title: Small-angle X-ray scattering and electron paramagnetic resonance study of the interaction of bovine serum albumin with ionic surfactants.
abstract: Small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) techniques have been used to monitor the interaction of bovine serum albumin (BSA) with ionic surfactants such as anionic sodium dodecyl sulfate (SDS), zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonium-1-propane sulfonate (HPS), and cationic cethyltrimethylammonium chloride (CTAC) at pH 7.0. The SAXS results have shown that in the presence of 5 mM SDS and HPS the radius of gyration (Rg) almost does not change as compared to the BSA free-surfactant solution; its value is ca. 30 Angstroms. In the presence of 5 mM CTAC the SAXS data indicate the presence of a particle with a Rg of at least 63 Angstroms, suggesting that in this case, a kind of protein aggregation takes place. In the presence of SDS and HPS surfactants at concentrations above 10 mM, a characteristic broad peak in the region of 0.12-0.18 Angstroms(-1) indicates the presence of micelle-like aggregates in solution. The SAXS curves are consistent with the "pearl necklace" model, where micelle-like aggregates are randomly distributed around the polypeptide chain. EPR results using 5-DSA and 16-DSA spin labels show that in the presence of BSA the EPR spectra are composed of two label populations, one contacting the protein and a second one due to label localization in the micelles. Evidence is also obtained for a competition of the surfactants with the spin labels for the high-affinity binding sites of the stearic acid spin labels as monitored by changes in the fractions of the two label populations as the surfactant concentration is increased. The effect of SDS seems to be stronger in the sense that increased SDS concentration leads to a complete transfer of spin labels from close protein contact sites to micelles, while for HPS, a significant immobilization of probe apparently remains even at higher surfactant concentrations. These two techniques are quite useful since SAXS monitors the overall properties of the scattering particle, while EPR gives information on the dynamics inside this particle and associated with label localization and motion.
authors.affiliation: Instituto de Química de São Carlos, Universidade de São Paulo, Cx. Postal 780, CEP 13560-970, São Carlos, SP, Brazil.
authors.forename: Emerson Luiz
authors.initials: EL
authors.lastname: Gelamo
authors.forename: Rosangela
authors.initials: R
authors.lastname: Itri
authors.forename: Antonio
authors.initials: A
authors.lastname: Alonso
authors.forename: Junaine Vasques
authors.initials: JV
authors.lastname: da Silva
authors.forename: Marcel
authors.initials: M
authors.lastname: Tabak
mesh_terms: D000818:Animals; D002417:Cattle; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D012542:Scattering, Radiation; D012710:Serum Albumin, Bovine / Q000737:chemistry*; D013499:Surface Properties; D013501:Surface-Active Agents / Q000737:chemistry*; D014961:X-Ray Diffraction; D014965:X-Rays
chemical_list: D013501:Surface-Active Agents; D012710:Serum Albumin, Bovine