Size and mobility of sodium dodecyl sulfate—bovine serum albumin complex as studied by dynamic light scattering and electrophoretic light scattering

Abstract

Electrophoretic light scattering and dynamic light scattering methods were applied to measure the electrophoretic mobilities and radii of the complexes of bovine serum albumin (BSA) with sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB). In the phosphate buffer of pH 7.0 and ionic strength 0.014, the mobility of BSA, μ BSA, was −1.7 × 10 −4 cm 2 s −1 V −1. The negative magnitude of μ BSA sharply increased at low SDS concentrations below 2 m M. The negative mobility sharply increased again above 5 m M SDS and reached −4.7 × 10 −4 cm 2 s −1 V −1 at 8 m M. In the DTAB solution, μ BSA remained negative below 6 m M. It crossed zero mobility at 6 m M DTAB and became positive beyond this concentration. The mobilities of BSA—SDS and BSA—DTAB complexes attained at high surfactant concentrations were appreciably smaller than those of the corresponding surfactant micelles. On the other hand, the effective hydrodynamic radius of BSA, R BSA, was estimated to be 3.1 nm. The magnitude of R BSA increased up to 6.0 and 5.2 nm with increases of SDS and DTAB concentrations, respectively. The changes in these μ BSA and R BSA values occurred in the surfactant concentration ranges where the secondary structure of BSA was disrupted. The secondary structural change of the protein appeared likely to accompany a large-scale tertiary structural change.