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Identification of a Novel Series of Benzohopanes and Their Geochemical Significance H. Peter Nytoft,† Nikola S. Vuković,‡ Geir Kildahl-Andersen,§ Frode Rise,∥ Dragana R. Ž ivotić,⊥ and Ksenija A. Stojanović*,‡ †

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350K Copenhagen, Denmark University of Belgrade, Faculty of Chemistry, Studentski trg 12-16, 11000 Belgrade, Serbia § University of Oslo, School of Pharmacy, P.O. Box 1068 Blindern, N-0316 Oslo, Norway ∥ University of Oslo, Department of Chemistry, P.O. Box 1033 Blindern, N-0315 Oslo, Norway ⊥ University of Belgrade, Faculty of Mining and Geology, Đušina 7, 11000 Belgrade, Serbia ‡

ABSTRACT: A series of novel C33−C35 hexacyclic benzohopanes (C33b-C35b) were identified in 39 samples of coal extracts and 39 crude oils of different ages from all over the world. C33b and C34b homologues were isolated, and their structures were determined by nuclear magnetic resonance. The structure of C35b benzohopane was proposed based on the mass spectrum and its similarity with the mass spectra of structurally defined C33b and C34b homologues. The structures of the C33b-C35b hexacyclic benzohopanes are closely related to isohopanes; both groups are typical for terrestrial organic matter and can be useful in the correlation analysis. A possible pathway of formation of these novel benzohopanes and their hopanoid precursors with an additional branch in the aliphatic side chain is proposed. C33b−C35b hexacyclic benzohopanes are stable up to the maturity level corresponding to random vitrinite reflectance (Rr) of ∼0.80%, which was demonstrated by analyzing the samples of different maturity and by the maturation simulation experiments: hydrous pyrolysis of two bituminous coals (Rr = 0.55 and 0.59%) and pyrolysis of an extracted bituminous coal (Rr = 0.56%) and its asphaltenes. This represents a confirmation that the formation of these novel benzohopanes is related to specific depositional conditions and microbial activity during diagenesis. Mature samples (Rr ≥ 0.8%) and hydrous pyrolysate of the bituminous coals (Rr < 0.60%) obtained at 330 °C show a distinct distribution of benzohopanes in comparison to immature and moderately mature samples, which is characterized by a low abundance of the “b” series benzohopanes and the presence of regular and numerous other benzohopane isomers. The latter most likely represent isomers of regular and novel benzohopanes with different substitution patterns on the aromatic ring. This isomerization of alkyl groups attached to the aromatic ring, leading to the formation of thermodynamically more stable isomers, is a well-known maturation scenario so far reported in the series of alkylated naphthalenes, phenanthrenes, and dibenzothiophenes. Therefore, in the same way, a distribution of benzohopanes can indicate thermal maturity. In addition to the novel benzohopanes, three series (2α, 2β, and 3β) of their methylated derivatives were identified in numerous samples. Finally, a novel C35 heptacyclic benzohopane with an additional cyclopentane ring was also observed in the studied samples, and its structure was tentatively identified based on the mass spectrum. Opposite to the hexacyclic C33b−C35b benzohopanes, the formation of the C35 heptacyclic benzohopane does not require a specific hopanoid precursor with two branches in the side chain. Therefore, this compound seems to have less geochemical significance than the new hexacyclic benzohopanes.

1. INTRODUCTION Regular benzohopanes (C32−C35) cyclized at the C-20 position1,2 are common constituents of coals, oil shales, source rocks, and oils of different ages from all parts of the world. These are the most abundant compounds with a benzohopane skeleton and can be easily detected in the corresponding aromatic fraction by means of gas chromatography−mass spectrometry (GC−MS) using the typical mass fragmentogram m/z 191. Some other aromatic hopane structures are also reported in the literature. Benzohopanes cyclized at C-16 cooccur with the regular benzohopanes and are widespread in immature sediments, particularly those deposited in carbonate or evaporitic environments.3 Several aromatic 8(14)-secohopanoids, including ring D monoaromatic compounds and diaromatic compounds with a fluorene or acenaphthene moiety, were also identified in the sedimentary organic matter (OM) and crude oils.2,4−8 Recently, Cong et al.9 identified © 2016 American Chemical Society

previously unknown benzohopanes in the Shengli lignite samples from China and proposed a novel type of cyclization of the hopanoid polyols at C-30. Moreover, the occurrence of hopanes with a thiophene moiety in the sedimentary OM is also well-documented.9−12 In the present study, we detected a series of novel C33−C35 hexacyclic benzohopanes (labeled as “b” series). C33b and C34b homologues were isolated, and their structures were determined by way of nuclear magnetic resonance (NMR) using 1D and 2D techniques. The structure of C35b benzohopane was proposed based on the mass spectrum and its similarity with the mass spectra of structurally defined C33b and C34b homologues. Implications related to their origin, Received: April 5, 2016 Revised: June 1, 2016 Published: June 6, 2016 5563

DOI: 10.1021/acs.energyfuels.6b00799 Energy Fuels 2016, 30, 5563−5575

Article

Energy & Fuels Table 1. Isohopane and Benzohopane Data for 39 Coals sample

country

area

age

9586 9587 9584 9583 9606 9603 9605 6375 OY4-24M OWK-01T 438853 438859 438861 438862 438866 Bogovina 26 Bogovina 30 4736A 4514A 4528A 4425Ac 4698A 4231A 9579 4430A 3473A 4285A 4244A 3609A 4257A 3247A 4529A 3586A 4086A 4256A 4242A 9589 9588 12247

Poland USA UK Germany China Australia Russia Denmark Nigeria Nigeria Greenland Greenland Greenland Greenland Greenland Serbia Serbia Colombia Colombia Colombia Colombia Colombia Colombia Colombia Colombia Colombia Colombia Venezuela Indonesia Indonesia Indonesia Indonesia Indonesia Indonesia Indonesia Indonesia Malaysia NZ Norway

