Eschenmoser Lecture 2023

May 8, 2023
«Exploring the Chemistry of Vitamin B12 Biology»

Prof. Dr. Bernhard Kräutler studied Chemistry at the ETH Zürich from 1966-​1970, completing his Diploma, with distinction, on «Der 17O-​Hyperfeintensor im ESR-​Spektrum von Kupfer(II)-​oxinat im Einzelkristall». He subsequently joined the group of Prof. Albert Eschenmoser, in which he completed his dissertation 1976, titled, «An electrochemical simulation of the light-​induced isomerization of A/D seco-​corrins to corrins».

As may be evident from the early work, Prof. Kräutler’s interests were already focusing on areas in which he would make long-​term and enduring contributions. In particular, the doctoral research project investigated an isomerization within the context of the vitamin B12 synthesis that had proceeded in the Eschenmoser group, but with electrochemical rather than photochemical initiation. The particular transformation did not occur in vivo, but the investigation nevertheless revealed an underlying relationship between photoexcitation and redox chemistry which foreshadows a number of subsequent developments in organic and organometallic chemistry.

With the award of his doctorate, and the granting of an SNF Postdoctoral Fellowship, Prof. Kräutler decamped to the United States, to Prof. Allen Bard at the UT, Austin, and then a year later, to Prof. Nicholas Turro at Columbia University. His studies on photoelectrochemistry, resulting in the photocatalytic Kolbe decarboxylation, an interesting reaction itself, but further, an assay for the synthesis and optimization of perovskite photocatalysts, and magnetic isotope effects, which appear in reactions in which an intersystem crossing step from a triplet to a singlet diradical or radical pair is kinetically significant for the overall reaction, demonstrated his careful mechanistic approach to chemical problems whose relevance has only increased.

Returning to the ETH in 1979, Prof. Kräutler initiated his independent research program, habilitating in 1985. His mechanistically-​driven approach generated already at this stage of his career a diversity of potential projects whose variety belied their common underlying physics and chemistry.

In 1991, Prof. Kräutler was called as Professor of Organic Chemistry to the University of Innsbruck, the position from which he formally retired in 2015, having served as Head of the Institute and Dean of the Faculty of Chemistry and Pharmacy. In this long period, Prof. Kräutler devoted himself to the chemistry of tetrapyrroles, specifically vitamin B12 plus derivatives, and chlorophyll and the products of chlorophyll catabolism, as well as some chemistry of fullerenes. He established himself as a leading expert on the reaction mechanisms of B12-​dependent enzymes in both radical and polar reactions. He showed that B12 interacts with RNA as a «reverse riboswitch», which implies fascinating, if not yet observed, biological function.

In recent years, Prof. Kräutler has prepared «anti-​vitamin» analogs of vitamin B12, with aryl or alkynyl functions in place of the adenosyl moiety, or analogs in which Rh and Ir replace the native Co. Prof. Kräutler investigated the mechanism for degradation of chlorophyll, being the first to identify the breakdown products, the phyllobilins, linear tetrapyrroles, and the cascade from fluorescent chlorophyll catabolites (FCCs) to non-​fluorescent derivatives (NCCs). The FCCs are responsible for the most curious observation that, unlike green bananas, ripe bananas fluoresce bright blue, the transformation of FCCs to NCCs having been blocked by esterification of a specific proprionic acid sidechain. One may speculate on the biological significance of the blue fluorescence.

In the various areas in which he has worked, Prof. Kräutler combines preparative methods with exacting analytical methods, always with an eye on the relationship of structure to function, whether that function be in chemical selectivity, materials, or biological transformations. One can regard his work as standing squarely in the center of modern Organic Chemistry, and rightly honored by the Eschenmoser Lectureship.

In the course of his career, Prof. Kräutler has received numerous awards. In his time at the ETH, aside from the Silver Medal for his dissertation, he received the Werner Award from the Swiss Chemical Society. In Austria, he received the Ernst-​Späh Award, the Erwin Schrödinger Award, and the Joseph Loschmidt Medal. A particular honor was the «Österreichisches Ehrenkreuz für Wissenschaft und Kunst, 1. Klasse». Subsequent to his retirement, Prof. Kräutler continues his research program in Innsbruck as Professor Emeritus.

All photos shown in the gallery below by Marius Lutz | LOC