2.B.66. The Urea-derivatized Phosphatidyl Choline Anion Carrier (UPC-AC) Family
A synthetic membrane transporter operates in vesicles by a relay mechanism (McNally et al. 2008). The transporter is a phosphatidylcholine derivative with a urea group appended to the end of its sn-2 acyl chain. The urea can bind a chloride ion at the membrane surface via hydrogen bonds and then relay it through the bilayer interior to an acceptor molecule located in the opposite membrane leaflet. Three phosphatidylcholine derivatives were studied, and transport rates increased with transporter affinity for chloride. An anion countertransport process using a relay mechanism and a kinetically active aggregate of two or four transporter molecules was proposed. Transport is inhibited if the transporter resides in only one leaflet of the membrane, if the bilayer is too thick, and if the counteranion is the sulfate dianion. The system facilitates membrane transport via a two-station relay mechanism (Shen et al. 2022).
Artificial membrane-active molecular machines include molecular swings, ion fishers, ion swimmers, rotors, tetrapuses and dodecapuses that permeabilize the membrane via swinging, ion-fishing, swimming, rotating, or swing-relaying actions, respectively (Shen et al. 2022). These systems span the membrane in a way akin to channels but with built-in flexible arms that can swing or bend in the dynamic membrane environment, transporting ions by constantly changing ion permeation pathways that are more defined than carriers but less defined than channels. Applying benzo-crown ether groups as the ion-binding and ion-transporting units, these transporters differ in ion transport property. For example, K+ transport activity is achieved by the molecular swing also termed a 'motional channel'. Molecular ion swimmers that contain 10 to 14 carbon atom alkyl linkers are highly active and selective (RK+/RNa+ = 8). Crown ether-appended molecular dodecapuses that establish the C60-fullerene core provide an excellent platform to allow for direct translation of solution binding affinity to transmembrane ion transport selectivity (Shen et al. 2022).