Stereo‐ and Regioselective Phyllobilane Oxidation in Leaf Homogenates of the Peace Lily (Spathiphyllum wallisii): Hypothetical Endogenous Path to Yellow Chlorophyll Catabolites

Abstract

In senescent leaves, chlorophyll typically is broken down to colorless and essentially photo-inactive phyllobilanes, which are linear tetrapyrroles classified as "nonfluorescent" chlorophyll catabolites (NCCs) and dioxobilane-type NCCs (DNCCs). In homogenates of senescent leaves of the tropical evergreen Spathiphyllum wallisii, when left at room temperature and extracted with methanol, the major endogenous, naturally formed NCC was regio-and stereoselectively oxidized (in part) to a mixture of its 15-hydroxy and 15-methoxy derivative. In the absence of methanol in the extract, only the 15-OH-NCC was observed. The endogenous oxidation process depended upon molecular oxygen. It was inhibited by carbon monoxide, as well as by keeping the leaf homogenate and extract at low temperatures. The remarkable "oxidative activity" was inactivated by heating the homogenate for 10 min at 70 8C. Upon addition of a natural epimeric NCC (epiNCC) to the homogenate of senescent or green Sp. wallisii leaves at room temperature, the exogenous epiNCC was oxidized regio-and stereoselectively to 15-OHepiNCC and 15-OMe-epiNCC. The identical two oxidized epiNCCs were also obtained as products of the oxidation of epiNCC with dicyanodichlorobenzoquinone (DDQ). Water elimination from 15-OH-epiNCC occurred readily and gave a known "yellow" chlorophyll catabolite (YCC). The endogenous oxidation process, described here, may represent the elusive natural path from the colorless NCCs to yellow and pink coloured phyllobilins, which were found in (extracts of) some senescent leaves.

Phyllobilins – Bioactive Natural Products Derived from Chlorophyll – Plant Origins, Structures, Absorption Spectra, and Biomedical Properties

“…Since the first report of PxBs 16 , an increasing number and variety of PxBs have been identified in various plant species, e.g., an unprecedented structural variety of PxBs has been demonstrated in senescent leaves of Echinacea purpurea 8 . Paper spray mass spectrometry showed that PxBs do not represent artifacts of an isolation procedure but do clearly occur in more plant species than previously assumed, e.g., in spinach and peace lily, for which only PleBs had been identified before 17 , 70 . Although PleBs remain the best-characterized phyllobilin structures, recent research demonstrates that, with regard to bioactivities, the PxBs significantly exceed their PleB precursors.…”

“…As their name implies, the phylloleucobilins are not colored. However, the phylloleucobilins can be oxidized by a still elusive ‘oxidative activity’ to colored pigments, the yellow chlorophyll catabolites (YCCs and DYCCs), also known as phylloxanthobilins (PxBs and DPxBs) 15 , 16 , 17 . The phylloxanthobilins and phylloleucobilins differ only in the position of a single double bond ( Fig.…”

“…Though the exact mechanism and the enzyme(s) involved are still unclear, PleBs and their C16 isomers are transformed by an endogenous oxidation process to polar PleB intermediates that carry either a hydroxyl group or a methoxy group at C15 depending on the solvent. These polar precursors are converted to the same ( Z )-PxB in the lab by acid-induced elimination of water or methanol, respectively 17 , but in the case of the hydroxylated compound, could also be detected naturally, e.g., in senescent leaves of basil ( 12 and 13 ) as inferred precursors of the PxB from basil leaves ( 37 ) 33 . Spectral effects caused by the hydroxylation are insignificant.…”

Structural Characterization, and Antioxidative and Anti-inflammatory Activities of Phylloxanthobilins in Tropaeolum majus, a Plant with Relevance in Phytomedicine

“…DMSO stocks of isolated substances were prepared from isolated or purchased substances and stored at − 20 °C. Concentrations of Tm -PxBs were calculated by log ε (426 nm) = 4.51, PleB by log ε (312 nm) = 4.23 using a Thermo Spectronic Genesys 5 (336 001) UV-Visible spectrophotometer, λ max [nm] ( ε rel) 3 , 4 . Peak purity was tested using Agilent OpenLab CDS software [UV peak purity (p) in parts per million]: Tm -PxB-1 p = 999.42; Tm -PxB-2 p = 867.07; Tm -PxB-3 p = 996.67; Tm -PxB-4 p = 999.86.…”

“…In terms of structure, PxBs are oxidation products of earlier discovered PleBs (or nonfluorescent chlorophyll catabolites (NCCs)] 1 , from which PxBs are formed in the plant cell by a yet unclarified mechanism ( Fig. 1 ) 4 . A structural feature of naturally occurring PleBs is a carboxyl group or methyl ester function at ring E. The loss of the carboxyl function has not been discovered in plants yet; in ferns, however, so-termed iso-phyllobilin s feature a similar structural motif from a rearrangement reaction 5 .…”