Abstract
In biology, beta-barrels, cylindrically rolled-up forms of beta-sheets, are ubiquitous structural motifs within various binding proteins, pores, and enzymes. This biological multifunctionality suggested that synthetic artificial beta-barrels would provide access to many different functions beyond the limitations of peptide chemistry. Unlike the relative ease of formation of synthetic (de novo) alpha-helix bundles, the synthesis of artificial beta-barrels remains a challenge. To bypass the folding problems involved, we have employed "unfoldable" rigid-rod scaffolds as privileged staves (staves are the wood strips that form the sides of macroscopic barrels); the resulting barrel-stave supramolecules exhibit their expected multifunctionality. Several "rigid rod" beta-barrels that act as receptors, ion channels, pores, catalysts, and sensors have been prepared and studied. The most recent topic of interest concerns the use of artificial beta-barrels as multicomponent sensors ("artificial tongues") in complex analyte matrices. For multicomponent sensing, we have designed artificial beta-barrels to form pores that can open and close in response to chemical stimulation within lipid bilayers. With use of fluorogenic vesicles, changes in pore activity are readily detectable with either the naked eye or multiwell screening formats. The varying responsiveness to substrates and products makes synthetic pores versatile detectors of chemical reactions, of the activity of the enzymes that catalyze these reactions, and of their inhibitors. In sensing applications, the "perfect" selectivity of enzymes is exploited to generate analyte-specific signals. Reactive signal amplifiers are then covalently linked to the products of enzymatic signal generation to enhance their pore blockage potency. With the help of signal generators and amplifiers, we have employed artificial beta-barrel pores to sense sweet (sucrose, lactose), sour (acetate, lactate, citrate), and umami ("deliciousness", glutamate) components in various food samples. This breakthrough naturally led us to design and synthesize refined pores for advanced sensing applications. We have developed methods to build guest-binding sites not only at internal and external barrel surfaces but also near the core or near the periphery of the pore. Further refinements include the introduction of asymmetric staves for voltage gating and anchoring of the pore at the membrane-water interface.
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