Chlorophyll catabolites from plants

Chlorophyll breakdown and the emergence of the fall colours in the foliage of deciduous trees is one of the most visible biological phenomena. In spite of the importance of chlorophyll breakdown, (bio-) chemical information on it has been very scarce and a few pioneering contributions to this subject have been made only in the last decade. The present project is planned to answer experimentally questions concerning the structure and occurrence of chlorophyll catabolites, as intermediary and terminal breakdown products in higher plants, the chemistry of key steps in chlorophyll breakdown and the availability of synthetic pathways for the preparation of chlorophyll catabolites. As the specific contributions of the planned work, it is proposed: (i) to explore and develop synthetic pathways for the preparation of the basic structural types of colourless chlorophyll catabolites and of related and refunctionalized compounds; (ii) to use the semisynthetic compounds available in this way as probes for examining in vitro key (enzymatic or nonenzymatic) transformations involved in senescence induced chlorophyll breakdown in green plants; (iii) to search for chlorophyll catabolites in senescent plants (from leaves and ripened fruit, in particular) and to elucidate their structures with emphasis on the constitutional and stereochemical variability of the tetrapyrrolic structures from various plant parts. The proposed work is planned to contribute to a subject of obvious ecological and economical importance and to expand our knowledge on one of the most fascinating natural phenomena.

Chlorophyll, the ubiquitous green pigment of the plants, is the central motor in plant photosynthesis. Its formation in spring and its disappearance in fall are indeed the most obvious visual signs of life on earth. These phenomena can be observed even from outer space. In spite of its obvious ecological importance, the annual breakdown of chlorophyll has been unexplored scientifically, and has remained a biological enigma, until a few years ago. We know today, that (in the plants) chlorophyll is rapidly degraded to colorless compounds - in contrast to all earlier speculations and expectations. The decisive contributions to this subject from chemical investigations were achieved in the past few years in the group of Prof. Bernhard Kräutler at the Institute of Organic Chemistry of the University of Innsbruck. This research work was made possible by projects which were financed by the Austrian National Science Foundation (FWF) and which were based on interdisciplinary and international collaborations. In the present FWF-project, colorless chlorophyll catabolites of plants were made available by chemical synthesis. Therefore a first non-biological path to such compounds has become available and - at the same time - a better understanding of their biologically relevant chemical properties could be developed. As a consequence of the improved knowledge it could be established that a spontaneous chemical reaction (but not an enzymatic transformation) achieves the last important step of chlorophyll breakdown in the leaves of higher plants (this was published recently in the Proceedings of the National Academy of Sciences, USA). Thus, chlorophyll catabolism in the senescing plant exploits the inherent "chemistry" of the intermediary catabolites in a very economic way. New colorless chlorophyll catabolites were found in experiments on chlorophyll breakdown in leaves of spinach and of other useful plants, a fact, which indicates different plant species to have found slightly divergent solutions for the "problem" of how chlorophyll is broken down (in a kind of natural "detoxification" process). A more general interest for chlorophyll breakdown in plants derives from the phenomenon of the fall colors. Aside of this, knowledge on chlorophyll breakdown is of considerable relevance from an economic and an agricultural, as well as from an ecological point of view. The present project thus represents a general contribution in support of innovation in Austria and to the specific strengthening of its international status in the natural sciences.