Lighting up the active site

10 February 2009

Belgian scientists have developed a system that could provide information about catalytic cycles at the single molecule level.

Single molecule fluorescence spectroscopy is an important tool for monitoring discrete events in catalytic reactions that would not normally be detected in bulk measurements. Usually this involves attaching a marker to a reactant that fluoresces upon conversion to the product. However, this route offers an indirect approach to obtaining information about the catalytic cycle. Now a team, led by Johan Hofkens and Dirk De Vos at the Catholic University of Leuven, has attached the fluorescent reporter group to the catalyst itself to provide a more direct method for monitoring the cycle.

The reporter fluoresces when a proton or metal ion (X) binds to the amine

The team linked a tertiary amine, the system's catalytic active site, to a perylene-based dye, which acts as the reporter group. They then anchored the dye to a microscope cover slide so they could test the catalyst under different conditions.

"Reporters such as this are certainly important for the development of this field"
- Philip Tinnefeld, Ludwig Maximilian University of Munich, Germany

Electron transfer from the free amine to the reporter quenched the reporter's fluorescence. But when the amine bound to a molecule or ion, such as a proton in a base-catalysed reaction, electron transfer was hindered and the reporter fluoresced. 'This opens perspectives on monitoring complexation by the amine group in a more direct way than present state of the art techniques,' says Hofkens.

'In recent years, beautiful tools have been developed to study catalytic reactions at the single molecule level,' says Philip Tinnefeld, who investigates fluorescence imaging techniques at the Ludwig Maximilian University of Munich, Germany. 'Reporters such as this are certainly important for the development of this field.'

Although the system worked well, Hofkens says that unexpected dark states, where the individual molecules' fluorescence was not detected irrespective of their binding state, could also be seen. The group aim to modify the system, for example by changing the link between the reporter and the cover slide, to eliminate this problem so that the system can be applied to the field of catalysis.

Bailey Fallon

http://www.rsc.org/Publishing/ChemTech/Volume/2009/03/lighting_active_site.asp