Fluorescing Chl-catabolites

Colourless, non-fluorescent Chl-catabolites (NCCs) have meanwhile been observed to accumulate in a variety of senescent vascular plants (1, 2, 3, 4, 5, 36, 48, 49, 50, 51, 52, 53, 54, 55, 56). All of them feature an annealed cyclopentanone unit, substituted by a carboxylate or methoxycarbonyl function (1), a hallmark of the natural chlorophyll derivatives (61). The molecular constitution of the NCCs revealed an intriguing and specific oxygenolytic ring-opening reaction at the a-meso position (rather than at the 8-meso carbon) of the chlorin macrocycle with retention of the a-meso carbon as a formyl group (1). 

A second fluorescent Chl-catabolite, Ca-FCC-2, was isolated from another in vitro system, based on enzymatic activity obtained from ripe (red) sweet pepper (Capsicum annuurri) and its structure was analyzed (67). The new fluorescent catabolite could be shown by mass spectrometry to be an isomer of 10 Further NMR-spectroscopic analysis revealed Ca-FCC-2 to have the same constitution and to differ from pFCC (10) only in the absolute configuration at C(l). Ca-FCC-2 was thus assigned as the epimeric primary I -epz-pFCC (epi-10) (67). 

As is delineated in more detail below (Section 2.2.3), the chiral center C(l) is introduced via the highly stereo-selective reduction step catalyzed by a reductase, present in the two plant species (67, 68, 69). These findings, identified the two FCCs (10 and epi-10) as direct products of these reductases and supported the earlier proposal to consider both of these fluorescent compounds as primary fluorescent Chl-catabolites (2, 3). 

The constitution of f/v-NCC-1 (2, 31,32,82-trihydroxy-l,4,5,10,15,20-(22//,24//)-octahydro-132-(methoxycarbonyl)-4,5-dioxo-4,5-seco-phyto-porphyrinate (see Scheme 2) gave first clues on the basic transformations involving the Chl-chromophore (1, 2, 4, 10). When, in addition, the structure of the fluorescent chlorophyll catabolite pFCC (10) was revealed, an isomerization of the chromophore of the FCCs into that of... 

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