Unusual stability of protein molecules in the presence of multivalent counterions

Evolution of the structure and interaction in the surfactant-dependent heat-induced gelation of protein

The addition of a surfactant and/or an increase in temperature disrupt the native structure of proteins, where high temperature further results in protein gelation. However, in a mixed protein-surfactant system, surfactant concentration and temperature have been observed to exhibit both mutually associative and counter-balancing effects towards heat-induced gelation of protein-surfactant dispersion. This study is conducted on globular bovine serum albumin (BSA) protein and cationic surfactant dodecyl trimethyl ammonium bromide (DTAB), which interact strongly owing to their oppositely charged nature. The findings reveal that the BSA-DTAB suspension undergoes gelation with increasing temperature but only at lower concentrations of DTAB, where the presence of the surfactant facilitates gelation (associative effect). Conversely, as the surfactant concentration increases beyond a critical value, temperature-driven gelation of the BSA-DTAB system is completely inhibited, despite surfactant-induced protein denaturation (counter-balancing effect). To conceptualize these results, we compared them with observations made in a system comprising protein and a similarly charged surfactant, sodium dodecyl sulfate (SDS). It has been further demonstrated that the anionic surfactant (SDS) can restrict protein gelation at much lower concentration compared to the cationic surfactant (DTAB). The evolution of the structure and interaction during gel formation/inhibition has been examined to understand the underlying mechanism guiding these sol-gel transitions. We present a comprehensive phase diagram, encompassing the solution/gel states of the protein-surfactant dispersion, with respect to the dispersion temperature, surfactant concentration, and ionic behavior (anionic or cationic) of the surfactants.

Molecular Insights into the Conformational and Binding Behaviors of Human Serum Albumin Induced by Surface-Active Ionic Liquids

Extensive research has been carried out to investigate the stability and function of human serum albumin (HSA) when exposed to surface-active ionic liquids (SAILs) with different head groups (imidazolium, morpholinium, and pyridinium) and alkyl chain lengths (ranging from decyl to tetradecyl). Analysis of the protein fluorescence spectra indicates noticeable changes in the secondary structure of HSA with varying concentrations of all SAILs tested. Helicity calculations based on the Fourier transform infrared (FTIR) data show that HSA becomes more organized at the micellar concentration of SAILs, leading to an increased protein activity at this level. Small-angle neutron scattering (SANS) data confirm the formation of a bead-necklace structure between the SAILs and HSA. Atomistic molecular dynamics (MD) simulation results identify several hotspots on the protein surface for interaction with SAIL, which results in the modulation of protein conformational fluctuation and stability. Furthermore, fluorescence resonance energy transfer (FRET) experiments with the intramolecular charge transfer (ICT) probe trans-ethyl p-(dimethylamino) cinnamate (EDAC) demonstrate that higher alkyl chain lengths and SAIL concentrations result in a significantly increased energy transfer efficiency. The findings of this study provide a detailed molecular-level understanding of how the protein structure and function are affected by the presence of SAILs, with potential implications for a wide range of applications involving protein-SAIL composite systems.

Elucidating the influence of electrostatic force on the re-arrangement of H-bonds of protein polymers in the presence of salts

Polyampholytes/proteins have an intriguing network of hydrogen bonds (H-bonds), especially their secondary structure, which plays a crucial role in determining the conformational stability of the polymer. The changes in protein secondary structure in the protein-salt system have been extensively deciphered by researchers, yet their pathways for breakage and recreation are unknown. Understanding the mechanism of protein conformational changes towards their biological activities, like protein folding, remains one of the main challenges and requires multiscale analysis of this strongly correlated system. Herein, salts have been used to reveal the re-arrangement behavior in the H-bond network of proteins under the influence of electrostatic interactions, as the strength of electrostatic forces is much stronger than that of H-bonds. At lower salt concentrations, there are negligible changes in the secondary structures as the electrostatic forces induced by the salt ions are less. Later, the existing H-bonds break and reconstruct new H-bonds at higher salt concentrations due to the influence of the stronger electrostatic interaction induced by the large number of salt ions. Molecular dynamics (MD) simulations and FTIR studies have been used rigorously to decipher the reason behind the re-arrangement of the H-bonds within gelatin (protein). The re-arrangement in the H-bond has also been observed with time from simulations and experiments. Thus, this study could provide a fresh perspective on the conformational changes of polyampholytes/proteins and will also influence the studies of protein folding-unfolding interaction in the presence of salt ions.

