and CA-MWCNTs, there was a slight increase in the fluorescence intensity. Circular dichroism spectral studies reflect the reappearance of the native band that was lost in the presence of CA-MWCNTs, which is a good indicator of the counteraction ability of DESs. Further, thermal fluorescence studies showed that the protein exhibited extremely great thermal stability in both DESs as well as in the mixture of DES-CA-MWCNTs compared to the protein in buffer. This study is also supported by dynamic light scattering and zeta potential measurements; the results reveal that DESs were successfully able to maintain the protein structure. The addition of CA-MWCNTs results in complex formation with the protein, which is indicated by the increased hydrodynamic size of the protein. The presence of DESs in the mixture of CA-MWCNTs and DESs was quite successful in eliminating the negative impact of CA-MWCNTs on protein structural alteration. DES-1 proved to be superior to DES-2 over counteraction against CA-MWCNTs and maintained the native conformation of the protein. Overall, both DESs act as recoiling media for both native and unfolded (denatured by CA-MWCNTs) Blg structures. Both the DESs can be described as potential co-solvents for Blg with increased structural and thermal stability of the protein. To the best of our knowledge, this study for the first time has demonstrated the role of choline-based DESs in the mixture with CA-MWCNTs in the structural transition of Blg. The DESs in the mixture successfully enhance the stability of the protein by reducing the perturbation caused by CA-MWCNTs and then amplifying the advantages of the DESs present in the mixture. Overall, these results might find implications for understanding the role of DES-CA-MWCNT mixtures in protein folding/unfolding and pave a new direction for the development of eco-friendly protein-protective solvents.

Ion-Mediated Protein Stabilization on Nanoscopic Surfaces

The emergence of nanoparticles in biomedical applications has made their interactions with proteins inevitable. Nanoparticles conjugated with proteins and peptide-based constructs form an integral part of nanotherapeutics and have recently shown promise in treating a myriad of diseases. The proper functioning of proteins is critical to achieve their biological functions. However, interface issues result in the denaturation of proteins, and the loss of orientation and steric hindrance can adversely affect the function of the conjugate. Furthermore, surface-induced denaturation also triggers protein aggregation, resulting in amyloid-like species. Understanding the mechanistic underpinnings of protein-nanoparticle interactions and controlling their interfacial characteristics are critical and challenging due to the complex nature of the conjugates. In this milieu, we demonstrate that ionic liquids can be suitable candidates for stabilizing protein-nanoparticle interactions by virtue of their excellent protein-preserving properties. We also probe the previously unexplored mechanism of ion-mediated stabilization of the protein molecules on the nanoparticle surface. The protein-nanoparticle conjugates consist of lysozyme and choline-based ionic liquids characterized by optical and electron microscopy techniques combined with surface-sensitive plasmon-enhanced Raman spectroscopy. Furthermore, atomistic molecular dynamics simulations of the conjugates delineate interfacial interactions of the protein molecules and the modulation by the ions, particularly the conformational changes and the dynamic correlation when the protein and specific ionic liquid molecules are adsorbed on the nanoparticle surface. The combined experimental and computational studies showed the synergistic behavior of the ions of the ionic liquids, specifically the orientation and coverage of the anions aided by the cations to control the surface interactions and hence the overall protein stability. These studies pave the way for using ionic liquids, particularly their biocompatible counterparts in nanoparticle-based complexes, as stabilizing agents for biomedical applications.

Analysis of the effect of 1-Allyl-3-Methylimidazolium chloride on thermodynamic stability, folding kinetics, and motional dynamics of horse cytochrome c

1-allyl-3-methylimidazolium chloride (AMIMCl) acts as a potential green solvent for proteins. The present work provides a possible pathway by which the structural, kinetic, thermodynamic, and folding properties of horse cytochrome c (cyt c) are affected in green aqueous-AMIMCl systems. Analysis of the effect of AMIMCl on thermodynamic stability, refolding/unfolding kinetics, and motional dynamics of cyt c provided important information, (i) AMIMCl decreases the thermodynamic stability of reduced cyt c and also strengthens the guanidinium chloride (GdmCl)-mediated decrease in thermodynamic stability of protein, (ii) AMIMCl reduces the thermal-fluctuation of Met80-containing omega-loop of natively-folded compact state of carbonmonoxycytochrome c (MCO-state) due to polyfunctional interactions between the AMIM⁺ and different groups of protein, (iii) AMIMCl shifts the kinetic chevron plot, ln k_obs[GdmCl] to the lower concentration of GdmCl, (iv) AMIMCl shifts the refolding and unfolding limps to vertically downwards and upwards, respectively, and (v) AMIMCl reducing the unfolding free energy estimated by both thermodynamic and kinetic analysis.

