Quantum chlorophyll

The simplest things in nature that make life possible are often the most difficult to comprehend. A recent theory on why plants are green threw up surprising results when it emerged that vegetation on earth works in the red and blue range of solar radiation and not the green. This was even more surprising given the fact that the sun pumps out maximum wattage in the green range. This is probably why the human eye is most sensitive to it. The next big surprise is that chlorophyll seems to use quantum effects to do its magic. In a recent announcement by Lawrence Berkeley National Laboratory & University of California (UC) at Berkeley:

We have obtained the first direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis," said Graham Fleming, the principal investigator for the study. “This wavelike characteristic can explain the extreme efficiency of the energy transfer because it enables the system to simultaneously sample all the potential energy pathways and choose the most efficient one.” The classical hopping description of the energy transfer process is both inadequate and inaccurate," said Fleming. "It gives the wrong picture of how the process actually works, and misses a crucial aspect of the reason for the wonderful efficiency."

Co-authoring the Nature paper with Fleming were Gregory Engel, who was first author, Tessa Calhoun, Elizabeth Read, Tae-Kyu Ahn, Tomas Mancal and Yuan-Chung Cheng, all of whom held joint appointments with Berkeley Lab’s Physical Biosciences Division and the UC Berkeley Chemistry Department at the time of the study, plus Robert Blankenship, from the Washington University in St. Louis.

The photosynthetic technique for transferring energy from one molecular system to another should make any short-list of Mother Nature’s spectacular accomplishments. If we can learn enough to emulate this process, we might be able to create artificial versions of photosynthesis that would help us effectively tap into the sun as a clean, efficient, sustainable and carbon-neutral source of energy.

Towards this end, Fleming and his research group have developed a technique called two-dimensional electronic spectroscopy that enables them to follow the flow of light-induced excitation energy through molecular complexes with femtosecond temporal resolution. The technique involves sequentially flashing a sample with femtosecond pulses of light from three laser beams. A fourth beam is used as a local oscillator to amplify and detect the resulting spectroscopic signals as the excitation energy from the laser lights is transferred from one molecule to the next. (The excitation energy changes the way each molecule absorbs and emits light.)

Fleming has compared 2-D electronic spectroscopy to the technique used in the early super-heterodyne radios, where an incoming high frequency radio signal was converted by an oscillator to a lower frequency for more controllable amplification and better reception. In the case of 2-D electronic spectroscopy, scientists can track the transfer of energy between molecules that are coupled (connected) through their electronic and vibrational states in any photoactive system, macromolecular assembly or nanostructure.

Fleming and his group first described 2-D electronic spectroscopy in a 2005 Nature paper, when they used the technique to observe electronic couplings in the Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting protein, a molecular complex in green sulphur bacteria.