Our mentor and friend Professor Dr. Klaus-Dieter Asmus died on Sunday, October 7, 2012, as a consequence of two heart attacks within one week during his vacation in Brittany, France. His sudden and unexpected death was a great shock for his family and friends.
Dieter, as he preferred to be called, was born on December 13th, 1937, in Breslau (which today is Wrocław). In 1938, his family moved to Marburg, where Dieter completed most of his elementary and high school education. Another move brought the family to Münster, where Dieter obtained his high school degree in 1957. Early on, Dieter developed an interest in the natural sciences; he also enjoyed competing in soccer and supporting his local soccer team, the VFL 1860 Marburg, enjoyed dancing and classical Russian music of the 19th century.
Dieter studied at the Technical University in Berlin, and obtained a degree as “Diplom-Ingenieur”, the equivalent of a master of science in engineering, from the Faculty of Engineering Sciences, Department of Chemistry, in 1963. He obtained the Silver Medal for “Excellence in Studies” from the Technical University Berlin in 1963. Dieter continued with his doctoral studies under Professor Armin Henglein and was awarded a Ph.D. degree in 1965 for his thesis, titled “The Investigation of Fast Reactions of Tetranitromethane with Radiation Chemically Generated Free Hydrated Electrons, Ions, and Radicals in Aqueous Solution” (“Die Untersuchung schneller Reaktionen des Tetranitromethans mit strahlenchemisch erzeugten freien hydratisierten Elektronen, Ionen und Radikalen in wäßriger Lösung”). During his entire study, Dieter was the recipient of a fellowship of the “German National Academic Foundation” (“Studienstiftung des Deutschen Volkes”).
In 1965, Dieter accepted his first position at the Hahn-Meitner Institute Berlin as a Research Assistant. Valuable experience in radiation chemistry was also obtained through working with Michael P. Ebert, John Keene, and colleagues at the Christie Hospital and Holt Radium Institute in Manchester, England. Between 1966 and 1968, he joined Carnegie Mellon University in Pittsburg, Pennsylvania, for postdoctoral work under Professor Robert H. Schuler. This fruitful period resulted in six publications, among which were papers describing the well-known “Warman-Asmus-Schuler (WAS) equation.” The WAS equation describes the scavenging of radiolytically generated electrons by appropriate scavengers in organic solutions of saturated hydrocarbons. Dieter returned to the Hahn-Meitner Institute Berlin as a Research Scientist in 1969, and in 1970 he defended his habilitation in physical chemistry at the Technical University Berlin with a thesis, titled “Nonhomogenous and Homogenous Kinetics of Ionic and Radical Reactions in γ-Irradiated Cyclohexane” (“Inhomogene und homogene Kinetik ionischer und radikalischer Reaktionen in γ-bestrahltem Cyclohexan”).
Dieter continued his work at the Hahn-Meitner Institute Berlin until 1995, where he led his research group but also accepted administrative responsibilities as the Chair of the Radiation Chemistry Department (1977–1981). For his achievements and scholarship, he received Research Awards by the Hahn-Meitner Institute in 1989 and 1991. During his tenure at the Hahn-Meitner Institute, Dieter trained 10 master's students and 20 Ph.D. students, and hosted numerous postdoctoral students and visiting scientists. Over these years, he was loyally supported by his long-time technical assistant Ms. Eva Darnstädt. Between 1974 and 1995, Dieter also held the position of Professor of Physical Chemistry at the Technical University Berlin. In 1995, Dieter became Director of the Radiation Laboratory at the University of Notre Dame, Indiana. He held this position until 1997, and continued as a Professor at the University of Notre Dame until 2005, when he became Emeritus Professor and returned to Berlin.
