Polymorphic Behavior in Protein-Surfactant Mixtures: The Water-Bovine SerumAlbumin-Sodium Taurodeoxycholate System

Mixtures containing water, bovine serum albumin (BSA), and sodium taurodeoxycholate (NaTDChave been investigated. Depending on the concentration of both solutes and the pH, solutions, precipitates, and gels are formed. At high concentrations, a gel, extending on both sides of the charge neutralization line, and two-phase regions are observed. The thermal gelation threshold, the temperature above which G> G, depends on BSA and NaTDC content and is concomitant to moderate heat effects, inferred by differential scanning calorimetry (DSC). Water self-diffusion in the gels is slightly slower than that in the bulk and poorly sensitive to composition: it is about 65% the value of neat H2O in a wide concentration range, irrespective of the BSA, or NaTDC, concentration. The 23Na T1 and T2 values, measured at 105.75 MHz on BSA-NaTDC gels, indicate that the motions determining the NMR relaxation of the sodium ions in the hydration layer of the protein-surfactant aggregates are not slow.