Azo derivatives of monoterpenes as selective anti-Helicobacter pylori agents and fine- tuning of membrane permeability by their reversible photoisomerization

Helicobacter pylori (Hp) infection affects nearly half of the global population. Current therapeutic options include the administration of a combination of antibiotics and proton pump inhibitors, although antimicrobial resistance rise remains a big concern. Phenolic monoterpenes, e.g., eugenol, vanillin, carvacrol, and thymol, have always attracted the scientific community for their antimicrobial activity and the possibility to be easily derivatized. The functionalization of such compounds through the conventional aryl diazotization reaction can generate two series of mono- and bis-azo derivatives (1–28). Moreover, the free phenolic moiety of the most active compounds was functionalized as well, through acetylation, (thio)carbamoylation and methoxylation reactions, leading to compounds 29-36, to investigate the antimicrobial activity of the group. The compounds were tested against four Hp strains including three clinical isolates, finding some potent and selective inhibitors of bacterial growth. The most active compounds 20 and 28, both nitro-azobenzene derivatives, displayed Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) values between 2 and 4 μg/mL, and were not cytotoxic against two normal cell lines (GES-1 and HIEC-6). Furthermore, to shed light on the selectivity of the compounds' antibacterial effect, the most potent anti-Hp derivatives 20 and 28 were tested against three additional bacterial species: two Gram-negative, e.g., Escherichia coli and Pseudomonas aeruginosa, and the Gram-positive Staphylococcus aureus. Given the presence of a photo-switchable azo moiety, compounds 30 and 34 were also embedded into liposomes to obtain light-sensitive membranes whose photo-responsiveness, release behaviour, and permeability towards Cl- ions were investigated. In liposomal bilayer the selected guests underwent reversible photoinduced isomerization upon irradiation with UV and visible light, alternately. Non-irradiated hybrid liposomes retained entrapped 5(6)-carboxyfluorescein (CF), slowing its spontaneous leakage, whereas UV-irradiation promoted CF release, due to guest trans-to-cis isomerization. Photoisomerization enhanced membrane permeability towards Cl- ions.