Cationic cyclocholamides; toroidal facial amphiphiles with potential for anion transport

Abstract

Cholic acid has been transformed into cyclotrimeric and cyclotetrameric toroidal amphiphiles with inward-directed ammonium substituents; the cyclotrimer effects the transport of chloride anions across vesicle bilayer membranes.

Using small molecules to facilitate exchange of bicarbonate and chloride anions across liposomal membranes

“…A subsequent assay in which a second anion (bicarbonate) was added to the external sulfate medium showed that chloride transport by compounds 1-4 was restored, clear evidence that these compounds facilitate HCO 3 -/Cl -exchange across phospholipid membranes. 13 C NMR assays provided direct evidence for transmembrane bicarbonate transport, as the intraand extra-vesicular carbon-13 labelled bicarbonate populations could be clearly distinguished. This NMR data, when combined with results from the Cl --selective electrode experiments, firmly establish that compounds 1-4 enable the transmembrane exchange of Cl -/HCO 3 -anions.…”

“…After addition of transporters (isophthalamide 3 in Figure 3a and prodigiosin 4 in Figure 3b), a sharp 13 C NMR signal for H 13 CO 3 -( ~161 ppm) was restored. This renewed 13 C NMR signal must be caused by ligand-mediated transport of HCO 3 -into the liposome since the paramagnetic Mn 2+ is impermeable to the phospholipid bilayer. Importantly, the control experiment in which DMSO was added without transporter, did not result in any restoration of 13 C NMR signal (Figure 3c).…”

“…We used 13 C NMR spectroscopy to verify that transporters 1-4 facilitate transmembrane HCO 3 -/Cl -anion exchange across phospholipid vesicles. We developed experiments that use paramagnetic Mn 2+ to bleach the 13 C NMR signal for extravesicular H 13 CO 3 -, allowing for ready discrimination of extravesicular and intravesicular H 13 CO 3 -.…”

“…13 C NMR spectra were recorded on a Bruker DRX500 instrument operating at 125.77 MHz, with chemical shifts reported in ppm. Egg yolk phosphatidylcholine (EYPC) lipids, nuclepore® polycarbonate membranes and membrane filters were purchased from Avanti Polar Lipids.…”

“…7,8 Despite the obvious importance of transmembrane bicarbonate transport, there have been no reports, to the best of our knowledge, of "small" molecules that are capable of promoting bicarbonate transport across lipid membranes, in contradistinction to the growing body of work on chloride and HCl transport across phospholipid vesicle membranes. [9][10][11][12][13][14][15] Phospholipid vesicles have been extensively investigated because of their usefulness as models for biological membranes. Unilamellar vesicles of a specific size are easily produced with control of the entrapped solution.…”