The porphyrins are macrocyclic compounds widely used as photosensitizers in anticancer photodynamic therapy. The binding of a tricationic meso-tris(N-methylpyridinium)-porphyrin, TMPyP, to poly(A)⋅poly(U) polynucleotide has been studied in neutral buffered solution, pH6.9, of low and near-physiological ionic strength in a wide range of molar phosphate-to-dye ratios (P/D). Effective TMPyP binding to the biopolymer was established using absorption spectroscopy, polarized fluorescence, fluorimetric titration and resonance light scattering. We propose a model in which TMPyP binds to the polynucleotide in two competitive binding modes: at low P/D ratios (< 4) external binding of the porphyrin to polynucleotide backbone without self-stacking dominates, and at higher P/D (> 30) the partially stacked porphyrin J-dimers are embedded into the polymer groove. Enhancement of the porphyrin emission was observed upon binding in all P/D range, contrasting the binding of this porphyrin to poly(G)⋅poly(C) with significant quenching of the porphyrin fluorescence at low P/D ratios. This observation indicates that TMPyP can discriminate between poly(A)⋅poly(U) and poly(G)⋅poly(C) polynucleotides at low P/D ratios. Formation of highly scattering extended porphyrin aggregates was observed near the stoichiometric in charge binding ratio, P/D = 3. It was revealed that the efficiency of the porphyrin external binding and aggregation is reduced in the solution of near-physiological ionic strength.
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