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Allosteric initiation and regulation of catalysis with a molecular knot

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Science  24 Jun 2016:
Vol. 352, Issue 6293, pp. 1555-1559
DOI: 10.1126/science.aaf3673

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Catalysis gets all tied up in knots

Over the past decade, chemists have used metal ion templating to prepare a wide variety of knotted molecular strands. Marcos et al. now show that one such pentafoil knot can be applied to catalysis. When held taut by zinc ions, the knot can capture a chloride or bromide ion from a halocarbon, thereby unleashing the reactivity of the residual cation for applications such as Lewis acid catalysis. Removing the zinc ions lowers the knot's affinity for the halides, offering a reversible modulation mechanism for the catalysis.

Science, this issue p. 1555

Abstract

Molecular knots occur in DNA, proteins, and other macromolecules. However, the benefits that can potentially arise from tying molecules in knots are, for the most part, unclear. Here, we report on a synthetic molecular pentafoil knot that allosterically initiates or regulates catalyzed chemical reactions by controlling the in situ generation of a carbocation formed through the knot-promoted cleavage of a carbon-halogen bond. The knot architecture is crucial to this function because it restricts the conformations that the molecular chain can adopt and prevents the formation of catalytically inactive species upon metal ion binding. Unknotted analogs are not catalytically active. Our results suggest that knotting molecules may be a useful strategy for reducing the degrees of freedom of flexible chains, enabling them to adopt what are otherwise thermodynamically inaccessible functional conformations.

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