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 °C. 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-OH-epiNCC 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.

Chlorophyll Catabolites in Senescent Leaves of the Plum Tree (Prunus domestica)

“…We suspected Pd ‐NCC‐54 as product of the formal addition of an O‐atom to Pd ‐NCC‐60 ( 5 ) from an endogenous oxidation process. Indeed, as shown recently, NCCs may undergo C(15) hydroxylation by endogenous, as well as by additional efficient adventitious oxidation during preparation of leaf homogenates and their extracts. From NCCs hydroxylated at their C(15) position, H 2 O may eliminate easily, resulting in corresponding YCCs .…”

“…Indeed, as shown recently, NCCs may undergo C(15) hydroxylation by endogenous, as well as by additional efficient adventitious oxidation during preparation of leaf homogenates and their extracts. From NCCs hydroxylated at their C(15) position, H 2 O may eliminate easily, resulting in corresponding YCCs . Indeed, a YCC was detected in the fresh plum leaf extracts, named Pd ‐YCC‐67, which showed mass spectral data ( pseudo ‐molecular ion with m / z 643.2) consistent with its formation as the formal product of an oxidative dehydrogenation of Pd ‐NCC‐60 ( 5 ).…”

“…For spectroscopic analysis of the most abundant NCCs in the leaves of P . domestica , 18.7 g of senescent plum tree leaves were extracted with cold MeOH (to avoid significant NCC oxidation, see ), and the extract was separated by semi‐preparative HPLC. A two‐stage purification procedure gave a uniform sample of 0.29 mg of Pd ‐NCC‐32 ( 1 ), analyzed by UV/VIS‐spectroscopy first (see Fig.…”

“…In the last 25 years, more than 20 structurally different NCCs from higher plants were, thus, detected, and their structures were characterized (see Table ) . Evidence for further oxidative transformation of NCCs in leaves was also provided more recently by the observation of yellow Chl‐catabolites (YCCs) and pink Chl‐catabolites (PiCCs) in senescent leaves of a variety of higher plants . These colored phyllobilins were identified as formal dehydrogenation products of corresponding tetrapyrrolic NCCs.…”

In Search of Bioactivity – Phyllobilins, an Unexplored Class of Abundant Heterocyclic Plant Metabolites from Breakdown of Chlorophyll

“…The phylloxanthobilin (PxB) 12 was the first detected yellow Chl catabolite (YCC), in which a π‐conjugated chromophore extends over the two ‘Western’ rings C and D and is structured like the one of bilirubin . In the absence of light, the PxB 12 is generated in leaf extracts in the 15 Z ‐form exclusively (but isomerizes to the E ‐form by light) . The PxB 12 is easily oxidized further, furnishing the pink colored Chl catabolite (PiCC) 13 , the first known example of a natural phylloroseobilin (PrB) .…”