Métrica de Grado de Corrección de Datos la Tabla

Hydrothermal petroleums/bitumens, which are products of rapid dia- genesis, catagenesis and metagenesis in aqueous systems (e.g. marine rift systems), have alkane and biomarker distributions analogous to

those of conventional crude oils.20,155–160 The carbon number distri-

butions, biomarker compositions, and other geochemical parameters of marine hydrothermal petroleums generally reflect the source organic mat- ter and the degree of thermal alteration or maturity.20,32,155,156,160–162 An example of the alkane and biomarker patterns representative of

Fig. 11. Annotated GC-MS TIC traces of the total extracts (TMS derivatives) of seed cones of: (a) Glyptostrobus oregonensis (Miocene), and (b) Taxodium distichum (extant).

hydrothermal petroleums is shown in Fig. 12, where alteration has pro- ceeded from immature natural product precursors to the fully mature petroleum biomarkers. Immature biomarkers occur in low-temperature regions of sedimentary hydrothermal systems, and maturation is observed with increasing temperature and depth below the seafloor (sub-bottom depth).159,163–167For example, sterols are major components in unaltered

Fig. 12. Salient features of the GC-MS data for a hydrothermal petroleum (Guaymas Basin, Gulf of California, Mexico): (a) TIC trace of total oil, (b) m/z 191 key ion for hopanes, and (c)m/z 217 key ion for steranes. Numbers refer to carbon chain (n-alkanes) or skeleton, UCM = unresolved complex mixture, Pr = pristane, Ph= phytane, asterisks = other isoprenoids, α, βα, αββ, R, S = configurations of biomarkers.

sedimentary sections and range from C27 to C29, with cholesterol domi-

nant or equal to sitosterol. Diagenetic alteration of sterols accelerated by thermal stress yields stenones and stanones with the same range from C27

to C29 and ultimately steranes and diasteranes (Fig. 12c). The hopanes

(Fig. 12b) are derived from the reductive alteration of hopanoid precursors primarily in bacterial detritus.

Novel biomarkers, i.e. tracer derivatives from unknown natural prod- ucts, are sometimes encountered in geological or environmental samples, typically as hydrocarbons. The detection and determination of these com- pounds are usually based on the interpretation of mass spectra in GC- MS analyses. The proofs of chemical structures are based on the pro- posed interpretation of the MS data, separation and purification of the unknown compounds, exact structure determination by NMR methods or X-ray crystallography (if the compound is a solid that can be crystal- lized), and finally, comparison with a synthetic standard.168–173The next question concerns the biological source of the biomarker precursor com- pound. Many biomarkers still have no proven natural product precursors nor known biological sources (e.g. perylene, tricyclic terpanes).174–176

The characterization of a novel series of biomarkers is illustrated with the gem-dialkylalkanes in bitumen from a hydrothermal system on the Mid- Atlantic Ridge.177 The total bitumen consists of hydrocarbons, a major UCM (unresolved complex mixture of branched and cyclic compounds) and mature biomarkers (e.g. hopanes) (Fig. 13a). The bitumen contains a series of cyclopentylalkanes (CnH2n) that range from n = 14 to 34,

with only even-chained pseudohomologs and a concentration maximum (Cmax) at n= 18. Their source is biogenic, based on the presence of only

even-carbon number homologs, but the precursors are unknown.177

Four other significant homologous series present are gem-diethyl sub- stituted n-alkanes. The gem prefix designates geminal-substituted com- pounds, i.e. two substituents on the same atom of a disubstituted compound. Several research groups have reported the presence of 5,5- diethylalkanes, as well as lesser amounts of other gem-dialkylalkanes, in sed- imentary rocks back to the Precambrian and in hydrothermal fluids.178–181 The correct structural assignments of the 3,3-diethylalkanes (XXIX) and the 5,5-diethylalkanes (XXX) were described recently by comparison with

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Fig. 13. Salient features of the GC-MS data for the saturated fraction of a hydrothermal bitumen sample from the Mid-Atlantic Ridge: (a) TIC - background trace, (b) m/z 85 fragmentogram (key ion for 3-ethyl-3-methylalkanes and n-alkanes), (c) m/z 99, key ion for 3,3- diethylalkanes with n-alkanes, and (d) m/z 127, key ion for 5,5-diethylalkanes with n-alkanes. (Numbers refer to total carbon number, dots over peaks are n-alkanes.)

synthetic standards.179The other gem-dialkylalkane series were tentatively identified based on the interpretation of their characteristic mass spec- trometric fragmentation patterns and gas chromatographic retention fac- tors.178,179The major homologous series can be visualized by key ion plots. The 3-ethyl-3-methylalkanes (i.e. 2,2-diethyl substitution) range from C14

to C34, with only even-chained pseudohomologs detectable and a Cmax at

18 (e.g. Fig. 13b). Their structures are interpreted from the mass spectra (e.g. Fig. 14a), which consist of a cleavage of C2-C3 to yield the base peak (C6H13, m/z 85), M-C2H5, minor alkane cleavage, and no molecular ion.

