How the Colourless ‘Nonfluorescent’ Chlorophyll Catabolites Rust

For a long time, the appearance of the fall colours has been associated with the enigmatic chlorophyll breakdown in higher plants. However, the extensive earlier search for coloured chlorophyll breakdown products has remained unsuccessful. When chlorophyll catabolites from higher plants were first tentatively identified, they were indicated to be colourless. These colourless compounds readily decomposed to rust-coloured materials upon analysis by thinlayer chromatography and were thus named “rusty pigments”, originally. The puzzling picture cleared up, when one of the presumed chlorophyll breakdown products was structurally characterized as a colourless linear tetrapyrrole, the type of which is meanwhile classified as a “nonfluorescent” chlorophyll catabolite (NCC, see Scheme 1). Indeed, the colourless NCCs are ubiquitous in various senescent leaves and have been considered to represent the major “final” products of chlorophyll breakdown in senescent plants. However, Cj-NCC-1 (1), a colourless NCC isolated from senescent leaves of the deciduous tree Cercidipyllum japonicum (Katsura tree) could be chemically oxidized to a yellow chlorophyll catabolite, named Cj-YCC, which has also been detected in fall leaves recently. Here, we analysed the major coloured products, when 1 decomposed to rust on silica gel. Application of a solution of Cj-NCC-1 (1) to a silica gel TLC plate first gave a nearly colourless “spot”, which acquired a brown colour (“rust”) within 2–5 min, when exposed to air and daylight. TLC analysis of such a “spot” of 1 separated off a yellow zone on the plate, as was observed earlier with “rusty pigments”, and an additional pink-red spot developed eventually. Thin-layer re-chromatography in a second dimension of the TLC trace originating from the NCC 1 revealed the yellow fraction to directly form on the