Engineering small-ion transporter channels

doi:10.1126/science.abh2618

GSTDTAP > 气候变化

DOI	10.1126/science.abh2618
	Engineering small-ion transporter channels
	Bruce J. Hinds
	2021-04-30
发表期刊	Science
出版年	2021
英文摘要	Protein channels that span lipid membranes are the primary regulators for the transport of chemical species into and out of cells. These gates and channels enable precise chemical selectivity and markedly enhanced transport speed ([ 1 ][1], [ 2 ][2]). Both selectivity and transport speed far exceed those of engineered membranes based on simplistic sieving and crude surface functionalization. The replication of protein-channel performance is especially challenging for mimicking potassium ion (K+) channels with their coupled activation and selectivity gates ([ 3 ][3]). These channels can pump against a concentration gradient and have a 1000:1 selectivity between K+ and sodium (Na+) ions, despite only a ∼0.38 Å difference in atomic radii. On page 501 of this issue, Xue et al. ([ 4 ][4]) created very fast ion channels between graphene two-dimensional (2D) sheets and the voltage-gating operation for an ionic transistor. Conventional microfabrication patterning and etching techniques produced lateral flow channels ∼5 µm long through a stack of ∼55 graphene layers. In principle, mimicking nature's complex channels in the form of mechanically robust engineered membranes would provide a keystone for nearly perfect and arbitrary chemical separations. One major distinction between protein channels and engineered membranes is the barrier thickness. The ∼4-nm-thick lipids are far too fragile compared to the ∼10 µm generally needed for mechanical robustness of engineered membranes. However, these short, nanometer-scale path lengths are a critical enabler of the excellent protein-channel performance, and there are promising approaches for stabilizing channels with block copolymers and micelle stacking ([ 5 ][5]). Relatively new classes of 2D materials, in sheet and tube geometries, have shown exceptional pressure-driven fluid flow that enables protein-channel–like transport rates over micrometer-thick length scales in mechanically stable materials ([ 6 ][6]–[ 8 ][7]). Near-perfect fluid slip over atomically smooth surfaces provides a generalizable approach for creating thin, chemically selective gates at the entrance of thick but fast channels that mimic protein function ([ 9 ][8]). The approach of Xue et al. is similar to lateral graphene channels reported by Gopinadhan et al. that demonstrated enhanced pressure-driven fluid flow rates ([ 8 ][7]), but in the Xue et al. study, the easier-to-process reduced graphene oxide (rGO) was used, and notably, voltage was directly applied to graphene to act as a gate electrode that draws ions between the graphene sheets and opens the channels. The driving force across the membrane is a concentration gradient for a net diffusional transport process. At a critical voltage, the channel opens wide and shows unusually fast transport. ![Figure][9] An expanding ion gate Xue et al. created a graphene multilayer mimic of voltage-gated protein ion channels. A gate electrode applies a bias voltage that drives potassium ions (K+) between the layers. GRAPHIC: C. BICKEL/ SCIENCE The effective diffusion coefficient is comparable to that through K+ protein channel selectivity gates; both are two orders of magnitude faster than diffusion in bulk water. The mechanism in this graphene channel can be visualized as a capacitative and ionic analog of “Newton's cradle,” where an inward swinging pendulum ball nearly instantaneously sends a ball swinging out at the opposite side of a row of hanging steel balls (see the figure). In the ion channel, capacitance creates a critical concentration of ions between graphene sheets, at which point incoming ions on the high-concentration side can force ions quickly out the other side of the membrane through concerted Coulombic motion. The nearly atomically flat 2D structures likely helped propagate the concerted ion motion without backscattering from the channel surface, as this effect has not been seen in conventional or non-2D materials. A moderate ion selectivity with a K+/Li+ ratio of about 9 was seen and was mechanistically based on a hydration sphere diameter or ion dehydration energies to insert ions between the graphene planes. Natural K+ ion channels are still far superior, given the 1000:1 ratio for the even more difficult K+-Na+ separation. The reported fast diffusion in many ways was unexpected, because normally, a large electrode bias creates a strong attractive interaction with interfacial ions. However, in this case, the slippery 2D surfaces counterintuitively enabled very fast coupled-ion transport through Coulombic forces. Voltage gating of the channel is also an important advancement and mimics the way that many protein channels regulate cellular chemistry by acting as chemical or charge-induced valves. Most approaches to date have used charged ligand chemistry at pore entrances or surfaces that modulate relatively large molecular species for applications such as drug delivery ([ 10 ][10], [ 11 ][11]). Gating small atomic ions is much more challenging, and generally only modest rectification in transmembrane currents are seen. In the work of Xue et al. , small ions were nearly completely blocked, and a sharp cutoff voltage was seen through a new coupled transport mechanism. Spatially addressable gates could find application in neural interfaces that require both voltage and chemical transport. 1. [↵][12]1. B. Hille, 2. C. M. Armstrong, 3. R. MacKinnon , Nat. 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/325020
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Bruce J. Hinds. Engineering small-ion transporter channels[J]. Science,2021.
APA	Bruce J. Hinds.(2021).Engineering small-ion transporter channels.Science.
MLA	Bruce J. Hinds."Engineering small-ion transporter channels".Science (2021).