Dieter's research interests were broad, with a central focus on the analysis of free radical oxidation and reduction reactions by radiation and photochemical methods. He was passionate about the application of pulse- and γ-radiolysis methods to the characterization of free radical reactions of biological relevance, especially to the then emerging field of “oxidative stress”. The field of “oxidative stress” had received an important stimulus through the discovery of the enzyme superoxide dismutase, and research groups throughout the world tested the hypothesis that free radicals might be important contributors to pathologies and aging.
In 1973, the Hahn-Meitner Institute´s early studies on the reduction of nitro- and halogen-containing hydrocarbons brought Dieter into contact with Robin L. Willson, who had just moved to the Brunel University Biochemistry Department (London) from the nearby Gray Laboratory. At the Gray Laboratory, Robin had been working on the pulse radiolysis of electron transfer reactions particularly in relation to radio-sensitizing, nitro-containing drugs and DNA constituents. At Brunel, the Head of Biochemistry, Professor Trevor F. Slater had a long standing interest in the liver toxicity of carbon tetrachloride (CCl4) and the possible role of free radical reactions. The drug promethazine had been shown to afford some protection, and the involvement of the trichloromethyl radical (•CCl3) had been proposed. During his last days at the Gray Laboratory, Robin had undertaken some initial studies in possible free radical-related actions of phenothiazine and had tentative evidence that the drug reacted rapidly with the radical and also might repair DNA by reacting with a radiation-induced guanine radical-cation. However, it would be another three years before the Brunel pulse radiolysis facility would be in operation. As a result of Dieter´s highly relevant chemical experience, the Hahn-Meitner Institute´s excellent pulse radiolysis conductivity analysis equipment, and Brunel´s access to biochemistry facilities and biological experience, working together was obvious. A formal collaboration was soon established. Every year qualified students of both institutions had the opportunity to participate in an exchange program with the goal to complete a research project abroad, either in London or Berlin. Studies on a series of nitroxide compounds related to the radiosensitizer, triacetoneamine-n-oxyl, and studies in collaboration with Peter Wardman at the Gray Laboratory on the guanine radical-cation and promethazine soon followed.
One of the many important achievements of this collaboration (with important contributions also from a collaboration with Professor John Packer from the University of Auckland, New Zealand) was a careful characterization of the reductive free radical degradation of carbon tetrachloride. An important milestone was the discovery that the resulting trichloromethyl radicals generated a powerful oxidant upon reaction with molecular oxygen, the trichloromethylperoxyl radical (CCl3OO•). Following the initial studies on carbon tetrachloride, Dieter, his students, and collaborators soon ventured into more complex molecules, targeting oxidative and reductive free radical reactions of halogenated anesthetics such as halothane, isoflurane, and enflurane (a few years later, these studies were expanded to the investigation of the photochemical degradation of halogenated compounds on semiconductor surfaces). Brunel University and the Hahn-Meitner Institute soon established biennial symposia, either in London or in Berlin, which gave all interested students and faculty of these institutions the opportunity for international exchange and discussion of research findings and new ideas. These symposia are fondly remembered by many participants not only for their scientific content but also the cultural and culinary programs, which, when in Berlin, included a dinner reception at Dieter's house, cheerfully hosted by Dieter and his wife, Sybille. Dieter himself participated in the formal exchange with Brunel University, spending a sabbatical in London as a visiting professor in 1975.
In 1982, the Society For Free Radical Research (SFRR) was founded in the UK and the first biennial meeting organized at Brunel University. Dieter played a vital role in the formation of the SFRR, and, right from the beginning, was a strong advocate for the SFRR being an interdisciplinary and international society. Dieter served the society as a committee member (1982–1984, 1988–1992), as treasurer (1988–1992), and as a member of the Presidential Advisory Board.
