Schulten also weighed in on why the team of Ritz and Damjanović made remarkable achievements. Xiche Hu's work on the light-harvesting protein was the first time in Schulten's group that someone had worked on structural biology. “This structure determination was very new for us; but particularly relative to the structural biology, we were really excellent quantum biologists,” notes Schulten. This second hat that the team wore, of quantum physicists, was a natural one for Schulten's group to embrace since they could draw on years of experience, and this resulted in explanation of the quantum physics behind light harvesting in photosynthesis in the near-record time of about three years.
This period of time in Schulten's group, from about 1995 to 2000, still stands out as one of the most impressive eras of achievement in Schulten's mind. It started with Xiche Hu and his relentless efforts to solve the structure of LH2, and then ended with Thorsten Ritz and Ana Damjanović. The pair basically sorted out the key physical characteristics that nature had designed to effectively absorb sunlight, to keep it for a short while and transfer it effectively to other pigment subsystems in the so-called photosynthetic light harvesting system.
Although Schulten published his first paper ever on photosynthesis in 1978 while at the Max Planck Institute, his next foray into the field would come only after a ten-year hiatus. But the serendipitous opportunity that presented itself a decade later would prove a treasure trove for a computational biophysicist like Schulten. The reason for the hiatus was because Schulten realized that for his theoretical purposes, the data was just not therethe structure of the reaction center was still unknown. “One could have done more,” says Schulten, “but it would have always been questioned.”
While the accomplishments of Förster and his famous equation came to Schulten naturally through Weller's lineage to Förster, Schulten is quick to point out that Förster was not the only one in the 1940s to wonder at the mechanism of excitation transfer between pigments in photosynthesis. The famous physicist Robert Oppenheimer, known for his work on the Manhattan Project during World War II, published in the minutes of the American Physical Society a brief abstract in 1941 entitled “Internal Conversion in Photosynthesis,” right before he got caught up with the war effort. In the abstract he addresses excitation transfer from certain pigments (namely, fluorescent dyes) to chlorophylls in photosynthetic algae, suggesting the transfer is a large-scale model of internal conversion of gamma rays. After the war, in a 1950 paper with William Arnold, the pair elaborated on the mechanism of excitation transfer between chlorophyll and phycocyanin, a pigment used as a dye. William Arnold had a background in physics and biology and had worked on experimental aspects of photosynthesis since the 1930s. Arnold had done some work to show that light absorbed by the pigment phycocyanin was transferred to chlorophylls instead of directly reducing carbon dioxide. When he told Oppenheimer about this finding, they set out to explain the mechanism of excitation transfer. While the pair discussed this idea of excitation transfer as internal conversion sometime around 1939 or 1940, they did not get around to publishing this explicitly until 1950, according to Arnold's autobiographical article.
There were many reasons why Schulten already knew enough about electron transfer in photosynthesis to suggest using the magnetic field effect as a yardstick in his 1978 paper. The electron transfer in photosynthesis occurred in a protein called the photosynthetic reaction center, which may be thought of as the heart of the photosynthetic unit. When plants and bacteria absorb energy from the sun, this energy is used by the reaction center to transfer an electron. The workings of the reaction center were not fully understood because the structure of this protein was not known in the late 1970s when Schulten was in Göttingen. “So people actually did very well,” recalls Schulten, “to conclude, from certain optical properties and from electron transfer rates, what the structure looked like. So it was amazingly good actually, but it was not firm.”
In the same year that he jump-started his return to photosynthesis by publishing computational papers that utilized the new reaction center structure, Schulten took a job at the University of Illinois at Urbana-Champaign. Soon after that, in 1989, Schulten founded the Theoretical Biophysics Group there at the Beckman Institute for Advanced Science and Technology, an interdisciplinary center at the university. Eventually the group at Beckman became known as the Theoretical and Computational Biophysics Group. It is through this group that Schulten and collaborators were able to start to piece together the many steps in photosynthesis and begin to understand it on a grand scale
In 1980 Schulten's formative time at the Max Planck Institute came to an end, as permanent positions there were few. He took a job 245 miles away in Bavaria, as a professor in the physics department of the Technical University of Munich. He was still eight years away from revisiting photosynthesis in his own work, but as the decade wore on he had a front row seat to some cutting edge research that would eventually bring him back to this topic.
During his time in Munich, Schulten got to know Hartmut Michel and Hans Deisenhofer, and was always eager to hear about their progress on the reaction center over the course of the decade. In fact, Schulten soon realized that with the elucidation of the structure of a reaction center, many new calculations could be done that would have otherwise been impossible before the structure was solved. The reaction center solution had two major effects on Schulten's professional life. First, he returned to the field of photosynthesis, which had fascinated him from boyhood through graduate school. While he briefly touched on this topic in Göttingen, there he also learned about many of the major players at the forefront of photosynthesis research. He was, therefore, in a prime position by 1987 to begin his own research on photosynthesis when his colleagues finally determined the reaction center structure. And the second major effect the structure determination had on him involved his long-term goals for working with large biomolecules. Since the size of the reaction center protein was so big, Schulten initiated an audacious plan to acquire a computer powerful enough to handle such a massive protein. While the building of a homemade parallel supercomputer was ultimately influenced by solution of the reaction center, that story is told in detail elsewhere (see this for a history of NAMD).Suffice it to say that it led to a software product that would help Schulten realize his dream of describing a living cell via its constituent molecules.
The research group of Professor Hiroaki Misawa and Assistant Professor Tomoya Oshikiri of the Research Institute for Electronic Science of Hokkaido University, by using a photoelectrode in which gold nanoparticles are loaded on an oxide semiconductor substrate, has worked to develop a method of artificial photosynthesis that has received attention as an ultimate light energy conversion system.
While he was studying for his diploma at the University of Frankfurt in Germany in the early 1990s, Thorsten Ritz heard Klaus Schulten give a seminar there about a possible mechanism for the magnetic compass in birds. Captivated by the talk, since it pointed out one of the ways in which quantum mechanics plays a role in biology, Ritz made sure to talk to Schulten while he was briefly doing research at the Center for Complex Systems Research in Urbana. Ritz decided to do dissertation research with Schulten in Urbana while getting his PhD from the University of Ulm. As Schulten did not have a candidate molecule yet for the avian magnetic compass, Ritz began his studies on photosynthesis instead.
One of the areas of study Karplus chose to pursue after his stay in Israel was the role of retinal in vision. Retinal is related to the carotenoids, for basically half a carotenoid molecule is vitamin A, and when oxidized vitamin A becomes retinal. Retinal also is one example of a polyene, a type of chemical compound with multiple carbon-carbon bonds. Polyenes play key roles in light absorption in vision as well as in photosynthesis. Studying polyenes would become a major focus of the Karplus group at Harvard.