Novel hopanoids from Frankia spp. and related soil bacteria - Squalene cyclization and significance of geological biomarkers revisited

Three series of hopanoids, differing by their configurations at C-17 and C- 21, have been identified in several Frankia spp. and other related soil bac teria. The widespread bacterial hopanoids of the 17 beta (H),21 beta (H) se ries were accompanied by their isomers of the 17 beta (H),21 alpha (H) (mor etane) and 17 alpha (H), 21 beta (H) series. The latter series has not prev iously been found in living organisms and is considered to be a result of t he abiotic isomerization of the thermodynamically less stable 17 beta (H),2 1 beta (H) hopanoids. This simultaneous presence of three isomeric hopanoid series highlights intriguing problems in the biogenesis of the bacteriohop ane skeleton and partly questions the significance of hopanic biomarkers in sediments.