Bile salts micellar size

The average size of bile salt micelles and the distribution of micellar sizes around the mean value are important physical-chemical characteristics of a bile salt solution [5,6]. Because bile salt micellar growth is sensitive to total detergent concentration within the micellar phase, with temperature and ionic strength, the physical-chemical conditions must be rigorously controlled and specified [5,6]. Further, most... 

At low concentrations just above the CMC and at low ionic strengths ( < 0.2 M NaCl), nearly all simple bile salt micellar solutions contain spherical or nearly spherical micellar particles [5,6,12,146]. Intrinsic viscosity measurements [162,170-172] are in agreement with this and also indicate that the micelles are highly hydrated, cholates2 DC [162,172]. The maximum size of these globular micelles falls in the range Ry, = 10-16 A with h = 10-12 [146]. In the case of NaTC, the water of hydration amounts to about 30 moles H20/mole of monomer in the micelle [162]. By employing the translational mobility of H20, Lindman et al. [173]... 

Preparation of monodisperse vesicles with variable size by dilution of mixed micellar solutions of bile salt and phosphatidylcholine, Biochim. Biophys. Acta, 775. 111-114. 

Many studies have been carried out on the micellar size of bile salt micelles (3, 4,12,13,17, 20, 25). The experimental conditions and results have been tabulated in a recent review 15). Because these studies were carried out with different bile salts in different laboratories using different techniques at different temperatures, pH, and salt concentrations, values have ranged from monomers (4) to aggregates having nearly 1000 associated molecules (12). This has tended to confuse investigators interested in biological and biochemical aspects of bile salts, who wanted to know the size of the bile salt micelle. The emphasis here is that the type of bile salt, the pH, the temperature, and the counterion concentration all can affect the size and probably the structure of bile salt micelles. 

For this water concentration, the micellar region for the bile salt mixture is large for all oleyl compounds except oleic acid. Oleic acid is distinguished from the other compounds in that it does not form a lyotropic liquid crystalline phase spontaneously in water and, similarly, is present as oil droplets in bile salt solution when its micellar solubility is exceeded. Figure 1 shows also that the micellar area of an equimolar mixture of monoolein and sodium oleate is considerably greater than that of an equimolar mixture of monoolein and oleic acid, indicating that fatty acid ionization also enhances micellar solubility when monoolein is present. The equimolar mixture of sodium oleate and oleic acid has a micellar area similar in size to that of monoolein, as does the equimolar combination of all three compounds. 

However, only slight differences in solubilizing capacity were observed among MM prepared from different bile salts, sodium cholate (SC) and SGC. The slight difference between SC/SPC-MM and SGC/SPC-MM is accounted for by the small difference in micellar size, as these two BS are trihydroxy bile salts and expected to form more or less similar micelles. 

For biorelevant dissolution media containing only bile salt and PL, dynamic light scattering determinations reveal that only one particle population in the micellar size range is present (Sunesen et al., 2005 llardia-Arana et al., 2006). Upon addition of free fatty acids and monoglycerides, the structure and size of the colloid phases in these media depend on absolute concentrations and the ratios between the amphiphiles. In general, a micellar fraction is still present but co-exists with particles of a vesicular size. This was also observed by Kossena and coworkers by the use of size exclusion chromatography (Kossena et al., 2003). 

Polydispersity of simple bile salt micelles can only be assessed by modem QLS techniques employing the 2nd cumulant analysis of the time decay of the autocorrelation function [146,161]. These studies have shown, in the cases of the 4 taurine conjugates in 10 g/dl concentrations in both 0.15 M and 0.6 M NaCl, that the distribution in the polydispersity index (V) varies from 20% for small n values to 50% for large n values [6,146]. Others [112] have foimd much smaller V values (2-10%) for the unconjugated bile salts in 5% (w/v) solutions. Recently, the significance of QLS-derived polydispersities have been questioned on the basis of the rapid fluctuation in n of micellar assemblies hence V may not actually represent a micellar size distribution [167-169]. This argument is specious, since a micellar size distribution and fast fluctuations in aggregation number are identical quantities on the QLS time scale (jusec-msec) [94]. 

Chemical relaxation methods [52] show evidence of a distribution of relaxation frequencies rather than a single one as found with classic ion detergents [193]. Thus, in agreement with the above conclusions, NaC and NaDC apparently self-associate over a whole range of concentrations and not at some critical micellar concentration. Further, the relaxation frequencies are strongly concentration dependent, suggesting that the distribution of aggregate sizes is wide and shifts upwards as bile salt concentration is increased [52]. 

Fig. 16 displays the Rf, values derived by QLS measurements of two bile salt-lecithin (BS-L) systems at 10 g/dl as fucntions of the L to BS ratio and 3 temperatures. As L/BS ratio is increased, micellar sizes vary markedly. In the... 

The classic biological example of these systems is bile salt (BS)-lecithin (L)-cholesterol (Ch) micelles which have been studied in detail by QLS [239], In TC-L-Ch systems, particle size and polydispersity were studied as functions of Ch mole fraction (= 0-15%), L/TC molar ratio (0-1.6), temperature (5-85°C), and total lipid concentration (3 and 10 g/dl) in 0.15 M NaCl. For values below the established solubilization limits (A )> added Ch has little influence on the size of simple TC micelles, on the coexistence of simple and mixed TC-L micelles, or on the growth of mixed disk TC-L micelles. For supersaturated systems >1), 10 g/dl simple micellar systems (L/TC = 0) exist as metastable micellar solutions even at = 5.3. Metastability is decreased in coexisting systems... 

