Alkanes isoprenoidal

Advances in petroleum characterization at the molecular structure level by GC-MS methods renewed interest in OSC. Within the past few years, at least one-thousand new and novel OSC that previously were not known to be present in petroleum and bitumens have been reported. Tentative molecular structures inferred from GC-MS and other techniques have been confirmed in many cases by synthesis of authentic reference-compounds. The difficult and time-consuming synthetic work has been crucial in validating many of the novel structures. Another key finding has been that immature bitumens and crude oils (samples that have not received significant thermal stress) differ markedly from the previously known OSC in that they have carbon-skeletons resembling ubiquitous biomarker hydrocarbons (e.g., n-alkanes, isoprenoid alkanes, steranes, and hopanes). This similarity, of course, suggests that the hydrocarbons and OSC have common biogenic precursors. 

The distribution of n-alkanes, isoprenoids and other branched hydrocarbons in the saturated/unsaturated hydrocarbons fraction of the modem microbial mat ranges mostly between C14 and C21, in extracts and pyrolysates (Figure 5A and 6C). It is similar to the distribution of the hydrocarbons described in the "top mat" of the Gavish sabkha in Israel (17). The main differences are the presence, in the extract of... 

A major goal in organic geochemistry is to reconstruct depositional environments of the geological past based on qualitative and quantitative analyses of biological markers in sediments. Classes of hydrocarbons such as n-alkanes, isoprenoid alkanes, steranes and hopanes are routinely used for this purpose (1). 

Figure 1 shows the absolute concentrations in dead oils and condensates of several major geo-chemically usable compound types in 351 petroleums covering the full range of maturities found in petroleum system fluids. Samples suspected of biodegradation were filtered out based on light hydrocarbon and n-alkane isoprenoid parameters. 

Recent advances in isotope analysis include the ability to analyse isotopically smaller proportions of individual compounds by the use of more sensitive online isotope ratio mass spectrometers (cf. Merritt Hayes 1994 Merritt et al. 1994) and stable carbon isotope determination of individual n-alkanes, isoprenoids and biomarkers in petroleums is now a standard tool. Despite early work on collection of chromato-graphically separated individual n-alkanes followed by combustion and isotope determination (Welte 1969) it was for a number of years impossible to analyse isotopically long chained alkanes with the same comfort and ease as the... 

Regular branched alkanes isoprenoids. Typical compound 2,6,10,14-tetra-methylpentadecane or pristane structure ... 

McKenna EJ, RE Kallio (1971) Microbial metabolism of the isoprenoid alkane pristane. Proc Natl Acad Sci USA 68 1552-1554. 

The H- and N-isoforms of Ras support the first (isoprenoid) hydrophobic modification by additional thioester formation with palmitoylic acids [18]. At physiological temperature (37°C) the dissociation of doubly modified lipo-peptides with an isoprenyl thioether and a palmitoyl thioester is very slow and characterized by half-times in the order of 50 h. Here, the relative effect of the carboxymethylation is significantly reduced. Palmitoyl groups with their C16 alkane chain length contribute more efficiently to membrane anchoring than the farnesyl modification. 

Susceptibility to n-alkane degradation is an inverse function of chain length. Branched alkanes are less susceptible than straight-chain n-alkanes, and the most resilient saturated components are the pristine and phytane isoprenoids (Wang et al. 1998). 

The solubility of fuel oil no. 2, particularly the alkane and isoprenoid fractions, in seawater is increased by the presence of fulvic acid, although the solubilities of phenanthrene or anthracene, both polycyclic aromatic hydrocarbons, are unaffected by the presence of humic materials (Boehm and Quinn 1973). Unfiltered Narragansett Bay water was able to dissolve 1,560 g/L of fuel oil no. 2, although removal of... 

The age of diesel oil in the subsurface soil environment can be determined by utilizing the fact that the composition of the diesel oil (the ratio between -alkanes and isoprenoids) changes due to biodegradation. In one study, the ratio of C , to pristane was highly correlated with the residence time of diesel fuel at 12 test locations (Christensen et al 1993). 

Fig. 9. GC-MS TIC traces for silylated total extracts of soil, river sediment and aerosol samples (a) Amazon Forest soil (Manaus, Brazil) (b) almond orchard agricultural field soil (CA, USA) (c) Harney River sediment in Everglades National Park (FL, USA), and (d) Gosan Island (Korea) aerosol during Asian dust event (April 27—28, 2001). Numbers refer to carbon chain length of homologous series ( = rj-alkane, o = rj-alkanol, A = rj-alkanoic acid, DHA = dehydroabietic acid, ik = isoprenoid ketone, S = sitosterol). 

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) mjz 191 key ion for hopanes, and ctmjz 217 key ion for steranes. Numbers refer to carbon chain (/ -alkanes) or skeleton, UCM = unresolved complex mixture, Pr = pristane, Ph = phytane, asterisks = other isoprenoids, ot, ot, R, S = configurations of biomarkers.

Acyclic isoprenoid Present in the marine en- Saturated forms reported alkanes vironment but often un- from recent marine sedi-... 

First reported from petroleum in 1963 (268, 269), acyclic isoprenoid alkanes have now been identified in crude oils from three continents (270-273), and in one crude oil all 12 isoprenoid hydrocarbons from C14 to C25 as well as head-to-tail conformations, have been found... 

Calvin, M., Occurrence of Isoprenoid Alkanes in a Precambrian Sediment, in Advances in Organic Geochemistry, pp. 41-74, Pergamon, Oxford, 1966. 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

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