Measuring anion binding at biomembrane interfaces

Abstract

The quantification of anion binding by molecular receptors within lipid bilayers remains challenging. Here we measure anion binding in lipid bilayers by creating a fluorescent macrocycle featuring a strong sulfate affinity. We find the determinants of anion binding in lipid bilayers to be different from those expected that govern anion binding in solution. Charge-dense anions H2PO4– and Cl– that prevail in dimethyl sulfoxide fail to bind to the macrocycle in lipids. In stark contrast, ClO4– and I– that hardly bind in dimethyl sulfoxide show surprisingly significant affinities for the macrocycle in lipids. We reveal a lipid bilayer anion binding principle that depends on anion polarisability and bilayer penetration depth of complexes leading to unexpected advantages of charge-diffuse anions. These insights enhance our understanding of how biological systems select anions and guide the design of functional molecular systems operating at biomembrane interfaces.

Single-Molecule Electrical Currents Associated with Valinomycin Transport of K+

“…The results of Figure D, however, suggest that the presence of ClO 4 – in the bulk electrolyte solution prevents the spontaneous insertion of val from the bulk solution into the bilayer. We speculate that ClO 4 – may block val from inserting into the bilayer due to ClO 4 – affiliating with the zwitterionic DPhPC phospholipid head-groups. − …”

Large anion binding in water

“…These common anions are strongly solvated in aqueous solution, which presents a challenge for the supramolecular design of the associated receptors. [1][2][3][4] In contrast, the binding of large anions such as I − , SCN − , ClO 4 − , and PF 6 − , which have a lower charge density and are weakly hydrated, has received comparably less attention, among others, due to the fact that they play, with the notable exception of I − , less critical natural roles. [5][6][7][8][9] On the other hand, the diametrically opposed effects that the two groups of anions can exert on biological systems have been recognized since long by the Hofmeister series, which conventionally classifies the character of ions as being salting-out (kosmotropic) or salting-in (chaotropic) in nature.…”

Transmembrane Transport of Fluoride Studied by Time-Resolved Emission Spectroscopy

“…The emission lifetime of the [Eu.L 1 ] + probe, which increases from 0.49 ms to 1.13 ms upon binding fluoride in water, 16 allows time-resolved measurements to be performed [18][19][20] to eliminate short-lived fluorescence arising from organic fluorophores, such as the biomolecules present in cells or synthetic ion transporters. 21,22 Here we present a new assay to directly monitor the influx of fluoride into liposomes using timeresolved emission spectroscopy. We demonstrate that the transport of anions by fluorescent anion transporters can be monitored without interference from the transporter itself.…”