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Energy & Fuels 1990,4,658-661
Origin of Petroporphyrins. 2. Evidence from Stable Carbon Isotopes Christopher J. Boreham* Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 378, Canberra, 2601, Australia
Christopher J. R. Fookes CSIRO, Division of Fuel Technology, Private Mail Bag 7,Menai, NSW, 2234, Australia
Brain N. Poppt and J. M. Hayes Indiana University, Biogeochemistry Laboratories, Bloomington, Indiana 47450-5101 Received April 27, 1990. Revised Manuscript Received August 2, 1990 Compared with the carbon-13 isotopic composition of the ubiquitous C,DPEP (DPEP, deoxophylloerythroetioporphyrin) the heavy but equivalent carbon-13isotopic composition for the porphyrin structures 15(2)-methyl-15,17-ethano-l7-nor-H-C&PEP and 15,17-butano-, 13,15-ethano-13(2),17suggests a common precursor, propano-, and 13(l)-methyl-13,15-ethano-13(2),17-propanoporphyrin presumably chlorophyll c, for these petroporphyrins isolated from the marine Julia Creek oil shale and the lacustrine Condor oil shale. Similarly, the heavy but variable carbon-13 isotopic composition of 7-nor-H-C,DPEP compared with C,DPEP is consistent with an origin from both chlorophyll b and chlorophyll ea. The equivalent carbon-13 isotopic composition for 13(2)-methyl-CaDPEP compared with C,DPEP suggests a common origin resulting from a weighted average of chlorophyll inputs.
Introduction Since the isolation and structural characterization ('H NMR)of the first petroporphyrin with a novel structure, the seven-membered (15,17-butano-) ringed porphyrin1t2 (Figure 1, structure l), numerous other unusual ring structures have been identified3-10(Figure 1). These in13(1)-methylclude the 15(2)-methyl-15,17-ethano-, 13,15-ethano-, and 13(2)-methyl-13,15-ethanoporphyrins with methyl-substituted five-membered rings (Figure 1, structures 2,3,s 3: and 4: respectively) and the 13,15ethano-13(2),17-propano-, 13(1)-methyl-13,15-ethano-13(2),17-propano-, and 13(1)-methyl-13,15-ethano-13(2),17prop- 13(2),15(2)-enoporphyrins with futed seven/ fivemembered rings (Figure 1, structures 5, 6,* and 7: respectively). While it is not difficult to imagine a product-precursor relationship between DPEP (Figure 1, structure 8) and etioporphyrin (Figure 1,structure 9) type structures and known chlorophyll^^^-'^ (a heme origin for 9 has also been suggested from stable carbon isotopic evidence13J4),the construction of pathways yielding some of the novel structures is more uncertain. Rearrangements involving the propionate substituent of chlorophyll a or b (after phytol loss) and the acrylic group of chlorophyll c have been evoked to explain the origins of structures 1' and 2,39srespectively. A chlorophyll c origin is also considered likely for the two porphyrins having structures related to 8 but with hydrogen (Figure 1, structure 10) and methyl at the 17-position5by way of degradation of the acrylic side chain. Chlorophyll b was evoked as the likely precursor of ~ - ~ O ~ - H - C , ~ D (Figure P E P ' ~1,structure 11) through loss of the 7-formyl group. Results and Discussion Vanadyl porphyrins from the marine Julia Creek oil shale (sample no. 3910) were demetalated with methanesulfonic acid, remetalated with N i ( a ~ a c )and ~ , ~then ana-
'
Present address: Department of Geology and Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822.
0887-0624/90/2504-0658$02.50/0
Table I. Carbon Isotopic Composition (% vs PDB)O of Single Porphyrins VO porphyrins Ni porphyrins structure 1 2 3 4 5 6
(no. 39101, %
(no. 4328), %
-22.0 -22.2
-16.0 -15.8 -17.8
8 9 10 11
-23.8 -22.8 -22.1 -23.1 -26.9 -21.9 -23.1
-19.6 -23.1 -16.8
OPee Dee Belemnite = 0%.
lyzed as nickel porphyrins (there appears to be no isotopic fractionation when vanadyl porphyrins are analyzed as such or converted into nickel porphyrin^'^). These nickel porphyrins and nickel porphyrins isolated as such from the (1) Fookes C. J. R. J.Chem. SOC.,Chem. Commun. 1983,1472-1473. (2) Wolff, G. A.; Murray, M.; Maxwell, J. R.; Hunter, B. K.; Saundere, J. K. M. J. Chem. SOC.,Chem. Commun. 1983,922-924. (3) Ocampo, R.; Callot, H. J.; Albrecht, P.; Kintzinger, J. P. Tetrahedron Lett. 1984,25, 2589-2592. (4) Ocampo, R.; Callot, H. J.; Albrecht, P. J. Chem. SOC.,Chem. Commun. 1985, 198-201. (5) Verne-Mismer, J.; Ocampo, R.; Callot, H. J.; Albrecht, P. Tetrahedron Lett. 1988,29,371-374.
