Subsequently, we assessed the diffusion of various carboxylates in NaNO3 solution to study the impact of the organic moiety on this process. Thus, a pulse of the different carboxylates (25 mM) was added to a liposome solution loaded with 0.8 mM lucigenin in 225 mM NaNO3, 5 mM HEPES at pH 7 (Figure ). The anion diffusion was followed by monitoring the changes in the lucigenin emission over 10 minutes. Then, liposomes were lysed with Triton X-100 solution. Consistent with the previous observations, aliphatic and aromatic carboxylates exhibited significant diffusion across the lipid membrane, except for terephthalate, which showed negligible permeability due to its double negative charge. For aliphatic carboxylates, we observed an increase in the diffusion by increasing methylene groups in the organic moiety (butyrate ≥ propionate > acetate) (Figure ), which is consistent with their calculated logP values and previous reports concerning the diffusion of carboxylic acids (Table ). Counterintuitively, aromatic carboxylates with -OH groups showed faster diffusion than their unfunctionalised analogues (Figure ). Nevertheless, recent reports describe that some small polar molecules can cross lipid membranes by forming intramolecular H-bonds, which reduces their hydrophilicity. Therefore, it is plausible to assume that hydroxyl-carboxylates also might reduce their hydrophilicity by forming H-bond interactions. We note that these experiments could not be performed for all carboxylates since anions such as citrate, DL-lactate, and anthracene 9-carboxylate resulted in unusual fluorescence behaviour (Figure ).