Molecular recognition of malto-oligosaccharides by free and metal tetrakis(4-sulfonatophenyl)porphyrins in basic aqueous solutions

In pH 10.0 buffers, tetrakis(4-sulfonatophenyl)porphyrin (TSPP) has been found to form complexes with malto-oligosaccharides from maltose to maltoheptaose and soluble starch. The peak of the electronic absorption of TSPP is monotonously shifted to longer wavelengths with an increase in the number Of D-glucopyranose residues in malto-oligosaccharide. Zn(II) TSPP (ZnTSPP) and Fe(III) TSPP (FeTSPP) also form complexes with malto-oligosaccharides. In the case of FeTSPP, the absorption peak is remarkably red shifted in pH 10.0 buffers that contain maltopentaose. The equilibrium constant of TSPP for the formation of the complex is increased with increasing numbers Of D-glucopyranose residues, whereas the equilibrium constants of ZnTSPP and FeTSPP for maltopentaose are extraordinarily large compared to maltotetraose and maltohexaose. Induced circular dichroism spectra of TSPP, ZnTSPP, and FeTSPP in malto-oligosaccharide solutions differ from one another, suggesting different association modes of the complexes.