The secret of a natural energy source

The authors used common or garden spinach from the local greengrocer for their research.

Without photosynthesis, higher life forms on earth would be inconceivable. A lot is known about this puzzling phenomenon, but there is still a great deal yet to be discovered.  Leiden researchers  make some revelations in PNAS.

Spinach
In the early edition of 24 July of the journal PNAS (Proceedings of the National Academy of Sciences of the United States of America), the researchers from the Leiden Institute of Chemistry (LIC), the  Leiden Institute of Physics (LION) and two researchers from Frankfurt and Konstanz describe the behaviour of electrons in the process of photosynthesis. For their investigations, the researchers used spinach bought from the local greengrocer in Oegstgeest.

Carbon dioxide
For a long period, the precise sequence in the process of photosynthesis was unknown.  The German biochemist Otto Heinrich Warburg, winner of the Nobel Prize for Medicine in 1931, assumed that the oxygen which is released during the process, originated from carbon dioxide.  Since then, it has become known that there are two steps in photosynthesis: the light and the dark reactions. With the light reaction, the combination of light and chlorophyll causes water to be split into oxygen and hydrogen. In the dark reaction, the energy from the light reaction is used to synthesise glucose from carbon dioxide.  For chemists, this process is very unusual, because a strong energy source is needed to drive the reaction in the opposite direction from what is normal.

Energy sources

The hinge model of the electron donor in PS2. Because of the tilt of the axial histidine toward pyrrool ring IV (arrow), the electron spin density pattern is inverted, and electron spin density is partially shifted on the axial histidine.

Nature has developed two energy sources for this purpose, which operate one after the other.  Photosynthesis II, the strongest natural oxidant, constitutes the stage in which water is formed, and photosynthesis I is the strongest reductant in living nature.  It is through the link between these two systems that the the flow of electrons from water is eventually able to form glucose.

Interchange
'The big question is how a protein can build up such an enormous electrical charge that it can split water, without destroying itself,' says Dr Jörg Matysik, one of the authors.  Using a new method  known as photo-CIDNP MAS NMR, the authors have studied the electronic structure of the reductant in photosynthesis II.  Matysik: 'Anna Diller, who will defend her dissertation on this subject in September, discovered that the density of electron spin resonance is very different from that in photosynthesis I. She also discovered that an electron spin of the amino acid histidine can be observed on a side chain. These two observations are clearly related. One of the effects of this interchange between histidine and chlorophyll is to lower the amount of energy required, thereby raising the reduction potential.

Solar cells
At the present time, this model is based on purely theoretical analysis.  Because natural energy sources are much more effective than artificial ones, the example of the reductant of photosynthesis II may give rise to the development of new artificial energy sources, for example better solar cells.  Energy can be produced in these solar cells using organic molecules, for example, or hydrogen can be produced. 

¹⁵N photochemically induced dynamic nuclear polarization magic-angle spinning NMR analysis of the electron donor of photosystem II
Anna Diller; Esha Roy; Huub J. M. de Groot; Jörg Matysik; A. Alia (LIC)
Peter Gast; Hans J. van Gorkom (LION)
Clemens Glaubitz (Johann Wolfgang Goethe Universität)
Gunnar Jeschke (Universität Konstanz)

(24 July 2007/SH)