A yellow chlorophyll catabolite is a pigment of the fall colours

Abstract

Here we describe the detection and identification of a yellow chlorophyll catabolite (Cj-YCC) in fresh extracts of senescent leaves of Cercidiphyllum japonicum. In addition, we report its partial synthesis by oxidation of Cj-NCC-1, the major (colourless) "nonfluorescent" chlorophyll catabolite (NCC) found in degreened leaves of C. japonicum. The spectroscopic analysis and structural characterization indicated Cj-YCC to be a simple dehydrogenation product of Cj-NCC-1 (by formal removal of a hydrogen atom at the C(20)- and C(1)-positions). Indeed, NCCs are easily oxidized and were first called "rusty pigments", as they had a tendency to turn brown upon storage on a dry silica gel plate. The yellow tetrapyrrole Cj-YCC may thus come about by oxidation of Cj-NCC-1 in the leaves. Its presence in the yellow leaves of a deciduous tree provides the first evidence for the contribution of a coloured chlorophyll catabolite to the fall colours.

Fig. 1

Outline of key steps in breakdown of chlorophylls in senescent leaves: structural formulae of pheophorbide a (Pheo a), of red chlorophyll catabolite (RCC), of primary fluorescent chlorophyll catabolite (pFCC) and constitutional formulae of Cj-NCC-1 (1),, a ‘nonfluorescent’ chlorophyll catabolite (NCC) from senescent leaves of Cercidiphyllum japonicum, and of Cj-YCC (2).

Fig. 2

Top: HPLC-analysis of a freshly prepared extract of senescent leaves of C. japonicum, with fractions of Cj-NCC-1 (1), Cj-YCC (2) and Cj-NCC-2 (3) highlighted (detection at 320 nm). Bottom: UV/Vis-spectrum of semi-synthetic Cj-YCC (2) in MeOH (c 15.9 = μM, full line) and of a sample of 2, isolated from freshly degreened leaves of C. japonicum (dashed line).

Fig. 3

Structural outline of the oxidation of Cj-NCC-1 (1) by dicyano-dichloro-quinone (DDQ) to give Cj-YCC (2).

Fig. 4

Left: Assignments for ¹H-signals from 500 MHz ¹H-NMR spectra of Cj-YCC (2) in CD₃OD and correlations from H,H-COSY- and ROESY-spectra (solid lines and arrows). Centre: Assignment of ¹³C-signals from ¹H,¹³C-heteronuclear correlations in HSQC-and HMBC-spectra. Right: Atom-numbering for 1 and 2, based on that proposed by IUPAC for the chlorophylls; however, simple 2- and 3-digit numbers are used here (e.g. 172) (in the text, an upper case number indicates the position on the side chain, e.g. 17², as suggested by IUPAC, see e.g. ref. 19).