Micellar electrokinetic chromatographic study of the interaction between enkephalin peptide analogs and charged micelles.

The relative hydrophobicity/hydrophilicity of the pentapeptides leucine enkephalin (LE), methionine enkephalin (ME) and five analogs, differing in their uncharged side chain and/or chirality, was investigated by micellar electrokinetic capillary chromatography (MEKC) employing anionic and cationic surfactants. The effect of sodium dodecyl sulfate (SDS) concentration on peptide mobility was studied at pH 8.8, a value that is well above the peptide isoelectric point, to minimize electrostatic interaction with the anionic micelles. Similarly, the effect of cetyltrimethylammonium bromide (CTAB) cationic micelles on peptide migration was studied at pH 4.1. The migration order from MEKC experiments was compared to the peptide hydrophobicity calculated from reversed-phase HPLC-derived hydrophobicity coefficients. Although relative peptide hydrophobicity was, in general, positively correlated with effective electrophoretic mobility, a tryptophan-containing analog showed only weak interaction with micelles compared to the less hydrophobic peptides. The enkephalins studied were zwitterionic in character from pH 3 to 8, and their migration as a function of pH under MEKC conditions demonstrated that electrostatic forces were at least as important as hydrophobic interactions in pentapeptide-micelle complexation.