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

Abstract

A β‐keto ester grouping is a characteristic of ring E of the chlorophylls (Chls). Its presence has also reinforced the original identification of nonfluorescent Chl catabolites (NCCs) as colorless, amphiphilic phyllobilins (PBs). Polar NCCs were also detected in higher plants, in which a free carboxyl group replaced the ring E ester group. Such NCCs are surprisingly resistant to loss of this carboxyl unit, and NCCs lacking the latter, that is, pyro‐NCCs (pyNCCs), have not been reported. Intrigued by the question of the natural occurrence of pyro‐phyllobilins (pyPBs), we have prepared a representative pyNCC by decarboxylation of a natural NCC. We also converted the pyNCC into its yellow oxidation product, a pyro‐YCC (pyYCC). The solution structures of pyNCC and of pyYCC, and a crystal structure of the pyYCC methyl ester (pyYCC‐Me) were obtained. pyYCC‐Me features the same remarkable H‐bonded and π‐stacked dimer structure as the corresponding natural yellow Chl catabolite (YCC) with the ring E methyl ester group. Indeed, the latter substituent has little effect on the structure, as well as on the unique self‐assembly and photoswitch behavior of yellow PBs.

Yellow Dioxobilin‐Type Tetrapyrroles from Chlorophyll Breakdown in Higher Plants—A New Class of Colored Phyllobilins

“…By using a homogenate of a Spatiphyllum wallisii leaf in a “green” oxidation protocol YCC 2 was prepared efficiently from the epimeric NCCs 1 or 1‐ epi (Scheme ). YCCs (and the analogous py YCCs, which lack the ring E methyl ester function) were found to self‐assemble reversibly to unique H‐bonded and π‐stacked homodimers in medium to low polarity solvents and to display exceptional medium‐dependent photochemistry …”

“…In contrast to the situation with the YCC 2 and its methyl ester, as well as with a py YCC, spectral data for DYCC 4 indicate little solvent and medium effects on spectra and structures, and no evidence for self‐assembly into dimeric structures was found for 4 . This pointed to the importance of the intact formyl‐pyrrole unit (ring A) of YCCs for their assembly to H‐bonded and π‐stacked homodimers in nonpolar solvents . Irradiation with sunlight of a solution of the DYCC 4 in MeOH converted 4 to a new and less stable DYCC cleanly and reversibly, identified as the 15 E ‐isomer 4 E .…”

Chlorophyll Breakdown in a Fern—Discovery of Phyllobilin Isomers with a Rearranged Carbon Skeleton

“…First representatives of phyllobilins that lack a carboxylic acid or ester function at C8 2 , so called pyro‐phyllobilins ( py PBs), have been the subject of a recent study 14. In that work, a pyro‐NCC ( py NCC) was prepared from a natural NCC with a ring E β‐ketocarboxylic acid group.…”

“…In that work, a pyro‐NCC ( py NCC) was prepared from a natural NCC with a ring E β‐ketocarboxylic acid group. The partial synthesis of the py NCC revealed the surprising resistance of the β‐ketocarboxylic acid group of the polar NCC against decarboxylation 14. Indeed, py NCCs are unknown as natural Chl catabolites and py PBs have only once been observed as the red products of an in vitro decarboxylation of natural red Chl catabolites excreted by the green alga A. protothecoides 10…”

“…Likewise, a β‐keto‐carboxylate, as present in some NCCs and FCCs, is activated (though only “modestly”14) for decarboxylation. Hence, enzyme catalyzed decarboxylation and pyrrole oxidation may presumably occur as separate steps during Chl breakdown in bracken ferns.…”