Construction of polar and hydrophobic pores and channels by assembly of peptide molecules

Abstract

Selected peptides and hybrid peptides (combinations of peptide sequences with organic moieties in a single molecule) self-assemble to form pores, channels and tubules. The assemblies occur in a variety of motifs. Various physiological functions, such as ion transport through cell membranes, and physical functions, such as solubilizing difficult-to-dissolve molecules, are facilitated by the tubes that are formed by molecular assemblies. Examples from nature are the ionophores zervamicin and antiamoebin that transport K⁺ ions through cell membranes. In the area of the constriction of the hour-glass shaped channel in the ionophores, the channel becomes quite convoluted and contains the double-gating mechanism that controls the ion passage. The formation of fairly straight tubules has been accomplished by designing cyclic peptides that have a relatively flat backbone, with extended side-chains, and with amide groups and carbonyl groups that are perpendicular to the plane of the backbone. Further, the amide groups and carbonyl groups have to be spaced so that they are in register from one peptide to another, stacked over or under it, in order to form intermolecular NH...O=C hydrogen bonds. Tubules of this type can be made if the amino acid residues alternate between α- and β-residues or between d- and l-residues. In order to obviate the register problem with all α-amino acid residues of the same hand, peptide segments have been interspersed with a number of different organic moieties, such as 1,3-adamantane dicarbonyl, norbornene dicarbonyl, 2,6-pyridyl dicarbonyl, cystine and –(CH₂)_n chains. Macrocycles that stacked vertically and formed tubules through hydrogen bonding, are hollow, open-ended, and continue to infinity. The inside diameter of the hollow tubules has varied to more than 10 Å. The hydrophobic tubules are able to accommodate highly lipophilic substances. Successes and failures to make tubules and crystal structures of a number of the tubules will be shown.