Capillary electrophoresis behavior of water-soluble anionic porphyrins in the presence of beta-cyclodextrin and its O-methylated derivatives

The electrophoretic behavior of water-soluble anionic porphyrins, such as meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP), meso-tetrakis(4-sulfonatophenyl) porphyrin (TSPP) and its zinc(ll) and copper(ll) complexes (ZnTSPP and CuTSPP, respectively) has been studied by capillary zone electrophoresis using fused-silica capillaries. The selectivity of the separation is strongly dependent on the type and concentration of beta-cyclodextrin (PCD) or the O-methylated derivatives added to the background electrolyte. CuTSPP and TSPP can be separated using a pH 2.5 aqueous sodium phosphate buffer in the presence of 1 mM PCD. Resolution is poorer or absent employing alkylated beta CDs, such as the heptakis (2, 6-di-O-methyl)-beta-cyclodextrin or the heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin, as additives. On the other hand, separation of TSPP from its copper and zinc complexes has been achieved using a pH 7.0 aqueous sodium phosphate buffer, in the presence of 0.75 mM PCD and 20% dimethyl sulfoxide (DMSO) as organic modifier. Under such conditions, the calibration curve for quantitative analysis of copper(ll) was obtained. A rationale for the observed behavior will be presented and discussed on the basis of binding and protonation equilibria and a simple mathematical model