Richard Willstätter unified thought concerning the material responsible for photosynthesis in plants and showed that this activity is the result of two basic chlorophyll molecules.
When Richard Willstätter fled from the Gestapo in Nazi Germany to Switzerland in 1939, one of the few items he took with him was his highly cherished 1915 Nobel Prize certificate honoring him for chemical research.
In hindsight, it is clear that Willstätter’s life as a chemist prior to this work was very direct preparation for the success he enjoyed. As a doctoral student under Adolf von Baeyer and Alfred Einhorn, Willstätter learned and practiced the techniques then available for structural characterization by working on cocaine, tropine, and other molecules extracted from plants. The general method available at the time involved a series of chemical reactions in which compounds of known composition were reacted with the original material (or a material derived previously from the original), with the goal of degrading the original into smaller fragments. These reactions were followed by tedious purification of the products, often formed in very low yields, until a simple molecule of known structure resulted.
Once these derivatives were identified, the researcher had to follow the series of reactions backward to determine the structure of the starting material that could be responsible for the observed results. For the most part, the reconstructed trail was neither direct nor clear, and the researcher had to rule out many sideline possibilities by performing further reactions before a definitive answer could be found. The answers were also very open to challenge and, unless the researcher exercised utmost care at each step of the way, could be easily refuted. The heart of Willstätter’s research for the remainder of his active career consisted of modifying, adapting, and refining these basic tools so as to establish a clearer trail leading to the structure of molecules.
After completing research for his doctoral degree, Willstätter continued in the area of organic structure research but shifted to applying the degradation and derivatization techniques to synthetic quinones, which formed the chemical basis of the German dye industry. Upon accepting a position as full professor at the Federal Institute of Technology in Zurich in 1905, Willstätter inaugurated a new research effort directed at understanding how photosynthesis (the conversion of light energy and inorganic material into plant tissue) occurs. The first step in this major research effort was the study of chlorophyll. Willstätter produced twenty-four journal articles in conjunction with his students as well as a book (now a classic), Investigations on Chlorophyll: Methods and Results, written with the collaboration of Arthur Stoll, another student.
At the time Willstätter began studying photosynthesis, very little was known about the source of color in plants; however, the generally accepted view was that each variation of green observed was the expression of a unique chemical molecule. Therefore, it was thought that a vast multitude of molecules must be responsible. These were, as a group, referred to as chlorophylls and were postulated to be the sites at which photosynthesis occurs.
Willstätter worked in an area where there was scant guidance from earlier research. Building on the meager knowledge available, he created techniques to extract plant material and to recover, unchanged, the constituent chemical substances. The raw material for developing these processes was the dried leaves of plants. With the raw extract in hand, he worked to invent, improve, and characterize methods that enabled him to separate and to purify the chemical substances resulting from the extractions. Some of these methods involved the use of enzymes, a then poorly understood class of biological catalysts; others used the technique of adsorption chromatography,
Of particular interest is Willstätter’s finding that, rather than the myriad forms previously projected, there are but two chlorophylls: a blue-green, or alpha, form and a yellow-green, or beta, form. All the hues of green observed result from only these two molecules. Willstätter demonstrated this simplicity in the presence of diversity by extracting and analyzing the green parts of more than two hundred species of plants. He found only the two types of chlorophyll. Further, he was able to show that the two forms always existed in a near constant ratio to each other, regardless of the plant source.
The extractive fractionation methods that Willstätter used can be described as depending on the selective solubility of the various plant components in different solvents at different acidities. For illustration, consider the final step in the separation of chlorophyll. This separation resulted when an alcohol solution containing the refined plant extract was treated with petroleum ether. Pure chlorophyll is not soluble in the petroleum ether, and it crystallized from the solution in good yield. Other components remained in either the alcohol solution or the petroleum ether solution.
Willstätter used the intensity of color in the various solutions as a means of following the course of the separations and also as a means of assessing the purity of the fractions. At this point in the purification, the chlorophyll was not a single substance, but rather a mixture of the two chlorophyll forms. One investigation began with 2 kilograms (4.4 pounds) of dried leaves and, following nearly a dozen extractive steps using a variety of solvents, produced slightly less than 1 gram (less than 0.03 ounce) of a mixture of the two chlorophylls. Further extraction using methanol and a low-molecular-weight hydrocarbon produced the separation of the two, with the alpha form being more soluble in the hydrocarbon and the beta in the alcohol.
Richard Willstätter.
