Pores barrel-hoop

Virtual vertical cutting of this unimolecular barrel gives the barrel-stave , horizontal cutting the barrel-hoop , horizontal and vertical cutting the barrel-rosette motif (Fig. 11.2). More complex motifs that include modification of the lipid bilayer are summarized as micellar pores. 

The barrel-stave architecture is a classic for both biological and synthetic ion channels and pores (Fig. 11.3). Whereas barrel-hoop motifs have received considerable attention, barrel-rosette ion channels and pores are just beginning to emerge. The more complex micellar (or toroidal ) ion channels and pores are, on the one hand, different from detergents because the micellar defects introduced into the lipid bilayer are only transient. Micellar pores differ, on the other hand, from membrane-spanning (i.e. transmembrane ) barrel-stave pores because (a) they disturb the bilayer suprastructure and (b) they always remain at the membrane-water interface. A representative synthetic barrel-stave pore is shown in Fig. 11.3 [3, 4] comprehensive collections of recently created structures can be found in pertinent reviews [2],... 

Figure 2 Transport of ions and molecules (fiUed circles) across lipid bilaya- manbranes (orange) by (a) ion channels and pores, (b) carriers, (c) endovesiculators, and (d) fusogens (empty blue ellipse) synthetic ion channels and pores are classified according to their (a) unimolecular, (e) barrel-stave, (f) barrel-hoop, (g) banel-rosette, or (h) miceUar (toroidal) active structures.

The simplest design is a unimolecular ion channel. Nevertheless, designing and synthesizing such macromolecules is complicated, and numerous examples of ion channels rely in the self-assembly of several structures within the membrane to form the functional pore. Depending on the type of molecules macrocycles, staves, or even smaller molecules can form barrel-hoop, barrel staves, or barrel rosette structures (Figure 7). 

Fig. 11.3. A representative synthetic barrel-stave pore self-assembled from four p-octiphenyl monomers (left). Molecular model with p-octiphenyl staves in grey, j -sheet hoops in yellow, external fullerene ligands in gold and an internal a-helix blocker in red (right, adapted from Ref. 4).

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