NCSA NEWS

A particle of light flies out of the sun, travels 8 minutes and 20 seconds, 93 million miles, down through Earth's atmosphere and hits a leaf on a plant. Within the leaf, a molecule of chlorophyll absorbs the energy, knocking one of its electrons into an excited state. A ring-shaped molecule called a quinone transfers the excited electron away from the chlorophyll and then shepherds it to a second quinone. The plant has now stored the sunlight energy in the electric field between the negatively charged quinone and the now positively charged chlorophyll -- a tiny battery, in other words.

Photosynthesis takes place inside chloroplasts within green plant cells, and occurs in two sets of reactions: light and dark. During light reactrions, sunlight and H20 are taken into the cell. Energy from sunlight powers the transformation of ADP and NADP+ into ATP and NADPH2, and O2 is realesed. During dark reactions, this chemical energy utilizes CO2 to produce CH2O, which the plant stores in the form of glucose and uses as a nutrient.

These are the first steps in photosynthesis, the energy factory of plants. These are also steps that Ralph A. Wheeler, a chemistry professor at the University of Oklahoma, would like to understand better using the computers at NCSA, the Alliance's leading-edge site.

Electron transfer -- transferring an electron from Point A to Point B to Point C -- sounds simple. "You can think of it like water running down a hill. The water wants to run down to the more stable point, and so do the electrons," says Wheeler. "The plant stores energy by using sunlight to pump electrons uphill and does this exquisitely well." Although scientists know the basics of photosynthesis, they have not yet replicated this lightweight, efficient feat of chemical engineering experimentally.

One reason is that the study of the movement of single electrons in electron transfer is still relatively new. Chemists traditionally have focused on pairs of electrons, which is how electrons generally arrange themselves as they seek the lowest, most stable energy states. For example, two atoms of hydrogen, each with one proton and one orbiting electron, will join into a dumbbell-shaped hydrogen molecule. With the two electrons now paired, the molecule has a lower energy state than the two separate hydrogen atoms.

That same electron craving makes molecules with an odd number of electrons highly unstable, likely to react with the first molecule that passes by, and thus difficult to study.

In some processes, like photosynthesis, the movement of single electrons is paramount. Beginning in the mid-1950s, chemist Rudolph A. Marcus pioneered this research in the field -- much of it performed while at the University of Illinois at Urbana-Champaign in the 1960s and 1970s. Marcus, now at the California Institute of Technology, explained how interactions between molecules momentarily increase the energy of a system, driving the electron to jump from one molecule to another. That work won him a Nobel Prize in 1992.

Wheeler's simulations examine the same essential puzzle -- the movement of single electrons -- but they look at specific reactions in detail. For photosynthesis, the subtle dynamics in thousands of atoms nudge electrons from one quinone to another. This degree of detail is needed for future breakthroughs, and they are why Wheeler requires computing resources like those at NCSA.

Access Online | Posted 11-2-1998