The culmination of Willstätter’s work was the establishment of the chemical composition of these two forms of chlorophyll. Magnesium, previously postulated by others to be an impurity, was shown to be an integral part of the molecule. Phosphorus, thought by others to be a part of the molecule, was shown to be an impurity. The two chlorophylls were shown to be magnesium complexes of dicarboxylic acids esterified with methanol and a new unsaturated alcohol, named phytol. Each chlorophyll form is a very large molecule. The alpha form contains fifty-five carbon atoms, seventy-two hydrogen atoms, four nitrogen atoms, five oxygen atoms, and a magnesium atom. The beta form differs by having one fewer oxygen atom and two fewer hydrogen atoms.
Willstätter also established the similarity of these molecules to hemoglobin, with the magnesium of chlorophyll and the iron of hemoglobin playing similar structural roles. This similarity had been intuitively conceived as early as 1851, but Willstätter’s work was the earliest to establish the idea as fact.
Willstätter’s research may be seen as the beginning of the understanding of the process by which simple sugars aresynthesized from carbon dioxide and water through utilization of the energy of sunlight. This is the process known as photosynthesis. Willstätter’s contribution also marks the point from which knowledge of the structure of large, biologically important molecules stems. The importance of this series of experiments is illustrated by the fact that several Nobel Prizes in Chemistry have been awarded for contributions depending directly on Willstätter’s earlier work. The 1930 award was given to Hans Fischer
Recognition of the uniqueness of the chlorophyll molecule, its composition, and the role it plays in the photosynthetic process has had significant impact on the human condition. Willstätter’s work, and the extensions made from it by others, has allowed an understanding of plant growth that has guided agriculturists in their search for greater plant productivity. Of most direct impact has been the understanding of the important role of magnesium as a plant nutrient. As an integral part of the chlorophyll molecule, magnesium must be readily available to any growing plant. This means that this essential element, along with others, must be supplied by fertilization in soils that show a magnesium deficiency. Understanding this connection has allowed successful crop production on land previously thought to be too poor for farming and, in consequence, has led to an increase in food available for the world’s increasing population.
The processes of photosynthesis and cellular respiration are complementary: Oxygen released into the atmosphere, a by-product of photosynthesis, is breathed in by animals, which in turn breathe out carbon dioxide, the gas that is essential for photosynthesis.
Although Willstätter is most recognized for research on chlorophyll, his contributions to the definition of the role of teacher and investigator should not go unmentioned. Each of his writings on chlorophyll was accomplished with a student as a collaborator. Willstätter was a member of a small group of chemists who pioneered the field of organic chemistry by breaking new ground and by devising new methods of experimentation. Because of this effort, Willstätter is often referred to as one of the fathers of organic chemistry.
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Armstrong, Henry E. “Scientific Worthies: XLV. Richard Willstätter.” Nature 120 (July 2, 1927): 1-5. One of a series of articles on the contributions and lives of scientists written by one who knew Willstätter personally and who conducted research in the same field. Summarizes Willstätter’s work on chlorophyll and other pigments as well as his life and scientific background. Provides a philosophical look at the impact of Willstätter and his work on the chemistry of the day. -
Hall, D. O., and K. K. Rao, Photosynthesis. 6th ed. New York: Cambridge University Press, 1999. Concise presentation of the process of photosynthesis aimed at undergraduate students in all fields of the biological sciences. Discusses the details of photosynthetic processes and also addresses the role of photosynthesis in food production and the global environment. Includes numerous line illustrations and color plates. -
Huisgen, Rolf. “Richard Willstätter.” Journal of Chemical Education 38 (January, 1961): 10-15. A biography emphasizing Willstätter’s chemical career. Sections deal with his graduate school research on alkaloids, his early work on quinones, his work with a variety of plant pigments and with photosynthesis, and his late work on enzymes. The chemistry presented is written at a level accessible to an interested layperson. -
Rabinowitch, Eugene, and Govindjee. Photosynthesis. New York: John Wiley & Sons, 1969. Provides a broad introduction to the physicochemical mechanism of the primary process in photosynthesis and the enzymatic mechanisms closely related to it. Also discussed is the role of photosynthesis in nature. Shows the place that Willstätter’s work holds in modern understanding of the subject. Suitable for interested readers with a high school background in science. -
Robinson, Robert. “Willstätter Memorial Lecture.” Journal of the Chemical Society (1953): 999-1026. Address presented to the Chemical Society in London on the tenth anniversary of Willstätter’s death. The chemistry included may be too complex for most laypersons, but the address also deals with Willstätter’s connections to others in the scientific community and presents several reminiscences that allow an understanding of the style of the man as a citizen and as a scientist. -
Willstätter, Richard. From My Life: The Memoirs of Richard Willstätter. Translated by Lilli S. Hornig. New York: W. A. Benjamin, 1965. Arthur Stoll collected partially completed notes that Willstätter had prepared for a biography and published them after Willstätter’s death. Presents a clear look at Willstätter’s scientific life and his life as a Jew in Germany during the early part of the twentieth century.
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