Searching for the fluorescence quenching mechanism of conjugated polymers by cytochrome c

The fluorescence quenching mechanism of the conjugated polymer poly[5- methoxy- 2- (3-sulfopropoxy)- 1,4-phenylenevinylene] by cytochrome c was investigated. Experiments with the ferrous and the ferric protein indicated that saturation of the accessible quenching sites of the fluorophore was reached at lower concentrations for the ferrous protein. The Stern-Volmer constants obtained were in the same order of magnitude (1.71 x 10(6) and 3.92 x 10(6) M-1), for ferrous and ferric protein, respectively, which seems to indicate that photoinduced electron transfer between the iron atom of the protein and the conjugated polymer is not the fluorescence quenching mechanism. Dynamic light scattering measurements indicated the formation of a complex, driven by Coulomb attractive interactions, between the positively charged protein and the negatively charged polymer. Furthermore, time-resolved fluorescence spectroscopy experiments together with the overlap of the fluorescent emission band of the polymer and the absorption band of the protein, revealed that the fluorescence quenching mechanism involved is mainly due to the formation of a ground state complex although resonance energy transfer may also occur. (C) 2016 Elsevier B.V. All rights reserved.