Complex Formation of Hematoporphyrin with Cyclodextrins and 1,1′-Diheptyl-4,4′-bipyridinium Dibromide in Aqueous Solutions

Abstract

At around 5×10-6 mol dm-3 of hematoporphyrin (HP), an HP dimer exists as well as an HP monomer. The equilibrium constant for the dimerization of HP in pH 10.0 buffer has been evaluated to be 1.70×105 mol-1 dm3 from the HP concentration dependence of the absorption spectrum. In aqueous solution, HP forms 1:1 inclusion complexes with β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD). The fluorescence of HP is significantly enhanced by the addition of CDs. From simulations of the fluorescence intensity changes, the equilibrium constants for the formation of the CD–HP inclusion complexes have been estimated to be 200, 95.7, and 938 mol-1 dm3 for β-CD, γ-CD, and TM-β-CD, respectively. HP forms a 1:1 complex with 1,1′-diheptyl-4,4′-bipyridinium dibromide (DHB) in aqueous solution. In contrast to the addition of CDs, the HP fluorescence is significantly quenched by the addition of DHB. The equilibrium constant for the formation of the HP–DHB complex has been evaluated to be 1.98×105 mol-1 dm3 from the fluorescence intensity change of HP. The addition of DHB to an HP solution containing β-CD induces a circular dichroism signal of negative sign, indicating the formation of a ternary inclusion complex involving β-CD, HP, and DHB. In contrast, there is no evidence for the formation of a ternary inclusion complex of HP with DHB and TM-β-CD.