Light switch: Autumn leaves

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  • Published: Dec 1, 2016
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Light switch: Autumn leaves

Mellow yellow

Light Switch in Autumn Leaves: Yellow chlorophyll decomposition products are environment-responsive photoswitches (Photo credit: David Bradley)

Yellow chlorophyll decomposition products are environment-responsive photoswitches researchers in Austria have found using UV-Vis and nuclear magnetic resonance (NMR) spectroscopy, and X-ray crystallography studies.

Before deciduous trees shed their leaves for the winter, they commonly present a bright autumnal display of colours, reds, oranges, and yellows. These colours arise as chlorophyll, the green, pigment of photosynthesis degrades and waste products are pumped into the leaves. As any high school biology teacher will tell you the autumnal colouring of leaves is a form of excretion as well as a survival tactic for avoiding wind and snow damage to twig and branch during the winter months. Among the decomposition products of chlorophyll are the yellow phyllobilins, which have some intriguing chemical properties. Austrian researchers explain in the journal Angewandte Chemie how they have demonstrated that the phyllobilins behave as four-step molecular "switches", for instances, these switches being operated by light in different ways depending on the environment.

Turning leaves

During the summer, green leaves use their chlorophyll to convert sunlight into chemical energy. Before they begin to lose their leaves and so excrete waste products at the onset of autumn and the cold season, the trees also extract important nutrients like nitrogen and minerals from their leaves and store them for the next spring. "The chlorophyll released in this process must be broken down because it has a damaging effect on the tree when it is irradiated by light while unbound," explains organic chemist Bernhard Kräutler of the University of Innsbruck. "Presumably, the chlorophyll decomposition products play a physiological role as well," he muses.

Most phyllobilins are colourless, but the yellow ones, known specifically as phylloxanthobilins are of more interest. Kräutler and his team have worked with colleagues at the Universities of Innsbruck and Graz, Austria, and Columbia University, in the USA, to demonstrate, with a range of techniques, that the compounds act as unique four-stage photoswitches.

Leafy behaviour

The different behaviour of these photoswitches is seen under different conditions. For instance, in polar media, such as the aqueous environment within a cell, the phylloxanthobilins are present as simple molecules. However, when they are irradiated with light, they switch reversibly between two forms that have slightly different spatial structures around a double bond; the undergo cis-trans isomerization. In non-polar media, such as the cell membrane systems, the cis (Z) isomers can pair up, held together through hydrogen bonds. Light irradiation leads to a chemical reaction between the two paired molecules, a cycloaddition, which forms a four-carbon ring to lock the pair together permanently, or at least until the application of heat reverses the cycloaddition and splits the pair again.

The team's NMR spectroscopy confirmed the four-carbon, lactam, ring structure, while UV-Vis fluorescence, and CD spectroscopy were used to corroborate the monomer versus dimer shuttling in different solvent systems, polar versus non-polar. "By using X-ray crystallographic analysis, we were able to determine the precise spatial arrangement (stereostructure) of a phylloxanthobilins and the hydrogen-bonded pair structure they adopt when crystallized," reports Kräutler. "The fascinating chemistry of these substances also suggests that phyllobilins may have important, unknown physiological roles, possibly in the photoregulation of plants. Our new insights will help to elucidate this role."

"Natural photo-switches are important gene regulatory elements in photosynthetic organisms (e.g. phytochromes in plants), which typically function on the basis of a double bond Z/E-isomerization," Kräutler told SpectroscopyNOW. "We hope to find interesting biological functions of the phyllobilins (possibly along the lines of photo-regulation). Phyllobilins are produced in nature at about 1000 million tons each year and, so far, appear to be waste!"

Related Links

Angew Chem Int Edn 2016, online: "Chlorophyll-Derived Yellow Phyllobilins of Higher Plants are Medium-Responsive, Chiral Photoswitches"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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