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Artificial ionic and water channels

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Angew. Chem. Int. Ed. 2011, 50(48), 11366-11372

Angew. Chem. Int. Ed. 2012, 51 11674-11676

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Water is fundamental to life, playing a variety of functions related to its complex dynamic behaviours at the supramolecular level. Most of the physiological processes depend on selective exchanges of ions or molecules between a cell and its environment and water play a crucial role on their translocation events. Artificial ion-channels have been extensively studied with the hope to facilitate the ionic conduction in the bilayer membranes. However, there has been less progress in the area of synthetic water channels.

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Water channel systems: a) cross section of the helical pore assembled from dendritic dipeptides, 1 and b) oriented dipolar water wires within chiral supramolecular I-quartet assembled from lipophilic ureidoimidazole, 2 selectively transporting water and protons against ions; c) hydrazide-pilar[5]arene, 3 functioning exclusively as highly selective single-molecular water channels. Water molecules and protonated water molecules are represented in red and white CPK models. Violet spheres are representing hydrated cations, non-penetrants for bilayer membrane.

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Barboiu et al. reported that imidazole I-quartets can be mutually stabilized by inner dipolar water-wires, reminiscent of G-quartets stabilized by cation templating.[8] The I-quartets are stable in solution, solid state and within bilayers leading to the tubular channel-type chiral superstructures. These systems have provided excellent reasons to consider that supramolecular chirality of I-quartets and water induced polarization within the channels may be strongly associated. The confined water wires, like in aquaporin channels, form one H-bond with the inner wall of the I-quartet and one H-bond with an adjacent water molecule. Moreover, the water molecules adopt a unique dipolar orientation and preserve the overall electrochemical dipolar potential along the channel.

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These results strongly indicated that water molecules and protons can permeate the bilayer membranes through I-quartet channels. The ion-exclusion phenomena are based on dimensional steric effects whereas hydrophobic and hydrodynamic effects appear to be less important. Water-free I-quartet-"off form" superstructure is reminiscent with closed conformation of the proton gate of the Influenza A M2 protein. The slight conformational adjustments allow the formation water assisted I-quartet-"open form" through protons can diffuse along dipolar oriented water-wire in the open state pore-gate region. These artificial I-quartet superstructures obtained by using a simple chemistry are in excellent agreement with structural X-ray and NMR results as well as theoretical results providing accurate structural issues for water/proton conductance mechanisms through Influenza A M2 proton channel.
 

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