Physicochemical Studies on Pepsin−CTAB Interaction: Energetics and Structural Changes

Physicochemical and conformational studies on BSA-surfactant interaction in aqueous medium

In this paper, results of physicochemical studies on the interaction of bovine serum albumin (BSA) with alkyltrimethylammonium bromide (ATAB), pentaethylene glycol mono-n-dodecyl ether (C12E5), and sodium dodecyl sulfate (SDS) under the experimental conditions of phosphate buffer at pH 7 in the presence of 10 mM sodium bromide (NaBr), maintaining the ionic strength of the overall solution at micro = 0.015 M, have been presented and discussed. Here, BSA-ATAB corresponds to a polyion-surfactant system bearing opposite charges. BSA precipitated out of the solution on addition of ATAB solution over a certain range of ATAB concentration, the concentration range being dependent on the particular member of the ATAB family. In our earlier reports on the precipitation of oppositely charged polymer-surfactant, the tensiometric profile for surfactant addition in polymer solution differed significantly from that expected from addition of surfactant in the dispersion medium. In the present study, the precipitation process could hardly affect the smoothness of the tensiometric profile. This indicates the interaction process is operative in bulk solution. Microcalorimetric profiles also evidenced an extra hump in the interaction profile at lower surfactant concentrations, without much affecting the dilution enthalpograms beyond micellization. This interaction appeared unimodal and the extent of interaction increased with increasing tail length of ATAB, evidencing the hydrophobic effect to be an important factor. Addition of salt (NaBr) also affected the nature of interaction: at lower concentration of NaBr, the interaction was mildly assisted, whereas 50 mM NaBr fairly assisted the interaction. The nonionic surfactant C12E5 modestly interacted with BSA. The anionic amphiphile SDS, on the other hand, interacted with BSA in two distinctly different stages, as evidenced from the tensiometric profile. The complexity of the BSA-SDS tensiometric isotherm compared to that of BSA-ATAB arose from the presence of cationic binding sites adjacent to hydrophobic patches of BSA in its native state, so that electrostatic and hydrophobic interactions can cooperatively operate side by side. The interfacial saturation occurred at a lower concentration in the presence of BSA compared to the normal cmc of SDS under identical solution conditions in the absence of BSA, which was slightly delayed for nonionic C12E5. The multitechnique approach evidenced that different experimental techniques probe different physicochemical phenomena and an attempt to show the concurrence of the break points in different techniques is only diluting the essence of this area.

Surfactant assisted synthesis and spectroscopic characterization of selenium nanoparticles in ambient conditions

In this work, an attempt has been made to synthesize well-distributed stable selenium (Se) particles of nanosize dimensions via an aqueous micellar solution by the assistance of surfactants of two different polarities (anionic, sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and cationic, hexadecyltrimethylammonium bromide (CTAB)). The morphology of the particles was examined with transmission electron microscopy (TEM). X-ray analysis reveals that the particles have a monoclinic structure. The band gap of the particles was determined from UV-visible optical spectroscopic results. The size variation was estimated by employing a quantum confinement effect equation. The evolution of the selenium nanoparticles in AOT and CTAB micellar media was corroborated with the time-dependent absorption spectra. The influence of hydrazine hydrate concentrations on the formation kinetics of Se nanoparticles was also investigated. The capping ability of the surfactants has been quantitatively evaluated from Fourier transform infrared (FTIR) studies.

Interfacial and bulk behavior of sodium dodecyl sulfate in isopropanol-water and in isopropanol-poly(vinylpyrrolidone)-water media

The surface activity of isopropanol (IP) and poly(vinylpyrrolidone) (PVP) at the air/water interface has been studied. The self-aggregation of sodium dodecyl sulfate (SDS) in IP-water as well as in IP-PVP-water media has been investigated using physical methods, viz., tensiometry, conductometry, calorimetry, and viscometry. The interaction of SDS with PVP in IP-water medium as well as its self-aggregation (or micellization) in the presence of PVP has been assessed. The results reveal a fair degree of surface activity of IP in aqueous medium, which is only moderate for PVP. The critical micellar concentration (CMC) of SDS passes through a minimum at (v/v) % IP = 6.62. SDS interacts with PVP, yielding a critical aggregation concentration (CAC) at a low [SDS], independent of IP content in the medium. At a higher [SDS], free micelle formation takes place in solution, which is lower in mixed solvent than in water and is independent of solvent composition by tensiometry, but not by conductometry and calorimetry. The viscosity of micelle-interacted PVP in solution takes a long time to stabilize, whereas, for non-interacting additives, such as NaCl and cetyltrimethylammonium bromide (CTAB), it is time independent.