Organoplatinum Compounds as Anion-Tuneable Uphill Hydroxide Transporters

Active transport of ions uphill, creating a concentration gradient across a cell membrane, is essential for life. It remains a significant challenge to develop synthetic systems that allow active uphill transport. Here, a transport process fuelled by organometallic compounds is reported that creates a pH gradient. The hydrolysis reaction of Pt-II complexes results in the formation of aqua complexes that established rapid transmembrane movement ("flip-flop") of neutral Pt-OH species, leading to protonation of the OH group in the inner leaflet, generating OH- ions, and so increasing the pH in the intravesicular solution. The organoplatinum complex effectively transports bound hydroxide ions across the membrane in a neutral complex. The initial net flow of the Pt-II complex into the vesicles generates a positive electric potential that can further drive uphill transport because the electric potential is opposed to the chemical potential of OH-. The OH- ions equilibrate with this transmembrane electric potential but cannot remove it due to the relatively low permeability of the charged species. As a result, effective hydroxide transport against its concentration gradient can be achieved, and multiple additions can continuously drive the generation of OH- against its concentration gradient up to Delta pH>2. Moreover, the external addition of different anions can control the generation of OH- depending on their anion binding affinity. When anions displayed very high binding affinities towards Pt-II compounds, such as halides, the external anions could dissipate the pH gradient. In contrast, a further pH increase was observed for weak binding anions, such as sulfate, due to the increase of positive electric potential.