ERROR 1
ERROR 1
ERROR 2
ERROR 2
ERROR 2
ERROR 2
ERROR 2
Password and Confirm password must match.
If you have an ACS member number, please enter it here so we can link this account to your membership. (optional)
ERROR 2
ACS values your privacy. By submitting your information, you are gaining access to C&EN and subscribing to our weekly newsletter. We use the information you provide to make your reading experience better, and we will never sell your data to third party members.
Biological Mechanisms
One of the elusive secrets of photosynthesis, the biological process that makes life on Earth possible, has been uncovered. Researchers have observed an oxidation state involved in generating oxygen. The work could lead to better ways to convert sunlight into other forms of energy.
In photosynthesis, water is oxidized to molecular oxygen, the source of the O2 we breathe and the atmospheric ozone that protects us from ultraviolet radiation. The electrons liberated are then used to synthesize carbohydrates, the source of food we eat. The process is carried out with nearly 100% efficiency and no toxic by-products in photosystem II, a multisubunit protein complex found in photosynthetic organisms. The catalytic center of photosystem II is a tetramanganese cluster coupled to a redox-active tyrosine residue called YZ.
In the water oxidation process, four successive photons of light induce photosystem II to traverse five oxidation states, S0 through S4. S0 to S3 had been observed spectroscopically, but not S4. It wasn't known if S4 is just a fleeting transition state or a more long-lived intermediate. Also, because O2 formation starts at the S4 state, the actual mechanism of O2 generation was not known.
Now, Michael Haumann, Holger Dau, and coworkers in the department of physics at the Free University of Berlin have observed S4 by using high-intensity, time-resolved X-ray absorption spectroscopy (Science 2005, 310, 1019). It's an intermediate.
The measurements would have been impossible without the high-brightness, third-generation synchrotron sources that provide higher X-ray flux than was previously available for such experiments, note enzyme specialists James E. Penner-Hahn and Charles F. Yocum of the University of Michigan in an associated commentary. With this demonstration of feasibility, a wide range of other applications of microsecond time-resolved X-ray absorption spectroscopy to chemically and biologically important reactions can now be imagined.
It had been proposed earlier that S4 might be created by electron transfer from photosystem II's Mn4 complex to a YZ radical (YZ•). The new study indicates that S4 forms instead by deprotonation. This suggests that the photosynthetic cycle includes a sixth oxidation state. The deprotonation must be followed by electron transfer to YZ•, thus implying an S4' state, the researchers note. S4' may also be either a transition state or an intermediate. With mysteries like that still to be solved, efforts to fathom the intricacies of photosynthesis will continue.
Join the conversation
Contact the reporter
Submit a Letter to the Editor for publication
Engage with us on Twitter