Upper Silesian Coal Basin Appalachian Basin Pennine Basin Ruhr Basin Shangong province Sydney Basin Kuznetsk Basin Øresund-18 well Anambra Basin Anambra Basin Qullissat, Nuussuaq Basin Qullissat, Nuussuaq Basin Qullissat, Nuussuaq Basin Qullissat, Nuussuaq Basin Qullissat, Nuussuaq Basin Bogovina Basin Bogovina Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Cesar and Rancheria Basin Maracaibo Basin Kutei and Barito Basins Kutei and Barito Basins Kutei and Barito Basins Kutei and Barito Basins Kutei and Barito Basins Kutei and Barito Basins Kutei and Barito Basins Kutei and Barito Basins Bintulu Sarawak West Coast Coalfield Svalbard

carboniferous carboniferous carboniferous carboniferous permian permian permian jurassic cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoiz cenozoic cenozoic

refa 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,

19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 13, 13,

14 14 14 14 14 14 14 15 16 16 17 17 17 17 17 18 18 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 13 14 14 13

Rr %

IHR 33b

0.73 0.73 0.81 0.82 0.67 0.71 0.78 0.51 0.55 0.59 n.a. n.a. 0.45 n.a. 0.48 0.42 0.56 0.54 0.55 0.55 0.57 0.57 0.59 0.61 0.62 0.63 0.63 0.66 0.37 0.42 0.43 0.44 0.45 0.50 0.52 0.55 0.44 0.61 0.72

0.20 0.19 0.16 0.23 0.40 0.05 0.07 0.65 0.83 0.95 0.46 0.64 0.57 0.92 0.52 0.35 0.29 0.49 0.55 0.46 0.83 0.79 0.49 0.39 0.45 0.46 0.43 0.49 0.40 0.39 0.39 0.39 0.48 0.44 0.53 0.37 0.25 0.12 0.13

IHR 34−31b IHR 34−32b 0.29 0.09 0.23 0.33 0.50 0.05 0.05 1.08 1.09 1.43 0.74 1.85 1.48 1.89 1.96 n.a. n.a. 0.76 0.78 0.71 1.59 1.52 0.79 0.66 0.72 0.71 0.67 0.57 0.69 0.77 0.74 0.79 0.85 0.72 0.92 0.68 0.36 0.09 0.09

0.72 0.21 0.50 0.66 0.60 0.64 0.94 0.16 0.32 0.24 0.26 0.30 0.25 0.24 0.32 n.a. n.a. 0.81 0.76 0.84 0.69 0.87 0.81 0.92 0.89 0.85 0.85 0.56 0.47 0.53 0.40 0.56 0.50 1.10 0.75 1.50 0.33 0.20 1.33

C34b/ C34a

C35b/ C35a

n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.61 0.71 0.93 0.40 0.57 0.51 0.67 0.58 0.32 0.68 0.54 0.93 0.84 1.12 1.13 0.58 0.51 0.71 0.59 0.50 0.45 0.93 0.66 1.26 0.93 1.16 0.64 0.44 0.51 0.39 0.51 n.a.

0.09 0.09 0.09 0.12 0.38 0.00 0.00 3.64 1.26 1.15 1.05 1.31 1.21 1.05 0.86 1.54 0.86 1.11 1.75 1.78 2.20 2.11 1.33 1.04 1.54 1.20 1.08 0.90 1.20 1.32 1.69 1.25 1.44 0.89 1.33 1.00 0.49 0.08 0.31

Refs: 13, Nytoft, 2011; 14, Petersen and Nytoft, 2006; 15, Petersen, 1994; 16, Akande et al., 2007; 17, Pedersen et al., 2006; 18, Ž ivotić et al., 2010; 19, Petersen et al., 2009; references in bold refer to articles containing additional data for the exact same samples. bIHR, isohopane ratios; see definitions in 13. cCoal used for the isolation of benzohopanes for NMR. a

free bitumen, oils, and liquid pyrolysates (see below) were separated into saturated, aromatic and polar (NSO) fractions by medium pressure liquid chromatography (MPLC) following the procedures modified from Radke et al.28 The aromatic fractions were analyzed by way of GC−MS using an Agilent 7890A Gas Chromatograph (HP-5MS column, 30 m × 0.25 mm, 0.25 μm film thickness, He carrier gas = 1.5 cm3/min) coupled to an Agilent 5975C Mass Selective Detector (electron ionization energy = 70 eV). The following column temperature program was used: 2 °C/min from 80 to 300 °C with 20 min hold at 300 °C followed by 10 °C/min from 300 to 310 °C with 1 min hold at the final temperature. The compound assignment was performed by an examination and comparison with the literature mass spectra, retention times, and elution order.1−3,5,6,9−12 Aromatic hydrocarbons (five fractions; Σ = 365 mg) were isolated as described by Nytoft13 from 134 g of a Colombian coal (4425A, Rancheria basin; Table 1). The least polar fraction (258 mg), containing the monoaromatic compounds, was separated into eight

thermal stability, and geochemical significance were discussed. Three series of methylated b-series benzohopanes and a C35 heptacyclic benzohopane were also tentatively identified. Finally, the regular C32 and C33 benzohopanes cyclized at the C-20 position (labeled as “a” series in this study) were isolated and fully characterized by NMR, thus completing the partial data available from their original isolation.1

2. MATERIALS AND METHODS 2.1. Materials. The list of studied samples (coals and crude oils), together with their age and maturity, is given in Tables 1 and 2. 2.2. Methods. Bitumen was extracted from the pulverized coal samples using Soxhlet or Soxtec extraction with dichloromethane/ methanol azeotrope (15:2, v:v) for 72 h. Asphaltenes were precipitated from the coal extracts and crude oils using n-pentane or n-heptane. Coal asphaltenes used for the pyrolysis experiments were further purified by Soxhlet extraction with n-heptane for 240 h. Asphaltene5564


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Denmark Denmark Denmark Denmark Denmark France France France USA Belize Belize Belize Belize Belize Guatemala Guatemala Guatemala Guatemala Thailand Thailand Thailand Thailand Thailand Thailand Thailand Thailand Nigeria Nigeria Nigeria Nigeria Nigeria Nigeria India India India India Indonesia Indonesia Indonesia

2008024-16349 2008024-16348 2008024-16347 A-799 A-771 2008025-16350 2008025-16351 2008025-16352 2003039-9281 17425 17426 17427 17428 18238 17429 17430 17431 17432 2005004-11059 2005004-11060 2005004-11061 2005004-11062 2005004-11063 2005004-11064 2005004-11065 2005004-11066 2007004-14448 2007004-14449 2007004-14590 2007004-14591 2007004-14593 2007004-14594 2007004-14597 2007004-14598 2007004-14600 2007004-14601 2007004-14450 2007004-14595 2007004-14596

