Interaction of antioxidants with depth-dependent fluorescence quenchers and energy transfer probes in lipid bilayers

Abstract

Three fluorescent, lipophilic, heterocyclic antioxidants were incorporated into lipid bilayers and exposed to depth-dependent nitroxyl fatty acid quenchers. The Stern-Volmer plots curved upward at low quencher concentrations. Quantitative analysis of the results showed that this behavior is consistent with complex formation between quencher and fluorescent antioxidant, where the complex is 2–3 times more fluorescent than the parent fluorophore. At higher quencher concentrations, both free antioxidant and ‘bright complex’ are quenched dynamically, albeit quenching of the latter is less efficient. The complex probably results from ionic, hydrogen bond and π-π interactions. Formation of such a ‘bright complex’ is also observable in a homogeneous solution of the reactants in cyclohexane. Additional evidence for the complexation of these antioxidants with fatty acids in lipid bilayers is provided by the fact that energy transfer from the antioxidants to anthroyloxy fatty acids occurs at surface concentrations where radiative energy transfer between free molecules should not be efficient. For directly probing the relative depths of these fluorophores in lipid bilayers we used the aqueous quenchers acrylamide and iodide. They showed that, in terms of increasing depth in the bilayer, the order was U-78,517f < U-78,518e < U-75,412e. Our results, in toto, demonstrate that the Lazaroid antioxidants are incorporated into the lipid bilayer where they occupy strictly defined positions and orientations. Complexation with fatty acyl chains should be mechanistically relevant, since it may enhance antioxidant activity by hindering free radical chain propagation.