Spectroscopic Studies on Binding of Cationic Pheophorbide-a Derivative to Model Polynucleotides

The binding of new cationic Pheophorbide-a derivative to double-stranded poly(A)-poly(U), poly(G)-poly(C) and four-stranded poly(G) was investigated in buffered solutions, pH6.9, of low ionic strength (2 mM Na+) by methods of absorption and polarized fluorescence spectroscopy in a wide range of molar phosphate-to-dye ratios (P/D). Two types of CatPheo-a binding to polynucleotides were revealed: (i) dye intercalation between the nucleic bases; (ii) cooperative binding with stacking of chromophores caused by electrostatic attraction of the dye to polynucleotide backbones. The last type is predominant for double-stranded polynucleotides. However in the case of four-stranded poly(G) containing sytem the external binding prevails only at low P/D, whereas under high P/D values the intercalative binding mechanism is mainly realized. Fluorescent techniques was revealed to be efficient for recognition of binding mode because of the dye emission increases upon chromophore intercalation and quenches strongly upon the outside binding accompanied with the dye stacking-association The spectroscopic characteristics of complexes formed were obtained.