Charged Porphyrin – dopa Melanin Interaction at Varied pH: Fluorescence Lifetime and Photothermal Studies

Abstract

The influence of pH on the spectral forms of negatively and positively charged porphyrin dyes: tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) and tetra(4-N, N, N, N-trimethylanilinium) porphyrin (TAP) in the presence of dopa melanin is investigated. The interaction between dyes and dopa melanin is shown to be dependent on the kind of dye spectral forms and on environmental pH. The creation of different forms, deactivation of their electronic excited states, and their interaction with dopa melanin have been monitored by absorption, fluorescence, and photoacoustic spectroscopy in the pH range of 2–9. This article demonstrates that the TAP–dopa melanin complex is destroyed at the lowest pH because of competition between positively charged peripheral groups of TAP and protons, which interact with negative centers of dopa melanin. Otherwise, dopa melanin affects photothermal and fluorescence properties of monomeric dications (TAP and TPPS4) rather weakly. It has also been suggested that the aggregated dicationic of TPPS4 can serve as an acceptor in the energy transfer from dopa melanin. Presented results seem to be applicable to a photodynamic therapy of cancer.