Bile is a mixed micellar solution of bile salt-lecithin-cholesterol which on dilution forms aggregates of much larger size than micelles indicating the formation at the phase limits of liposome-like bodies [9]. In intestinal content during lipid digestion in man, saturated mixed micelles and vesicles or liposomes containing the... 

In the lumen of the small intestine, dietary fat does not only meet bile salt but the much more complex bile in which bile salts are about half saturated with lecithin in a mixed micellar system of bile salt-lecithin-cholesterol. On dilution in the intestinal content, the micelles grow in size as the phase limit is approached and large disk-like micelles form which fold into vesicles [49]. These changes are due to the phase transition that occurs when the bile salt concentration is decreased and the solubility limit for lecithin in the mixed micelles is exceeded. The information is mostly derived from in vitro studies with model systems but most probably is applicable to the in vivo situation. What in fact takes place when the bile-derived lamellar bile salt-lecithin-cholesterol system meets the partly digested dietary fat can only be pictured. Most probably it involves an exchange of surface components, a continuous lipolysis at the interphase by pancreatic enzymes and the formation of amphiphilic products which go into different lamellar systems for further uptake by the enterocyte. Due to the relatively low bile salt concentration and the potentially high concentration of product phases in intestinal content early in fat digestion, the micellar and monomeric concentration of bile salt can be expected to be low but to increase towards the end of absorption. 

Fig. 9. Phase equilibria for the bile salt (bile acid)-fatty acid-water system at constant water concentration in relation to temperature (see Fig. 5). Six mixtures varying in molar ratios of bile salt (bile acid) and palmitic acid with total concentration of micellar bile acid plus palmitic acid equal to 40 mM were examined. Fatty acid has a finite solubility in the micellar bile acid solution, the excess being crystalline at body temperature. At 50-60 C, there is a marked increase in micellar solubility, and the fatty acid melts. At higher fatty acid/bile acid ratios, the micellar solubility is exceeded, and an immiscible oil phase occurs. The melting point of fatty acid in the presence of water is nearly identical to that in the anhydrous state (38), in contrast to the behavior of monoglyceride (Table I). As shown in Fig. 3, the size of the micellar area decreases with increasing chain length. Unsaturated fatty acids (not shown) behave similarly to saturated fatty acids, but their micellar solubility is greater, and at most experimental temperatures a crystalline phase will not occur.

Fig. 12. Solubility and behavior at 37°C of mixtures of oleic acid (HA), sodium oleate (A ), and mono-olein (MG) the experimental design is as indicated in Fig. 11. Solid black line separates dispersions of large aggregates from dispersions of micellar size—turbid dispersions from clear dispersions. In 150 misi NaCl, fatty acid is present as oil droplets (black with white stippling) and mono-olein as a nondispersed liquid crystalline phase (horizontal hatching) or a viscous water-in-oil emulsion (cross-hatching). Increased ratios of sodium oleate result in a dispersed phase (white with black stippling), and at 10 and 15 mM sodium oleate alone is present in micellar form. In bile salt, fatty acid is also present as oil droplets (black with white stippling), and at higher concentrations mono-olein and fatty acid form a dispersed liquid crystalline phase (white with dots). In 20 mM bile salt, most of the lipid mixtures are now present in micellar solution (clear). From Hofmann (60), with the publisher s permission.

MLC uses micellar mobile phases with classical RPLC columns. This chapter expands the field to include some mobile phases that can be considered close to micellar phases, such as normal and reverse microemulsions, bile salt solutions, and surfactant solutions in supercritical fluids. Also, this chapter rapidly surveys the use of micellar mobile phases with non-RPLC stationary phases such as size exclusion or gel permeation polymer phases. Allied techniques using micellar phases such as ion-exchange chromatography and capillary electrophoresis are also briefly presented. 

The term MLC is usually given to the use of micellar mobile phases with RPLC columns. A few examples have been described in other chromatographic modes that use similar mobile phases, with normal or reverse microemulsions, bile salts, and surfactants in supercritical fluids. Also, studies using non-RPLC stationary phases and micellar mobile phases have been reported in size exclusion and gel permeation chromatography. These topics are beyond the scope of this article. 

Micellar solutions and microemulsions are also used for parenteral delivery of hydrophobic drugs. They are usually stabilized by large amounts of hydrophilic surfactants, such as bile salts and polysorbates. Owing to their small size, micelles are less readily recognized by the immune system, resulting in prolonged circulation [36]. An example of micellar formulation is Taxol , in which the active antitumor agent paclitaxel is solubilized in micelles of polyoxyl castor oil. 

Under these considerations, the analysis of the energetics of size and shape of the micelles becomes of interest. The spherical shape would be the most stable structure if the monomers aggregate with a minimum of other constraints needed to satisfy the forces as described under Chap. 2.3, because this gives the minimum surface area of contact between the micelle and the solvent. On the other hand, if large constraints exist, other possible shapes, e.g. ellipsoids, cylinders or bilayers would be present [1,4]. It is obvious that micelles as formed by non-linear surfactants, e.g. bile salts etc., can not be analyzed by these theories, because steric hinderance gives rise to rather small aggregation numbers [1,3,4, 12,32,33,34,35,36,37,38,39,40]. In the case of spherical micelles of linear alkyl chain surfactants, with aggregation numberm, the radius, R, and total volume, V, and micellar surface area, A, we have ... 

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