pH-Induced Biophysical Perspectives of Binding of Surface-Active Ionic Liquid [BMIM][OSU] with HSA and Dynamics of the Formed Complex

The influence of pH on the human serum albumin (HSA) interaction with ionic liquid (IL)1-butyl 3-methylimidazolium octyl sulfate ([BMIM][OSU]) at its sub-micellar concentration of 5 mM (well below CMC ∼31 mM at 25 °C) in aqueous solution has been monitored employing different methods, viz., circular dichroism (CD), fluorescence, electrokinetic determination of the zeta potential (ZP), nuclear magnetic resonance (NMR), small-angle neutron scattering (SANS), and molecular docking (MD). CD analysis indicated a noticeable reduction of the α-helical content of HSA by IL at pH 3. A significant interaction of the anionic part of IL with HSA was evident from the ¹H chemical shifts and saturation transfer difference (STD) NMR. A strong binding between IL and HSA was observed at pH 3 relative to pH 5, revealing the importance of electrostatic and hydrophobic interactions assessed from global binding affinities and molecular correlation times derived from STD NMR and a combined selective/nonselective spin-relaxation analysis, respectively. ZP data supported the electrostatic interaction between HSA and the anionic part of IL. The nature of IL self-diffusion with HSA was assessed from the translational self-diffusion coefficients by pulse field gradient NMR. SANS results revealed the formation of prolate ellipsoidal geometry of the IL-HSA complex. MD identified the preferential binding sites of IL to the tryptophan centers on HSA. The association of IL with HSA was supported by fluorescence measurements, in addition to the structural changes that occurred in the protein by the interaction with IL. The anionic part of IL contributed a major interaction with HSA at the pH levels of study (3, 5, 8, and 11.4); at pH > 8 (effectively 11.4), the protein also interacted weakly with the cationic component of IL.

Tuning of silica nanoparticle-lysozyme protein complexes in the presence of the SDS surfactant

The structures of the complexes of anionic silica nanoparticle (size ∼ 16 nm)-lysozyme (cationic) protein, tuned by the addition of the anionic surfactant sodium dodecyl sulfate (SDS), have been investigated by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The unique advantage of contrast variation SANS has been used to probe the role of individual components in binary and ternary systems. The cationic lysozyme protein (at pH ∼ 7) adsorbs on the anionic silica nanoparticles and forms mass fractal aggregates due to the strong attractive interaction, whereas similarly charged SDS does not interact physically with silica nanoparticles. The presence of SDS, however, remarkably affects the nanoparticle-protein interactions via binding with the oppositely charged segments of lysozyme. In general, the SDS-lysozyme complexes possess a variety of structures (e.g., insoluble complexes of Ly(DS)₈, crystalline structure, or micelle-like structure) depending on the surfactant-to-protein molar ratio (S/P). In the ternary system (HS40-lysozyme-SDS), lysozyme preferentially binds with SDS, instead of directly to nanoparticles. At low S/Ps (0 ≤ S/P ≤ 10), the SDS concentration is not enough to fully neutralize the charge of lysozyme, leading to the formation of cationic SDS-lysozyme complex-mediated nanoparticle aggregation. The morphology of the nanoparticle-(lysozyme-SDS) complexes is also found to be mass fractal kind where the fractal dimension increases with increasing SDS concentration. At S/P > 10, there is sufficient SDS to fully neutralize the lysozyme in the absence of competing charges from the particle but it is at S/P = 50 before all lysozyme desorbs from the particle and binds completely to the overwhelming amount of SDS, creating an oppositely charged lysozyme-SDS complex, which is repelled from the particle.

Unravelling Polyethylenimine Mediated Non-monotonic Stability Behaviour of Silica Colloids: Role of Competing Electrostatic and Entropic Interactions

Polymer-mediated interactions play an important role in the stability of the colloids and therefore, are paramount for both fundamental as well as scientific interests. The stability of the colloids in...