Abiotic and biological hopanoid transformation: towards the formation of molecular fossils of the hopane series

Tritium labeled hopane and bacteriohopane were incubated in the presence of the bacterium Arthrobacter simplex. In both experiments, the corresponding 17(21)-olefins and 17,21-epoxides were formed from the saturated hydrocarbon. In addition, hop-17(21)-en-20-one, 17,21-secohopane-17,21-dione and trinorhopan-21-one were found in the case of the hopane incubation. Control experiments were performed in the absence of the bacteria either with the culture medium alone or with the culture medium containing the non-ionic surfactant Emulphogenefi. Tritium labeled hop-17(21)-ene and 17,21-epoxyhopane were incubated under the same conditions. Only the 17(21)-olefins and probably to some extent the 17,21-epoxides, resulted from the bioconversion of the corresponding saturated hopanoids. The other hopanoids were formed by the abiotic oxidation of hop-17(21)-ene. Their formation was however favored by the presence of the bacteria or of the non-ionic surfactant Emulphogenefi, suggesting that dispersion of the hydrophobic hopanoid in the aqueous phase was essential for the oxidative degradation. Surprisingly, no degradation of the n-alkyl chain of bacteriohopane was observed.