North Sea North Sea North Sea North Sea North Sea Paris Basin Paris Basin Paris Basin Texas, Giddings area Corozal Basin Corozal Basin Corozal Basin Corozal Basin Corozal Basin South Petén Basin South Petén Basin South Petén Basin North Petén Basin Phitsanulok Basin Suphan Buri Basin Suphan Buri Basin Fang Basin Fang Basin Fang Basin Fang Basin Fang Basin Niger Delta Niger Delta Niger Delta Niger Delta Niger Delta Niger Delta Assam Assam Assam Assam Kutei Basin Kutei Basin Kutei Basin

area Mona-1 Olaf-1 Lone-1 N-1X M-2 n.a n.a. n.a Austin Chalk Calla Creek Eagle Never Delay Spanish Lookout Canal Bank Caribe Rubelsanto Tierra Blanca Xan Sirikit U Thong Sang Kajai Ban Thi Pong Nok Mae Soon Nang Yao San Sai n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.

field or well jurassic jurassic jurassic jurassic jurassic jurassic jurassic jurassic cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cretaceous cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic cenozoic

age marine marine marine marine marine marine marine marine marine marine marine marine marine marine marine marine marine marine lacustrine lacustrine lacustrine lacustrine lacustrine lacustrine lacustrine lacustrine terrestrial/marine terrestrial terrestrial/marine terrestrial/marine terrestrial/marine terrestrial/marine terrestrial terrestrial terrestrial terrestrial terrestrial terrestrial terrestrial

source

13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,

25, 25, 25, 25, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26,

13, 20, 13, 13, 13, 13, 13, 13, 13, 13,

refa 21 20 20 20 20 22 22 22 23 24 24 24 24 24 24 24 24 24 26 26 26 26 26 26 26 26 27 27 27 27 27 27 27 27 27 27 27 27 27

0.03 0.04 0.04 0.05 0.05 0.05 0.06 0.06 0.07 0.11 0.17 0.15 0.15 0.16 0.13 0.06 0.05 0.16 0.11 0.07 0.07 0.07 0.08 0.07 0.09 0.07 0.06 0.11 0.07 0.08 0.09 0.09 0.24 0.22 0.21 0.23 0.19 0.15 0.32

IHR 33b 0.03 0.04 0.04 0.06 0.05 0.03 0.04 0.03 0.09 0.06 0.09 0.08 0.07 0.13 0.03 0.02 0.02 0.16 0.16 0.11 0.11 0.09 0.12 0.13 0.14 0.13 0.08 0.13 0.09 0.13 0.11 0.11 0.25 0.22 0.24 0.26 0.20 0.17 0.35

IHR 34−31b 0.11 0.11 0.18 0.14 0.14 0.15 0.14 0.15 0.15 0.03 0.06 0.06 0.08 0.06 0.01 0.01 0.01 0.04 0.20 0.20 0.21 0.17 0.19 0.17 0.19 0.18 0.19 0.29 0.19 0.21 0.23 0.19 0.18 0.22 0.19 0.16 0.30 0.33 0.28

IHR 34−32b 0.09 0.12 0.09 0.07 0.09 0.10 0.10 0.11 0.16 0.10 0.11 0.14 0.13 0.13 0.07 0.04 0.06 0.18 0.21 0.19 0.19 0.15 0.19 0.17 0.18 0.20 0.20 0.24 0.23 0.20 0.27 0.38 0.38 0.40 0.36 0.38 0.32 0.36 0.70

C34b/C34 a

0.03 0.09 0.04 0.09 0.07 0.06 0.09 0.12 0.05 0.05 0.09 0.07 0.07 0.05 0.03 0.02 0.01 0.08 0.19 0.19 0.16 0.04 0.13 0.15 0.16 0.20 0.38 1.29 0.50 0.45 0.52 0.53 0.57 0.67 0.75 0.53 0.51 0.72 1.14

C35b/C35 a

a Refs: 20, Østfeldt, 1987; 21, Nytoft et al., 2000; 22, Espitalié et al., 1987; 23, Robison, 1997; 24, Petersen et al., 2012; 25, Petersen et al., 2007; 26, Nytoft et al., 2009; 27, Nytoft et al., 2010; references in bold refer to the articles containing additional data for the exact same samples. bIsohopane ratios; see definitions in 13.

country

sample

Table 2. Isohopane and Benzohopane Data for 39 Crude Oils

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Energy & Fuels subfractions using reverse-phase high-pressure liquid chromatography (HPLC; Phenomenex C18, 250 × 10 mm, 5 μm particle size; mobile phase: acetone/acetonitrile, 70:30, v:v, at 2 cm3/min; ∼20 mg/ injection). Fraction eight (9.1 mg) containing mainly benzohopanes was separated again (same column; mobile phase: acetone/ acetonitrile, 80:20, v:v, at 2 cm3/min) into 20 fractions. The fractions containing C33−C35 benzohopanes were separated using a column having a different selectivity (Vydac 201TP C18, 250 × 4.6 mm, 5 μm particle size; mobile phase: acetone/acetonitrile, 80:20, v:v, at 0.8 cm3/ min) to isolate the novel benzohopanes sufficiently pure for NMR characterization and free of coeluting compounds on GC. The regular benzohopanes C32a (0.3 mg) and C33a (0.6 mg) were obtained >95% pure (GC−MS, full scan) by recrystallization from acetone. The novel benzohopanes C33b (∼0.1 mg) and C34b (0.5 mg) were obtained 90 and 60% pure, respectively, without recrystallization. GC−MS characterization of the isolated compounds and GC−MS/ MS analysis of the aromatic fractions of studied samples (Tables 1 and 2) were performed using an Agilent 6890 N Gas Chromatograph (Agilent HP-5 column, 30 m × 0.25 mm, 0.10 μm film thickness) coupled to a Waters Micromass Quattro Micro tandem quadrupole mass spectrometer. The injection temperature was 70 °C with 2 min hold. The temperature program was 30 °C/min from 70 to 100 °C and 4 °C/min from 100 to 308 °C with an 8 min hold at the final temperature. Argon was used as collision gas for the MS/MS experiments. NMR spectra of the isolated benzohopanes were recorded on a Bruker Avance 600 FT-NMR spectrometer operating at 600 MHz for 1 H and 150 MHz for 13C. The instrument was equipped with a 5 mm TCI CryoProbe. The samples were dissolved in approximately 0.5 mL of CDCl3 (99.8% D), and 1H and 13C chemical shifts were calibrated against residual CHCl3 (1H, 7.26 ppm) or CDCl3 (13C, 77.00 ppm). For all compounds, a sequence of 1D and 2D experiments was performed comprising 1D 1H NMR, DEPT135, 2D NOESY, 1H−1H COSY, 1H−13C HSQC, and 1H−13C HMBC using pulse programs from Bruker BioSpin’s standard library. Decoupled 1D 13C data were obtained using Z-restored spin−echo experiments as detailed previously.29,30 The pyrolysis experiments were conducted on (1) an extracted coal sample (Bogovina 30; Table 1; approximately 6 g), which contained kerogen with native minerals, and (2) asphaltenes (approximately 0.2 g) isolated from the coal extract. The pyrolyses were performed in an autoclave under a nitrogen atmosphere (the initial pressure at 25 °C was 600 kPa) for 4 h at two different temperatures: 250 and 400 °C. Liquid pyrolysis products were extracted using chloroform by combining a thorough rinsing of the autoclave interior with hot solvent and Soxhlet extraction from the solid residue for 36 h. The hydrous pyrolysis of coals was carried out in stainless-steel HPLC columns (Knauer A0021, 120 mm × 4 mm).21 The columns were filled with 0.7 g of finely ground coals (OY4-24M and OWK01T; Table 1) and 1 mL of water. The sealed columns were heated for 72 h at 270, 300, 330, and 345 °C in a block of aluminum equipped with temperature sensors and heating cartridges. After cooling, the columns were thoroughly washed out with water. Free oil was not observed. The pyrolyzed coal was obtained using a PTFE membrane filter and dried at room temperature. The water phase was not used.

