Abstract
The properties of sodium fusidate micelles were determined by a spectral shift technique, surface tension measurements, and ultracentrifugal analysis. The critical micellar concentrations, mean molecular areas, and apparent aggregation numbers were estimated as a function of the concentration of counterion (0.001-1.0 m Na(+)) at 20 degrees C. The critical micellar concentrations were studied over a temperature range of 10 degrees C to 40 degrees C at one counterion concentration (0.001 m Na(+)), and from these data the standard thermo-dynamic functions of micellization were calculated. The ability of sodium fusidate solutions to solubilize the insoluble swelling amphiphiles, lecithin and monoolein, was investigated, and the results were compared with the solubilizing properties of sodium taurocholate. The critical micellar concentrations of sodium fusidate approximated those of sodium taurocholate. The values fell in the range of 1.44-4.56 mm, varying with the technique used, counterion concentration, and temperature. The percentage of counterions bound to fusidate micelles in water, calculated from the log critical micellar concentration-log Na(+) curve, was estimated to be negligible, which compares with sodium taurocholate micelles. The critical micellar concentration of sodium fusidate exhibited a minimum at 20 degrees C, a phenomenon observed with other ionic detergents and with bile salts. Micelle formation in sodium fusidate solutions was shown to be primarily entropy-driven at 10 degrees and 20 degrees C, whereas at 30 degrees and 40 degrees C the enthalpy factor predominated. From the surface tension measurements the molecular areas of sodium fusidate and sodium taurocholate were calculated. The mean molecular area of fusidate was 101 A(2), whereas sodium taurocholate possessed a molecular area of 88 A(2). It was demonstrated that the sodium fusidate molecule, like a bile salt molecule, lies with its longitudinal axis horizontal at an air-water interface. The apparent aggregation number of sodium fusidate micelles increased from 5 to 16 as the concentration of counterion increased from 0.01 to 0.60 m Na(+). These values are slightly larger than the corresponding aggregation numbers of sodium taurocholate micelles.
Highlights
Concentration of counterion (0.001-1.0 M Na+) at 20°C
Micellar charge . mixedmicelles with lecithin and monoolein . gallstones intherange of 1.44-4.56 mM, varyingwiththetechnique used,counterionconcentration,andtemperature.Thepercentage of counterionsboundtofusidate micelles in water, calculatedfromthe log critical micellarconcentration-log
The critical micellar concentra- Tubaki )w, a s originally developed in 1962 by Leo tion of sodium fusidate exhibited a minimum at 2OoC, a phe- Research (Denmark) a s an antibiotic active against the nomenon observed with other ionic detergents and with bile salts
Summary
CAREYAND SMALLProperlies of Sodium Fusidale Micelles 605 rings exist as "chair" conformational isomers. Whenthe space-filling (Stuart-Briegleb)molecular models of both steroids are studied (Fig. 1, e and f),the polar groups of each molecule lie onone side of their bulky hydrocarbonparts. The present paper dealswiththeproperties of pure sodium fusidate micelles, and a comparison is made between these properties and the micellar properties of a trihydroxy bile s d t , sodiumtaurocholate (NaTC). Furthert,hreemarkable ability of sodiumfusidate solutions to solubilize lecithin and monoolein as mixed micelles is comparedwiththe solubilizing capacity of bile salts (NaTC) for these lipids. Sodium fusidate was of the highest purity (>99%), as assessed by TLC, calculation of theequivalent weight by potentiometric titration, and the absence of minima in surface tension vs log concentration curves. T L C plates, sodium fusidate produced a purple fluorescence quite distinct from the fluorescence of cholesterol (pink-violet) or bile salts (yellow) under similar conditions. ThpeH of all fusidate solutions was 10.0 f 0.2.This was attained either by direct adjustment of the p H of each solution withtheaddition of stock solutions of NaOH (dye titrationand solubilization studies) or by preparingthe fusidate solutions cinarbonate-bicarbonate buffer, pH 10.0 [12], to which weighed amounts of NaCl had been added to achieve the desired ionic strength (surface tension and ultracentrifugal studies)
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