In this week's issue of Nature (30 May 2002) researchers from Glasgow, Amsterdam and Munich show how specially tailored ultrashort laser pulses can be used to control how light is used in photosynthesis. Photosynthesis is the process whereby plants and some bacteria harness light energy for their own use. Food chains depend ultimately on this process.

Molecules vibrate, bend and wiggle. When an ultrashort laser pulse is used to start a chemical reaction, it sets molecules in motion; with a second pulse researchers can then watch them 'dance'. The 1999 Nobel Prize for Chemistry recognised the profound impact of femtosecond (extremely fast) lasers on the understanding of chemical reaction dynamics.

Recent developments allow the structure of the femtosecond laser pulses to be tailored in such a way that changes the molecular motion and steers the reaction dynamics down specific pathways. The power of this approach has been demonstrated in this study by Dr Jennifer Herek from the Free University of Amsterdam, Professor Richard Cogdell from the University of Glasgow and scientists of the Max Planck Institute for Quantum Optics in Munich, where these experiments were carried out.

Light harvesting is the first step in photosynthesis. Pigment molecules such as carotenoids and chlorophylls collect sunlight and transfer the absorbed solar energy to specific proteins that convert it into fuel. (Carotenoids are pigments that give the orange colour found in carrots). As this energy is passed from molecule to molecule within the photosynthetic machinery, some of it is inevitably lost along the way. When femtosecond lasers are used instead of the sun to excite these pigments scientist can watch, in real time, how the energy flows in these pathways.

The system Herek et al. studied is the carotenoid-bacteriochlorophyll donor-acceptor network of the LH2 antenna complex from a purple bacterium. The crystal structure of this LH2 complex was determined in Glasgow in 1995 by Professor Cogdell and Professor Neil Isaacs (Department of Chemistry).

The function of this complex is to harvest sunlight and make the light energy available for photosynthesis. The efficiency of the energy transfer, however, is not 100%: only 50% of the light harvested by the carotenoid donor molecules is delivered to the bacteriochlorophyll acceptor sites. The remaining energy is lost as heat via intra-molecular loss channels. The aim of the Herek et al. experiments was to control the branching ratio between these two channels by using shaped femtosecond pulses that exploit the quantum mechanical properties of the carotenoids.

The pulses are shaped by first breaking them down into their composite colour components - much like the individual piano keys that struck together create a chord. The phase and amplitude of each component can be independently adjusted such that upon recombination, the femtosecond pulse has a new shape. In other words, the chord has become a melody.

To find the tune that resonates with the molecule, the laser must become 'smart'. The pulse-shaping apparatus is coupled to a computer program based on an evolutionary algorithm. Pulse after pulse is generated and tested: the two competing pathways of energy loss and energy transfer are probed simultaneously to determine the branching ratio. The pulse shapes that succeed to change the branching ratio are then used as parents to create the next generation of pulses. In this closed-loop experiment, the melody that emerges has been refined by repetition to affect the specific molecular motions that determine energy flow.

The outcome of these experiments was a pulse shape that substantially increased the energy loss channel, thereby lowering the efficiency of light harvesting. Ironically they could not do better that nature itself. Furthermore they could show that the control effect exploits quantum phenomena in the molecules, as the phase of the light field was of crucial importance.

This is the first time that 'coherent control' has been demonstrated for such a complex molecular system, and these results have important implications to understanding the molecular mechanisms that drive biological reactions. The ability to control these processes may yield insight to design criteria for new devices that mimic natural photosynthesis such as solar cells or strategies for molecular computing.

This study illustrates the power of interdisciplinary research where first class biology from Glasgow in combination with physics from Amsterdam and Munich has produced unique scientific results.

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High-resolution graphics that illustrate this work can be found at the following web-site

http://www.mpq.mpg.de/lachem/reaction-dynamics/research/LH2/LH2project.html

Jennifer L.Herek, Wendel Wohlleben, Richard J Cogdell, Dink, Zeidler and Marcus Motzkus 'Quantum control of energy flow in light harvesting' Nature 417, 533-535 (2002)

First published: 30 May 2002