DETERMINATION OF THE BINDING ISOTHERM AND SIZE OF THE BOVINE SERUM ALBUMIN-SODIUM DODECYL-SULFATE COMPLEX BY DIFFUSION-ORDERED 2D NMR

Diffusion ordered 2D-NMR spectroscopy (DOSY), a technique based on PFG-NMR, was applied to the study of equilibria involving sodium dodecyl sulfate (SDS), SDS micelles, and bovine serum albumin (BSA). DOSY results for the SDS/BSA mixtures confirm the binding of SDS to the protein and show rapid chemical exchange of SDS between monomers, micelles, and BSA binding sites. The binding isotherm for the BSA-SDS complex was computed from DOSY data, and the binding ratio at saturation was found to be about 1.9 g of bound SDS per gram of BA in a solution with a total SDS to BSA weight ratio of 3 g/g. Stokes-Einstein radii were obtained for the BSA-SDS complex as a function of the binding number (n), and chemical shifts were measured for the SDS protons over the complete binding range. The diffusion and chemical shift data are consistent with a model in which micelle-like aggregates of SDS form at hydrophobic regions of the unfolded protein at high binding ratios. The solubilization capacity of SDS aggregates on BSA was found to be similar to that of SDS micelles.