DISULFIDE-BOUND OR POLYSULFIDE-BOUND BIOMARKERS IN SULFUR-RICH GEOMACROMOLECULES AS REVEALED BY SELECTIVE CHEMOLYSIS

Three types of sulphur-rich high-molecular-weight material in the alkylsulphide, the polar, and the asphaltene fractions isolated from the bitumen of an immature bituminous shale from the Vena del Gesso basin (Italy) were desulphurised using Raney Ni and were treated with MeLi/MeI, a chemical degradation method which cleaves selectively and quantitatively di- or polysulphide linkages. The products formed were characterised by gas chromatography-mass spectrometry. Raney Ni desulphurisation revealed that these S-rich macromolecules are in substantial part composed of sulphur-linked biomarkers with linear, branched,isoprenoid, steroid, hopanoid, and carotenoid carbon skeletons. MeLi/MeI treatment provided evidence that a major part of the total amount of macromolecularly bound biomarkers are linked via di- or polysulphide moieties to the macromolecular network. Since the di- or polysulphide linkages are attached at specific positions of the bound biomarkers it is proposed that they are formed by intermolecular incorporation reactions of HS(x)- into low-molecular-weight functionalised biological lipids during early diagenesis. The different properties (solubility and molecular weight) of the sulphur-rich macromolecules in the alkysulphide, the resin, and the asphaltene fractions can be explained simply by differences in degree of sulphur cross-linking.