Highly permeable artificial water channels that self-assemble into two-dimensional arrays

Yue-xiao Shen, Wen Si, Mustafa Erbakan, Karl Decker, Rita De Zorzi, Patrick O. Saboe, You Jung Kang, Sheereen Majd, Peter J. Butler, Thomas Walz, Aleksei Aksimentiev, Jun-li Hou, and Manish Kumar
Proceedings of the National Academy of Science 112(32) 9810-9815 (2015)
DOI:10.1073/pnas.1508575112  BibTex


Artificial nanoscale water channels are the future of cheap, low-power water filtration and desalinization. Biological water channels such as the aquaporin membrane protein selectively pass water across the cell membranes of living creatures, yet they are difficult to use in technological applications. Over the past decade a number of artificial channels such as carbon nanotubes have been designed to imitate the function of aquaporins. A new single-molecule water channel called pillar[5]arene, or PAP, bears the potential to outshine all artificial water channels known to date with its high permeability, ease of precision manufacture, and ready assembly into high-throughput membranes. A PAP channel has  a single benzyl ring at its center (shown in blue in the image) and ten peptide-like arms (purple) that extend from the ring accross the membrane (green). Water (red and white) passes  single file through the carbon nanotube-like ring of the channel while the peptide arms anchor the channel into a lipid bilayer. 


Bioinspired artificial water channels aim to combine the high permeability and selectivity of biological aquaporin (AQP) water channels with chemical stability. We characterized a new architecture of artificial water channels, peptide-appended pillar[5]arenes (PAPs). The average single-channel osmotic water permeability for PAPs is 1.0(± 0.3)×10−14 cm3/s or 3.5(± 1.0)×108 water molecules/s, which is in the range of AQPs (3.4~40.3×108 water molecules/s) and their current synthetic analogs, carbon nanotubes (CNTs, 9.0×108 water molecules/s). This permeability is an order of magnitude higher than first-generation artificial water channels (20 to ~107 water molecules/s). Furthermore, within lipid bilayers, PAP channels can self-assemble into two-dimensional arrays. Importantly for permeable membrane design, the pore density of PAP channel arrays (~2.6×105 pores/μm2) is two orders of magnitude higher than CNT membranes (0.1~2.5×103 pores/μm2). PAP channels thus combine the advantages of biological channels and CNTs and improve upon them through their relatively simple synthesis, chemical stability and propensity to form arrays.

Using dynamic light scattering on lipid vesicles at high membrane tension, experimental collaborators measured PAP’s permeability to water as top-tier among first-generation water channels. But rather than confirm this impressive result, our molecular dynamics (MD) simulations instead revealed permeability a hundred times greater. Over the course of only a few nanoseconds (left), a PAP channel (cutaway, side-on, blue spheres) can switch from a wetted, water-permeable state to a dry, nonpermeable state in which waters (red and white spheres) remain outside the inner channel ring. This transition is reversable; channels in simulation were seen to spend as much as 80% of their time in a water-permeable state. Understanding the reasons for this transition was key to understanding the extreme permeability of PAP.

Seeking for the reason behind the discrepancy between simulation and experiment, the Kumar group of Penn State University repeated their dynamic light scattering experiments on lipid vesicles at low membrane tension instead of high. Membrane tension turned out to be the key. Experiment verified the simulation predictions that PAP in low-tension lipid vesicles passed water two orders of magnitude faster than in high-tension vesicles. Further analysis of simulation demonstrated that PAP channel arms sometimes come together to close the channel, and the channel itself can suffer invasion by membrane lipid tails. The movie to the right depicts such an invasion by a lipid molecule (brown spheres) into a PAP channel (purple licorice with transparent grey surface); the event takes only 16 ns from start to finish. Increased membrane tension could increase the frequency of both forms of blockage, providing an explanation for the two order of magnitude difference in permeability at low versus high membrane tension.

Molecular dynamics simulations predicted one more useful feature of the system: spontaneous channel aggregation. In simulation, the phenylalanine arms of PAP form temporary hydrogen bonds among their amide groups, not only within the same channel, but between channels as well. A 250 ns simulation was sufficient to demonstrate aggregation of a patch of 25 channels into 2-3 rafts.  The movie to the left illustrates this process: PAP channels (purple) move stochastically within the place of a lipid bilayer (green) to form the rafts, which extend across the system's periodic boundaries. At length, cryo-EM experiments performed by the Walz group at Harvard Medical in Boston verified that PAP channels at sufficient density in the membrane form a hexagonal grid with cell size of 21 Angstroms. This cell size matched exactly with average center-to-center distance between aggregated PAP channels. Simulation results led directly to discovering a self-aggregation feature that will allow easy concentration of water-permeable channels into a highly area-efficient osmotic membrane.