Editorial pubs.acs.org/JPCL
The Dream Microscope for a Biophysical Chemist
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works, from harvesting sunlight to producing charge pairs to stepping though the sequence of chemical processes that split water. When one gets down to details, this is an extremely complicated problem. The Perspectives published in this issue evoke speculations of the dream experiment to relate structure to function. The great challenge is that it is not sufficient to watch atoms move during photosynthesis, even if that could be done in three dimensions at high resolution. We also need to gather maps of subtle changes in electron distribution and excitation energy. However, still we do not have enough detail to elucidate the mechanism! We need to have ways of correlating processes, for example, dielectric fluctuations of the environment to excitation, charge, or proton transfer. We would certainly need ways of mapping dynamical changes that reveal how quantum probability laws correct those of classical physics. Of course, no single experiment will expose all these details, but this speculation gives us a sense of the kinds of things that a physical chemist’s microscope would need to “image”.
he most basic light-initiated reactions are electronic energy transfer, electron transfer, and proton transfer. Despite their apparent simplicity, deep understanding of these processes has, and continues to, challenge physical chemists. There are a great many examples of chemical systems where light-initiated dynamics are important, and similarly, much work has been directed to biological systems. Counterintuitively, many biological examples serve as wonderful model systems1 because rather than being messy and illdefined, high-resolution structures have been elucidated that show the precise organization of the chemical ingredients involved. Three examples of such research are presented in this issue of the The Journal of Physical Chemistry Letters.2−4 The common theme of these articles is photosynthesis, nature’s solar cell that not only provides the foundation of most food chains on the planet but also oxygenates our atmosphere in the process. Nature is powered by the cleanest fuels possible, water, carbon dioxide, and sunlight. The Perspective from Strümpfer et al.2 shows how much we now know about photosynthetic light harvesting. It is explained, using purple photosynthetic bacteria as an example, that function is specialized in various proteins (light-harvesting complexes and reaction centers), and the workings of each of these components has been studied in detail. It is further shown how elucidating the cooperation of these units further enriches our understanding of the overall photosynthetic machinery involved in the “light reactions”. The molecular-level view of photosynthesis is a truly remarkable achievement for the field, considering where it started. Stokes, for example, was puzzled that mustard seedlings grew well in red light, but the growth was less vigorous under blue and violet illumination, those wavelengths “which act so powerfully on most photographic preparations”.5 Eventually, as an understanding of molecular spectroscopy developed, Stokes’s question was resolved. Later, one critical realization was that most of the chlorophyll in a photosynthetic unit was not used to produce separated charge pairs; it was employed simply to harvest light. A breakthrough in the modern era was the discovery of the crystal structure of the primary protein involved in this light harvesting for purple bacteria at atomic resolution.6 Cogdell’s work not only revealed the exquisite ring structure of the complex, but it stimulated an important partnership among theory and much more incisive experimental studies that enabled detailed structure−function relationships to be discovered. The other Perspectives featured in this issue highlight some of the phenomena being investigated in complexes central to photosynthesis in oxygen-evolving organisms. This photosystem II (PSII) enzyme is truly remarkable in its proficiency at splitting water.7 PSII has long been a landmark in photosynthesis, owing to this central role it plays in solar fuel production. Again, determination of the three-dimensional structure of PSII at atomic resolution8 heralded a breakthrough. What is shown in the Perspectives of Lewis et al.3 and Barry et al.4 is that there still remains the challenge of explaining how the PSII structure © 2012 American Chemical Society
Gregory D. Scholes, Senior Editor
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Department of Chemistry, Institute for Optical Sciences, and Centre for Quantum Information and Quantum Control, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
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REFERENCES
(1) Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; van Grondelle, R. Lessons from Nature about Solar Light Harvesting. Nat. Chem. 2011, 3, 763−774. (2) Strümpfer, J.; Sener, M.; Schulten, K. How Quantum Coherence Assists Photosynthetic Light Harvesting. J. Phys. Chem. Lett. 2012, 4, 536−542. (3) Lewis, K. L. M.; Ogilvie, J. P. Probing Photosynthetic Energy and Charge Transfer with Two-Dimensional Electronic Spectroscopy. J. Phys. Chem. Lett. 2012, 4, 503−510. (4) Barry, B. A.; Chen, J.; Keough, J.; Jenson, D.; Offenbacher, A.; Pagba, C. Proton-Coupled Electron Transfer and Redox-Active Tyrosines: Structure and Function of the Tyrosyl Radicals in Ribonucleotide Reductase and Photosystem II. J. Phys. Chem. Lett. 2012, 4, 543−554. (5) Stokes, G. G. On Light; MacMillan and Company: London, 1887. (6) McDermott, G.; et al. Crystal Structure of an Integral Membrane Light-Harvesting Complex from Photosynthetic Bacteria. Nature 1995, 374, 517−521. (7) Moore, G. F.; Brudvig, G. W. Energy Conversion in Photosynthesis: A Paradigm for Solar Fuel Production. Annu. Rev. Condens. Matter Phys. 2011, 2, 303−327. (8) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Architecture of the Photosynthetic Oxygen-Evolving Center. Science 2004, 303, 1831−1838.
Published: February 16, 2012 555
dx.doi.org/10.1021/jz300101u | J. Phys. Chem. Lett. 2012, 3, 555−555