Figure 1. Total ion current (TIC) chromatogram of the aromatic fraction of Bogovina 30 coal extract. TMNs, trimethylnaphthalenes; Ed, eudalene; Cd, cadalene; P, phenanthrene; MPs, methylphenanthrenes; Py, pyrene; C9-N, n-nonylnaphthalene; BNFu, benzonaphthofuran; C11-N, n-undecylnaphthalene; ■, aromatized des-Adegraded nonhopanoid triterpenoids; ⧫, methyl esters of fatty acids; Per, perylene; AH, aromatized hopane (7-methyl, 3′-ethyl, 1,2cyclopentanochrysene); ●, benzohopane cyclized at C-16; ○, diaromatic 8(14)-secohopanoid with a fluorene moiety; C32a−C35a, regular benzohopanes; C34b and C35b, novel benzohopanes.

moiety. Moreover, two other peaks were also observed, labeled here as C34b and C35b. Typical m/z 191 fragmentogram confirmed the presence of the novel compounds mentioned above (C34b and C35b) as well as one more compound labeled as C35/7 (Figure 2). Their mass spectra are shown in Figure 3

Figure 2. GC−MS mass chromatogram of benzohopanes, m/z 191, in the aromatic fraction of Bogovina 30 coal extract. C35BH, unidentified C35 benzohopanes; C36 BH, unidentified C36 benzohopanes; C35/7, novel heptacyclic C35 benzohopane; αβ, βα, and ββ designate configurations at C-17 and C-21 in C35 hopanes with a thiophene moiety (C35T). For other peak assignments, see the legend of Figure 1.

(e, g, h). In accordance with the existence of hopanoids above C35,2,9,13,31 C36 benzohopanes with molecular weight of 488 were also observed in our samples (Figure 2). More sensitive GC−MS/MS analysis, using the typical transitions M+ → 191 (Figure 4), allowed us to identify the C34b and C35b benzohopanes in numerous samples from all over the world, as listed in Tables 1 and 2. Moreover, the C33 member of this series was also observed (C33b; Figures 5−7). Its mass spectrum is given in Figure 3f. The novel C33b−C35b benzohopanes have the same molecular masses as the regular C33a−C35a benzohopanes and are characterized by relatively prominent m/z 191 ion (Figure 3f−h). They have longer GC retention times than the regular benzohopanes (Figures 2 and 4). C33b benzohopane differs from C33a benzohopane by a higher m/z 158/191 ratio (Figure 3b, f), whereas C34b and C35b benzohopanes differ from C34a and C35a benzohopanes by distinctively higher m/z 172/191 and 186/191 ratios (Figure 3c, d, g, h) in the

3. RESULTS AND DISCUSSION 3.1. Novel Benzohopanes−Insights from GC−MS and GC−MS/MS Analysis. The novel series of benzohopanes was first observed in the aromatic fraction of Bogovina 30 bituminous coal extract (Bogovina basin, Serbia; Table 1) and in the aromatic fraction isolated from the bituminous 4425A coal (Rancheria basin, Colombia; Table 1). The total ion current (TIC) chromatogram of the aromatic fraction of Bogovina 30 coal extract is shown in Figure 1. It is characterized by relatively abundant regular C32a−C35a benzohopanes, minor C31 benzohopane cyclized at C-16, and minor C31 aromatized 8(14)-secohopanoid with a fluorene 5566


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Figure 3. Mass spectra of regular (a−d) and novel (e−h) benzohopanes.

8) as well as the interpretation of mass spectra (not shown) of HPLC-purified compounds. As with methylated hopanes, methylated regular benzohopanes, and methylated isohopanes, the novel series of methylated benzohopanes comprise compounds with the methyl group attached to ring A at the C-2 (α and β) or C-3 (β) position. The benzohopanes methylated at C-2 elute close to the nonmethylated compounds

corresponding mass spectra. This result suggests that the C33b− C35b benzohopanes belong to the same series. The existence of methylated derivatives of hopanes, regular benzohopanes, and isohopanes in oils and sedimentary organic matter has been previously confirmed.13,32−36 In this study, we identified three novel series of methylated benzohopanes of the b-series using the GC−MS/MS transitions M+ → 205 (Figure 5567


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Figure 4. GC−MS/MS transitions of C33 (446 → 191; a), C34 (460 → 191; b) and C35 (474 → 191; c) benzohopanes. C33a, regular C33 benzohopane; C33b, novel C33 benzohopane; C35BH, unidentified C35 benzohopanes. For other peak assignments, see the legend of Figure 1.

