Fluorescence quenching of anthracene by indole derivatives in phospholipidbilayers

The quenching of anthracene fluorescence by indole, 1,2-dimethylindole (DMI ), tryptophan (Trp) and indole 3-acetic acid (IAA) in palmitoyloleoylphosph atidylcholine (POPC) lipid bilayers was investigated. A very efficient quen ching of the anthracene fluorescence in the Lipid membrane is observed. Ste rn-Volmer plots are linear for DMI but present a downward curvature for the other quenchers. This was interpreted as an indication of the presence of an inaccessible fraction of anthracene molecules. By a modified Stern-Volme r analysis the fraction accessible to the quenchers and the quenching const ant were determined. The changes in the fluorescence emission spectrum of i ndole and DMI have been used to calculate the partition constants of these probes into the membranes, and bimolecular quenching rate constants were de termined in terms of the local concentration of quencher in the lipid bilay er. The rate constants are lower than those in homogeneous solvents, which may be ascribed to a higher viscosity of the bilayer. No changes in the emi ssion spectra of Trp and IAA are observed in the presence of vesicles, indi cating that these probes locate preferentially in the aqueous phase, or in close proximity to the vesicular external interface in a medium resembling pure water. In these cases quenching rate constants were determined in term s of the analytical concentration. In the quenching by DMI a new, red shift ed, emission band appears; it is similar to that observed in non-polar solv ents and it is ascribable to an exciplex emission. The exciplex band is abs ent in the quenching by IAA and Trp and only very weakly present when the q uencher is indole. From the position of the maximum of the exciplex emissio n, a relatively high local polarity could be estimated for the region of th e bilayer where the quenching reaction takes place. (C) 2001 Elsevier Scien ce B.V. All rights reserved.