In 1974, Dieter published his first paper on the free radical oxidation of sulfur-containing organic molecules. This paper set the stage for a life-long interest in sulfur-centered reactive intermediates. The combination of UV- and conductivity-detection with pulse radiolysis allowed for the characterization of short-lived radical cations and their stabilization by two-center, three-electron bond formation with heteroatoms. Dieter developed a passion for the thermodynamic, kinetic, and theoretical characterization of such three-electron bonds, represented by a “∴” sign between the connected atoms (e.g., [> S∴S<]+ for a three-electron bond between a one-electron oxidized and a non-oxidized dimethyl sulfide). It was realized that three-electron bonds are not restricted to radical cations, but also form between neutral radicals and anions, e.g. in the reaction between thiyl radicals and thiolate. Later, Dieter would also look into three-electron bond formation of radical ions not only derived from organic compounds containing sulfur, but also of molecules containing nitrogen, oxygen, selenium, phosphorus, and halogens. For example, through a collaboration with Professor Stephen F. Nelsen of the University of Wisconsin, Dieter had the opportunity to study three-electron bonded intermediates generated from hydrazines.
In 1981, Dieter published his first paper on three-electron bonded intermediates in the biologically significant one-electron oxidation of the amino acid methionine. Synthetic organic sulfides were now required to study geometrical prerequisites of three-electron bond formation, and a new collaboration was established between Dieter and Professors George S. Wilson and Richard S. Glass of the University of Arizona, Tucson. Since 1984, several seminal publications originated from this collaboration, specifically involving three-electron bond formation in a conformationally restricted methionine analogue embedded within a rigid norbornane frame. In the fall of 1986, Dieter spent a sabbatical as visiting professor at the University of Arizona. Every one of Dieter's students (and probably Dieter, too, even though his office was located a few doors away) remembers the distinct odors associated with the synthesis and handling of some of these organic sulfides. On several occasions, some of the physicists of the Hahn-Meitner Institute, whose offices were directly affected by the ventilation of Dieter's laboratory hoods, went on “strike” playing cards in the canteen until the experiments were completed.
Dieter's interests in the oxidation chemistry of sulfur-containing organic molecules were shared by Professor Christopher S. Foote from the University of California in Los Angeles (UCLA), who studied singlet oxygen reactions. Dieter spent a summer as visiting professor at UCLA in 1980, and Professor Foote visited Dieter at the Hahn-Meitner Institute in 1986 (later, both continued their collaboration in another area, the chemistry of Buckminsterfullerene).
Dieter's international collaborations on three-electron bonded sulfur-containing intermediates were expanded through a visit of Dr. (now Professor) Krzysztof Bobrowski from the Institute for Nuclear Chemistry and Technology in Warsaw, Poland, to the Hahn-Meitner Institute in 1987. Dr. Bobrowski returned to Berlin every year between 1988 and 1991 to study three-electron bonded intermediates and decarboxylation reactions in methionine-containing peptides. Until the fall of the Berlin Wall in 1989, travel between Poland and Berlin (West) was sometimes difficult, with visa frequently issued only shortly before train departures, and on one occasion, Dr. Bobrowski arrived unexpectedly late and, after a 12-hour journey, was directly accompanied to Dieter's birthday dinner rather than to his hotel room. Dieter maintained his collaboration with Professor Bobrowski (which later also included Dr. Gordon L. Hug from the Radiation Laboratory at the University of Notre Dame and Professor Bronisław Marciniak from Adam Mickiewicz University in Poznań, Poland) until recently. He was instrumental as chairman in the inauguration and organization of the “European Young Investigator Conference” at Słubice, Poland, in 2003, a highly successful conference which has since continued and will be organized for the 6th time in 2013. In 2004, Dieter was awarded the Marie-Skłodowska-Curie Medal of the Polish Radiation Research Society. To celebrate Dieter's 60th birthday, the Faculty of Chemistry, Adam Mickiewicz University in Poznań and the Institute of Nuclear Chemistry and Technology in Warsaw co-organized a symposium in Poznań in 1998, titled “Reactive Intermediates in Sulfur Chemistry”. The proceedings were published in a special issue in Nukleonika, edited by Professors Marciniak and Bobrowski.
