Chemists must consider engineering principles when designing molecules after news that tough carbon-carbon bonds break easily under mechanical strain.
Chemists must consider engineering principles when designing molecules following news that tough carbon-carbon bonds break easily under mechanical strain.
Sergei Sheiko at the University of North Carolina at Chapel Hill, US, was investigating shape changes in polymer molecules absorbed on substrates when he discovered that what was regarded as a strong covalent bond - the carbon-carbon bond - was breaking because of the tension created in the molecule after it was adsorbed.
It is accepted that molecules change their structure when they are bound to a surface - for example proteins that flatten out. ’Nobody thought the same molecules could also change their primary - or chemical - structure,’ Sheiko told Chemistry World. In his system, brush-like macromolecules were adsorbed to different surfaces via their ’bristles’, or side chains. At certain points, the polymeric backbone was stretched so much that it snapped. This discovery was shocking at first, said Sheiko, ’Breaking covalent bonds was a huge step in our minds,’ he said. ’The molecular origin of life is based on assumptions that covalent bonds don’t break easily.’
This is a purely physical phenomenon and chemists will now need to use engineering principles to make sure their molecules don’t fall apart, Sheiko predicts. ’If molecules are going to work on interfaces, [chemists] are going to have to take into account mechanical properties,’ he said.
Sheiko calls the concept ’molecular mechanical engineering’. Just as walls in buildings take different loads, sites in adsorbed molecules have different induced tensions. This increases reactivity at those sites and opens up a number of possibilities: ’you can design molecules where specific bonds are under tension,’ said Sheiko, ’you can induce chemistry at specific sites.’
Steve Granick, from the University of Illinois, US, sees the wider chemical implications for this mechanical effect. ’There is a general proof of concept here - that slow or even forbidden chemical reactions can be activated by mechanical stress,’ said Granick.
Sheiko has used computer simulations to measure tension distributions in different bonds of branched molecules. He predicts a range of applications - including drug delivery, where changes in the local environment of a structure cause it to break open.
et alNature440, 191