The Role of the Organic Moiety in the Diffusion and Transport of Carboxylates into Liposomes

Understanding the carboxylate transport through lipid membranes under physiological conditions is critical in biomedicine and biotechnology, as it allows for the emulation of biological membrane functions and can enhance the absorption of hydrophobic carboxylate-based drugs. However, the structural diversity of carboxylates has made it challenging to study their transport, and the limited available examples do not provide a comprehensive understanding of the role of the organic moiety in this process. Here, we present an in-depth analysis of the diffusion and transport of various aliphatic and aromatic carboxylates into liposomes. We assessed the influence of their size, shape, number of carboxylate groups, and presence of hydroxyl groups. To monitor the diffusion and transport of carboxylates, we used fluorescence spectroscopy with the probes lucigenin and HPTS, as well as the chloride ion-selective electrode assay (Cl-ISE). Our findings from the fluorescence assays revealed that most carboxylates can spontaneously diffuse across liposomes suspended in NaNO3 solutions at pH 7, facilitated by the efflux of HNO3. The Cl-ISE assay showed chloride/carboxylate exchange in presence of an anion transporter. Clear trends were observed when the organic moiety was systematically varied, with a particular enhancement of anion transport by the presence of hydroxyl groups in the aromatic carboxylates. Our findings provide insights into the internalisation process of carboxylates into liposomes and could contribute to understanding the transport of other biologically relevant organic anions.