Quantum carotenoids shine light on photosynthesis

The mechanism by which carotenoids help chlorophyll turn light into chemical energy has been uncovered by researchers at the Universities of Glasgow and Toronto.

Carotenoids, the pigments that give carrots their orange colour, are also fundamental for photosynthesis. Their role in absorbing light and transferring it to chlorophyll to be converted to energy has been known for a long time but the exact mechanism for this process has remained poorly understood.

Professor Richard Cogdell, Director of the Institute and Hooker Professor of Botany at 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.”

The researchers demonstrated 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.

Scholes and colleagues aimed to characterise in more detail the whole sequence of quantum mechanical states of carotenoids.

Using a technique called broadband two-dimensional electronic spectroscopy, the researchers were able to measure the electronic structure and its dynamics in atoms and molecules of light-harvesting proteins from purple bacteria.

The findings revealed a signature of a special ‘dark-state’ where the carotenoid cannot absorb or emit light which plays an important role in mediating energy flow from carotenoid to chlorophyll.

The existence of ‘dark-states’ has been speculated about for decades. The team’s work published in Science is the clearest evidence to date of their existence and importance.

“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, more, and puts those states to use in such diverse ways” said Scholes.

E. E. Ostroumov, R. M. Mulvaney, R. J. Cogdell, G. D. Scholes. Broadband 2D Electronic Spectroscopy Reveals a Carotenoid Dark State in Purple Bacteria.