Coloured Bilin-Type Chlorophyll Catabolites

The disappearance of the green plant pigment chlorophyll in fall leaves is an ever fascinating phenomenon. In the course of the last two and a half decades, important molecular aspects of the enigmatic breakdown of chlorophyll in higher plants were eventually elucidated. Discovery and structure elucidation of colourless chlorophyll catabolites, as well as studies on their chemical reactivity, have contributed to achieving decisive advances in this field. Nowadays, the occurrence of colourless chlorophyll catabolites, known to accumulate temporarily in fall leaves and in ripening fruit, is well documented. Recently, we have also detected yellow and pink-red products of chlorophyll breakdown in senescent leaves, suggested to represent oxidation products of colourless products of chlorophyll breakdown. The present project will tackle, and investigate by scientific means, the question whether, and in what way, such coloured chlorophyll- catabolites are likely to play a relevant role in further breakdown of chlorophyll in nature. Coloured chlorophyll catabolites are to be prepared by systematic chemical syntheses from colourless precursors and will be characterized structurally by spectroscopic means. Coloured chlorophyll catabolites represent a unique new group of natural heterocyclic compounds whose chemical reactivity is to be investigated in the context of chlorophyll breakdown. The natural temporary occurrence of such coloured chlorophyll catabolites in senescent leaves and in fruit will also be studied, and their molecular structures will be analyzed. Findings from these investigations are expected to provide additional evidence of the way coloured catabolites are formed from their presumed colourless precursors in senescent leaves and in ripening fruit, and how they disappear there naturally, as well. The present project deals with chlorophyll catabolites and their coloured oxidation products in senescent leaves and in ripening fruit. These are considered to play a major role in chlorophyll- degradation. Earlier studies have rationalized chlorophyll breakdown as a process of natural detoxification serving the recuperation of important metabolic components. However, beyond the earlier consideration of their mere role as detoxification products, chlorophyll catabolites are now presumed to also have valuable physiological functions. In view of the expected new findings, the present project, thus, aims at finding answers to the questions of how and why the still largely enigmatic chlorophyll breakdown does occur in higher plants.

Chlorophyll (Chl), the characteristic green pigment of plants, is broken down enzymatically in ripening fruit and in fall leaves of deciduous plants, accompanied by fascinating color changes. Important insights into the long puzzling phenomenon of Chl-breakdown could be obtained thanks to the groundbreaking contributions on the structures of the bilin-type Chl-catabolites, named 'phyllobilins (PBs). The key reaction of Chl-breakdown in flowering plants is the ring opening of the Chl-macrocycle that specifically generates a formyloxo-bilin type PB, as precursor of the original line of type-I PBs. Dioxobilin-type PBs were recently observed as an important secondary group of Chl-catabolites that are formed from specific type-I PBs and, thus, are classified as type-II PBs. The main topic of the present project was concerned with colored PBs, yellow and pink type-II PBs, in particular, that were proposed to occur as 'late products of Chl-breakdown in senescent plant tissues. The colored PBs are plant pigments, which absorb part of the visible range of solar light. The photochemical properties of the colored PBs, as well as their anti-oxidative properties, may be directly beneficial in the plant tissues. Strongly blue 'fluorescent PBs' also are formed, sensitizing the formation of singlet oxygen. The here prepared and analyzed colored and fluorescent dioxobilin-type PBs display largely similar structural, photochemical and coordinative properties as their known type-I analogues. However, the dioxobilin-type PBs relate remarkably more closely structurally to the important alternative class of the bilins, which are heme-derived. Hence, the type-II PBs, in particular, are alternative candidates for important specific functions of bilins from heme. This structural link may give them relevant physiological effects not only in higher plants, but also in other evolved domains of life, such as plant eating animals and humans. The findings from this project reinforce today's view that the massively produced open chain tetrapyrrolic catabolites of Chl should not be considered as mere waste products resulting from a biological Chl-detoxification process, as done earlier. On the contrary, the natural colored PBs have unique structures and exceptional chemical reactivity and are very promising heterocyclic natural products that may make further biological and biomedical studies with them a highly attractive area of research.