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Mitochondria-targeted antioxidants (MTAs) have been studied quite intensively in recent years as potential therapeutic agents and vectors for the delivery of other active substances to mitochondria and bacteria. Their most studied representatives are MitoQ and SkQ1, with its fluorescent rhodamine analog SkQR1, a decyl ester of rhodamine 19 carrying plastoquinone. In the present work, we observed a pronounced antibacterial action of SkQR1 against Gram-positive bacteria, but virtually no effect on Gram-negative bacteria. The MDR pump AcrAB-TolC, known to expel SkQ1, did not recognize and did not pump out SkQR1 and dodecyl ester of rhodamine 19 (C12R1). Rhodamine 19 butyl (C4R1) and ethyl (C2R1) esters more effectively suppressed the growth of Δ tolC Escherichia coli , but lost their potency with the wild-type E. coli pumping them out. The mechanism of the antibacterial action of SkQR1 may differ from that of SkQ1. The rhodamine derivatives also proved to be effective antibacterial agents against various Gram-positive species, including Staphylococcus aureus and Mycobacterium smegmatis . By using fluorescence correlation spectroscopy and fluorescence microscopy, SkQR1 was shown to accumulate in the bacterial membrane. Thus, the presentation of SkQR1 as a fluorescent analogue of SkQ1 and its use for visualization should be performed with caution.

Mitochondrial uncouplers are actively sought as potential therapeutics. Here, we report the first successful synthesis of mitochondria-targeted derivatives of the highly potent uncoupler 3,5-di tert -butyl-4-hydroxybenzylidene-malononitrile (SF6847), bearing a cationic alkyl(triphenyl)phosphonium (TPP) group. As a key step of the synthesis, we used condensation of a ketophenol with malononitrile via the Knoevenagel reaction. SF-C 5 -TPP with a pentamethylene linker between SF6847 and TPP, stimulating respiration and collapsing membrane potential of rat liver mitochondria at submicromolar concentrations, proved to be the most effective uncoupler of the series. SF-C 5 -TPP showed pronounced protonophoric activity on a model planar bilayer lipid membrane. Importantly, SF-C 5 -TPP exhibited rather low toxicity in fibroblast cell culture, causing mitochondrial depolarization in cells at concentrations that only slightly affected cell viability. SF-C 5 -TPP was more effective in decreasing the mitochondrial membrane potential in the cell culture than SF6847, in contrast to the case of isolated mitochondria. Like other zwitterionic uncouplers, SF-C 5 -TPP inhibited the growth of Bacillus subtilis in the micromolar concentration range., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)