Inclusion Complexes of Cyclodextrins with Tetrakis(4-carboxyphenyl)porphyrin and Tetrakis(4-sulfonatophenyl)porphyrin in Aqueous Solutions

Abstract

In neutral and alkaline solutions, tetrakis(4-carboxyphenyl)porphyrin (TCPP) and tetrakis(4-sulfonatophenyl)porphyrin (TSPP) form 1:1 and 2:1 host–guest inclusion complexes with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), and 6-deoxy-6-diethylamino-β-CD (DEA-β-CD), except for DEA-β-CD in alkaline solution. On the other hand, TCPP and TSPP form only 1:1 inclusion complexes with 6-deoxy-6-dihexylamino-β-CD (DHA-β-CD). The limited solubilities of DEA-β-CD in alkaline solution and DHA-β-CD are likely responsible for no observation of the 2:1 inclusion complex containing DEA-β-CD in alkaline solution and that containing DHA-β-CD. The equilibrium constants (Ks) of TCPP and TSPP for the formation of the inclusion complexes have been estimated from the absorption and/or fluorescence intensity changes in neutral and alkaline solutions. The K2 values, which are the equilibrium constants for the formation of the 2:1 host–guest inclusion complex from the 1:1 inclusion complex, are about one tenth the corresponding K1 values, except for the α-CD–TSPP system in alkaline solution. In neutral solution, where DEA-β-CD and DHA-β-CD are in protonated forms, the electrostatic force operates between DEA-β-CD (DHA-β-CD) and TCPP (TSPP), leading to the greater K values than those in alkaline solution, where DEA-β-CD and DHA-β-CD exist as neutral species.