Chlorophyll f: UV sheds some light on infrared molecule

Red-shifted molecule

Chlorophyll f is the most red-shifted natural chlorophyll and is made by the blue-green alga Halomicronema hongdechloris. Researchers have now carried out a structural assignment UV-Vis spectroscopy and other techniques. New insights into chlorophyll and its variants is important for understanding photosynthesis as well as for research into energy conversion, electron transfer applications.

Five different forms of chlorophyll - chlorophylls a, b and c were first identified by 19th century scientists, chlorophyll d was reported in 1943. Then, in August 2010, University of Sydney scientists and their colleagues announced that they had discovered the first new form of chlorophyll in more than six decades. They reported chlorophyll f in the journal Science and explained how it was a serendipitous discovery uncovered in stromatolites from Western Australia's Shark Bay. The new pigment can convert light of a much longer wavelength (lower frequency) than any other known chlorophyll. F can convert light at 720 nanometres. In terms of survival, chlorophyll f allows the cyanobacteria that build deep, dark stromatolites to carry out photosynthesis using the low-energy infrared light that impinges on them as it is the only light that can penetrate these rock-like structures. The 2010 discovery pushed back the boundaries of what biologists consider to be the physical limits of photosynthesis.

Structural work

Now, the team - Yaqiong Li and Min Chen of the University of Sydney, Robert Willows of Macquarie University, Sydney and Hugo Scheer of Universität München, Germany - has published details of the UV/vis absorption, circular dichroism (CD), nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) and high-performance liquid chromatography (HPLC) studies of chlorophyll f, to which they assign the structure [2-formyl]-chlorophyll a (C₅₅H₇₀O₆N₄Mg).

The successful isolation and propagation of the cyanobacterium, H hongdechloris, allowed the team to produce sufficient quantities of this new chlorophyll to allow them to carry out the necessary analyses and structural tests. They could "force" the microbe in the horticultural sense by bathing it in far-red light culture conditions, which pushed up the proportion of chlorophyll f relative to the cyanobacterium's chlorophyll a supplies to 20 percent. Nevertheless, 120 micrograms of this chlorophyll was sufficient for complete structural assignment.

Speechless

At the time of the 2010 discovery of chlorophyll f, Chen is on record as saying: "It is amazing that this new molecule, with a simple change to its chemical structure, can absorb extremely low energy light. This means that photosynthetic organisms can utilise a much larger portion of the solar spectrum than we previously thought and that the efficiency of photosynthesis is much greater than we ever imagined."

He pointed out that chlorophyll f and its ability to function under infrared light might have various applications for plant biotechnology and bioenergy but the next step will be to figure out the precise function of chlorophyll f in photosynthesis. "Is its job to capture additional red light and pass it on to another chlorophyll, like chlorophyll a, in the reaction centre for photosynthesis?" asked Chen. "Or is it the only chlorophyll responsible for photosynthesis in the cyanobacterium? And if it is, then we will be speechless wondering how this molecule can get enough energy from infrared light to [extract] oxygen from water."

"We have now sequenced the genome of this novel organism and are in the process of annotating it," Willows told SpectroscopyNOW. "Ultimately, we want to work out how this organism makes the chlorophyll f, how the chlorophyll f is used and which genes are involved in making chlorophyll f."