Control of a redox reaction on lipid bilayer surfaces by membrane dipole potential

Nitro-2,1,3-benzoxadiazol-4-yl (NBD) group is a widely used, environment-se nsitive fluorescent probe. The negatively charged dithionite rapidly reduce s the accessible NBD-labeled lipids in liposomes to their corresponding non fluorescent derivatives. In this study both the phospholipid headgroup and acyl chain NBD-labeled L-alpha -1,2-dipalmitoyl-sn-glycero-3-phospho-[N-(4- nitrobenz-2-oxa-1,3-diazole)-ethanolamine] (DPPN) and 1-acyl-2-[12-[(7-nitr o-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (N BD-PC), respectively, were employed. The correlation of both the rate coeff icient k(1) of the redox reaction and the fluorescence properties of the tw o probes with the membrane dipole potential Psi in fluid dipalmitoylglycero phosphocholine (DPPC) liposomes is demonstrated. When Psi of the bilayer wa s varied (decreased by phloretin or increased by 6-ketocholestanol), the va lue for k(1) decreased for both DPPN and NBD-PC with increasing Psi. For bo th fluorophores a positive correlation to Psi was evident for the relative fluorescence emission intensity (RFI, normalized to the emission of the flu orophore in a DPPC matrix). The relative changes in emission intensity as a function of Psi were approximately equal for both NBD derivatives. Changes similar to those caused by phloretin were seen when dihexadecylglycerophos phocholine (DHPC) was added to DPPC liposomes, in keeping with the lower di pole potential for the former lipid compound compared with DPPC. These effe cts of Psi on NBD fluorescence should be taken into account when interpreti ng data acquired using NBD-labeled lipids as fluorescent probes.