Figure 5. GC−MS/MS of C33−C35 benzohopanes in hydrous pyrolysis products of OY4-24M coal (Rr = 0.55%) obtained at 270 °C (a−c) and in pyrolysis products of Bogovina 30 coal asphaltenes obtained at 250 °C (d−f). For peak assignments, see the legends of Figures 1 and 4.

An additional C35 benzohopane (C35/7 in Figure 2) was also found in our samples. To the best of our knowledge, this is the first report of this compound. C35/7 differs from the C35b

as is also the case for the corresponding saturated hopanes and methylhopanes,37,38 whereas the compounds having methyl at C-3 have much longer retention times. 5568


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Figure 6. GC−MS/MS of C33−C35 benzohopanes in 9606 coal (Rr = 0.67%) (a−c), in hydrous pyrolysis products of OY4-24M coal (Rr = 0.55%) obtained at 300 °C (d−f) and in pyrolysis products of Bogovina 30 coal (Rr = 0.56%) obtained at 250 °C (g−i). For peak assignments, see the legends of Figures 1 and 4.

Figure 7. GC−MS/MS of C33−C35 benzohopanes in 9583 coal (Rr = 0.82%) (a−c) and in hydrous pyrolysis products of OY4-24M coal (Rr = 0.55%) obtained at 330 °C (d−f). C33BH, unidentified C33 benzohopanes; C34BH, unidentified C34 benzohopanes; C35BH, unidentified C35 benzohopanes. For other peak assignments, see the legends of Figures 1 and 4. Note: unidentified C34 benzohopane (C34BH) coeluting with C34b in mature samples exhibits a different mass spectrum than C34b (see Appendix and Figure 3g).

benzohopane by a shift of m/z 186, 253, and 268 fragments in the mass spectrum to m/z 184, 251, and 266, respectively,

indicating the presence of an extra 5-membered ring (Figure 3e). 5569


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Figure 8. GC−MS/MS transitions of methylated regular and novel C33 (446 → 205; a), C34 (460 → 205; b), and C35 (474 → 205; c) benzohopanes; 2- and 3- indicate the position of the methyl group in the A ring of methylated benzohopanes, and α- and β- designate the configuration at C-2 or C-3 in methylated benzohopanes. For other peak assignments, see the legends of Figures 1 and 4.

3.2. Determination of the Structure of Novel Hexacyclic Benzohopanes by NMR. The structures of the novel benzohopanes were further studied in detail by NMR analysis of the isolated pure compounds C33b and C34b using 1D and 1H-detected 2D experiments. In addition, the isolated benzohopanes from the regular a-series (C32a and C33a) were reanalyzed with modern techniques, expanding on the partial data already available for these compounds.1 Carbon−carbon connectivity was mainly established from 2J and 3J correlations from methyl groups in the HMBC spectrum, giving an outline of the carbon framework, as shown in Figure 9a for a general benzohopane structure. The five remaining methylene groups were assigned based on 1H−1H correlations either using through-space interactions in the NOESY spectra or via scalar couplings from the COSY spectra, leading to the chemical shift data provided in Table 3. For the usually ambiguous assignment of C-8 and C-14 (for numbering, please see Figure 9b), we were able to identify a correlation in HMBC from H-16α to C-14 in compound C34b, and this formed the basis for the assignment of C-8 and C-14 in all the other compounds as well. However, we will admit that having larger variations in the shifts of C-8 compared to C-14 in these data sets immediately looks suspicious, and the assignment should thus be treated with some caution. For compound C33b, of which we had only a very limited amount of material available, C-8, C-20, and C-21 could not be observed in the 13C spectrum. Although we were able to determine a chemical shift for C-21 from the HMBC experiment, the same could not be done unequivocally with C-8 or C-20 due to a combination of too low resolution and spectral overlap. When comparing our new NMR data for compounds C32a and C33a to the available partial 1H data,1 there are some minor

Figure 9. Carbon connectivity established from HMBC data (bold bonds) (a) and numbering of carbon atoms in C34b (b).

differences in some of the reported shifts, in particular, for H-30 and H-31 in compound C33a, which is off by 0.04 ppm (Table 5570


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Table 3. 1H and 13C NMR Chemical Shifts (ppm) for Benzohopanes C32a, C33a, C33b, and C34b (600/150 MHz, CDCl3)a C32a 13

1 2 3 4 5 6 7 8b 9 10 11 12 13 14b 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 a

C

40.26 18.73 42.10 33.28 56.48 18.48 33.10 42.24 50.97 37.50 21.34 24.33 42.01 41.45 32.07 24.91 51.95 44.75 50.58 142.43 145.40 134.20 33.42 21.61 15.95 16.41 20.49 23.88 18.77 127.54 126.18 122.13

C33a 1

H

0.79; 1.67 1.39; 1.59 1.14; 1.36 0.73 1.51; 1.31 1.43; 1.35 1.29 1.61; 1.29 1.54 (2H) 1.81 1.04; 1.85 1.74; 1.40 2.68 2.72; 2.87

0.86 0.80 0.81 0.87 1.11 0.99 2.29 6.92 7.02 6.97

13

C

40.28 18.74 42.11 33.29 56.51 18.48 33.10 42.29 51.01 37.52 21.37 24.36 42.35 41.47 32.42 25.44 52.16 44.28 49.54 140.89 145.24 131.28 33.43 21.63 15.95 16.44 20.77 24.35 18.38 127.71 127.05 131.09 18.83

C33b 1

13

H

C

0.81; 1.69 1.39; 1.59 1.14; 1.36 0.74 1.52; 1.34 1.44; 1.38 1.28 1.64; 1.33 1.58 (2H) 1.85 1.05; 1.89 1.73; 1.33 2.67 2.72 (2H)

0.86 0.80 0.82 0.89 1.12 0.99 2.25 6.85 6.85

40.28 18.74 42.11 33.29 56.50 18.49 33.12 not observedb 51.00 37.52 21.37 24.34 42.07 41.45 32.10 25.15 51.59 44.89 50.51 not observedb 142.6b 133.91 33.43 21.62 15.96 16.43 20.51 23.95 18.67 128.47 135.97 122.86

C34b 1

H

0.79; 1.67 1.39; 1.58 1.13; 1.35 0.73 1.51; 1.33 1.43; 1.36 1.26 1.61; 1.32 1.53 (2H) 1.81 1.04; 1.85 1.73; 1.38 2.63 2.67; 2.82

