A new way to control reactions

A group of scientists at The Scripps Research Institute (TSRI) have demonstrated complex system behaviour among small, reacting organic molecules by putting them in and out of a nanocapsule.

In the current issue of the journal Nature, a report from a group led by Dr. Julius Rebek, Jr demonstrated that they could achieve chemical amplification (speeding up the reaction as it proceeded) without the presence of an autocatalyst (a product of the reaction that acts as a catalyst for more product).

'[The findings] show a different way of controlling reactivity,' says Rebek, who is Director and Professor of The Skaggs Institute for Chemical Biology. 'This is a way of turning a reaction on and off.'

The Rebek group's autoamplification was made possible by carrying out a reaction in the presence of dimeric nanocapsules, which are like two identical half eggs that can close around a reactant molecule, sequestering it from the rest of the molecules in solution. The nanocapsules encapsulate molecules of the proper size and shape and inhibit their reactivity.

In the reaction, the sequestered molecules are dicyclohexylcarbodiimide (DCC), one of the reactants. When DCC molecules are mixed with molecules of p toluic acid and p-ethyl-aniline, they react to form an amide and a urea molecule.

But when the DCC molecules are mixed with the nanocapsules first, the reaction proceeds much more slowly, since the nanoencapsulation effectively reduces the concentration of DCC in solution.

However, the reaction proceeds nonlinearly: it speeds up over time because the products of the reaction fit inside the nanocapsules. And, as the amide and urea molecules are produced, they displace the DCC molecules, freeing them to react.

In fact, the reaction resembles a chain reaction as the reaction proceeds, more and more of the product is made, and the end result is exponential growth. One molecule makes two, and these both displace another DCC, which make two more, etc.

These kinetics are unlike anything that has been seen in the absence of catalysis. Rebek plans to next set up a system with different capsules and reagents inside them that can be triggered to react. He adds that this control is inherent within the reaction a self-regulatory control, not unlike the ones that living systems use in nature.

'To study the dynamics of living systems, which are really very complex,' he says, 'you have to look at systems such as these.'

The research article 'Amplification from compartmentalized reagents' is authored by Jian Chen, Steffi Körner, Stephen L. Craig, Dmitry M. Rudkevich, and Julius Rebek Jr and appeared in the January 24, 2002 issue of Nature.

The research was funded by The Skaggs Institute for Chemical Biology, the NASA Astrobiology Institute, and the National Institutes of Health.