Media Releases

University of Toronto-led study provides new insight into photosynthesis

April 4, 2013

TORONTO, ON – Pig­ments found in plants and pur­ple bac­te­ria employed to pro­vide pro­tec­tion from sun dam­age do more than just that. Researchers from the Uni­ver­si­ty of Toron­to and Uni­ver­si­ty of Glas­gow have found that they also help to har­vest light ener­gy dur­ing pho­to­syn­the­sis.

Carotenoids, the same pig­ments which give orange col­or to car­rots and red to toma­toes, are often found togeth­er in plants with chloro­phyll pig­ments that har­vest solar ener­gy. Their main func­tion is pho­to­pro­tec­tion when rays of light from the sun are the most intense. How­ev­er, a new study pub­lished today in Sci­ence shows how they cap­ture blue/green light and pass the ener­gy on to chloro­phylls, which absorb red light.

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

A series of exper­i­ments 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.

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 state in this sequence that was pre­dict­ed decades ear­li­er, and sought ever since. The results point to this state’s role in medi­at­ing ener­gy flow from carotenoid to bac­te­ri­ochloro­phyll.

“It is utter­ly counter-intu­itive that a state not par­tic­i­pat­ing in light absorp­tion is used in this man­ner,” says Scholes. “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 for decades and that the report by Scholes and his col­leagues is the clear­est evi­dence to date of their exis­tence.

“We found a smok­ing gun for the state pre­dict­ed decades ago and argued about ever since,” says Scholes.

“The ener­gy trans­fer process­es 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 par­tic­u­lar mys­tery,” says Richard Cogdell, the Hook­er Pro­fes­sor of Botany at the Uni­ver­si­ty of Glas­gow, co-author of the report.

“It makes us look dif­fer­ent­ly at the poten­tial of mol­e­cules as build­ing blocks,” Scholes says. “Just imag­ine one mol­e­cule, a carotenoid, that can be used to har­vest light, pho­to­pro­tect, con­vert to a ‘safe­ty valve’ in bright light to dis­si­pate exci­ta­tions, or even be employed as a heat trans­duc­er by pur­ple bac­te­ria such as are found in the black hole on the island of San Andros in the Bahamas.”

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Note to media: Vis­it for an illus­tra­tion of the research described here.

For more infor­ma­tion, con­tact:

Greg Scholes
D.J. LeRoy Dis­tin­guished Pro­fes­sor
Depart­ment of Chem­istry
Uni­ver­si­ty of Toron­to
416–333-0044 (mobile)
416–946-7532 (office)

Sean Bet­tam
Fac­ul­ty of Arts & Sci­ence
Uni­ver­si­ty of Toron­to