The 3,3-diethylalkanes (XXIX) range from C15to C38, with a Cmaxat 27

and only odd-carbon numbered pseudohomologs (Fig. 13c). The structures are based on an interpretation of the mass spectrometric fragmentation pat- terns (e.g. Fig. 14b) and coinjection of authentic 3,3-diethylpentadecane.

The mass spectra have a base peak at m/z 57, an intense ion at m/z 99

(C7H15, key ion) from C3-C4 cleavage, M-29 (C2H5), minor M-57, typ-

ical alkane fragments, and no molecular ion.

The 5,5-diethylalkanes (XXX) also range from C15 to C39 with odd-

carbon numbered pseudohomologs and a Cmax at 29 (Fig. 13d). The

structures are based on the interpretation of the mass spectrometric frag- mentation patterns, and on prior reports of the occurrence of those

compounds in sedimentary sulfides and rocks.178–180 The mass spectra

generally exhibit a base peak at m/z 57 (C4H9), an intense key ion at

m/z 127 (C9H19), and M-29 (C2H5), M-57 (C4H9) and other general

alkane fragments, but no or a low-intensity molecular ion (e.g. Fig. 14c). The 6,6-diethylalkanes range from C16to C38, with a Cmaxat 30 and only

even-carbon numbered pseudohomologs. The structures are based on the analogous interpretation of the mass spectra (e.g. Fig. 14d), which have a base peak at m/z 57 (C4H9), an intense key ion at m/z 141 (C10H21),

and M-29 (C2H5), M-71 (C5H11), general alkane fragments, but no

molecular ion.

The branched gem-alkane series are biomarkers because of the locale of their occurrence and the presence of only alternate pseudohomologs (even- or odd-carbon numbers only). Their inferred origin is from probable microbial precursors of unknown species, where methylation and ethyla- tion, diethylation, butylation and ethylation occurred during biosynthesis

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Fig. 14. Representative mass spectra of gem-dialkylalkanes in a hydrothermal bitumen sample: (a) 3-ethyl-3-methylpentacosane, C28H58, (b) 3,3-diethyltricosane, C27H56, (c) 5,5-diethyltricosane, C27H56, and (d) 6,6-diethyltetracosane, C28H58.

at the C-2, C-3, C-5, or C-6 positions of odd- or even-carbon chained substates (i.e. C11-C33). These compound series occur in samples of relict

or weathered hydrothermal talus at the base of active vent systems, consis- tent with a lower input of detritus from archea and with an enrichment of lipid residues from microbial mats and/or sulfide-oxidizing bacteria as proposed for the ancient examples178,180,181 and hydrothermal fluids on the Juan de Fuca ridge flank.179

4.4 PROGNOSIS

This chapter has presented an insight with some key literature about the elucidation of natural products for applications as biomarkers and molec- ular tracers for numerous environmental and geological processes. Natural product biomarker elucidation and analysis has been illustrated with var- ious examples to clarify the concepts, applications, and procedures. The number of novel compounds, as well as the application of natural products as biomarker tracers, is expected to increase, especially with the use of mass spectrometry methods in the contemporary interdisciplinary sciences.

Most analytical methods for current natural product/biomarker appli- cations usually deal with individual compounds in the concentration range of pg toµg g−1, small sized samples (5–1000µl solution), and in complex mixtures. At those low concentrations, the contamination by extraneous organic matter during sample acquisition, preparation, extraction, and analysis can be a major problem and analysts need to take precautions. The interpretation by an organic mass spectrometrist/chemist should be sought when mass spectra of unknown compounds are encountered. This may lead to the characterization of novel biomarkers with the ultimate structure proof by other chemical methods (e.g. NMR). Finally, I encour- age the classical natural product chemists to continue to collaborate with environmental and geological organic chemists by providing their expertise and invaluable standards.