1.D.54 The potassium-selective Hexyl-Benzoureido-15-Crown-5-Ether Channel (HBEC) Family
Artificial ion-channels of H-bonded hexyl-benzoureido-15-crown-5-ether are selective for K+ over Na+. K+-channel conductance arises from the formation of oligomeric cooperative channels, resulting in cation-induced membrane polarization and enhanced transport rates. These channels are selectively responsive to the presence of K+ cations, even in the presence of a large excess of Na+ (Gilles and Barboiu 2015).
Self-assembled alkyl-ureido-benzo-15-crown-5-ethers are selective ionophores for K+ cations, which are preferred to Na+ cations. The transport mechanism is determined by the optimal coordination rather than classical dimensional compatibility between the crown ether hole and the cation diameter. Li et al. 2018 demonstrated that systematic changes of the structure lead to unexpected modifications in the cation-transport activity and suffice to produce adaptive selection. They showed that the main contribution to performance arises from optimal constraints on the conformational freedom, which are determined by the binding macrocycles, the nature of the hydrogen-bonding groups, and the hydrophobic tails. Simple changes to the flexible 15-crown-5-ether lead to selective carriers for Na+. Hydrophobic stabilization of the channels through mutual interactions between lipids and variable hydrophobic tails appears to be an important cause of increased activity. However, restricted translocation is achieved when constrained hydrogen-bonded macrocyclic relays are less dynamic in a pore superstructure (Li et al. 2018).