This project aims to make a movie of molecules at work. The molecule of interest is called Photosystem I (PSI), and is present in all green plants. Together with another molecule (Photosystem II), these are responsible for photosynthesis, which maintains our biosphere by degrading carbon dioxide (an important contributor to Global Warming) and by splitting water to provide the oxygen in our atmosphere. Making this movie will entail use of the world's first X-ray laser, at Stanford (the DOE's Linac Coherent Light Source(LCLS)), which produces extremely brief "snap-shot" X-ray diffraction images of groups of molecules. This project is a study of "nanocrystals" of the molecules, which are periodically arranged clusters containing perhaps 20 molecules on a side. The molecules, which must be wet, are sprayed in a vacuum across the X-ray beam in a special liquid jet of micron dimensions. This gas-focussing jet (somewhat similar to an ink-jet printer) was developed under NSF support in a previous grant. In this first attempt to record atomic motion by snap-shot diffraction from membrane protein nanocrsystals, a red laser beam will be used to initiate the separation (or "undocking") of a second molecule (ferredoxin) from PSI, then, after a brief delay, an X-ray snapshot will be recorded. Frames of a movie will be accumulated by varying the delay time. Several snapshots, read out from the LCLS detector 60 times per second, will be added together for each frame.
Broader Impacts
The broader impact of the project includes the training of postdoctoral researchers and a graduate student, and the dissemination of the results in journals and on the web. In addition the project and results will be actively promoted in a number of outreach programs, including the Lawrence Berkeley Laboratory Outreach program, the Open Days at ASU Physics, the Arizona Science Fair, the Outreach program of the Eyring Center at ASU and in the Arizona High School Teacher Training Program. There will be continued participation in these programs, where the methods and results of the research are explained to students in a pedagogically sound manner. This work is directly relevant to efforts to reduce global warming since it provides an atomic mechanism for a crucial step in the process of photosynthesis.
Life on our planet is made possible by photosynthesis, the splitting of water which occurs when sunlight falls on all green plants, releasing practically all the oxygen we breathe. This research project aims to make a movie using X-ray snapshots, showing how this water splitting occurs. The recent invention of the X-ray laser makes this possible. The world's first hard X-ray laser was commissioned by the US Department of Energy in 2009, and we were among the first users. The machine, which occupies a two-mile long tunnel near Stanford, generates extremely breif pulses of X-rays, about 120 times every second. These pulses are so brief (50 femtoseconds duration) that they end, before the delicate biological molecules which perform photosynthesis, can be damaged. Yet because this is a laser, we can pump an enormous intensity of X-rays into the molecules, and still not damage them, if we do it quickly enough. This is a very important breakthrough - previously throughout the histroy of microscopy in biology it had always been believed that individual molecules could not be seen by X-rays, because the high dose needed to do so would destroy them. Not if you are quick enough ! Our work was successful. We imaged part of the molecules involved in photosynthesis - Photosystem 1 - ferredoxin. To get a few frames of a 3D movie , we used submicron protein crystals, and illuminated them with visible laser-light to simulate the sunlight, then took their X-ray snap-shot a short delay (microseconds) later. In addition to that work, we obtained images of another photosynthetic molecule (reaction center) using not small crystals but just molecules in solution. We also invented the scientific instruments needed to make a sinlgle-file stream of micron-sized droplets, needed to carry the molecules or crystals across the pulsed X-ray beam. There are many research groups around the world trying to improve on Nature's natural methods of photosynthesis , and the better we understand this, the more chance we have of making artificial photosynthesis devices, which could convert the carbon dioxide in the atomosphere into useful hydrogen fuel, using just sunlight and water.
