Propuesta de Métricas de Proyectos

Many natural products, i.e. homologous aliphatic (lipids), polar (sug- ars, amino acids), and cyclic (terpenoids) compounds can be utilized as biomarkers. The following is a brief overview of the classical biomarkers commonly used, namely lipids and terpenoids.

4.1.4.1 Lipids

Lipids are important biomarkers because they carry a strong carbon- number predominance that is inherited from their biosynthesis (e.g. buildup from acetate), and their homolog distribution can reflect biogenic origin (i.e. marine vs. terrestrial origin).8,17They are generally derived from the epicuticular waxes and related lipids of higher plants. They consist pri- marily of n-alkanes (C27, C29, C31, C33,), n-alkanols, and n-alkanoic acids

(both homologous series have dominant C24, C26, C28, or C30), with lesser

amounts of other oxygenated homologous species. 4.1.4.2 Terpenoids

The terpenoid biomarkers from higher plants and other flora are the natural and derivative products from the reductive and oxidative alteration of the precursors. That alteration can occur during transport, by diagenesis in sed- imentary environments, or by thermal transformation processes.3,18–21,83 Reductive alteration generally yields the parent compound skeleton with various isomerizations of chiral centers and, in some cases, loss of carbon due to decarboxylation and other reactions. Oxidative alteration occurs mainly by successive ring aromatizations that usually commence from a

ring that has a functional group (e.g. OH, C= C, C = O, typically on

ring-A), by direct dehydrogenation, dehydration, ring rearrangement, or ring opening and subsequent loss.

Mono-, sesqui-, and some diterpenoids are found in marine and terres- trial flora. They are, therefore, not always unambiguous tracers for higher plant sources. However, diterpenoids with the abietane and pimarane (Fig. 1), and less common phyllocladane and kaurane, skeletons are predominant constituents in resins and supportive tissue of coniferous vegetation (Coniferae), which evolved in the late Paleozoic (200–300 million years ago). Diterpenoid biomarkers have been characterized in

Fig. 1. Scheme for the alteration of diterpenoid natural product precursors to saturated and aromatic derivatives (natural product examples are boxed, stereochemistry is indicated where known).

ambers, coals, sediments, contemporary environments, and anthropogenic emissions.3,6,11,23The product-precursor relationship for diterpenoids has been presented by many authors7–9,11,24–26and is summarized in Fig. 1. Reductive preservation retains the C20skeletons, which are the major diter-

pane biomarkers in the geological record. Decarboxylation of resin acids with subsequent reduction yields biomarkers with structures≤ C19.

Triterpenoids are a major group of natural products in higher plants. The tetracyclic triterpenoids, based on the lanostane, euphane, onocer- ane, and dammarane skeletons, are found mostly in vascular plants and

Fig. 2. Scheme for the alteration of triterpenoid precursors from higher plants to saturated and aromatic derivatives (natural product examples are boxed, stereochemistry is indicated where known).

have been reported in sedimentary rocks.11,27 Many pentacyclic triter- penoids [e.g. the oleanane, ursane, taraxerane, lupane, friedelane, serratane, or bauerane skeletons (Fig. 2)] are characteristic natural product tracers for an origin from terrestrial higher plants. They occur as functionalized (e.g. alcohols, acids, ketones, esters) precursors but are not necessarily specific to

individual classes of biota. The most frequently encountered compounds in sedimentary or contemporary environments are those derived from α- and β-amyrins (I, II, ursanes and oleananes, respectively, structures of

key compounds are shown in Appendix I).11,23The reductive and oxida-

tive alteration of the amyrins is illustrated in Fig. 2. It should be noted that the presence of a functionality at C-3 in those triterpenoids makes them more susceptible to microbial or photochemical degradation to yield ring-A opened products and ultimately compounds without ring-A (e.g. des-A- oleanane). Reductive alteration of triterpenoids yields mainly the parent skeleton with epimerization of key chiral centers. Aromatic triterpenoids were first isolated and characterized from brown coal extracts, and were inferred to be derived from triterpenoids based on the structural similarity and the origin of the coal.6,28,29

Steroids are natural product derivatives from triterpenoids and are ubiquitous in the geosphere and the ambient environment. The phytos- terols (mainly C29, minor C28 skeletons) have been used as indicators

for higher plant sources, although many marine algae also biosynthesize C28 and C29 sterols with the same or different alkyl substituents on the

side chain.11,30The reductive and oxidative alteration of steroids is illus- trated in Fig. 3, and shows the geosteranes, biosteranes, aromatic steroid hydrocarbons, and thermal cracking derivatives. Geological reductive pro- cesses produce steranes and diasteranes (geosteranes) with typically the parent skeletons and various epimerizations of chiral centers. Products from dealkylation of the side chain are also encountered. The steroid aromatization reactions have been elucidated in the geological record.31–33 Biological reductive processes produce epi-stanols and related derivatives (Fig. 3,34).

Tetraterpenoids and polyterpenoids are minor components of higher plants and are generally overwhelmed by the input of those com- pounds from microbial biomass in marine and lacustrine environments or sedimentary rocks. The natural cyclic tetraterpenoids have a maximum of two alicyclic rings, and thus the saturated and aromatic derivatives are limited. The common parent skeltons are lycopane, carotane, 1-(2’, 2’,6’-trimethylcyclohexyl)-3,7,12,16,20,24-hexamethylpentacosane, and biphytane.35,36

SP I-B530 Selected T opics in the Chemistr y of N atural P roducts Ch04 B.R.T . Simoneit

Fig. 4. Scheme for the alteration of hopenoids to aromatic hopanoids and hopane (natural product examples are boxed).

The hopane series are the natural product biomarkers elucidated ini- tially as attributable to bacteria.37–40The 17α(H),21β(H)-hopanes rang-

ing from C27 to C35 (no C28) were encountered in numerous ancient

sediments and petroleums, and diagenesis and maturation of the micro- bial precursors (e.g. bacteriohopanepolyol and diploptene, Fig. 4) were elucidated.40,41The diagenesis of diploptene in contemporary sediments

proceeds by double bond migration from ∆22,29 via ∆21,22 to ∆17,21

and possibly to neohopene (Fig. 4). Oxidation of bacteriohopanetetrol yields mainly the homohopanoic acids ranging from C31to C34and minor

homohopanols.

Aromatization of hopanoids, when derived from bacterial detritus in sediments, is inferred to proceed via diagenetic alteration or bond- ing of the precursors to organic matter (e.g. diploptene to neohopene,

bacteriohopanepolyol), subsequent cracking and then dehydrogenation from ring-D to ring-A (Fig. 4.42,43).

4.2 METHODS

The experimental and analytical methods used in the numerous other reports cited in this chapter should be consulted directly if it is of use. Here, I provide a brief summary of the experimental and analytical methodology generally used for the studies discussed here as examples.