The catalyst buried deep within

Catalysts with similar properties to the very specific allosteric enzymes found in nature have been synthesised, allowing scientists greater control over chemical reactions [Yoon et al., Science. (2010) 330, 66].  

Allosteric enzymes are Nature’s way of initiating and controlling reactions. They are triggered into action by the presence of a specific molecule or ion whereby they change their shape to expose a catalytic centre. Once the reaction is over, it is terminated by removal of the trigger and the enzyme returns to its original shape.

 

Until now, attempts to make synthetic allosteric catalysts have relied on the formation of tweezers which work in a bimetallic fashion, but researchers from the US and Japan have now synthesised a catalyst with a monometallic structure and a single catalytic site. "One of our challenges as synthetic chemists has been learning to synthesize structures inspired by biology, but that have nothing to do with biology other than the fact we'd like such complex functions realized in man-made systems," explains Chad Mirkin of Northwestern University.

 

The new allosteric supramolecular structure is a triple layer complex of two transition metal nodes, two chemically inert blocking exterior layers, and a single catalytically active Al(III)-salen complex buried deep within the sandwich. It is kept inactive in an ‘off’ state until triggered by the presence of chloride ions which reversibly expose the catalytic site. The chloride ions bind to the allosteric binding site and the reaction, in this case a ring opening polymerisation, begins. Polymerisation stops on addition of the chloride abstracting agent NaBArF and the complex closes up again.

 

The percentage of substrate conversion to product varies linearly with the number average molecular weight of polymer and an increase in weight average molecular weight of the polymers is observed on reactivation of the catalyst. This could help to determine the molecular weight of the polymer produced.

Such a catalyst should provide greater control over chemical reactions, regioselectivities, and maybe even stereoselectivities.

 

Mirkin adds, “We’d like to be able to detect tiny amounts of targets important to medicine and the environment, opening avenues to new types of diagnostic tools, just as polymerase chain reaction (PCR) did for the modern fields of medical diagnostics and forensics. Our new catalysts could make that possible.”