Heat-set bovine serum albumin-sodium dodecyl sulfate gels studied by fluorescence probe methods, NMR, and light scattering

In this work, concentrated protein-surfactant solutions and their corresponding heat-set gels; were studied by fluorescence probe methods, NMR, and light scattering. Bovine serum albumin (BSA) was used as the protein, and sodium dodecyl sulfate (SDS) as the surfactant. Heating concentrated BSA solutions gives turbid gels. Heat-setBSA-SDS gels are transparent. From fluorescence measurements it was concluded that SDS forms micelle-like clusters on BSA, both in solution and in the corresponding heat-set gel. Aggregation numbers were found to be similar in solution and gel. Also, I-1/I-3 values in solution and gel were similar. H-2 NMR relaxation measurements of specifically deuterated SDS at the ct-carbon position next to the headgroup were performed, and the longitudinal relaxation rates R-1 were found to be the same in solution and gel. High values for the transverse relaxation rate R-2 (indicating slow motions of SDS bound to large aggregates) were obtained, and the largest R-2 value was found for the gel. Dynamic light scattering on BSA-SDS gels was used to obtain the correlation length xi, which defines a mean distance between two points of entanglements. The decrease of xi with increasing [SDS]/[BSA] molar ratio was explained by the size of the BSA-SDS complex and the possibility that micelle-like structures might form cross-links between different BSA molecules. With static light scattering the extent of inhomogeneities in BSA and BSA-SDS gels was found to decrease with increasing SDS concentration. Also, the gel region in the ternary phase diagram BSA-SDS-3.1 mM NaN3 at room temperature and constant pressure (1 atm) was determined.