Hopanes, Geological Record of

Definition

Hopanes is a class of pentacyclic compounds derived from bacteriohopanepolyols (hopanoids) via diagenetic degradation and thermal maturation. The hopane structure is a saturated hydrocarbon (Fig. 1). Hopanes are molecular fossils of bacterial lipids that serve to adjust membrane permeability and fluidity as well as other adaptation to extreme habitats. Some hopanoids and derived hopanes are regarded as specific to certain groups of bacteria, such as 2-methyl hopanoids from photosynthetic bacteria (cyanobacteria and purple sulfur bacteria) and 3-methylhopanoids from Type I aerobic methylotrophic bacteria.

Fig. 1

Core structure of hopanes composed of C-C bonds (lines) and saturated with hydrogen atoms (not shown). The key stereochemical variants occur at C-17, C-21, and C-22 positions. Additional methylation is common at C-2 and C-3. Other structural variants are missing methyl groups. R = H, methyl, ethyl, n-propyl, n-butyl, or n-pentyl group