In one-quadrillionth of a second a plant can take the sun’s light and transfer it to the chlorophyll molecules (which give the plant its green pigmentation) in its light-harvesting centre. This process, a critical component of photosynthesis, is the most efficient energy-transfer process known, yet in many ways it is still poorly understood.
One of the most mysterious aspects of photosynthesis is the mechanism behind its efficiency. It is this mechanism that Professor Peter Hannaford and Dr Jeff Davis from Swinburne University of Technology are rying to expose.
They are particularly interested in a pigment called lycopene, which gives tomatoes their red colour but which is also an important molecule in photosynthesis. Lycopene absorbs light from the sun, which excites an electron (a sub-atomic particle that is negatively charged); the energy of this electron makes its way to the plant’s light-harvesting centre, and the light energy is converted to chemical energy.
“Based on that it sounds like we know what is happening in photosynthesis and these light-harvesting complexes, but that is not strictly true,” Dr Davis says. “We know roughly where the energy is absorbed and where it goes to, but we don’t know the precise mechanisms.
“We want to know what’s happening in the light-harvesting complex. We want to know this because photosynthesis is fundamental to all forms of life on Earth but also, if you can understand how photosynthesis works, some time in the future you might be able to use that knowledge to more efficiently capture energy from the sun.”
In 2007 the world’s two leading scientific journals – Science and Nature – published research that suggested a phenomenon known as quantum coupling was a major contributor to the efficiency of energy transfer in photosynthesis.
Professor Hannaford, director of Swinburne’s Centre for Atom Optics and Ultrafast Spectroscopy, explains that at the subatomic level, the level at which photosynthesis occurs, an electron can be in two internal quantum states. The laws of quantum mechanics allow the electron to be in a superposition of the two states, such that it is in multiple locations simultaneously. When this occurs across different molecules, they are said to be quantum coupled.
In October 2008 Professor Hannaford, Dr Davis, Professor Lap Van Dao and colleagues at the University of Melbourne were awarded a $280,000 Australian Research Council (ARC) Discovery Project grant to investigate quantum coupling in photosynthesis using ultrafast laser spectroscopy.
They will use a technique – called Noninterferometric Two-Dimensional Fourier-Transform Spectroscopy – developed by Dr Davis, Professor Hannaford, Professor Dao and Dr Tra My Do from Swinburne and Professor Keith Nugent and Dr Harry Quiney at the University of Melbourne. The technique, which evolved through a collaboration within the ARC Centre of Excellence for Coherent X-ray Science, uses laser pulses to probe different states within molecules and identify if there is any quantum coupling occurring and, if so, the dynamics behind it. Details of the technique were published in June 2008 in the leading physics journal Physical Review Letters.
The conventional view of photosynthesis has been that sunlight is absorbed by a molecule, which excites an electron; this energy then passes between the different molecules in the light-harvesting centre looking for the molecule that can convert it to chemical energy through photosynthesis.
“But if quantum coupling is playing a role, the molecule absorbs the light energy, excites one electron and the electron may exist in all the different states at the same time,” Dr Davis says. “In this way the electron can identify the molecule with the lowest energy state, which means it can go there much more rapidly and much more efficiently without having to hop around the molecules looking for the molecule at the ‘bottom of the hill’ of energy states.”
The transfer of energy from lycopene to a plant’s reaction centre occurs on a timescale of hundreds of femtoseconds. (One femtosecond is a thousand trillionths of a second.)
“All the experiments we’re doing in ultrafast spectroscopy revolve around using light to measure dynamics on this sort of timescale,” Dr Davis says. “We use pulses of light to measure small timescales because nothing travels faster than light. Since the development of the laser, the length of these pulses has been getting shorter and shorter and now people are producing pulses of duration even less than femtoseconds.”
Dr Davis’s experiments use pulses that last 100 femtoseconds and have already provided evidence of quantum coupling occurring during photosynthesis. “We’re seeing long-lived quantum coupling between energy states in lycopene, which makes it possible that you then get efficient energy transfer to the rest of the light-harvesting complex,” he says.
As more experimental data is gathered it will be sent to Professor Yasushi Koyama from Kwansei Gakuin University in Japan, an expert in carotenoids (plant pigments that capture sunlight for photosynthesis). Professor Koyama is collaborating on the ARC project, contributing lycopene samples and assisting in result interpretation.
Dr Davis says if it is quantum coupling in lycopene that is causing the efficient energy transfer from these molecules to the rest of the light-harvesting complex, this would be the first step in understanding the mechanisms behind photosynthesis and could ultimately be used to design more efficient solar cells or as a more efficient way of generating hydrogen as an energy source.
A story provided by Swinburne Magazine. This article is under copyright; permission must be sought from Swinburne Magazine to reproduce it.
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