Bile salts, reversed micelles

MEKC is also performed using cationic, nonionic, and zwitterionic surfactants. Widely employed are cationic surfactant consisting of a long chain tetralkylammonium salt, such as cetyltrimeth-ylammonium bromide, which causes the reversal of the direction of the EOE, due to the adsorption of the organic cation on the capillary wall. Other interesting options include the use of mixed micelles resulting from the simultaneous incorporation into the BGE of ionic and nonionic or ionic and zwitterionic surfactants. Chiral surfactants, either natural as bile salts [207] or synthetic [208] are employed for enantiomer separations. 

The differing adjuvant activities of various bile salt species relate to their differing capacities to penetrate and self-associate as reverse micelles within the membrane. In reverse micelles, the hydrophilic surfaces of the molecules face inward and the hydrophobic surfaces face outward from the lipid environment. The formation of reverse micelles within the cell membranes may create an aqueous pore, through which drag moieties can pass. 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

Bile salts sodium deoxycholate sodium glycocholate sodium fusidate sodium taurodihydrofusidate denaturation of proteins decrease of mucus viscosity decrease of peptidase activity solubilization of peptides formation of reversed micelles... 

Bile salts Sodium deoxycholate Sodium glycocholate Sodium taurocholate Formation of reverse micelles, solubilization of proteins/peptides Removal of epithelial cells and formation of transient pores Inhibition of proteolytic activity Reduction of mucus viscosity... 

Micelles and cyclodextrins are the most common reagents used for this technique. Micellar electrokinetic capillary chromatography (MECC or MEKC) is generally used for the separation of small molecules [6], Sodium dodecyl sulfate at concentrations from 20 to 150 mM in conjunction with 20 mM borate buffer (pH 9.3) or phosphate buffer (pH 7.0) represent the most common operating conditions. The mechanism of separation is related to reversed-phase liquid chromatography, at least for neutral solutes. Organic solvents such as 5-20% methanol or acetonitrile are useful to modify selectivity when there is too much retention in the system. Alternative surfactants such as bile salts (sodium cholate), cationic surfactants (cetyltrimethy-lammonium bromide), nonionic surfactants (poly-oxyethylene-23-lauryl ether), and alkyl glucosides can be used as well. 

Bile salts are natural and chiral anionic surfactants which form helical micelles of reversed micelle conformation. The first report on enantiomer separation by MEKC using bile salts was the enantioseparation of... 

The various esters of cholesterol, retinoyl esters, esters of vitamin D and E, probably depend solely on the activity of carboxylester lipase for their hydrolysis. They are, before their uptake by the enterocytes, transformed into the corresponding alcohols to form mixed micelles with bile salts. Carboxylester lipase catalyzes not only the cleavage of esters but also their formation. The most studied example is the reversible esterification of cholesterol, which is favored by low bile salt concentrations and pH [42]. Additionally, the laige specificity of carboxylester lipase probably functions as a first-line detoxification mechanism for a broad variety of orally ingested xenobiotics. 

The classic X-ray diffraction work of Small et al. [5,207,208] pointed out the existence of inverted (reverse) bile salt micelles within mixed bile salt-phospholipid liquid crystalline bilayers. The aggregates were considered to consist of 2-4 molecules (of cholate) with their hydrophilic sides facing inwards bound by hydrogen bonds between the hydroxyl groups, leaving their hydrophobic sides facing outwards to interact with the acyl chains of the phospholipid. At saturation, about 1 molecule of cholate was present for every 2 molecules of lecithin. Appreciably more bile salts... 

Fig. 17. Longitudinal and cross-sectional views of the proposed molecular models for the structure of bile salt-lecithin (BS-L) mixed micelles. All recent experimental data for BS-swelling amphiphile micelles are consistent with model B. In this model, reverse BS aggregates are present in high concentrations within the hydrophobic domains of L or other swelling amphiphile bilayers. BS also coat the perimeter of the disks as a bilayered ribbon . (From ref. 102 with permission.)...

The nature of the aggregates formed from amphiphiles in dipolar aprotic solvents is not well established, probably because these systems have been less thoroughly studied than the normal micelles in water and the reverse micelles in apolar solvents. Bile salts (Scheme 2) have one face apolar, (hydrophobic), and the other face is hydrophilic because of the hydroxyl groups, therefore substrates could associate with the hydrophihc face by hydrogen bonding which could influence reactivity. 

Tnng, S.-H., Y.-E. Hnang, and S.R. Raghavan, Self-assembled organogels obtained by adding minute concentrations of a bile salt to AOT reverse micelles. Soft Matter, 2008. 4(5) 1086-1093. 

Bile salts are natural and chiral anionic surfactants which form helical micelles of reversed micelle conformation. The first report on enantiomer separation by MEKC using bile salts was the enantioseparation of dansylated DL-amino acids (Dns-o,L-AAs) and, since then, numerous papers have been available. Nonconjugated bile salts, such as sodium cholate (SC) and sodium deoxycholate (SDC), can be used at pH > 5, whereas taurine-conjugated forms, such as sodium taurocholate (STC) and sodium taurodeox-ycholate (STDC), can be used under more acidic conditions (i.e., pH > 3). Several enantiomers, such as diltiazem hydrochloride and related compounds, carboline derivatives, trimetoquinol and related compounds, binaphthyl derivatives, Dhs-dl-AAs, mephenytoin and its metabolites, and 3-hydroxy-l,4-benzodiazepins have been successfully separated by MEKC with bile salts. In general, STDC is considered as the the most effective chiral selector among the bile salts used in MEKC. 

H. Yang, K. Erford, D. J. Kiserow, and L.B. McGown 2003 Effects of bile salts on percolation and size of AOT reversed micelles, J. Colloid Interf. Sci. 262, 531-535. 

Yang, H., Kiserowb, D.J., McGown, L.B. 2001. Effects of bile salts on the solubility and activity of yeast alcohol dehydrogenase in AOT reversed micelles. J. Mol. 

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