Binding of a tetracationicmeso-porphyrin to polyadenylic acid: a spectroscopic study

Binding of a tetracationic porphyrin (TMPyP4+) to poly(rA) has been studied in neutral buffered solution of low ionic strength in a wide range of molar phosphate-to-dye ratios (P/D) using absorption spectroscopy, polarized fluorescence and fluorimetric titration. Two competitive binding modes were identified: partial intercalation of porphyrin chromophores between adenine bases prevailing atP/D> 20 and its outside binding to poly(rA) backbone dominating atP/D< 6. Both of them were accompanied by enhancement of the porphyrin emission. Absence of the emission quenching near stoichiometricP/Dratios allowed us to assume that external binding occurs without the self-stacking of the porphyrin chromophores.