FORMATION OF C-20 ISOPRENOID THIOPHENES IN MODERN SEDIMENTS

Three isomers of C20 isoprenoid thiophenes were identified in the solvent extracts from diverse modern sediments including brackish water lake and inland bay sediments, that were under highly reducing saline conditions. Based on the field observations and some laboratory experiments, this paper demonstrates that the likely formation reaction of these thiophenes is from chlorophyll-derived phytol and hydrogen sulfide (H2S) via phytadiene intermediates. The reactions of phytol sealed with H2S-saturated seawater, lacustrine freshwater and distilled water all produced two of the three thiophene isomers above. The molecular structure of the thiophenes deter-mined by their mass spectra suggested an incorporation of sulfur into the conjugated s-cis diene moiety of phytadienes, which can be formed by dehydration of phytol. An acid dehydration experiment of phytol in aqueous media yielded four phytadiene isomers. Subsequent reaction of the phytadiene mixtures in the H2S-saturated seawater gave the same two thiophene isomers. These results provided substantial evidence for a mild reaction to produce the limited number of C20 isoprenoid thiophene isomers which occur during the very early stages of diagenesis, not only under saline but also under freshwater conditions. In fact, the three C20 isoprenoid thiophene isomers were identified in the surface sediment of a freshwater lake of low pH owing to the inflow of sulfide-rich hot-spring water (Lake Usori-ko, Japan).