PAPs water channels for biomimetic membrane applications

Since I am working on bio-inspired membranes for water purification such as aquaporin containing membranes, I would like to share the main findings of a related and recently published paper.

An international research team comprising scientists from the Pennsylvania State University, the Harvard Medical School and other institutions published a paper on the utilization of peptide-appended pillar[5]arenes (PAPs) in artificial bilayer membranes useful for several application such as water purification. The water permeability of those structures comes very close to those of aquaporins, which are protein channels that are known for their remarkable water transport properties and thus have been in focus of membrane scientists during the last decade. The drawback of embedding actual aquaporins in artificial membranes are the high costs of production, their low stability and the complicated membrane manufacturing process. PAPs, in contrast, are easier to manufacture and are also more stable than aquaporins. Like aquaporins, PAPs also have the characteristics allowing them to self assemble in the right orientation in artificial membranes. When using carbon nanotubes for instance, which are also stable materials that mimic the water transport characteristics of aquaporins, the biggest problem is aligning them vertically to allow water transport across the membrane. This problem could be solved when using self-orienting PAPs in artificial membranes. While the first generation of PAPs showed a six order of magnitude lower water permeability in comparison to aquaporins, new generation PAPs contain more hydrophobic regions improving their water permeability. Another advantage of using PAPs is the high packing density that is even higher than when aquaporins are inserted into artificial membranes. However, the biggest drawback of using PAP channels is their low selectivity towards small solutes such as salts. A narrow constriction of around 30 pm such as the width of the aquaporin pore can not be manufactured by any technology yet, thus, impermeability of solutes that are only slightly larger than a single water molecule is difficult to achieve. So, for desalination devices, the direct utilization of aquaporins still seems to be the most promising route. However, for other purification processes, involving larger solutes, PAPs could be an effective alternative. More details can be found in the referenced paper by Kumar et al.

Reference: Kumar et al., 2015: Highly permeable artificial water channels that can self-assemble into two-dimensional arrays. PNAS, vol. 112, no. 32, 9810-9815.

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