Interaction of a tricationic meso-substituted porphyrin with guanine-containing polyribonucleotides of various...

The interaction of a tricationic water-soluble meso-(N-methylpyridinium)-substituted porphyrin, TMPyP³⁺, derived from classic TMPyP4, with double-stranded poly(G)  ⋅  poly(C) and four-stranded poly(G) polyribonucleotides has been studied in aqueous buffered solutions, pH 6.9, of low and near-physiological ionic strengths in a wide range of molar phosphate-to-dye ratios (P/D). To clarify the binding modes of TMPyP³⁺ to biopolymers various spectroscopic techniques, including absorption and polarized fluorescence spectroscopy, Raman spectroscopy, and resonance light scattering, were used. As a result, two competitive binding modes were revealed. In solution of low ionic strength outside binding of the porphyrin to the polynucleotide backbone with self-stacking prevailed at low P/D ratios (P/D  <  3.5). It manifested itself by the substantial quenching of porphyrin fluorescence. Also the formation of large-scale porphyrin aggregates was observed near the stoichiometric binding ratio. The spectral changes observed at P/D  >  30 including emission enhancement were supposed to be caused by the embedding of partially stacked porphyrin J-dimers into the polymer groove. TMPyP³⁺ binding to poly(G) induced a fluorescence increase 2.5 times as large as that observed for poly(G)  ⋅  poly(C). In solution of near-physiological ionic strength the efficiency of external porphyrin binding was reduced substantially due to the competitive binding of Na⁺ ions with the polymer backbone. The spectroscopic characteristics of porphyrin bound to polynucleotides at different conditions were compared with those for free porphyrin.