A highlight of the celebration of Dieter's interests in sulfur-centered reactive species was the co-organization (with Dr. Chryssostomos Chatgilialoglu from the National Research Council, Bologna, Italy) of a NATO-sponsored conference on Sulfur-Centered Reactive Intermediates in Chemistry and Biology in Maratea, Italy, in the summer of 1989. This two-week conference brought together experts in the organic, biologic, and physical chemistry of sulfur-centered reactive species and resulted in numerous scientific collaborations and a frequently cited book published in the NATO ASI Series in 1990. Besides the scientific program, a soccer game was organized between the conference attendants and the hotel staff. The scientists lost 0:3 and the excitement of the hotel staff about this victory resulted in an especially well-prepared dinner for the scientists on the evening of the game. Dieter maintained his collaboration with Dr. Chatgilialoglu and spent two months in 2003 as a visiting professor at the National Research Council in Bologna.
Dieter's election to chair the organization of the 2000 Gordon Research Conference on Radiation Chemistry represented another highlight in his career as a radiation chemist. During his entire tenure at the Hahn-Meitner Institute Berlin, Dieter fostered scientific interactions with other German institutions specializing in radiation chemistry, especially with the Max Planck Institute for Radiation Chemistry in Mülheim. These institutions organized important and well-attended biennial conferences in order to advance radiation chemistry research in Germany, and Dieter maintained very close personal contact especially with Professors Dietrich Schulte-Frohlinde and Clemens von Sonntag.
In the early 1990s, Dieter developed an interest in the redox chemistry of fullerenes and derivatives, and his first paper on these reactions was published in 1991. He continued to study these reactions after he moved to the Radiation Laboratory at the University of Notre Dame in 1995. An important result of this research was the demonstration that electron transfer reactions between fullerenes and arene radical cations occur in the “Marcus-inverted region”, i.e. a region where increasing exergonicity is associated with a deceleration of the reaction kinetics. During his tenure at the University of Notre Dame, Dieter also rediscovered his interest in the oxidation of simple amino acids (glycine and glycine derivatives), resulting in a series of important papers with his collaborators Dr. Marija Bonifačić, Dr. Gordon L. Hug, Professor Bronisław Marciniak, and the late Professor David A. Armstrong. After his return to Europe in 2005, Dieter continued his scientific work through lectures at Adam Mickiewicz University in Poznań and courses at Collegium Polonicum in Słubice (a branch campus of Adam Mickiewicz University), which resulted in the publication of a trilingual (Polish, English, German) textbook, entitled “Pollution and Environmental Protection. Chemical Aspects and Related Considerations”. He also formally supervised graduate students and postdocs in the laboratory of Professor Marciniak after Professor Marciniak's election as Rector of Adam Mickiewicz University.
Dieter always showed a keen interest in the development of students and people. His office was always open, and students, postdocs, and staff could ask Dieter for advice at any time. He tremendously enjoyed personal interactions, and his students, postdocs, collaborators, and staff fondly remember the traditional lunch meetings at the Hahn-Meitner Institute, especially the annual cheese fondue. These personal interactions created a perfect environment not only for people to grow, but also for many excellent scientific findings.
Dieter welcomed every member of his group as if he/she was part of his family. In fact, many of Dieter's group members and collaborators became very close friends of his family and spent many memorable hours in Dieter's house, either enjoying a superb meal prepared by Sybille, watching an important soccer match, or playing pool. Dieter was a very optimistic person; there was always a good side to any occasion, and frequently unforeseen events were accepted with a good sense of humor. While in the hospital in France after his first heart attack, Dieter prepared his slides for a lecture he planned to present at the upcoming Miller Conference on Radiation Chemistry in Israel (March 2013). When the hospital invoice arrived before his release from the clinic, he remarked that “despite the significant cost, he had not been served croissants for breakfast...” Dieter will be dearly missed by his friends, students, colleagues, and collaborators and all of us will do our best to carry forward his legacy.