De novo designed α-helical barrels as receptors for small molecules.

We describe de novo designed α-helical barrels ( HBs) that bind and discriminate between lipophilic biologically active molecules. HBs have five or more α helices arranged around central hydrophobic channels the diameters of which scale with oligomer state. We show that pentameric, hexameric and heptameric HBs bind the environmentally sensitive dye, 1,6-diphenylhexatriene (DPH) in the µM range and fluoresce. Displacement of the dye is used to report the binding of non-fluorescent molecules: palmitic acid and retinol bind to all three HBs with sub-µM inhibitor constants; farnesol binds the hexamer and heptamer; but ß-carotene only binds to the heptamer. A co-crystal structure of the hexamer with farnesol reveals oriented binding in the centre of the hydrophobic channel. Charged side chains engineered into the lumen of the heptamer facilitate binding of polar ligands: a glutamate variant binds a cationic variant of DPH; and introducing lysine allows binding of the biosynthetically important farnesol diphosphate.