Catalysis in a cavity

Chemists in the US have created a molecular vase that mimics an enzyme's catalytic activity.

Chemical catalysts are chemoselective - they are able to recognise and transform a particular functional group - but few are capable of differentiating differently sized or shaped molecules with the same functional groups. This is where enzymes have an advantage. They are usually very specific about which reactions they catalyse: the shape of the starting material and its interaction with the enzyme being important factors in that specificity.

Julius Rebek, Jr. and Richard Hooley from the Scripps Research Institute, La Jolla, California, have set themselves the goal of creating chemical systems that match the selective catalytic abilities of enzymes whilst retaining the properties of normal chemical catalysts.

An imide molecule fits snugly in the molecular cavity, perfectly primed to react with an alcohol

Rebek's system mimics an enzyme in that the cavitand has a cavity in which only an appropriately sized starting material can fit. A hydrogen bonding network at the rim of the cavity, similar to that found in enzymes, activates the starting material - the unsaturated imide - by sucking electron density from the double bond. This increases its reactivity and accelerates its Diels-Alder reaction with the aromatic alcohol. The product is too big to fit in the cavity and is ejected, leaving the cavity free to activate another molecule of starting material.

Rebek aims to improve the cavitand so that its hydrogen bonding network is positioned directly at the bound starting material. This would allow more challenging reactions to be accelerated and catalysed.

Nicola Burton