Transmembrane water transport by self-assembled metal-organic cages can be controlled by guest binding

Abstract

Biology makes use of specialised proteins, the Aquaporins, to facilitate rapid water transport across cell membranes whilst strictly excluding salt, enabling maintenance of homeostasis.
Whilst attempts have been made to use these properties in water purification and desalination, issues with stability, scalability, and cost have restricted their impact. Recent attention has focussed on building artificial water channels, typically formed from folded or selfassembled polymers. Herein, we disclose the first metal-organic cage which enables water transport in liposomes, and showcase how host-guest chemistry enables control of water flux. We find that a self-assembled pseudo-octahedral Pd6L12 cage acts as a stable and eXective transmembrane channel for water flow at high rates (up to 1.4 × 108 water molecules per second per channel, in the range of natural Aquaporins), whilst strictly excluding the transport of salt. We show
that the properties that make metal-organic cages so fascinating in solution – namely their host-guest chemistry – are maintained, but transformed, in the membrane environment. We demonstrate that addition of molecules that bind to the metal-organic cage modulates the rate
of water transport across the membrane. Further, rather than blockage being driven by a simple relationship with aXinity or size, instead a complex interplay of membrane partition propensity and binding constant dictate the ability of guest molecules to impede water flow. Our work begins
to unpick the rules determining how metal-organic cages can be used as next generation artificial water channels, with novel properties.