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Summary: In this study sodium cholate (NaC) was used as a representative bile salt for the competitive binding between NaC and sodium dodecyl sulfate (SDS) in bovine serum albumin (BSA), in 0.02 M tris-HCl buffer solution at pH 7.50 and 25 8C. The NaC and SDS associations with BSA were monitored at low surfactant concentrations where only this specific binding process can develop. The applied method to monitor the binding was based on the analysis of the effect of SDS and NaC concentrations and their mixtures upon the fluorescence intensity of the BSA tryptophan residues. This consists of the measurement of the surfactant monomer partitioning between the dispersion medium and the microaggregates on the protein molecule where the binding is indicated by the quenching of the fluorescence chromophores. Experimentally, varying the protein concentration, the surfactant concentration needed to reach a given I o /I ratio (I o and I are the intensities with and without protein, respectively) was measured. The analyses, based on the average number of surfactant molecules bound on the protein, indicated that the SDS is a more efficient quencher than the bile salt. The need for 4-6 NaC bound molecules to give the same protein quenching efficiency by a single molecule of SDS was estimated. We concluded that the differences in the competitive binding on the protein are exclusively related to the quenching efficiency in the formation of the nonfluorescent fluorophorequencher complex via a physical contact and static quenching process.

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A formação de complexos supramoleculares em solução aquosa pela associação do polieletrólito poli(etilenoimina) (PEI) com misturas do biossurfactante colato de sódio (NaC) e o surfactante aniônico dodecil sulfato de sódio (SDS) foi aqui investigado usando as técnicas de condutivimetria, tensiometria, fluorimetria, espalhamento de raios X a baixos ângulos (SAXS) e medidas de pH. Os resultados de fluorimetria, condutivimetria e medidas de pH levaram à conclusão de que os monômeros de NaC e SDS ligam-se primeiramente em sítios específicos das cadeias do polieletrólito PEI via interação eletrostática e posteriormente através de associação cooperativa. A interação do NaC com o PEI é mais fraca do que a interação do SDS com o PEI, porém, a adição de SDS ao sistema NaC-PEI levou à formação de micelas mistas SDS-NaC que interagiram fortemente com o polieletrólito PEI. Os resultados de SAXS sugeriram que o complexo supramolecular possui característica elipsoidal e essa forma não depende da concentração de surfactante nem da χ NaC .The formation of supramolecular complexes produced by association of poly(ethyleneimine) (PEI) and mixtures of sodium cholate (NaC) and sodium dodecyl sulfate (SDS) was investigated by pH, electrical conductivity, fluorescence spectroscopy and small angle X-ray scattering (SAXS) measurements. The fluorescence linked to previously measured values of pH and conductivity led to the conclusion that NaC and SDS molecules firstly bind to specific sites of the PEI chains via electrostatic interaction and secondly undergo self-assembly through regular cooperative association. The interaction of NaC with the polyelectrolyte PEI is weaker than that of SDS and the addition of SDS to the NaC-PEI system led to the formation of mixed NaC-SDS micelles which stronger interact with PEI. The SAXS results suggested that the micellar aggregates have a considerably ellipsoidal characteristic and the micellar shape is not affected by the surfactant concentration nor by χ NaC .Keywords: sodium cholate, sodium dodecyl sulfate, poly(ethyleneimine), polymer-surfactants interaction IntroductionBile salts are naturally-occurring amphiphilic molecules. They are physiologically important in the solubilization and transport of fats and lipids. The structure of bile salts in water has been extensively investigated. Although they are comparable to common surfactants, the general conclusion is that biosurfactants self-assemble in a different way than the standard surfactants. [1][2][3][4][5][6][7][8][9][10][11][12][13] Poly(ethyleneimine) (PEI) is a member of a large family of water-soluble polyamines having different molecular weights (M w ) and polymer architectures. Polyamines are weak bases and exhibit a cationic character depending on the degree of protonation. PEI has been extensively studied (particularly the branched form) due to its intense use in the formulation of drugs, thickeners, flocculating agents, personal care products, food products, detergents and adhesives. It has also been used for biological proposes to p...