Sulphur-binding in recent environments. I. Lipid by-products from Ni2B desulphurization

Selected geochemically relevant model compounds were subjected to the nickel boride desulphurization method. Poor hydrogenating properties of this agent were observed in the case of sterically hindered double bonds. Apart from its desulphurizing properties, a significant number of functionalities present in lipid compounds were transformed on treatment with nickel boride. Ketones were reduced to their alcohols whereas aldehydes were partly converted to their ketals. Although alcohols and esters were unaffected by this agent, a nearby unsaturation (i.e., position α) resulted in the loss of the alcohol group and cleavage of the ester bond, respectively. The significance of these results was demonstrated by desulphurization experiments on polar fractions from two recent sediment extracts. The main compound among the apolar products obtained, phytane, was found to be quantitatively derived from phytol and not, as may be presumed, from its macromolecularly sulphur-bound precursor. The results of this work show that compounds generated by nickel boride desulphurization of polar fractions from lipid extracts, particularly from recent sediments, are not necessarily derived from their sulphur-bound precursors. Conclusions with regard to the palaeoenvironmental significance of the hydrocarbons formed, therefore, should be made with caution.