化石ポルフィリンのタフォノミー : 分子レベル同位体指標としてのポテンシャルを引き出すために(2009年度田口賞受賞,<特集>有機化合物の安定同位体比を用いた有機地球化学的研究の発展とその応用(Part I))
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概要
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Herein I introduce "taphonomy" as a new methodology in organic geochemistry. Organogeochemical taphonomy treats analyses of both chemical transformation processes leading precursory biomolecules to fossil molecules as well as analyses of provenances of these fossilized molecules in space and time. As an introductory example, present paper analyzes the former "organic geochemical" taphonomy, focusing in particular on the early transformation reactions of chlorophylls leading to fossil porphyrin preservation. In chloropgments and their derivatives, the carbon atom located in the exocyclic five-membered ring adjacent to the carbonyl group is exceptionally nucleophilic, being a "weak point" of these molecules. This reaction center represents both α-position of the β-carbonyl ester system and benzyl position of the chloron/porphyrin aromatic system; hence its acidic proton is quite amenable to deprotonation in natural environment leading to generation of chlorin/porphyrin enolate anion. This stable enolate anion with delocalized paired electron over the aromatic macrocycle is subjected to extraction of an electron by triplet-state oxygen molecule, leading radical chain oxidation reactions and opening the exocyclic ring. In absence of molecular oxygen and other oxygen radical species, the enolate anion can undergo intramolecular Claissen condensation with electrophilic C-17 alkoxycarbnylethyl or carboxyvinyl functions. The resulted compounds, 13^2, 17^3-cyclopheoforbide a enol and isochlorin e_6 esters, respectively, are conclusive precursors of certain fossil porphyrins that retains carbon skeletons of those rearranged exocyclic rings. Furthermore, demethoxycarbonylation at this reaction center reduces the reactivity of the original exocyclic ring, preventing formation of reactive enolate anion in natural environments. Interestingly, the demethoxycarbonylation is suggested to proceed in fact from chlorine/porphyrin enolate anion. Such transformations divert the fate of chloropigments to be preserved in the sedimentary record; inversely, intactness and rearrangement of the exocyclic ring found in the fossil porphyrins should reflect their process immediately after the death of the phototrophs in the geological timescale, hence possibly retaining environmental conditions or food-web structures in the water-sediment system at the time of sedimentation. Such taphonomical approaches in organic geochemistry should be increasingly important since compound-specific isotopic analyses of molecular fossils have recently been developing rapidly. Understanding of the taphonomy will bring out further potentials of compound-specific isotopic studies as paleoproxies for the past biological processes.
- 2010-12-24