Organic Geochemistry [Biogéochimie moléculaire]

Abstract:

The widely distributed aromatic di- and triterpenoids from higher plants occurring in sediments are formed by diagenetic microbial processes affecting their precursor plant lipids. These compounds and their stable isotopic composition (δ¹³C and δ²H) have the potential to be used for palaeoenvironmental and palaeoclimatic studies. In the present study, the isotopic composition of di- and triterpenoids aromatized to different extents has been measured to specifically examine the isotopic effects associated with the aromatization reaction. To overcome the possibility of multiple higher plant sources, as is generally the case with sedimentary lipids, the δ¹³C and δ²H values of aromatized di- and triterpenoids from lipid extracts recovered from conifer and angiosperm buried wood have been determined, allowing an unambiguous substrate/product relationship along the aromatization pathway to be assured. The results show that the δ¹³C compositions of both di- and triterpenes do not seem to be significantly affected by progressive aromatization, whereas the situation is different for δ²H values. In the case of diterpenoids related to abietic acid, a significant increase of the δ²H values by up to 86‰ with ongoing aromatization was measured. This is in contrast to what is expected for dehydrogenated compounds which should be globally more ²H-depleted than their precursor molecules (e.g., biosynthesized unsaturated fatty acids vs. their saturated precursors). This ²H enrichment of aromatized diterpenoids could indicate that they represent only minor residual intermediates, the majority having been further degraded by processes favoring the degradation of the ¹H-containing substrates and having a moderately pronounced ¹²C/¹³C selectivity. With triterpenoids, preservation of the δ²H values was observed whatever the nature and extent of the aromatization process considered, which may be related to enzymatic reactions showing a limited influence on the hydrogen stable isotopic composition of the aromatized compounds.

Reference :

The effects of diagenetic aromatization on the carbon and hydrogen isotopic composition of higher plant di- and triterpenoids: evidence from buried wood

P. Schaeffer, L. Bailly, E. Motsch, P. Adam

Organic Geochemistry, 2019, 136, 103889 - DOI: https://doi.org/10.1016/j.orggeochem.2019.06.010

Contact chercheur :

Pierre ADAM, équipe Biogéochimie moléculaire, Institut de Chimie (UMR 7177).