Toward catalytic rigid-rod beta-barrels: a hexamer with multiple histidines

Rigid-rod beta-barrels are composed of interdigitating, short, amphiphilic peptide strands that are flanked by stabilizing rigid-rod "staves." As a first step toward the construction of catalytic rigid-rod beta-barrels, we here report synthesis and study of a new barrel designed to comprise alternating leucine and histidine residues at the inner and lysine and glutamate residues at the outer barrel surface. Synthesis of p-octiphenyls with lateral tripeptide strands followed procedures described previously. Barrel formation by programmed assembly of complementary tripeptide-p-octiphenyl rods was monitored by circular dichroism (CD). CD-mixing curves (Job-plots) were consistent with 1:1-stoichiometry. Guanidinium chloride denaturation experiments gave a DeltaG(H20) = -1.8 kcal mol(-1) with a C(50) = 1.9 M. Size exclusion chromatography suggested quantitative formation of a hexamer. Facile barrel deconstruction by acid and divalent cations demonstrated the presence of internal, nonproximal histidines. Inclusion complex formation with fluorescent guests corroborated internal hydrophobicity of beta-barrel hosts and potential for intratoroidal catalysis.