On the Nature of Isomeric Nonfluorescent Chlorophyll Catabolites in Leaves and Fruit – A Study with a Ubiquitous Phylloleucobilin and its Main Isomerization Product

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 80th birthdayThe typical main products of chlorophyll (Chl) breakdown in higher plants are non-fluorescent, colorless phyllobilins, named phylloleucobilins. These long elusive Chl-catabolites are linear tetrapyrroles, whose structure elucidation has required thorough spectroscopic analyses. Interestingly, in recent LC/MS studies of leaf extracts, isomeric forms of phylloleucobilins were detected. The existence of isomeric phyllobilins may suggest incomplete stereo-selectivity of catabolic processes, or isomerization processes in plant cells or in the analytes. Here we report a study with the phylloleucobilin NCC-1, a basic Chl-catabolite in extracts of leaves and fruit. NCC-1 and its main isomerization product in aqueous solution were identified as 8 2 -epimers. Formation of 8 2 -epi-NCC-1 from NCC-1 implies an unstable enol(ate)-intermediate, which reverts to NCC-1 or converts to 8 2 -epi-NCC-1. Such reversible epimerization reactions are a non-biological in vitro feature of typical phylloleucobilins, and probably also take place in vivo.

Pyro‐Phyllobilins: Elusive Chlorophyll Catabolites Lacking a Critical Carboxylate Function of the Natural Chlorophylls

“…[5,6] Hence, natural NCCs (as well as YCCs and Chls) exist as C8 2 -epimeric pairs exhibiting significantly different relative stabilities. [28,31,32] However, as shown here, the carboxylate substituent at C8 2 has little effect on the flexible structure of NCCs and on the photoreactivity of YCCs. Thus, pyPBs are close biosimilars of the abundantn atural PBs, and it may be rewarding, indeed, to search routinely for pyNCCsi nextracts of plant materials.…”

“…Typical NCCs carry a β‐keto carboxylate grouping at ring E, which is inherited from their Chl precursors, and which renders their C8 2 asymmetric and prone to solvent‐assisted loss of a proton . Hence, natural NCCs (as well as YCCs and Chls) exist as C8 2 ‐epimeric pairs exhibiting significantly different relative stabilities . However, as shown here, the carboxylate substituent at C8 2 has little effect on the flexible structure of NCCs and on the photoreactivity of YCCs.…”

A Blue Zinc Complex of a Dioxobilin‐Type Pink Chlorophyll Catabolite Exhibiting Bright Chelation‐Enhanced Red Fluorescence

“…[14] The type-II PBs (DNCCs, DYCCs and DPiCC) feature the configurationally stable asymmetric methine carbon C4 (tentatively assigned R-configuration in the DPiCC 1). [14] Hence, the observation of two epimers of 2 (as well as of 1) in MeOH solution can be rationalized on the basis of the formation of diastereoisomers in a roughly (1 : 1)-ratio by configurational equilibration at the exchange-labile asymmetric carbon C8 2 [19] in conjunction with the stable asymmetric C4 of the type-II PBs.…”

Chlorophyll breakdown—Regulation, biochemistry and phyllobilins as its products

“…The adjacent asymmetric C8 2 has S-configuration in the predominant forms of the NCCs. However, C8 2 is part of a β-ketoester grouping at ring E, which promotes slow equilibration of the main isomer with the minor, C8 2 -R-epimer (Moser, Scherzer, & Kräutler, 2017). Scheme 5 The typical natural NCCs are generated by a non-enzymatic isomerization of FCCs that carry the free propionic acid substituent derived from Pheide a and RCC (Kräutler, 2016).…”