Glycerol configurations of environmental GDGTs investigated using a selective sn2 ether cleavage protocol

The glycerol configurations of glycerol dialkyl glycerol tetraethers (GDGTs) in environmental samples were investigated using a selective sn2 ether cleavage protocol. Using this procedure, GDGTs with a parallel glycerol configuration afford two types of derivatives, diols and diallylethers, whereas only one kind, monoallylethers, originate from their antiparallel isomers. Isoprenoidal GDGTs from a marine sediment are shown to be predominately parallel based on the distributions of these ether cleavage products. Crenarchaeol and its so-called regioisomer both have parallel configurations with the cyclohexane ring located on the sn3,3 ether bonded tricyclic biphytanyl moiety. A Messel shale sample containing isoprenoidal GDGTs contributed mainly by methanogenic archaea has a substantial portion with the antiparallel configuration. Branched (non-isoprenoidal) GDGTs in both the Messel shale and the marine sediment are mainly antiparallel. This selective sn2 ether cleavage approach provides a potentially powerful analytical tool to investigate not only the exact molecular structures of GDGT constitutional isomers and their biosynthetic pathways but also the heterogeneous inputs of sedimentary GDGT and their isotopic signatures, if different source species synthesize GDGTs with unique glycerol configurations. Further analyses of this type will reveal the glycerol configurations of the GDGTs of a broad range of microbial cultures and environmental samples. (C) 2018 Elsevier Ltd. All rights reserved.