Chiral separations using a polypeptide and polymeric dipeptide surfactant polyelectrolyte multilayer coating in open-tubular capillary electrochromatography.

A polyelectrolyte multilayer (PEM) coating consisting of the polypeptide, poly(l-lysine) hydrobromide, poly(l-lysine) and the polymeric dipeptide surfactant, poly(sodium undecanoyl-l-leucyl-alaninate), poly(l-SULA), is investigated as a new medium for the separation of chiral analytes in open-tubular capillary electrochromatography (OT-CEC). In this approach, a stable PEM is constructed in situ by alternative rinses of the cationic polymer poly(l-lysine) and the anionic polymer poly(l-SULA). In previous studies, the PEM coating has been constructed by use of the cationic polyelectrolyte poly (diallydimethylammonium chloride), PDADMAC. In this study, we investigate the use of a biopolymer as the cationic polyelectrolyte. The results reported here indicate an increase in selectivity and resolution when poly(l-lysine) is used as the cationic polymer in place of PDADMAC. To evaluate the chromatographic performance of the PEM coating as a chiral stationary phase, the separation of the beta-blockers, labetalol and sotalol, and the binaphthyl derivatives, 1,1'-bi-2-naphthyl-2,2'-dihydrogen phosphate, 1,1'-bi-2-naphthol, and 1,1-binaphthyl-2,2'-diamine, are investigated. In addition, the effect of varying the amino acid order of the polymeric dipeptide surfactant on resolution is investigated. The number of bilayers also significantly influences the separation efficiency and resolution of enantiomers. The run-to-run and capillary-to-capillary reproducibilities are evaluated by calculating the relative standard deviations (RSDs) of the electroosmotic flow. These RSD values were found to be less than 1%. The coating is also stable and allows more than 290 runs to be performed in the same capillary. In addition, coupling of this chiral OT-CEC column with mass spectrometry is investigated.