Origins and significance of benzoporphyrins in sediments

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  • 地層ベンゾポルフィリンの起源と地球化学的意義
  • チソウ ベンゾポルフィリン ノ キゲン ト チキュウ カガクテキ イギ

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Abstract

Benzoporphyrins have been found as minor components of sedimentary porphyrins. They are evidently diagenetic products, and several hypotheses have been proposed to account for their origins. The present review is mainly concerned with the formation mechanism of sedimentary benzoporphyrins, since it is important for assessing significance of geomolecules to relate diagenetic products to the original structures of biomolecules on the basis of chemistry. The Diels-Alder hypothesis proposes [2 + 4] cycloaddition of natural dienophiles with a methylvinylpyrrole moiety of divinylchlorophyll a and subsequent aromatization of the resulting cyclohexene ring, which can occur at the early stage of diagenesis. The reaction of vinylpyrrole moieties of vinylporphyrins was confirmed by heating experiments to generate a benzopyrrole structure without considering the intervention of any other reactant compounds. Another experimental evidence was given in support of diagenetic transformation of alkylporphyrins into benzoporphyrins. The heating experiments using etioporphyrin afforded phthalimides, after oxidation of the heating products. The compositions of phthalimide homologs and isomers of methyl- and dimethylphthalimides obtained in the experiments were shown to be in accord with those in the natural sediments, indicating the possibility of diagenetic transformation of geoporphyrins into benzoporphyrins in mature stratigraphic zones.

Journal

  • Chikyukagaku

    Chikyukagaku 44 (4), 221-232, 2010

    The Geochemical Society of Japan

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