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Photosystem II 3

Photosystem II is crucial to life as we know it. This process is the only natural process capable of forming O2 from water and sunlight (Siegbahn, 2009).This capability is used to convert light energy to chemical energy in plants. This process also provides the energy to drive the conversion of carbon dioxide into the oxygen critical to animal life. Animals also derive benefit from chemical energy stored in the form of sugars within the cells of the plants through their consumption. This may also occur indirectly via the consumption of other animals who have previously consumed these plants.

The overall process is divided into light reactions which need light to occur, and dark reactions, also known as the Calvin Cycle, which do not directly need light. (Campbell, 2005) Electrons and protons are passed from the light reactions to NADPH which allows for their transport to the Calvin Cycle. (Campbell, 2005).

At the heart of this indispensable system is the protein p680 which drives the light reaction, and thereby the entirety of the process (Campbell, 2005). At the heart of p680 is the Oxygen Evolving Complex (OEC), shown below, which is a cluster of inorganic atoms. The precise form of the OEC continues to elude researchers, and many models have been proposed. The Dangler Model (Siegbahn, 2009) has been selected for this assignment because it gives the best opportunity to delve into the symmetrical aspects of the OEC. This model of the OEC consists of four manganese atoms, four oxygen atoms, and one calcium atom. A chlorine atom becomes involved in the reactions but is not directly a part of this cluster. (Vrettos, 2002)


 OEC.JPG

Figure 1: Oxygen Evolving Complex. (simplified† from distorted cubic layout)


But what of the chemistry drives this vital process? We find that inorganic chemistry, most specifically that of manganese, plays the key role in this process. In many of its oxidation states, those above (II), manganese is known to have great oxidizing power (Housecraft, 2008). The Manganese atoms in the OEC have been found to be the (III) and (IV) oxidation states (Kulik, 2007). Using the oxidative power of these manganese atoms the OEC is able to drive the process through a series of reactions divided into steps S0-S4. Overall, the Manganese drives the general reaction: 2H2O + Light → O2 + 4H+ + 4e- (Micklitz, 1998)

Seeing the oxygen evolving complex arranged in the manner of the Dangler Model provides an excellent opportunity to assess the symmetry of this structure, at least locally. Knowledge of the symmetry is of importance when predicting or interpreting the results of IR and Raman Spectroscopy. Using the simplified depiction of the OEC in figure 1 it can be seen that the cluster is of a cubic shape with one magnesium stretching away from a corner (Siegbahn, 2009) the symmetry is as follows:
 

symmetry.JPG
 

The complex without the blue Mn would be C3v

With the blue Mn the complex is C1

The entire protein also has C1 symmetry

Through knowing the structure and employing the character tables, the IR and Raman spectroscopic results can be predicted for this complex. In the event that the simplified version of the distorted cubic structure were are to be the precise layout of the OEC, we could can expect to see the following.

  • The sigmas bonds of the blue Mn reduce to 1Σ+ which yields one Raman and one IR band
  • The sigma bonds of the black Mn's as well as the Ca are reduced to 1A + 1E which yields 2 IR and 2 Raman Bands.
  • The d-orbitals of all 4 Mn's are reduced to 3Ag + 3Au which yield 3 Raman and 3 IR bands.
  • The d-orbitals of the Ca are reduced to 1Σg+ which yields 1 Raman band.

These results can also be run through a quick visual check to see that point group symmetries that have a center of inversion have irreducible forms in which each individual term does not yield both a Raman and IR band. These predicted bands meet that criteria. Another useful application of the fundamentals of inorganic chemistry to understanding the form of the oxygen evolving center is to look at the bonding energy diagram of the manganese to an adjacent oxygen.


 

MnO Bonding.JPG
 

     The primary item of interest is that the electron bonding configuration leads to 6 bonding electrons and 3 non bonding electrons. There are no anti-bonding orbitals filled. This shows that the bond is likely to occur and will in fact be relatively strong.
 

Through an understanding of the basics of inorganic chemistry and looking at ongoing research we can see the pivotal role inorganic chemistry plays in the chemistry of life. Looking at a periodic table with each atom marked as to whether it is essential to all investigated species yields 5 elements in the which reside in the d-block, metals. The same table shows an additional 5 metals being essential to at least one biological species (Schore, 2007)

Through this example, and likely many, many more, it becomes obvious that there is no distinct chasm that inorganic and organic chemistry can not bridge. Life is a powerful and diverse process rooted in chemistry and as such takes advantage of opportunities not provided by the typical carbon, hydrogen, oxygen, nitrogen arsenal of organic chemistry.

†: The Dangler Model does not predict all atoms for the OEC to be equidistant, not for all the angles to perfectly fit the 90º required of a perfect cube. It can be described as distorted cubic. By representing it as a simple cube, there is access to a greater variety of symmetries to evaluate. The image is a simplified depiction of that found in the Siegbahn paper in the references section.

References

  • Campbell, Neil A., Reece, Jane B. Biology, Seventh Edition. San Frincisco: Pearson Education, Inc., 2005.
  • Housecraft, Catherine E., Sharpe, Alan G. Inorganic Chemistry, Third Edition. Harlow: Pearson Education Limited, 2008.
  • Kulik, Leonid V., Epel, Boris, Lubitz, Wolfgang, Messinger, Johannes. “Electronic Structure of the Mn4OxCa cluster in the S0 and S2 States of the Oxygen Evolving Complex of Photosystem II base on Pulse 55Mn-ENDOR and EPR Spectroscopy.” Journal of the American Chemistry Society. 129 (2007): 13421-13435
  • Micklitz, Wolfgang, Bott, Simon G., Bentsen, James G., Lippard, Steven J. “Characterization of a novel µ4-Peroxide Tetrairon Unit of Possible Relevance to Intermediates in Metal Catalyzed Oxidations of Water to Dioxygen.” Journal of the American Chemistry Society. 111 (1989): 372-374
  • Schore, Neil E., Vollhardt, K. Peter C. Organic Chemistry: Structure and Function: Fifth Edition. New York, W. H. Freeman and Company, 2007
  • Siegbahn, E.M. “An Energetic Comparison of Different Models for the Oxygen Evolving Complex of Photosystem II.” Journal of the American Chemistry Society. 131 (2009): 18238
  • Vrettos, John S., Brudvig, Gary W. “Water Oxidation Chemistry of Photosystem II.” Philosophical Transactions of The Royal Society ((Biological Sciences). 357 (2002): 1395-1405

Contributors

  • Kenneth King