Tuning the hydrophobicity overcomes unfavorable deprotonation making octylamino-substituted 7-nitrobenz-2-oxa-1,3-diazole (n-octylamino-NBD) a protonophore and uncoupler of oxidative phosphorylation in mitochondria.

The environmentally sensitive fluorescent probe 7-nitrobenz-2-oxa-1,3-diazole (NBD) is generally utilized to monitor dynamic properties of membrane lipids and proteins. Here we studied the behavior of a homologous series of 4-n-alkylamino-substituted NBD derivatives (NBD-Cn; n=4, 6, 8, 9, 10, 12) in planar lipid bilayers, liposomes and isolated mitochondria. NBD-C10 induced proton conductivity in planar lipid membranes, while NBD-C4 was ineffective. The NBD-Cn compounds readily provoked proton permeability of neutral liposomes being less effective in negatively charged liposomes. NBD-Cn increased the respiration rate and reduced the membrane potential of isolated rat liver mitochondria. Remarkably, the bell-shaped dependence of the uncoupling activity of NBD-Cn on the alkyl chain length was found in mitochondria in contrast to the monotonous dependence in liposomes. The effect of NBD-Cn on the respiration correlated with that on proton permeability of the inner mitochondrial membrane, as measured by mitochondria swelling. Binding of NBD-Cn to mitochondria increased with n, as shown by fluorescence correlation spectroscopy. It was concluded that despite a pKa value of the amino group in NBD-Cn being about 10, i.e. far from the physiological pH range, the expected hindering of the uncoupling activity could be overcome by inserting the alkyl chain of a certain length.