Some biomass species are able to reduce C02 via the initial sugars produced in photosynthesis to higher energy hydrocarbons, most of which have terpene structures. Because the energy values of terpene hydrocarbons can be as high or higher in some cases than conventional motor fuel components, and because some of the terpenes have been used as motor fuels and chemical feedstocks, a somewhat more detailed discussion of the biochemical pathways of hydrocar­bon production in biomass is worthwhile. The terpenes are isoprene adducts having the generic formula (C5H8)„, where n is 2 or more. A large number of terpene derivatives in various states of oxidation and unsaturation (terpenoids) may also be formed. Perhaps the best-known example of natural hydrocarbon production is high-molecular-weight polyisoprene rubber having very high stereospecihcity from the cambium of the hevea rubber tree (Hevea braziliensis), a member of the Euphorbiaceae family that grows in Brazil and in other tropical climates. In contrast, the Brazilian tree, Copaifera multijuga, a member of the Caesalpiniaceae family, produces relatively pure liquid sesquiterpene hydrocar­bons (C15H24), not in the cambium but in the heartwood from pores that run vertically throughout the tree trunk (Calvin, 1983). In some members of the Euphorbiaceae family such as E. lathyris, the biosynthetic pathway leads mainly to the acyclic dihydrotriterpene squalene, Сз0Н50, which then undergoes inter­nal cyclization to form C30 terpenoid alcohols and sterols. In still other biomass species, lower molecular weight acyclic and alicyclic isoprene adducts are formed as monoterpenes (C10Hi6) and diterpenes (C20H48). Certain aquatic, unicellular biomass such as the green microalgae Botryococcus braunii are reported to accumulate terpene-type hydrocarbon liquids within the cells, sometimes in large amounts depending on the growth conditions.

The major steps in the mechanisms of terpene and polyisoprene formation in plants and trees are known, and this knowledge should help improve the natural production of terpene hydrocarbons (Fig. 3.4). Mevalonic acid (1), a key intermediate derived from plant sugars via acetylcoenzyme A, is succes-

-co2

-h2o

■ C10Monoterpenes

CH,

V.

+ 111.

-OPP

FIGURE 3.4 Biochemical pathways to terpenes.

sively transformed into 5-diphosphomevalonic acid (II) and the five-carbon intermediate isopentenylpyrophosphate (III) via phosphorylation, dehydra­tion, and decarboxylation. Isomerization of a portion of III then occurs to form dimethylallylpyrophosphate (IV). The isomers III and IV combine by head-to-tail condensation to form another allylic pyrophosphate containing 10 carbon atoms (V), which can be converted to monoterpenes. Inorganic phosphate is released in the process. Continuation of this process leads to all other terpenes. The chain can successively build up by five-carbon units to yield sesquiterpenes and diterpenes containing 15 and 20 carbon atoms via VI and VII by additional head-to-tail condensations. The same structural unit

is inserted in each step. A single enzyme, farnesyl pyrophosphate cyclase, is reported to be involved in the cyclization of the farnesyl units in C. multijuga to yield many sesquiterpenes (Calvin, 1983). Alternatively, tail-to-tail conden­sation of two C15 farnesyl units can yield the 30-carbon compound squalene followed by a large number of cyclizations and rearrangements to yield an array of natural triterpenoids, as already mentioned. Similar condensation of two C20 units yields phytoene, a precursor of carotenoids. This information is expected to help in the development of genetic engineering methods to control the hydrocarbon structures and yields.