Retention of bile salts in micellar electrokinetic chromatography: relation of capacity factor to octanol-water partition coefficient and critical micellar concentration

The capacity factors of 16 anionic cholates (from six bile salts, including their glyco- and tauro-conjugates) were determined in a micellar electroki netic chromatography (MEKC) system consisting of buffer, pH 7.5 (phosphate- boric acid, 20 nmol/l) with 50 mmol/l sodium dodecyl sulfate (SDS) as micel le former and 10% acetonitrile as organic modifier. The capacity factors of the fully dissociated, negatively charged analytes (ranging between 0.2 an d 60) were calculated from their mobilities, with a reference background el ectrolyte (BGE) without SDS representing "free" solution. For comparison, t he capacity factors were derived for a second reference BGE where the SDS c oncentration (5 mmol/l) is close to the critical micellar concentration (CM C). The capacity factors are compared with the logarithm of the octanol-wat er partition coefficient, log P-ow, as measure for lipophilicity. Clear dis agreement between these two parameters is found especially for epimeric cho lates with the hydroxy group in position 7. In contrast, fair relation betw een the capacity factor of the analytes and their CMC is observed both depe nding strongly on the orientation of the OH groups, and tauro-conjugation a s well. In this respect the retention behaviour of the bile salts in MEKC s eems to reflect their role as detergents in living systems, and might serve as model parameter beyond lipophilicity. (C) 2001 Elsevier Science B.V. Al l rights reserved.