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Ring‐Closure on the Rocks in a Prebiotic Environment

Von Wiley-VCH zur Verfügung gestellt

Did analogues of metabolic reactions occur in a geochemical context on the early Earth? We have attempted to reproduce the cyclisation step of the orotate pathway of pyrimidines biosynthesis on mineral surfaces chosen for their type of reactivity and similarity to enzymes that carry out the same reaction. Diverse outcomes were observed, with extensive cyclization on some supports, favouring however 5-cycles (hydantoins) rather than the biological 6-cycle.


Ring-closure is a key step in current pyrimidine anabolism and one may wonder whether cyclisation reactions could be promoted in the geochemical context at the origins of life, i. e. with the help of minerals. Various prebiotic minerals were tested in this work, including silica, carbonates, microporous minerals. In particular, the role of zinc ions supported on minerals was investigated in view of its presence in the catalytic site of cyclic amidohydrolase enzymes. Based on in situ (TGA: ThermoGravimetric Analysis, ATR-IR: Attenuated Total Reflectance-InfraRed) and ex situ (1H NMR- Nuclear Magnetic Resonance) characterisations, we identified the products of thermal activation of NCA (N-carbamoyl-aspartic acid) in wetting-and-drying scenarios on the surface of minerals. NCA can cyclize extensively only on some surfaces, with the predominant product being 5-carboxymethylhydantoin (Hy) rather than dihydroorotate (DHO), while there is a competition with hydrolysis on others. Replacing the enzymes with heterogeneous catalysts also works with other reactions catalysed by enzymes of the cyclic amidohydrolases family. The role of the hydrophilicity/hydrophobicity of minerals as well as the regioselectivity of the cyclisation (5-carboxymethylhydantoin versus dihydroorotate) are examined.

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