Hopanoids systems

Fig. 4. 25 Some important biomarker hydrocarbons in crude oils (n.b. numbering system for cheilanthanes is based on the C25 parent sesterterpane and so is different from the hopanoidal system).

On the other hand, in penta- and hexa(spirotetrahydrofuranyl)cyclohexyl systems the chair conformation of the central cyclohexane ring is still preserved (96JA4504). The boat conformation of the cyclohexane ring can be stabilized also by careful substitution. A series of hopanoid hydrocarbons, the D cyclohexane ring prefers the boat conformation by 1.3-2.5 kcal/mol (95JA6532). 

Fig. 2.20 (a) Ring numbering conventions for steroids and hopanoids (b) examples of all-chair conformations (with trans ring junctions) for steroids and hopanoids (c) examples of application of hopanoidal nomenclature system. (Ts may also be called 17-methyl-18,22,29,30-tetranor-17a,18a-hopane.)... 

Sterols comprise a very important class of compounds in plant and animal systems. Bacteria and other organisms utilize cholesterol or related sterols in the synthesis and maintenance of their cell walls. True sterols are present in cyanobacteria to only a very small degree they are largely replaced with a steroid-like class of triterpenoid known as the hopanoids. The carbon frameworks of sterols and bacterial hopanoids are synthesized in essentially the same manner as other terpenes, however, the cyclization reactions to yield their polycyclic skeletons are unique. A variety of sterols and hopanoids are found in cyanobacteria and they are believed to have important cellular functions in cell wall structure and function. " As discussed in Section 2.06.5, many of the hopanoids also possess a polyalcohol chain derived from a sugar. ... 

In specific natural products, double bonds often occur within the ring system, (c) Squalene folded so as to demonstrate its relationship to the pentacyclic ring systems in stractmes d, e, and f (d) The hopane carbon skeleton, (e) Tetrahymanol. (f) Bacteriohopane tetrol, an extended hopanoid that includes five carbons deriving from the C5 sugar ribose. (g) p-Amyrin, a typical pentacyclic isoprenoid produced by higher plants. 

Hopanoids are membrane stabilizers in certain procaryotes (E 2.2). Their hydrophilic, polyhydroxylated side-chain is directed towards the aqueous face the lipophilic ring system is integrated in the lipophilic core of the membranes. Lanosterol is used as emulsifier in the pharmaceutical industry. Cucurbitacins due to their bitter taste may discourage animal predators, but attract certain beetles, which use Cucurbitaceae as food plants (E 5.5.3). 

A few more complex systems should be mentioned. An interesting one is the very peculiar lipid A-type pentasaccharide 75 whieh has been identified in the Bradyrhizobium strain BTAil lipid eontent, showing a unique structure in which the lipopolysaccharide (LPS) lipid A is covalently substituted by an hopanoid structure. This eompound is built on a pentasaccharidic backbone, with several fatty ehains eonnected as amides on the aminodeo groups of two internal residues. One of the fatty chains is substituted at its other end with an hopanoid backbone through an ester bond. One galacturonic and one dimannosyl residue complete the structure, that we could consider as a SFCX system, for sure a very complex one. The discovery of this unique molecule by Molinaro and co-workers and the accompanying studies revealed the influence of structural variations in lipid A type eompounds on the properties of cell membranes, notably in the eontext of plant-microbe symbioses. ... 

Jurgens, U.J., Simonin, P., and Rohmer, M. (1992) Localization and distribution of hopanoids in membrane systems of the cyanobacterium Synechocystis PC6714. FEME Microbiol. Lett., 71, 285-288. 

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