0.86 0.80 0.81 0.87 1.11 0.99 2.25 6.75 6.80

2.17 21.14

2.27

13

C

40.28 18.75 42.11 33.29 56.52 18.49 33.10 42.30 51.02 37.53 21.38 24.36 42.42 41.45 32.48 25.76 52.04 44.40 50.04 141.16 142.92 130.90 33.43 21.63 15.96 16.45 20.81 24.44 18.20 129.49 134.10 129.55 15.51 19.49

1

H

0.80; 1.68 1.39; 1.59 1.13; 1.35 0.73 1.51; 1.33 1.43; 1.37 1.27 1.63; 1.33 1.57 (2H) 1.85 1.04; 1.87 1.71; 1.31 2.65 2.74 (2H)

0.86 0.80 0.82 0.89 1.12 0.99 2.213 6.77

2.09 2.207

Methylene protons are reported as α-H; β-H. For carbon numbering, please see Figure 9b. bPlease see comment in text.

membered rings appear in chair conformation. On the basis of this, we have assumed a ring D boat conformation also for C33b and C34b. Our NOESY spectrum for compound C33a was of superior quality compared to the rest of the set, and assignments for this compound formed the basis for the other compounds as well. From this, we were able to assign all the diastereotopic protons de novo with the exception of CH2−11, where we relied on data for the methylhopane mentioned above. Support for our assignment of rings A−C could also be found from NMR data on hopane (17β(H),21β(H)-hopane),40 except for CH2-7, where the boat conformation of ring D in our compounds puts these two protons in a slightly different environment compared to the all-chair conformation of hopane in the reference compound. For the two compounds with dissimilar chemical shifts on H-19, data for C32a showed evidence for NOE between H-13 and H-19β and from H-12β to H-19α, leading to the complete assignment of 1H chemical shifts as provided in Table 3. 3.3. Possible Origin of Novel Benzohopanes and Their Geochemical Significance. Formation of the novel hexacyclic benzohopanes requires the presence of an additional methyl or ethyl group in the side chain of hopanoid precursors

3). This small discrepancy could easily be explained by differences in solvent (CDCl3 vs. CDCl3 + CCl4) and differences in calibration. The remainder is in agreement, leaving us in no doubt that we have isolated the same compounds as Hussler et al.1 and thus also with isomerization of the benzylic H-17β to H-17α. For compounds C33b and C34b, the substitution pattern of the aromatic ring was established based on HMBC correlations and NOE cross peaks from methyl groups on the aromatic ring to either H-17 or H-19. For the assignment of diastereotopic protons, we needed to establish the most probable conformation for the compounds, in particular, for rings D and E. During NMR assignment of 3βmethyl-17α(H),21α(H)-hopane, a boat conformation of ring D was suggested based on the coupling constants of H-16β.39 However, because of the influence of the aromatic ring in the current set of compounds, H-16β is shifted to a crowded spectral region, and no conclusions of the conformation of ring D could be made based on NMR data alone. Fortunately for us, X-ray data for C32a and C33a were published together with the NMR data1 (please note that C-23 and C-24 are assigned erroneously in Figure 1 in this reference), clearly indicating a boat conformation of ring D also in this case. The remaining 65571


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Energy & Fuels besides the one at C-22.41 Although natural biohopanoids with this structure have not been reported, they could be formed via the reactions of C32 hopanoid aldehydes/acids with other organic compounds during diagenesis. Namely, C32 hopanoid aldehydes/acids are the main products of oxidation of bacteriohopanepolyols in coal sources.41,42 In addition, isohopanoic acids with an additional methyl group in the side chain were tentatively identified in the liquid products obtained by the ruthenium-catalyzed oxidation of extracted coals (which in turn contained isohopanes in their extracts)14 and corresponding asphaltenes isolated from the coal extracts.13 In addition to the methyl group at C-22, a branch in the side chain of hopanoid precursors could be formed in two ways: (1) via substitution of the OH group with a methyl group (which can originate from numerous sources), as proposed by Cong et al.,9 or (2) by dehydration and formation of a double bond followed by the addition of a methyl group. Further cyclization and aromatization followed by a cleavage of the side chain and the loss of three, two, or one carbon atoms result in the formation of C33b−C35b benzohopanes. On the other hand, dehydration, reduction, and cleavage of the side chain, without its cyclization, result in the formation of isohopanes. The existence of structures with an additional methyl group in the hopanoid side chain is well-documented by the identification of a full series of C33−C35 isohopanes in numerous samples (40 coals and 73 crude oils).13 The comprehensive study of isohopanes performed by Nytoft13 clearly showed that these biomarkers are generally much more abundant in the terrestrial OM than in the OM from lacustrine and marine sources. By means of GC−MS/MS analysis of the saturated fraction, isohopanes were also identified in the samples from the present study (Tables 1 and 2). Generally, the samples containing higher amounts of isohopanes also contain higher amounts of the novel benzohopanes; this is documented by a positive correlation of the C34 isohopane/C34 regular hopane (IHR34-31) and C34b/C34a ratios observed for the investigated oils (Table 2, Figure 10). Marine oils are poor in isohopanes and almost free of the b-series benzohopanes. Lacustrine oils contain small amounts of isohopanes and benzohopanes of the b-series, whereas coals and terrestrial oils are enriched in both hopanoid series. Therefore, we assume that the novel hexacyclic benzohopanes, along with the structurally related isohopanes,13 are typical for the terrestrial OM and can be useful in the correlation analysis. On the other hand, the series of C32a−C35a benzohopanes is generally more abundant in the samples from carbonate and evaporitic environments.2,41,43 The existence of isohopanes with 33 and more carbon atoms and the hexacyclic b-series benzohopanes in the same range clearly indicates that alkylation of the hopanoid side chain occurs on initial bacteriohopanepolyols or during the reactions of elongation of their C32-functionalized products of diagenetic oxidation. Unlike the hexacyclic C33b−C35b benzohopanes, the formation of the novel heptacyclic compound reported in this study (C35/7) does not require a specific hopanoid precursor with two branches in the aliphatic side chain. It can be produced via the cyclization of both C35 benzohopane structures (C35a or C35b; Figure 11). Therefore, this compound seems to be less specific for certain depositional conditions than the previously discussed novel hexacyclic benzohopanes. 3.4. Thermal Stability of Novel Benzohopanes. Thermal behavior of the b-series benzohopanes was examined

