Akshay J. Chinchalikar

Small-angle neutron scattering study of structure and interaction of nanoparticle, protein, and surfactant complexes.

Small-angle neutron scattering (SANS) measurements have been carried out from the multicomponent system composed of Ludox HS40 silica nanoparticle, bovine serum albumin (BSA) protein, and sodium dodecyl sulfate (SDS) surfactant in an aqueous system under the solution condition that all the components are negatively charged. Although the components are similarly charged, strong structural evolutions among them have been observed. The complexes of different components in pairs (nanoparticle-protein, nanoparticle-surfactant, and protein-surfactant) have been examined to correlate the role of each component in the three-component nanoparticle-protein-surfactant system. The nanoparticle-protein system shows depletion interaction induced aggregation of nanoparticles in the presence of protein. Both nanoparticle and surfactant coexist individually in a nanoparticle-surfactant system. In the case of a protein-surfactant system, the cooperative binding of surfactant with protein leads to micelle-like clusters of surfactant formed along the unfolded protein chain. The structure of the three-component (nanoparticle-protein-surfactant) system is found to be governed by the synergetic effect of nanoparticle-protein and protein-surfactant interactions. The nanoparticle aggregates coexist with the structures of protein-surfactant complex in the three-component system. The nanoparticle aggregation as well as unfolding of protein is enhanced in this system as compared to the corresponding two-component systems.

Small-angle neutron scattering study of temperature vs. salt dependence of clouding in charged micellar system

Clouding is studied by small-angle neutron scattering (SANS) on a charged micellar system of sodium dodecyl sulphate (SDS) and tetrabutylammonium bromide (TBAB) with varying temperature and salt NaCl. We show that the clouding occurs as a result of increase in the attractive potential between the micelles mediated by the dehydrated TBA+ counterions on increasing temperature and in the presence of salt. Both micelles and clusters coexist at cloud point temperature (CP) and beyond CP. The addition of salt can be used to obtain CP at room temperature (30 ° C). The relative effect of different salts on clouding has been found in the order CaCl2 > MgSO4 > Na2SO4 > NaF > NaCl > KCl > CsCl > NaBr > NaNO3 . This order is explained on the basis of two important roles played by salt ions: i) counterion condensation that increases the size of the micelles and ii) dehydration of TBA+ counterions by salt ions for bridging the micelles.

SANS study of understanding mechanism of cold gelation of globular proteins

Small-angle neutron scattering (SANS) has been used to probe the evolution of interaction and the resultant structures in the cold gelation of globular proteins. The cold gelation involves two steps consisting of irreversible protein deformation by heating followed by some means (e.g. increasing ionic strength) to bring them together at room temperature. We have examined the role of different salts in cold gelation of preheated aqueous Bovine Serum Albumin (BSA) protein solutions. The interactions have been modeled by two Yukawa potential combining short-range attraction and long-range repulsion. We show that in step 1 (preheated temperature effect) the deformation of protein increases the magnitude of attractive interaction but not sufficient to induce gel. The attractive interaction is further enhanced in step 2 (salt effect) to result in gel formation. The salt effect is found to be strongly depending on the valency of the counterions. The gel structure has been characterized by the mass fractals.

Tuning of adsorption vs. depletion interaction in nanoparticle-polymer system

Small-angle neutron scattering (SANS) has been used to study the interaction of anionic silica nanoparticles with poly ethylene glycol (PEG) in dilute aqueous solution. The measurements were carried out for fixed concentrations of silica nanoparticles (1 wt%) and polymer (0.5 wt%) without and with salt (0.2 M NaCl). It is found that PEG molecules adsorb on the nanoparticles when the interaction between nanoparticles is dominated by electrostatic repulsion. On the other hand with screening of electrostatic repulsion between nanoparticles in presence of salt, PEG molecules induce depletion attraction leading to the clustering of nanoparticles.

Co-existence of monomers and clusters in concentrated protein solutions

Small-angle neutron scattering (SANS) measurements have been performed on concentrated protein solutions in order to study aggregation of lysozyme molecules at different pH. The variation of correlation peak in concentration (C) dependent SANS data shows deviation from C1/3 behavior suggesting the aggregation phenomena in these systems. The aggregates or clusters coexist along with monomers with cluster fraction proportional to protein concentration. The clustering is also favored at higher pH approaching isoelectric point (pI) because of decrease in charge on the protein molecule.

SANS Study of Liquid‐Liquid Phase Transition in Protein Electrolyte Solution

Small‐angle Neutron Scattering (SANS) measurements have been performed on lysozyme protein solution to examine liquid‐liquid phase transition with the addition of NaCl. We show that the liquid‐liquid phase transition is governed by the increase in the attractive interaction between protein molecules as tuned by the salt concentration. This attractive interaction is modeled by the Baxter’s sticky hard sphere potential. It is found that when the attractive potential becomes significantly larger than the thermal energy protein molecules coalesce to form gel.

SANS Study of Clustering of Charged Micelles in Aqueous Electrolyte Solution

Small‐angle neutron scattering studies have been carried out for the charged micellar system of 50 mM sodium dodcyle sulphate (SDS) and 50 mM tetrabutylammonium bromide (TBAB) in presence of salts. The system undergoes clouding (turbid) at 0.75 M NaCl. We show that clouding takes place as a result of clustering of micelles and the required attraction is modeled by the Baxter’s sticky hard sphere potential. The clouding is also examined for different salts and is governed by the counterion condensation and dehydration properties of salts ions.