Complex formation of tetrakis(4-sulfonatophenyl)porphyrin with γ-cyclodextrin, phenylalanine, and tryptophan in aqueous solution

Abstract

In pH 10.3 buffer, in which phenylalanine and tryptophan are in an anionic form, the interactions of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) with γ-cyclodextrin (γ-CD) and phenylalanine (or tryptophan) have been examined by means of absorption and fluorescence spectroscopy. A 1:1 inclusion complex is formed between TSPP and γ-CD. TSPP forms 1:2 complexes with l- and d-phenylalanine (LPhe and DPhe), while TSPP forms 1:1 complexes with l- and d-tryptophan (LTrp and DTrp). In TSPP solution containing γ-CD and LPhe (or DPhe), the 1:1:2 γ-CD–TSPP–LPhe (or –DPhe) inclusion complex is formed, while the 1:1:1 γ-CD–TSPP–LTrp (or –DTrp) inclusion complex is formed for LTrp (or DTrp). The equilibrium constants for the formation of the complexes have been evaluated from the fluorescence intensity change of TSPP. The equilibrium constants are nearly the same for the optical isomers of phenylalanine and tryptophan, respectively, indicating that the optical isomers are not discriminated through the complexation. For the LPhe complexes, the equilibrium constants have also been evaluated at a fixed ionic strength of 0.2 mol dm−3 using NaCl. 3-Phenyl-1-propanol and l-phenylalaninol, which are analogous to phenylalanine in molecular structure, have been examined for the complexation with TSPP and γ-CD. In contrast to phenylalanine, the stoichiometries of their binary complexes with TSPP and their ternary inclusion complexes with γ-CD and TSPP are 1:1 and 1:1:1, respectively.