Figure 10. Correlation C34 isohopane/C34 hopane (IHR34-31) ratio vs C34b/C34a benzohopane ratio for the investigated oils (Table 2).

by GC−MS/MS analysis of the coal samples showing different maturities, by hydrous pyrolysis of the Nigerian OY4-24M and OWK-01T coals (Rr = 0.55 and 0.59%, respectively; Table 1) rich in the novel benzohopanes as well as by pyrolysis of the extracted Bogovina 30 coal (with prominent novel benzohopanes and Rr = 0.56%; Table 1) and asphaltenes isolated from its bitumen. The immature samples (Rr < 0.60%, Figure 4), the hydrous pyrolysates of the Nigerian coals obtained at 270 °C, and the pyrolysate of Bogovina 30 asphaltenes obtained at 250 °C show “immature distributions” of benzohopanes, which are characterized by the presence of both a- and b-series of benzohopanes (Figure 5). The early to moderately mature samples (Rr in the range of 0.60−0.80%), the hydrous pyrolysis products of the Nigerian coals obtained at 300 °C, and the pyrolysate of Bogovina 30 coal at 250 °C (which pyrolysis residue indicated Rr of 0.78 ± 0.04%) contain some other minor peaks of the benzohopane series in addition to the a- and b-series (Figure 6). The mature coal samples (Rr > 0.80%) and the hydrous pyrolysates of the Nigerian coals obtained at 330 °C exhibit a different, “mature” distribution of benzohopanes (Figure 7). They contain low amounts of b-series benzohopanes (particularly C34b and C35b homologues). However, the corresponding GC−MS/MS transitions indicate the presence of a-series benzohopanes and some other isomers (Figure 7), which were not observed in the immature samples (Figures 4 and 5) or were present in traces in the moderately mature samples (Figure 6). We assume that these compounds represent isomers of the a- and b-series benzohopanes with different substitution patterns on the aromatic ring. The isomerization of alkyl groups attached to the aromatic ring is a well-known scenario during maturation, so far reported in the series of alkylated naphthalenes, phenanthrenes, and dibenzothiophenes.44−47 However, a detailed determination of benzohopane isomers found in the mature samples needs 5572


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Figure 11. Cyclization of C35 regular (C35a) and novel (C35b) hexacyclic benzohopanes into the C35 heptacyclic benzohopane (C35/7).

clearly shows that the novel hexacyclic benzohopanes, along with structurally related isohopanes,13 are typical for the terrestrial OM.

further investigations. Nevertheless, our results unambiguously show that distributions of benzohopanes can also indicate maturity. Table 2 contains isohopane and benzohopane data for 39 oils. Thirteen of them have a terrestrial or mixed terrestrial/ marine source. They all have an “immature” distribution of benzohopanes, which is surprising because some of them (Assam Basin oils27) have the C29 sterane 20S/(20S + 20R) ratio around 0.5, indicating at least peak oil generation. It is expected that more mature source rock generates oil with low concentrations of benzohopanes and still high concentrations of steranes. However, this late contribution may not significantly alter the original (early generated) distribution of benzohopanes in oil. Finally, benzohopanes were almost absent in the hydrous pyrolysates of the Nigerian coals (OY4-24M and OWK-01T; Table 1) obtained at 345 °C and in the aromatic fraction of Bogovina 30 pyrolysates produced at 400 °C (obtained from asphaltenes and extracted coal, which pyrolysis residue indicated Rr = 1.83 ± 0.04%). The absence of benzohopanes at this OM maturity level is consistent with the results of Tian et al.,36 who found that all benzohopanes and methylbenzohopanes became minor components at the maturity levels corresponding to vitrinite reflectance of ∼0.97%. Abundant novel hexacyclic benzohopanes in the immature samples and the liquid pyrolysates obtained at temperatures up to 330 °C are in accordance with the observation of Nytoft13 that hopanes with an iso-skeleton are attached to kerogen by relatively weak bonds and therefore can be released under mild conditions. A decrease in the content of b-series hexacyclic benzohopanes during the maturation suggests that their formation is related rather to specific depositional conditions and microbial activity during diagenesis than to thermal stress. This result is in agreement with the data from Figure 10, which

4. CONCLUSIONS A series of novel C33−C35 hexacyclic benzohopanes (labeled as “b” series in this study) was identified in numerous samples of coal extracts and crude oils of different ages from all over the world. The structures of C33b and C34b homologues were assigned by NMR, whereas the structure of C35b benzohopane was proposed based on the mass spectrum. The b-series hexacyclic benzohopanes are structurally related to isohopanes, and their formation requires the presence of a methyl group in the side chain of hopanoid precursors in addition to the methyl group at C-22. This group can be introduced via substitution of a OH group or by dehydration followed by the addition of a methyl group. Moreover, hopanoid precursors with two methyl groups in the side chain can be formed via the reactions of C32 aldehyde and/or acids produced by the oxidation of bacteriohopanepolyols with other organic compounds during diagenesis. The existence of isohopanes with 33 and more carbon atoms and the b-series hexacyclic benzohopanes in the same range clearly indicates that alkylation of the hopanoid side chain occurs on initial bacteriohopanepolyols or during the reactions of elongation of their C32-functionalized products of diagenetic oxidation. Further cyclization and aromatization of hopanoid precursors with an additional methyl group produce the novel hexacyclic benzohopanes. On the other hand, dehydration and reduction of the same precursors, without cyclization of the side chain, produce isohopanes. Both groups are typical for the terrestrial organic matter and can be useful in the correlation analysis. GC−MS/MS analyses of the samples of different maturities and the maturation simulation experiments revealed different 5573


