Full elucidation of the transmembrane anion transport mechanism of squaramides using in silico investigations.

A comprehensive experimental and theoretical investigation of the transmembrane chloride transport promoted by four series of squaramide derivatives, with different degrees of fluorination, number of convergent N-H binding units and conformational shapes, is reported. The experimental chloride binding and transport abilities of these small synthetic molecules in liposomes were rationalised with quantum descriptors and molecular dynamics simulations in POPC bilayers. The tripodal tren-based compounds, with three squaramide binding motifs, have high chloride affinity, isolating the anion from water molecules within the membrane model and preventing its release to the aqueous phase, in agreement with the absence of experimental transport activity. In contrast, the symmetrical mono-squaramides, with moderate chloride binding affinity, are able to bind and release chloride either in the aqueous phase or at the membrane interface level, in line with experimentally observed high transport activity. The PMF profiles associated with the diffusion of these free transporters and their chloride complexes across phospholipid bilayers show that the assisted chloride translocation is thermodynamically favoured.

Transmembrane Fluoride Transport: Direct Measurement and Selectivity Studies.

Fluoride has been overlooked as a target in the development of synthetic anion transporters despite natural fluoride transport channels being recently discovered. In this paper we report the direct measurement of fluoride transport across lipid bilayers facilitated by a series of strapped calix[4]pyrroles and show that these compounds facilitate transport via an electrogenic mechanism (determined using valinomycin and monensin coupled transport assays and an additional osmotic response assay). An HPTS transport assay was used to quantify this electrogenic process and assess the interference of naturally occurring fatty acids with the transport process and Cl- over H+/OH- transport selectivity.

Anion binding and transport properties of cyclic 2,6-bis(1,2,3-triazol-1-yl)pyridines.

A series of cyclic 2,6-bis-(1,2,3-triazolyl)-pyridine anion receptors with thiourea functionalities were synthesized by click reaction of 2,6-diazidopyridine with protected propargylamine followed by condensation of a bisthiocyanate derivative with a series of diamines. Their chloride binding affinities as well as their transport properties in POPC bilayers were examined. These receptors were found to function as anion carriers, which can mediate both Cl(-)/NO3(-) antiport and H(+)/Cl(-) symport, and the transport activity of these hosts were dominated by their lipophilicity.

QSAR analysis of substituent effects on tambjamine anion transporters.

The transmembrane anion transport activity of 43 synthetic molecules based on the structure of marine alkaloid tambjamine were assessed in model phospholipid (POPC) liposomes. The anionophoric activity of these molecules showed a parabolic dependence with lipophilicity, with an optimum range for transport efficiency. Using a quantitative structure-transport activity (QSAR) approach it was possible to rationalize these results and to quantify the contribution of lipophilicity to the transport activity of these derivatives. While the optimal value of log P and the curvature of the parabolic dependence is a property of the membrane (and so similar for the different series of substituents) we found that for relatively simple substituents in certain locations on the tambjamine core, hydrophobic interactions clearly dominate, but for others, more specific interactions are present that change the position of the membrane hydrophobicity parabolic envelope.

Fluorescent transmembrane anion transporters: shedding light on anionophoric activity in cells.

A series of fluorescent anion transporters consisting of a urea or thiourea group linked to a naphthalimide fluorophore have been synthesised and their anion transport properties studied. The compounds possess similar anion transport properties to (thio)urea-based anionophores that have previously been reported. Fluorescence studies in cells show all anionophores cross the plasma membrane and localise within the interior of cells. The most lipophilic, aromatic substituted transporters localise homogeneously throughout the cell and are toxic towards cancer cells with the highly fluorinated compound 6 being the most effective. The least lipophilic, alkyl substituted transporters localise in specific vesicles and are non-toxic to cells. This work provides new insight to the actions of anionophores in cells and may be useful in the design of novel antineoplastic agents.