A comparison between electrokinetic capillary chromatography and absorption spectroscopy for the analysis of peptide-micelle association by weak hydrophobic interactions

Micellar electrokinetic capillary chromatography (MEKC) was compared to absorption spectroscopy to estimate equilibrium association constans (K(as)) for peptide-micelle systems involving three peptides (leucine-enkephalin, methionine-enkephalin and leucine-phenylalanine (LF)) and two surfactant micelles (sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB)). Buffer pH was chosen to minimize purely electrostatic interactions between peptides and micelles that could not be interrogated by absorption spectroscopy. Viscosity-corrected MEKC mobilities gave reasonably similar estimates of K(as) between the two methods for all three peptide-SDS micelle systems, with K(as) values ranging from 13.7 +/- 0.3 to 49 +/- 1 M(-1). For CTAB, estimates of K(as) for LF-CTAB micelle association were of the same order of magnitude as the SDS micelle by the two methods of estimation. On the other hand, enkephalin-CTAB micelle binding was about 10 times stronger (K(as) = 122 +/- 3 M(-1) to 311 +/- 9 M(-1)) than the enkephalin-SDS micelle binding. In addition, MEKC underestimated the K(as) values relative to spectroscopy by a factor of 2-3 for the enkephalin-CTAB system.