Selective time-resolved binding of copper(ii) by pyropheophorbide-a methyl ester

Abstract

The complexation behavior of pyropheophorbide-a methyl ester (PPME) with transition metal ions as well as other biologically relevant metal ions has been investigated in water–DMF (2 : 1 v/v) solution. PPME was found to selectively complex Cu²⁺ ions, which leads to a distinct change in its absorption spectrum as well as efficient fluorescence quenching. The degree of fluorescence quenching by Cu²⁺ depended on concentration and time. Upon addition of Cu²⁺, the fluorescence showed a time-resolved decay on the time scale of minutes to hours, with the decay rate being dependent on the cation concentration. Fitting according to a bimolecular reaction rate law provided a rate constant of 650 ± 90 M⁻¹ s⁻¹ at 298 K for metallochlorin formation. The potential implications of Cu²⁺ binding for the use of PPME in photodynamic therapy are discussed, along with its use as a fluorescent sensor for detection of micromolar concentrations of Cu²⁺.