Diffusion Coefficient of a Complex Composed of Bovine Serum Albumin and Sodium Dodecyl Sulfate

Abstract

A complex was formed from bovine serum albumin (BSA) monomer, fractionated by liquid chromatography, with sodium dodecyl sulfate (SDS) by adding 2-mercaptoethanol.
The translational diffusion coefficient D of the complex was measured by dynamic light scattering in two phosphate buffer solutions of different concentration. The linear relationships between D and the solute concentration c were indicated for BSA monomer, SDS micelle and BSA-SDS complex. D=D₀(1+k_Dc)The diffusion coefficient at infinite dilution D₀ of BSA and SDS is larger in 20 mmol·dm^-3than in 100 mmol·dm^-3 solution. But a little smaller D₀ was found for the complex in 20 mmol·dm^-3 than in 100 mmol·dm^-3. The hydrodynamic radius of the complex was 5.76 nm, which is sufficiently large compared with 3.50 nm of BSA monomer and 2.49 nm of SDS micelle in 20 mmol·dm^-3 solution. The inter-particle interaction parameter k_D decreases with the electrolyte concentration. In 20 mmol·dm^-3 phosphate solution k_D value of the complex is markedly large compared with that of BSA or SDS. The variation of the inter-particle interactions, thermodynamic and hydrodynamic, by complexation is attributable essentially to the conformational change from BSA prolate ellipsoid to random coil chain of the complex backbone. In addition, the localization of electric charge of SDS micelles bound onto the backbone chain contributes to k_D, value of the complex in 20 mmol·dm^-3 buffer solution, beyond the effect by the molecular weight or hydrodynamic dimen sion.