Spectroscopic features of Pheophorbide-a binding to poly-L-lysine

Binding of Pheophorbide-a of to poly-L-lysine (PLL) have been studied in neutral aqueous buffered solutions of low and near-physiological ionic strengths with low ethanol content (2.4-5.9%) in a wide range of molar polymer-to-dye ratios (P/D) using absorption and polarized fluorescence spectroscopy, fluorimetric titration, fluorescence melting. The binding has highly cooperative character (cooperativity parameter q approximate to 1000) and results in 40-fold quenching of the dye fluorescence as well as in increase in the fluorescence polarization degree up to 0.16, that evidences formation of pi-stacked dye aggregates on the polypeptide exterior. The spectroscopic properties of these aggregates were established. The cooperative biding constants were estimated by Schwarz's method.