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(8) Oliveira, C. R.; Oliveira, C. J. F.; Ferreira, A. A.; Azevedo, D. A.; Aquino Neto, F. R. Org. Geochem. 2012, 53, 131−136. (9) Cong, X.-S.; Zong, Z.-M.; Wei, Z.-H.; Li, Y.; Fan, X.; Zhou, Y.; Li, M.; Zhao, Y.-P.; Wei, X.-Y. Energy Fuels 2014, 28, 6745−6748. (10) Valisolalao, J.; Perakis, N.; Chappe, B.; Albrecht, P. Tetrahedron Lett. 1984, 25, 1183−1186. (11) Sinninghe Damsté, J. S.; Rijpstra, W. I. C.; de Leeuw, J. W.; Schenck, P. A. Geochim. Cosmochim. Acta 1989, 53, 1323−1341. (12) van Kaam-Peters, H. M. E.; Köster, J.; de Leeuw, J. W.; Sinninghe Damsté, J. S. Org. Geochem. 1995, 23, 607−616. (13) Nytoft, H. P. Org. Geochem. 2011, 42, 520−539. (14) Petersen, H. I.; Nytoft, H. P. Org. Geochem. 2006, 37, 558−583. (15) Petersen, H. I. DGU series A; Geological Survey of Denmark: Copenhagen, Denmark, 1994; Vol. 33. (16) Akande, S. O.; Ogunmoyero, I. B.; Petersen, H. I.; Nytoft, H. P. J. Pet. Geol. 2007, 30, 303−324. (17) Pedersen, G. K.; Andersen, L. A.; Lundsteen, E. B.; Petersen, H. I.; Bojesen-Koefoed, J. A.; Nytoft, H. P. J. Pet. Geol. 2006, 29, 3−26. (18) Ž ivotić, D.; Jovančićević, B.; Schwarzbauer, J.; Cvetković, O.; Gržetić, I.; Ercegovac, M.; Stojanović, K.; Šajnović, A. Int. J. Coal Geol. 2010, 81, 227−241. (19) Petersen, H. I.; Lindström, S.; Nytoft, H. P.; Rosenberg, P. Int. J. Coal Geol. 2009, 78, 119−134. (20) Østfeldt, P. In Petroleum Geology of North West Europe; Brooks, J., Glennie, K., Eds.; Graham & Trotman: London, UK, 1987; pp 419−429. (21) Nytoft, H. P.; Bojesen-Koefoed, J. A.; Christiansen, F. G. Org. Geochem. 2000, 31, 25−39. (22) Espitalié, J.; Maxwell, J. R.; Chenet, Y.; Marquis, F. Org. Geochem. 1988, 13, 467−481. (23) Robison, C. R. Int. J. Coal Geol. 1997, 34, 287−305. (24) Petersen, H. I.; Holland, B.; Nytoft, H. P.; Cho, A.; Piasecki, S.; de la Cruz, J.; Cornec, J. H. J. Pet. Geol. 2012, 35, 127−164. (25) Petersen, H. I.; Nytoft, H. P.; Ratanasthien, B.; Foopatthanakamol, A. J. Pet. Geol. 2007, 30, 59−78. (26) Nytoft, H. P.; Samuel, O. J.; Kildahl-Andersen, G.; Johansen, J. E.; Jones, M. Org. Geochem. 2009, 40, 595−603. (27) Nytoft, H. P.; Kildahl-Andersen, G.; Samuel, O. J. Org. Geochem. 2010, 41, 1104−1118. (28) Radke, M.; Willsch, H.; Welte, D. H. Anal. Chem. 1980, 52, 406−411. (29) Xia, Y.; Moran, S.; Nikonowicz, E. P.; Gao, X. Magn. Reson. Chem. 2008, 46, 432−435. (30) Nytoft, H. P.; Kildahl-Andersen, G.; Šolevic-Knudsen, T.; Stojanović, K.; Rise, F. Org. Geochem. 2014, 77, 89−95. (31) Wang, P.; Li, M.; Larter, S. R. Org. Geochem. 1996, 24, 547−551. (32) Seifert, W. K.; Moldowan, J. M. Geochim. Cosmochim. Acta 1978, 42, 77−95. (33) Summons, R. E.; Jahnke, L. L.; Hope, J. M.; Logan, G. A. Nature 1999, 400, 554−557. (34) Summons, R. E.; Jahnke, L. L.; Roksandic, Z. Geochim. Cosmochim. Acta 1994, 58, 2853−2863. (35) Neunlist, S.; Rohmer, M. Biochem. J. 1985, 231, 635−639. (36) Tian, H.; Cheng, P.; Zhou, Q.; Xiao, X.; Wilkins, R. W. T. Org. Geochem. 2013, 63, 139−144. (37) Summons, R. E.; Jahnke, L. L. Geochim. Cosmochim. Acta 1990, 54, 247−251. (38) Summons, R. E.; Jahnke, L. L. In Biological Markers in Sediments and Petroleum; Moldowan, J. M., Albrecht, P., Philp, R. P., Eds.; Prentice Hall: Englewood Cliffs, NJ, USA, 1992; pp 182−200. (39) Kildahl-Andersen, G.; Nytoft, H. P.; Johansen, J. E. Magn. Reson. Chem. 2010, 48, 951−954. (40) Ageta, H.; Shiojima, K.; Suzuki, H.; Nakamura, S. Chem. Pharm. Bull. 1993, 41, 1939−1943. (41) Peters, K. E.; Walters, C. C.; Moldowan, J. M. The Biomarker Guide, Vol. 2: Biomarkers and Isotopes in the Petroleum Exploration and Earth History; Cambridge University Press: Cambridge, UK, 2005; pp 567, 588, 589.

distributions of benzohopanes depending on the thermal maturity. The same experiments indicated that the C33b− C35b benzohopanes are stable up to the maturity level corresponding to vitrinite reflectance of ∼0.80%. Three novel series of methylated benzohopanes were identified in numerous samples. Finally, a novel C35 heptacyclic benzohopane with an additional cyclopentane ring was also observed in the studied samples, and its structure was tentatively identified based on the mass spectrum. It can be produced via the cyclization of regular or novel C35 hexacyclic benzohopanes and therefore does not require a specific hopanoid precursor with two branches in the side chain. Consequently, this compound is probably less useful in the correlation analysis than the new hexacyclic benzohopanes.

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APPENDIX Mass spectrum of the unknown C34 benzohopane (C34BH) coeluting with C34b in Figure 7.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +381 11 3336 776. Fax: +381 11 2636 061. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The study was financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project No. 176006). The Danish Research Council for Nature and Universe (FNU; grant 21-04-0605) is also thanked for support. We are also grateful to the anonymous reviewers, whose constructive comments and suggestions greatly improved this manuscript.

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ABBREVIATIONS NMR = nuclear magnetic resonance Rr = random vitrinite reflectance GC−MS = gas chromatography−mass spectrometry OM = organic matter MPLC = medium-pressure liquid chromatography HPLC = high-pressure liquid chromatography GC = gas chromatography GC−MS/MS = gas chromatography−tandem mass spectrometry TIC = total ion current

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REFERENCES

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