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Quantum carotenoids – how pigment’s ‘dark state’ helps turn light into energy

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Posted April 17, 2013

Scientists at the Universities of Glasgow and Toronto have finally uncovered the mechanism by which carotenoids – the same pigment that gives carrots an orange colour – help chlorophyll turn light into useful chemical energy.

Carotenoids are important light-absorbing pigments in photosynthesis and their role in absorbing light and transferring it to chlorophyll to be turned into energy has been extensively studied for 60 years. However, while the role of carotenoids has been known for a long time the exact mechanism by which this key energy transfer reaction takes place has remained obscure.

Now, researchers at the Institute of Molecular, Cell and Systems Biology, have shed some light on the workings of carotenoids.

Professor Richard Cogdell, Director of the Institute and Hooker Professor of Botany at the University of Glasgow, said: “The energy transfer processes involving carotenoids in natural light-harvesting systems have been intensively studied for the last 60 years, yet certain details of the underlying mechanisms remain controversial. Our work really clears up this mystery.”

A series of experiments carried out by researchers in Glasgow and Toronto showed that a special ‘dark state’ of the carotenoid – a hidden level not used for light absorption at all – acts as a mediator to help pass the energy it absorbs very efficiently to a chlorophyll pigment.

“This is an example of how nature exploits subtleties that we would likely overlook if we were designing a solar energy harvester,” said Greg Scholes, the D.J. LeRoy Distinguished Professor in the Department of Chemistry at the University of Toronto.

The researchers performed broadband two-dimensional electronic spectroscopy – a technique used to measure the electronic structure and its dynamics in atoms and molecules – on light-harvesting proteins from purple bacteria.

The aim was to characterize in more detail the whole sequence of quantum mechanical states of carotenoids that capture light and channel energy to bacteriochlorophyll molecules.

The data revealed a signature of a special ‘dark state’ – where the carotenoid cannot absorb or emit light – in this sequence that was predicted decades earlier, and hunted for ever since. The results point to this state’s role in mediating energy flow from carotenoid to bacteriochlorophyll.

Prof Scholes said: “It is utterly counter-intuitive that a state not participating in light absorption is used in this manner. It is amazing that nature uses so many aspects of a whole range of quantum mechanical states in carotenoid molecules, moreover, and puts those states to use in such diverse ways.”

The other significant aspect of the work is that the existence of these dark states has been speculated about for decades and that the report by Professors Cogdell, Scholes and their colleagues is the clearest evidence to date not only of their existence but also of their importance.

Source: University of Glasgow

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