Molecular taphonomy of membrane constituents the search for the most primitive membranes

Abstract

Taphonomy is the study of fossilization processes, and includes "reconstruction" of past animals from their fossils. At the molecular level, the study of the organic constituents of sediments (crude oils and coals, but also clays, shales, etc) has yielded hundreds of structures, essentially in the hands of P.Albrecht and his co-workers in Strasbourg. These are sometimes hardly modified molecular fossils of known natural products. For instance, cholestane "must" come from cholesterol : reductions are frequent molecular taphonomic processes. Diacholestane, also frequently present in sediments, might be derived from "diasteror', an unknown but biosynthetically reasonable rearranged cholesterol ; however, the ease of rearrangement of cholesterol to diasterane derivatives by moderate heating with clays suggests rather that diacholestane derives also from cholesterol ; Wagner-Meerwein rearrangements are reasonable taphonomic processes. To interpret data from organic geochemistry, one must therefore collect trustworthy precise structural data, rely on the extensive catalogs of natural products structures and of biosynthetic process, and know which molecular taphonomic processes are plausible. Known molecular fossils belong to several natural products families, including porphyrins obviously derived from bacterial pigments ; by far most of them are derived from terpenoid precursors, as expected as biodegradation of highly branched structures is notoriously difficult. It was also not unexpected to find, frequently, molecular fossils of some arch~eal lipids : phytane and pristane, which could also have come from chlorophyll or from tocopherols, but also bisphytane, the structure of which became meaningful only once the C40 chains of phospholipid ethers of thermophilic and of methanogenic arch~ea became known. In other cases, while the structures of the precursors could be derived without doubt, they did not tally with any known one. The most spectacular case of these "orphan fossils" has been that of the ubiquitous geohopanoids, triterpene derivatives carrying an additional n-C 5 chain, for which more than 250 structures have been fully demonstrated ; their ubiquity and abundance in sediments make them probably the most abundant natural products on Earth ! Their precursors, the bacterial hopanoids, were discovered initially in two particular bacteria, but later, by M.Rohmer, in many very varied bacteria, with very varied structures (more than 40 established so far, including the fantastic adenosylhopane). Based on the molecular dimensions and amphiphilic character of bacterial hopanoids, we postulated that they reinforce bacterial membranes like