Microstructure of protein-surfactant complexes in gel and solution -: An NMR relaxation study

The regions formed upon addition of the anionic surfactant sodium dodecyl sulfate (SDS) to the oppositely charged protein, lysozyme, in aqueous solutions are, in succession, solution phase, precipitation region, bluish gel phase, and solution phase, within 20 wt % protein. The dodecyltrimethylammonium chloride (DOTAC)-lysozyme system, where both the protein and surfactant are positively charged, only gives a solution phase. In the lysozyme-SDS-water system, the longitudinal(R-1) and the transverse (R-2) relaxation rates are determined for the gel and its transformation into the colorless solution via a region of bluish solution (gel dispersed in solution). The results are compared with those of the binary surfactant-water and the lysozyme-DOTAC-water systems. H-2 NMR relaxation data of selectively deuterated surfactant, next to its headgroup, show that R-2 much greater than R-1 in bluish solutions. Larger values of R-2, and in most cases of R-1, are obtained for bluish solutions compared with colorless solutions that are formed in both protein-surfactant-water and surfactant-water systems. H-2 NMR spectra of the gel and bluish solution, obtained with perdeuterated SDS, show one broad peak, which is resolved into three peaks in the colorless solution. R-2 values deduced from line width measurements are in agreement with results from selectively deuterated surfactant. For the gel and bluish solution,H-1 R-2 measurements of lysozyme exhibit a biexponential relaxation, which is not observed in the DOTAC-lysozyme system for corresponding surfactant concentrations. In the oppositely charged system the results obtained from the gel and bluish solution are explained by the presence of large structures, in which the surfactant is present in a more restricted environment than that of a micelle. Upon increasing the surfactant concentration in the colorless solution, the large aggregates dissolve into smaller aggregates and an increasing fraction of surfactants is found in a micellar environment. In the lysozyme-DOTAC-water system only micellar interactions with the protein are detected.