Unusual stability of protein molecules in the presence of multivalent counterions.

Proteins are known to undergo denaturation and form different phases with varying physicochemical parameters. We report unusual stability of bovine serum albumin protein against commonly used denaturants (temperature and surfactant) in the charged reversal reentrant phase, caused by the multivalent counterions. Unlike monovalent counterions, which promote the denaturants' induced protein unfolding, the unfolding is restricted in the presence of multivalent ions. The observations are beyond the scope of general understanding of protein unfolding and are believed to be governed by ion-ion correlations driven strong condensation of the multivalent ions.

Evolution of Interactions in the Protein Solution As Induced by Mono and Multivalent Ions.

The evolution of interactions in the bovine serum albumin (BSA) protein solution on addition of mono and multivalent (di, tri and tetra) counterions has been studied using small-angle neutron scattering (SANS), dynamic light scattering (DLS) and ζ-potential measurements. It is found that in the presence of mono and divalent counterions, protein behavior can be well explained by DLVO theory, combining the contributions of screened Coulomb repulsion with the van der Waals attraction. The addition of mono or divalent salts in protein solution reduces the repulsive barrier and hence the overall interaction becomes attractive, but the system remains in one-phase for the entire concentration range of the salts, added in the system. However, contrary to DLVO theory, the protein solution undergoes a reentrant phase transition from one-phase to a two-phase system and then back to the one-phase system in the presence of tri and tetravalent counterions. The results show that tri and tetravalent (unlike mono and divalent) counterions induce short-range attraction between the protein molecules, leading to the transformation from one-phase to two-phase system. The two-phase is characterized by the fractal structure of protein aggregates. The excess condensation of these higher-valent counterions in the double layer around the BSA causes the reversal of charge of the protein molecules resulting into reentrant of the one-phase, at higher salt concentrations. The complete phase behavior with mono and multivalent ions has been explained in terms of the interplay of electrostatic repulsion and ion-induced short-range attraction between the protein molecules.

Interactions in reentrant phase behavior of a charged nanoparticle solution by multivalent ions.

The interactions following a reentrant phase transition of charged silica nanoparticles from one phase to two phases and back to one phase by varying the concentration of multivalent counterions have been examined. The observations are far beyond the framework of Debye-Hückel or even nonlinear Poisson-Boltzmann equations and demonstrate the universal behavior of multivalent counterion-driven charge inversion. We show that the interplay of multivalent counterion-induced short-range attraction and long-range electrostatic repulsion between nanoparticles results in reentrant phase behavior.

High-Precision Determination of the Neutron Coherent Scattering Length.

The neutron coherent scattering length b c has been determined interferometrically to an uncertainty of about 5 × 10(-5) by measuring the nondispersive phase. We propose improving the uncertainty to about 10(-6) by optimizing various parameters of the interferometric experiment. Any uncertainty in the b c determination arising from possible variations in the constitution of the ambient air can be eliminated by performing the experiment in vacuum. When such uncertainty is attained, it becomes necessary to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correct b c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimate b c. The refractive index for neutrons can thus be determined to a phenomenal uncertainty of about 10(-12).