A spectroscopic study on binding of a tricationic meso-substituted porphyrin to poly(A)·poly(U)
The binding of a tricationic meso-tris(N-methylpyridinium)-porphyrin, TMPyP3+, to ds-poly(A)·poly(U) has been studied in neutral buffered using various spectroscopic techniques. Two competitive binding modes were established: (i) external binding of the porphyrin to polynucleotide backbone without self-stacking at P/D < 4, and embedding of the partially stacked porphyrin J-dimers into the polymer groove at P/D > 30. 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 TMPyP3+ 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.