During the first year of my PhD I spent most of my time working up sediments from various ODP cores and came across the same compounds time and time again. I had some n-alkanes. I had some sterols. I even had some GDGTs now and again. But it was not until I started working on lignites (1) until I started finding weird little compounds.
One of the compounds eluted around 55 minutes and was characterised by a strong m/z 191 fragment. This particular ion is characteristic of hopanoids but this was coming out way too late to be a normal hopane. A bit of investigation into some old literature (Valisolalao et al., 1984) showed that what I was looking at was a regular hopane with a thiophene ring attached (2).
A hopane is a pentacyclic triterpenoid produced by most bacteria and a thiophene ring is a heterocyclic compound with 4 carbons, 4 hydrogens and 1 sulphur. As sulphur-containing compounds are rarely reported in living organisms, it seemed that sulphur was being incorporated into bacterial lipids during the early stages of diagenesis (3).
A C35 hopane with a thiophene group on the left and a C35 hopanoid on the right (Valisolalao et al., 1984)
This was developed further by Jaap Sinninghe-Damsté during his PhD thesis where he identified countless compounds including isoprenoid, alkyl and steroidal thiophenes. In many settings, the presence of organic sulphur compounds were assosciated with hypersaline conditions and led to the development of a useful paleosalinity proxy based upon the distribution of C20 isoprenoid thiophenes (4). However, it is always worth bearing in mind that “…no single biological marker property is sufficient to characterise and assess a specific environment of deposition” (Mello et al., 1988).
A selection of lipids with thiophene rings (Sinninghe-Damsté et al., 1996)
And then, to the best of my knowledge, people just kinda stopped publishing on them. However, I think they are due a revival. Have a look in your samples for them. While they are unlikely to be Nature or Science-worthy , they might tell you something neat!
(1) Lignites are somewhere between peat and coal.
(2) If you can, check out the font! It is better than Comic Sans.
(3)The sterochemistry of the hopane was BB and further supports this observation.
(4) This can be combined with another palaeosalinity proxy which uses methylated chromans (a tetrahydropyran ring attached to a benzene ring…I think. Citation needed)
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