Quaternary Diffusion Coefficients in a Protein−Polymer−Salt−Water System Determined by Rayleigh Interferometry

Determination of Critical Micelle Concentration from the Diffusion-Driven Dilution of Micellar Aqueous Mixtures

Micellization is a phenomenon of central importance in surfactant solutions. Here, we demonstrate that the diffusion-based spreading of the free boundary between a micellar aqueous solution and pure water yields a one-dimensional spatial profile of surfactant concentration that can be used to identify the critical micelle concentration, here denoted as C*. This can be achieved because dilution of micelles into water leads to their dissociation at a well-defined position along the concentration profile and an abrupt increase in the diffusion coefficient. Rayleigh interferometry was successfully employed to determine C* values for three well-known surfactants in water at 25 °C: Triton X-100 (TX-100), sodium dodecyl sulfate (SDS), and poly(oxyethylene)(4)Lauryl Ether (Brij-30). The dependence of C* on salt concentration was also characterized for TX-100 in the presence of Na₂SO₄, NaCl, and NaSCN. Accurate values of C* can be directly identified by visual inspection of the corresponding concentration-gradient profiles. To apply the method of least squares to experimental concentration profiles, a mathematical expression was derived from Fick's law and the pseudophase separation model of micellization with the inclusion of appropriate modifications. While Rayleigh interferometry was employed in our experiments, this approach can be extended to any experimental technique that yields one-dimensional profiles of surfactant concentration. Moreover, diffusion-driven surfactant disaggregation is precise, noninvasive, requires single-sample preparation, and applies to both nonionic and ionic surfactants. Thus, this work provides the foundation of diffusion-driven dilution methods, thereby representing a valuable addition to existing techniques for the determination of C*.

Macromolecule Diffusiophoresis Induced by Concentration Gradients of Aqueous Osmolytes

Diffusiophoresis is the migration of a particle in a fluid induced by the concentration gradient of another solute. We have experimentally investigated diffusiophoresis of a neutral macromolecule, poly(ethylene glycol) (PEG; molecular weight, 20 kg mol^-1), in water induced by concentration gradients of osmolytes. Three osmolytes were examined: trimethylamine- N-oxide (TMAO), diethylene glycol (DEG), and urea. PEG diffusiophoresis coefficients were obtained from measurements of multicomponent-diffusion coefficients at 25 °C using Rayleigh interferometry. Osmotic diffusion coefficients, characterizing osmolyte diffusion from high to low PEG concentration, were also extracted. PEG diffusiophoresis was found to occur from high to low osmolyte concentration in all cases, with magnitude increasing in the order urea < DEG < TMAO. This ranking is consistent with that of osmolyte effectiveness in stabilizing protein native state. Osmotic diffusion coefficients, which allowed us to determine preferential-interaction coefficients, revealed that TMAO and DEG are preferentially excluded from the vicinity of PEG whereas urea was found to preferentially bind to this macromolecule. A novel model for macromolecule diffusiophoresis, which allowed us to examine the roles of preferential hydration, hydration, solute binding, and frictional dragging in this transport process, was developed. Our experimental results suggest that TMAO concentration gradients may be exploited to direct the motion of PEG and PEG-functionalized particles such as micelles, PEGylated proteins, and PEG-coated inorganic nanoparticles with potential applications to separation and adsorption technologies.

Analysis of main- and cross-term diffusion coefficients in bile salt mixtures

Mutual diffusion coefficients have been measured for several average compositions of the system sodium cholate-sodium deoxycholate-water at 25 °C. The experiments have been grouped in different sets having constant concentration of one component and variable concentration of the other one. Following this approach, it has been found that the trends of the main- and cross-term diffusion coefficients can be interpreted on the basis of the diffusion and equilibrium results of similar experiments performed on the two binary systems sodium cholate-water and sodium deoxycholate-water. Implications of the presented results in the transport of lipids operated by bile salt aggregates are mentioned. The method proposed in this work, able to connect the diffusivities of an n-component system to those of the related n-1 subsystems, can be extended to obtain qualitative prediction on the diffusion coefficient trends for mixtures of other surfactants, of both industrial and biological interest.

Diffusion and sedimentation interaction parameters for measuring the second virial coefficient and their utility as predictors of protein aggregation

The concentration-dependence of the diffusion and sedimentation coefficients (k(D) and k(s), respectively) of a protein can be used to determine the second virial coefficient (B₂), a parameter valuable in predicting protein-protein interactions. Accurate measurement of B₂ under physiologically and pharmaceutically relevant conditions, however, requires independent measurement of k(D) and k(s) via orthogonal techniques. We demonstrate this by utilizing sedimentation velocity (SV) and dynamic light scattering (DLS) to analyze solutions of hen-egg white lysozyme (HEWL) and a monoclonal antibody (mAb1) in different salt solutions. The accuracy of the SV-DLS method was established by comparing measured and literature B₂ values for HEWL. In contrast to the assumptions necessary for determining k(D) and k(s) via SV alone, k(D) and ks were of comparable magnitudes, and solution conditions were noted for both HEWL and mAb1 under which 1), k(D) and k(s) assumed opposite signs; and 2), k(D) ≥k(s). Further, we demonstrate the utility of k(D) and k(s) as qualitative predictors of protein aggregation through agitation and accelerated stability studies. Aggregation of mAb1 correlated well with B₂, k(D), and k(s), thus establishing the potential for k(D) to serve as a high-throughput predictor of protein aggregation.