Absorption of the [bmim][Cl] Ionic Liquid in DMPC Lipid Bilayers across Their Gel, Ripple, and Fluid Phases

Lipid bilayers are a key component of cell membranes and play a crucial role in life and in bio-nanotechnology. As a result, controlling their physicochemical properties holds the promise of effective therapeutic strategies. Ionic liquids (ILs)─a vast class of complex organic electrolytes─have shown a high degree of affinity with lipid bilayers and can be exploited in this context. However, the chemical physics of IL absorption and partitioning into lipid bilayers is yet to be fully understood. This work focuses on the absorption of the model IL [bmim][Cl] into 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers across their gel, ripple, and fluid phases. Here, by small-angle neutron scattering, we show that (i) the IL cations are absorbed in the lipid bilayer in all its thermodynamic phases and (ii) the amount of IL inserted into the lipid phase increased with increasing temperature, changing from three to four IL cations per 10 lipids with increasing temperature from 10 °C in the gel phase to 40 °C in the liquid phase, respectively. An explicative hypothesis, based on the entropy gain coming from the IL hydration water, is presented to explain the observed temperature trend. The ability to control IL absorption with temperature can be used as a handle to tune the effect of ILs on biomembranes and can be exploited in bio-nanotechnological applications.

Recent Advances in Ionic Liquids in Biomedicine

The use of ionic liquids and deep eutectic solvents in biomedical applications has grown dramatically in recent years due to their unique properties and their inherent tunability. This review will introduce ionic liquids and deep eutectics and discuss their biomedical applications, namely solubilization of drugs, creation of active pharmaceutical ingredients, delivery of pharmaceuticals through biological barriers, stabilization of proteins and other nucleic acids, antibacterial agents, and development of new biosensors. Current challenges and future outlooks are discussed, including biocompatibility, the potential impact of the presence of impurities, and the importance of understanding the microscopic interactions in ionic liquids in order to design task-specific solvents.

Correlating Bromelain's activity with its structure and active-site dynamics and the medium's physical properties in a hydrated deep eutectic solvent

Deep eutectic solvents (DESs) are emerging as new media of choice for biocatalysis due to their environmentally friendly nature, fine-tunability, and potential biocompatibility. This work deciphers the behaviour of bromelain in a ternary DES composed of acetamide, urea, and sorbitol at mole fractions of 0.5, 0.3, and 0.2, respectively (0.5Ac/0.3Ur/0.2Sor), with various degrees of hydration. Bromelain is an essential industrial proteolytic enzyme, and the chosen DES is non-ionic and liquid at room temperature. This provides us with a unique opportunity to contemplate protein behaviour in a non-ionic DES for the very first time. Our results infer that at a low DES concentration (up to 30% V/V DES), bromelain adopts a more compact structural conformation, whereas at higher DES concentrations, it becomes somewhat elongated. The microsecond conformational fluctuation time around the active site of bromelain gradually increases with increasing DES concentration, especially beyond 30% V/V. Interestingly, bromelain retains most of its enzymatic activity in the DES, and at some concentrations, the activity is even higher compared with its native state. Furthermore, we correlate the activity of bromelain with its structure, its active-site dynamics, and the physical properties of the medium. Our results demonstrate that the compact structural conformation and flexibility of the active site of bromelain favour its proteolytic activity. Similarly, a medium with increased polarity and decreased viscosity is favourable for its activity. The presented physical insights into how enzymatic activity depends on the protein structure and dynamics and the physical properties of the medium might provide useful guidelines for the rational design of DESs as biocatalytic media.

Effect of imidazolium based ionic liquids on CO-association dynamics and thermodynamic stability of Ferrocytochrome c

Analysis of kinetic and thermodynamic parameters measured for CO-association reaction of Ferrocytochrome c (Ferrocyt c) under variable concentrations of 1-butyl-3-methylimidazolium with varying anion ([Bmim]X) (X = Cl^-, I^-, Br^-, HSO₄^-) at pH 7 revealed that the low concentration of [Bmim]X (≤0.5 M) constrains the CO-association dynamics of Ferrocyt c and typically follows the order: [Bmim]HSO₄ > [Bmim]Cl > [Bmim]Br > [Bmim]I. At relatively higher concentrations (>0.5), the chaotropic action of [Bmim]⁺ dominates which consequently increases the thermal-fluctuations responsible to denature the protein and thus accelerates the speed of CO-association reaction. Analysis of thermal denaturation curves of Ferrocyt c measured at different concentrations of [Bmim]X revealed that the [Bmim]X decreases the thermodynamic stability of protein and typically follows the order: [Bmim]I > [Bmim]Br > [Bmim]Cl > [Bmim]CH₃COO > [Bmim]HSO₄, demonstrating that the effect of [Bmim]X on thermodynamic stability of protein is not in accordance to Hofmeister series effect of anions because instead of increasing the kosmotropic anion carrying [Bmim]X ([Bmim]CH₃COO and [Bmim]HSO₄) also decreases the thermodynamic stability of protein.