Coherent Coupling: A Photosynthesis Mystery Solved

By Scholes Group, Princeton University
Tuesday, January 16 2018

Scientists studying photosynthesis have solved a puzzle that has troubled researchers for a decade. The findings, reported on January 15 in Nature Chemistry (DOI: 10.1038/NCHEM.2910) by Gregory Scholes and his group at Princeton University together with Robert Blankenship’s group at Washington University in St. Louis, support the role of coherent coupling in photosynthesis but change the prevailing view of how long-lived quantum coherence is detected.

The controversial question concerns the apparent preservation of a fragile quantum “superposition” state in the Fenna–Matthews–Olson (FMO) complex, a protein found in green sulfur bacteria. In previous ultrafast laser experiments using FMO researchers detected oscillations --- or beats --- that showed retention of order, or phase and frequency, of waves lined up in step.

"What we discovered in the new work was that those oscillations are not directly representing the preserved quantum state --- of two or more chlorophyll molecules handling the sun’s energy like a team," said Scholes. "Instead they are in-step vibrations of the molecules."

The breakthrough, however, came when the authors found that they could set up vibrations on one bacteriochlorophyll molecule in the protein with a pump pulse, and then they could probe a separate bacteriochlorophyll in that same protein and generate an overall signal from the combined effect of the pump and probe pulses. As Scholes explains, "There is an unexpectedly significant interaction between these molecules that allows us to drive one with the pump laser, then later generate signal from another molecule, which we then probe."

"This information is the coherent coupling," he added.

The findings finally resolve the controversy of the assignment of the oscillations and show that the molecules in the FMO protein are coupled in a special way that may aid energy transport by directing it or making it faster. A more complete understanding of this phenomenon will help scientists apply the mechanisms of photosynthesis to emerging solar energy technologies.

"Energy transfer in biology and in the beaker results from perturbations that drive dynamics," noted Gregory Engel of the University of Chicago. "This work reveals that biology drives energy transfer by exploiting a robust, structured, and coupled vibrational environment within the FMO photosynthetic complex." Engel, whose group has published groundbreaking work in this field, believes that Scholes's article provides an explanation of previous findings while offering a path to future discoveries. 

"These observations may explain many of the past observations and suggest a blueprint for a novel approach to directing energy transfer," said Engel.

The article, titled "Coherent wavepackets in the Fenna–Matthews–Olson complex are robust to excitonic-structure perturbations caused by mutagenesis," was first published online by Nature Chemistry on January 15, 2018.