While untold numbers of plants have carried out photosynthesis for hundreds of millions of years, the nature of this subtle process continues to elude us. Scientists are, however, focusing increasingly sophisticated tools on this problem, raising hopes that this fundamental biological activity will eventually reveal its secrets.

Among those tools has been the advent of 2D spectroscopy, which adds a second frequency axis to measure correlations between the different electronic resonances observed in more traditional linear transmission spectroscopy. This additional dimension provides a new perspective on molecular dynamics, including intramolecular vibrations and the transfer of energy during a reaction.

Such spectroscopic measurements are carried out with short laser light pulses; as those pulses become shorter, a larger section of the electromagnetic spectrum can be investigated. With the advent of lasers capable of femtosecond (10-15 second) pulses, it became possible to probe the visible wavelengths, including the energy redistribution between pigments involved in photosynthesis as plants turn sunlight into biochemical energy. The resulting data could be interpreted as showing that these pigments act cooperatively, making use of quantum effects to facilitate a much more efficient flow of that energy. That flow would be quite different from the step-wise fashion of a classical chemical reaction, where intermediate steps are well described by kinetic rate constants.

“It’s a very complicated network by which the energy gets moved around within photosynthetic proteins,” says Alexei Halpin, a PhD student in the departments of chemistry and physics at the University of Toronto. “You have anywhere from a dozen to a few hundred identical pigments all sitting there like little antennas,” Halpin says. “They’re packed very closely together and roughly absorb at the same frequency of light, leading to very congested optical spectra. With 2D spectroscopy you can directly probe how pinging one of those transitions affects the response of another — at a different wavelength — elsewhere in the pigment aggregate.”

The imagination of many observers was captured by the notion that the foundation of photosynthesis adhered to the unusual rules of quantum mechanics. It also raised the possibility of a new technology for harvesting the energy in light using this same kind of reaction. However, Halpin’s research has led him to question whether these quantum effects are significant. “Every molecule behaves like a quantum system but only for a very short period of time,” he says. 

Working with colleagues in Canada, the Netherlands and Germany, Halpin synthesized a model system for testing a single set of coupled pigments, allowing more direct analysis and interpretation than larger biological aggregates. The findings, published in Nature Chemistry, show that the quantum signatures of pigments acting in concert are rapidly washed out and that prolonged oscillations may nevertheless occur, but these will mostly reflect vibrations localized on a single pigment. This finding does not directly nullify the earlier studies that have been linked to quantum effects, Halpin adds, but rather underscores just how much room for interpretation remains in the data from these experiments. 

“This was the first experiment of this type on this kind of very simple coupled pigment system,” says Halpin. “We have to do more on other synthetic model systems that have specific properties more in common with the aggregates that we were actually looking to mimic in photosynthesis.”