New Technology Applied to Study by Photosynthesis

David E. Budil

All of life on earth is ultimately linked to the radiant energy of the sun by the process of photosynthesis. In photosynthetic organisms, light is harnessed to drive the cellular metabolism in special pigment-containing proteins called reaction centers. These proteins reside in inner membranes of the electron organism and carry out an electron transfer reaction that is triggered by light. The separation of an electronic charge across the membrane creates an electrical potential — a microscopic cellular battery — that can be used to carry out the chemical work of the cell.

The reaction center carries out its function with astounding efficiency, many times greater than that of even the best semiconductor-based solar cells. Clearly, it would be of great value to understand how nature has designed this “organic solar cell” to capture and convert light energy so effectively. The quest to unlock the secrets of the reaction center have given rise to a long-standing and intensive international research effort. In 1988, Hartmut Michel and his colleagues at the Max Planck Institute in Germany won the Nobel Prize in chemistry for solving the molecular structure of the reaction center from a lake-dwelling bacterium. This structure, shown in Figure 1 (see back cover), turns out to be quite similar to reaction centers from more familiar photosynthetic organisms, such as plants and algae.

The colored wire frame structures shown in Figure 1 (see page 16) are the pigment molecules that actually carry out the light reaction of photosynthesis, and the grey ribbonlike structure represents the backbone of the surrounding protein. When the reaction center is photoexcited, the topmost “special pair” of chlorophyll molecules (red) lose a single electron, which appears on the green molecule (pheophytin) within a few trillionths of a second. The electron is then very rapidly transferred to the yellow molecule labeled Q (quinone), which completes its transit across the photosynthetic membrane.

Even given this detailed “resting state” structure of the reaction center, its active working states remain poorly understood. One of the most tantalizing mysteries is why, despite the fact that the reaction center appears to have two symmetric branches as shown in Figure 1, the light reaction only proceeds along the right-hand side. The answer seems to lie in the details of the interactions between the protein and the pigments contained within it, which could yield important clues to how an efficient artificial organic-based solar cell could be designed.

Breakthroughs in our understanding of biology on a molecular level have frequently required the development of novel physical methods to elucidate increasingly microscopic details of biomolecules. The method under development in my laboratory at Northeastern takes advantage of the intrinsic property of electrons known as spin. Each individual electron behaves as if it were a spinning globe of charge that causes it to act like a tiny bar magnet, in much the same way the circulating current in an electric motor coil produces a magnetic force. In most stable chemical compounds, electrons are paired so that their magnetic moments perfectly cancel each other, and there is no net magnetism. During a reaction in which chemical bonds are broken or electrons are transferred from one molecule to another, however, it often occurs that the electrons become unpaired and exhibit magnetic effects.

When unpaired spins are placed in a strong external magnetic field, they line up with it, just as a compass needle points north in the earth’s magnetic field. The spins may be tipped away from this alignment by the application of electromagnetic radiation at a frequency that resonates with the spin’s motion. The particular frequency needed to tip the electron spin depends on the effective magnetic field at the electron, which is in turn determined by its chemical surroundings. Thus, the electrons that participate in the light reaction of photosynthesis can be made to serve as sensitive internal magnetic probes of the reaction center if we observe the frequency of the radiation they absorb.

The measurement of the spectrum of frequencies absorbed by electron spins is known as electron magnetic resonance, or EMR, spectroscopy. It is directly analogous to the magnetic-resonance method used in medical imaging (which, however, relies upon the nuclear spin property of such elements as hydrogen, carbon, and phosphorus). The reaction center has been studied by EMR for well over two decades. The major innovation at Northeastern has been to boost the frequency used to excite the spins over twentyfold compared to conventional EMR. The advantage of high frequency is that one obtains a better resolution of spectra from electrons that are in slightly different chemical environments, providing a powerful new means of “fingerprinting” reactive chemical species that contain an unpaired electron.

Such high frequencies require the application of correspondingly high magnetic fields, which must be generated by superconducting magnets. Our group has been generously funded by the Biophysics Program at the National Science Foundation to construct a unique high-field EMR spectrometer (see photo, page 3) and apply it to study the detailed interactions between the light reactants and the reaction center protein.

We are examining the unpaired electrons formed during the primary light reaction to characterize the details of their interaction with the protein, with the ultimate goal of understanding how the protein shapes the light reaction of photosynthesis. The high resolution of our spectrometer will reveal subtle new details about the symmetries of the various working states in photosynthesis, which may themselves shed light on nature’s marvelous engineering of the reaction center.

An alternative approach, which we will be carrying out in collaboration with Professor Colin Wraight at the University of Illinois, is to use site-directed mutagenesis to introduce an electron “spin-label” into selected locations on the reaction center protein. This approach, combined with the sensitivity of high-field EMR, will allow us to build an “electrostatic map” of the reaction center that will enable us to determine the forces that influence the electron’s path during the light reaction. Such a detailed understanding of the reaction center will ultimately lead to new technologies for designing efficient organic-based optical and electronic devices on a molecular level.

Although the instrument at Northeastern is currently one of only a handful of custom-built high-frequency EMR spectrometers in the world, the field is growing rapidly, and commercialization of the method is not far off. The advent of high-frequency methods will usher in a new era in EMR, in much the same way that extension to high fields revolutionized the sister field of nuclear magnetic resonance (NMR).

I feel particularly fortunate to be situated in the Egan Center, which affords our group the opportunity to promote the general utility of high-field EMR by exploring areas outside our own expertise. One such area is materials science. In collaboration with our new colleagues, we have very recently undertaken high-field EMR studies to elucidate the radical reactions that form carbon composites for aerospace and medical applications. This knowledge can then be used to tailor the chemistry to improve the mechanical strength and heat resistance of carbon composite materials. The fertile ground provided by the Egan Center for future interdisciplinary research initiatives makes it a truly exciting time in which to do research at Northeastern.

David Budil is an assistant professor of chemistry. He received his B.S. from Yale University and his Ph.D. from the University of Chicago. He was the recipient of a CAREER Award from the National Science Foundation, which provides the funding for his research.