Personal account: Heinz Falk’s contributions to geosciences

Abstract

Heinz Falk is best known for his contributions to organic chemistry, but actually, his scientific interests are much broader and include geology/paleontology among other disciplines (Fig. 1). During my diploma thesis (geology) on inorganic and organic pigments of fossils at the University of Heidelberg, I stumbled upon a paper of Heinz on color measurements of the pink Jurassic red alga Solenopora using simple diffuse reflectance UV–Vis spectroscopy, suggesting that the fossil pigments are organic compounds related to the so-called fringelite pigments (Fig. 2) [1]. The fringelites were discovered in fossil crinoids with distinct violet coloration from northern Switzerland and were described as hydroxylated phenanthroperylene quinones [2]. The work on the Solenopora pigments resulted from a cooperation with his friend Andreas E. Richter who asked him about the nature of the pigments causing the pink color of the fossils. A subsequent call in the journal “Fossilien”, directed to the fossil collector community, to look for further colored fossils from different geological periods had a very good feedback and led to many further indications for the occurrence of these fossil pigments [3]. The results of the survey performed by his PhD student Elisabeth Mayr were also published in “Fossilien” [4]. To apply the method of color measurement to my samples, I phoned him and asked for a reference sample of a fossil crinoid containing fringelite pigments. He was very friendly, and soon, I received such a sample (that he also donated to me, Fig. 2, left). In further studies on the fringelites, he looked for an explanation for the incredible stability of the fossil pigments that remained colorful after more than 150 million years. He found that fringelites can be stabilized by the formation of phenolates with bay hydroxy groups and the formation of peri chelate complexes [5]. Other studies on fossil material included the chemical investigation of droplet inclusions in amber that turned out to be water droplets [6]. For my PhD thesis on the topic of phenanthroperylene quinone pigments in fossil crinoids, he provided support by sending an educt required for the synthesis of a polycyclic aromatic hydrocarbon that was needed as a reference compound [7]. However, even more important, he provided help by numerous constructive discussions on the chemistry of quinone pigments and during the preparation of a joint publication. An essential result of the work obtained by liquid chromatography, complementary mass spectrometry, and comparison to reference compounds was that the main pigment of fossil crinoids of Jurassic and Triassic age is fringelite F (1) and a minor one is hypericin (2), the latter typically known from the medicinal plant St. John’s wort (Hypericum perforatum). With the finding of hypericin and the reported discovery of related phenanthroperylene quinone pigments in a present-day crinoid (e.g., gymnochrome A, 3) [8], it was clear that the fossil pigments represent almost unchanged natural products that were biosynthesized millions of years ago [9, 10] (Scheme 1). After several years with contact only by e-mails or phone, my first actual encounter with Heinz was in 2003 when I visited him at the Johannes Kepler University in Linz. After my PhD, Heinz suggested a postdoctoral stay in the group of his colleague Prof. Wolfgang Buchberger from the Institute of Analytical Chemistry, whose laboratory was equipped with modern LC–MS instrumentation. This was fortunately supported by Prof. Buchberger and a DFG grant. During my postdoc time at the University of Linz, I had the opportunity to investigate the Solenopora pigments in detail while being mentored by Heinz. One day, we received the first high-resolution mass spectra from Dr. Jürgen H. Gross, a former colleague from the University of Heidelberg, who concluded from the isotopic pattern that the fossil pigments contain the element boron. This was a huge surprise for all of us, since boron is only rarely found in natural products and has never before been reported from a fossil organic compound. Now, it was also obvious that the Solenopora pigments were different to the * Klaus Wolkenstein [email protected]