Control of a Redox Reaction on Lipid Bilayer Surfaces by Membrane Dipole Potential

AbstractNitro-2,1,3-benzoxadiazol-4-yl (NBD) group is a widely used, environment-sensitive fluorescent probe. The negatively charged dithionite rapidly reduces the accessible NBD-labeled lipids in liposomes to their corresponding nonfluorescent derivatives. In this study both the phospholipid headgroup and acyl chain NBD-labeled l-α-1,2-dipalmitoyl-sn-glycero-3-phospho-[N-(4-nitrobenz-2-oxa-1,3-diazole)-ethanolamine] (DPPN) and 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC), respectively, were employed. The correlation of both the rate coefficient k1 of the redox reaction and the fluorescence properties of the two probes with the membrane dipole potential Ψ in fluid dipalmitoylglycerophosphocholine (DPPC) liposomes is demonstrated. When Ψ of the bilayer was varied (decreased by phloretin or increased by 6-ketocholestanol), the value for k1 decreased for both DPPN and NBD-PC with increasing Ψ. For both fluorophores a positive correlation to Ψ was evident for the relative fluorescence emission intensity (RFI, normalized to the emission of the fluorophore in a DPPC matrix). The relative changes in emission intensity as a function of Ψ were approximately equal for both NBD derivatives. Changes similar to those caused by phloretin were seen when dihexadecylglycerophosphocholine (DHPC) was added to DPPC liposomes, in keeping with the lower dipole potential for the former lipid compound compared with DPPC. These effects of Ψ on NBD fluorescence should be taken into account when interpreting data acquired using NBD-labeled lipids as fluorescent probes