The enigmatic structure of the crenarchaeol isomer

Isolation of crenarchaeol and its isomer from marine surface sediments, followed by ether cleavage and GC-MS characterization using supersonic molecular beam (SMB) ionization of the biphytanes formed, revealed that the crenarchaeol isomer comprises a tricyclic biphytane that is stereochemically different from the tricyclic biphytane of crenarchaeol. This isomeric tricyclic biphytane was also released from the crenarchaeol isomer in extant Thaumarchaeotal biomass. Reinterpretation of previously obtained 13C NMR data of the crenarchaeol isomer suggested that the cyclopentane moiety adjacent to the cyclohexyl moiety of the tricyclic biphytane of the crenarcheaol isomers possesses the unusual cis stereochemistry in comparison to the trans stereochemistry of all cyclopentane moieties in crenarchaeol. This stereochemical difference likely affects the packing of lipid membranes of Thaumarchaeota and therefore provides a biophysical explanation for the role of the crenarchaeol isomer in the TEX86 palaeothermometer based on fossilized Thaumarcheotal lipids.