Photosynthesis is vital to the continued survival of life on Earth because it produces most of Earth’s supply of oxygen. For a long time, scientists have been convinced that if they could completely understand how photosynthesis works, they could apply it to synthetic systems to create clean energy from water and sunlight, with the only emissions being oxygen and hydrogen.
Researchers at Georgia Institute of Technology have shown the importance of a hydrogen bonding water network in the photosynthesis substructure called photosystem II. They extracted photosystem II from ordinary spinach and replaced water with ammonia to test the idea that a network of hydrogen-bonded water molecules would provide a catalyst for the process that produces oxygen.
“By substituting ammonia, an analog of the water molecule that has a similar structure, we were able to show that the network of hydrogen-bonded water molecules is important to the catalytic process,” said Bridgette Barry, a professor at Georgia Tech’s School of Chemistry and Biochemistry. “Substituting ammonia for water inhibited the activity of the photosystem and disrupted the network. The network could be re-established by addition of a simple sugar: trehalose.”
Photosynthesis is controlled by the chloroplasts of green plants. Oxygen is produced through the illumination of calcium and manganese ions in the oxygen-evolving complex (OEC) of the chloroplast. Short laser flashes can be used to step through the reaction cycle, which involves four sequential light-induced oxidation reactions. For the oxygen to separate from the OEC, it forms an electrostatic network with the ions in the OEC and a protein called amide carbonyl (C=O). Oxygen’s hydrogen bonds are used as a catalyst component to split off the oxygen.
Georgia Tech graduate student Brandon Polander prepares for a Fourier transform infrared spectroscopy experiment. The green laser light is used to photoexcite the spinach photosystem II sample. Courtesy of Gary Meek.
The researchers used Fourier transform infrared spectroscopy to observe how the hydrogen-bonded network reacted to pulses from an Nd:YAG laser. After each pulse, they analyzed the photosystem’s transition and measured the bond strength of the C=O groups, which were, in turn, used as markers of hydrogen bond strength. With the flash, there was an observed increase in C=O hydrogen bond strength; however, when ammonia was added, the C=O hydrogen bonds weakened. Trehalose blocked the ammonia’s effects.
“This research helps to clarify how ammonia inhibits the photosystem, which is something that researchers have been wondering about for many years,” Barry said. “Our work suggests that ammonia can inhibit the reaction by disrupting this network of hydrogen bonds.”
Barry hopes that her research can be used to harness or imitate energy and oxygen production.
“We are only looking at a single part of the overall reaction now, but we would like to study the entire cycle, in which oxygen is produced, to see how the interactions in the water network change and how the interactions with the protein change,” Barry said. “The work is another step in understanding how plants carry out this amazing series of photosynthetic reactions.”
The research was published in the April 2 online edition of Proceedings of the National Academy of Sciences