Interactions of sanguinarine and chelerythrine with molecules containing a mercapto group

Capillary zone electrophoresis was applied to the investigation of the interaction of sanguinarine and chelerythrine with mercapto compounds including albumins at pH 7.4. Mercaptoethanol and L ‐cysteine were chosen for the identification of the type of interaction and for the identification of the interacting chemical form of these alkaloids. It was evidenced that sanguinarine and chelerythrine do not react chemically with these mercapto compounds at pH 7.4 and that non‐covalent products form in this interaction. Their interaction is a fast and reversible complexation and is based on non‐bonding intermolecular interactions. Conditional binding constants measured at pH 7.4 and 5.0 indicate that only uncharged forms of sanguinarine and chelerythrine (pseudobases) participate in complexation. A negatively charged group, either bound to the mercapto ligand or supplied by the solution, enters in the complexation. The simple 1 : 1 interaction scheme holds, therefore, only for mercapto compounds bearing an anionic group. Constants corrected for the abundance of the uncharged alkaloid form are of the order of magnitude of 10 4  l mol −1 and depend on the chemical composition of buffer. Interaction of sanguinarine and chelerythrine with human or bovine serum albumins is qualitatively identical with interaction of these alkaloids with simple mercapto compounds. Constants for the binding of uncharged form of sanguinarine with human and bovine serum albumins in sodium phosphate buffer at pH 7.4, corrected for abundance of the interacting uncharged form, are 332 000±38 400 and 141 000±14 400 l mol −1 , respectively. The former agrees well with the value K =385 000 (or log K =5.59) reported from static experiments. For the uncharged form of chelerythrine, the constants are 2 970 000±360 000 and 1 380 000±22 600 l mol −1 for human and bovine serum albumins, respectively. Copyright © 2003 John Wiley & Sons, Ltd.