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

Abstract

In senescent leaves chlorophyll (Chl) catabolites typically accumulate as colorless tetrapyrroles, classified as formyloxobilin‐type (or type‐I) or dioxobilin‐type (type‐II) phyllobilins (PBs). Yellow type‐I Chl catabolites (YCCs) also occur in some senescent leaves, in which they are generated by oxidation of colorless type‐I PBs. A yellow type‐II PB was recently proposed to occur in extracts of fall leaves of grapevine ( Vitis vinifera ), tentatively identified by its mass and UV/Vis absorption characteristics. Here, the first synthesis of a yellow type‐II Chl catabolite (DYCC) from its presumed natural colorless type‐II precursor is reported. A homogenate of a Spatiphyllum wallisii leaf was used as “green” means of effective and selective oxidation. The synthetic DYCC was fully characterized and identified with the yellow grapevine leaf pigment. As related yellow type‐I PBs do, the DYCC functions as a reversible photoswitch by undergoing selective photo‐induced Z / E isomerization of its C15=C16 bond.

A pink colored dioxobilin-type phyllobilin from breakdown of chlorophyll

“…For the partial synthesis of DPiCC 2 from DYCC 1 (Fig. 2), we used the method developed for the preparation of the PiCC 3 from YCC [33,35] [21]) [34] in DMF was treated with an excess of Zn(OAc) 2 at room temperature and in the presence of O 2 , the color of the reaction mixture changed slowly from yellow to blue. After 18 h, a TLC showed complete disappearance of the DYCC 1 and the blue reaction mixture was worked up (Experimental Section).…”

“…The dioxobilin-type NCCs, named DNCCs [5,21,24], have been observed recently in some leaves (and leaf extracts), to be oxidized regio-selectively, similar to NCCs, generating dioxobilin-type yellow Chl catabolites (DYCCs) [34]. Furthermore, the availability of the semisynthetic DYCC 1 (a type-II phylloxanthobilin) invited the exploration of a partial synthesis of its pink colored analog, the DPiCC 2, a type-II phylloroseobilin (PrB).…”

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

“…A related set of PBs, as represented by the type‐I phyllochromobilins 12 and 13 , is available from oxidation of DNCCs, the type‐II phyllobilanes. Hence, oxidation of the DNCC 14 by the ubiquitous ‘oxidative activity’ furnished the DYCC 15 , a proto‐type‐II yellow PB, which is readily oxidized further in the presence of air to the corresponding type‐II PrB, the DPiCC 16 (see Scheme ).…”

“…When dissolved in less polar solvents or in membrane mimetic detergent solutions the YCC 12 Z undergoes a clean photo‐induced [2+2]‐cycloaddition reaction at its C15=C16 double bond, to furnish the unstable, strained C2‐symmetric homodimer 1 8 (Scheme ) . The type‐II PxB 15 also undergoes the photo‐induced 15 Z /15 E isomerization but, in contrast to 12 , 15 does not dimerize by [2+2]‐cycloaddition at its C15=C16 double bond …”

Chlorophyll Breakdown – How Chemistry Has Helped to Decipher a Striking Biological Enigma

“…51 Furthermore, phyllochromobilins feature structures strikingly related to those of some (heme-derived) natural bilins, specifically when taking into account some colored type-II or dioxobilin-type PBs. 5,76 Hence, phyllochromobilins may play crucial roles as inhibitors of bilin-dependent enzymes, e.g., in photoregulation, 91 or as effectors, in their own right, in such enzymes. Clearly, the discovery of the presumed, but still elusive plant-biological roles of phyllobilins is an exciting scientific quest, as is the possible use of phyllobilins in pharmacological applications.…”