(6) Verne-Mismer, J.; Ocampo, R.; Callot, H. J.; Albrecht, P. J. Chem. Soc., Chem. Commun. 1987,1581-1583. (7) Ocampo, R.; Callot, H. J.; Albrecht, P. Tetrahedron 1984, 40,
4033-4039. (8) Chicarelli, M. I.; Kaur, S.; Maxwell, J. R. Metal Complexes in
Fossil Fuels; Filby, R. H., Branthaver, J. F., EMS.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 41-67. (9) Boreham, C. J.; Fookes, C. J. R. J. Chromotogr. 1989,467,195-208. (10) Prowse, W. G.; Chicarelli, M. I.; Keely, B. J.; Kaur, S.; Maxwell, J. R. Geochim. Cosmochim. Acta 1987,51, 2875-2877. (11) Fookes, C. J. R. J. Chem. Soc., Chem Commun. 1983,1474-1476. (12) Fookes, C. J. R. J. Chem. SOC.,Chem. Commun. 1985,706-708. (13) Boreham, C. J.; Fookes, C. J. R.; Popp, B. M.; Hayes, J. M. Ceochim. Cosmochim. Acta 1989,53,2451-2455. (14) Ocampo, R.; Callott, H. J.; Albrecht, P.; Popp, R. N.; Horowitz, M.; Hayes, J. M. Naturwissenschaften 1989, 76, 419-421. (15) Chicarelli, M. I.; Maxwell, J. R. Tetrahedron Lett. 1984, 4701-4704.
Published 1990 by the American Chemical Society
Origin of Petroporphyrins /
Energy & Fuels, Vol. 4, No. 6, 1990 659
/
/
/
H
*Tentative structure (lower half of molecule is definite)
Figure 1. Porphyrin structures referred to in the text.
lacustrine Condor oil shale (sample no. 4328) were separated by TLC and reversed-phase HPLC into various single s p e c i e ~ . ~The J ~ carbon isotopic compositions determinedls on 30-100 pg of nickel porphyrin are listed in Table I; a 2a error in the isotopic determination may be taken as approximately k0.36Y60.'~ It has been previously shown that the chemical preparation steps do not lead to significant isotopic fra~ti0nation.l~ The carbon isotopic composition of the nickel butanoporphyrin (1) from the lacustrine Messel shale was reported to be 2% heavier (i.e., enriched in carbon-13) than Ni-DPEP (8) from the same deposit.I7 This isotopic difference was considered to be too large to be explained in terms of diagenetic, or any other postsynthetic, fractionations at a few carbon positions, and was taken as evidence for a unique origin for 1, apparently from an unknown chlorophyll or porphyrin s t r u c t ~ e . 'The ~ carbon isotopic compositions (Table I) of the butanoporphyrins 1 from both sediments in this study were also isotopically heavier than Ni-DPEP (8). For the Julia Creek vanadyl porphyrins, 1 was 1.7% heavier while for the Condor nickel porphyrins it was 3.6% heavier. The carbon isotopic compositions for 2 from both sediments and 10 from Julia Creek do not differ significantly from those of the respective butanoporphyrins 1. Although (16) Popp, B. N.; Boreham, C. J.; Hayes, J. M. Submitted for Dublication in Energy Fuels. (17)Hayes, J. M.; Takigiku, R.; Ocampo, R.; Callot, H. J.; Albrecht, P. Nature 1987, 329, 48-51.
the isotopic similarity may be coincidential, we suggest that 1,2, and 10 are closely related and have a common precursor, presumably chlorophyll c. In the Messel shale, 1 was 2.4% heavier than 8 while 2 had a carbon isotopic composition 0.2% lighter than 8.17 The latter finding is different from the results presented here. The Messel sediment contains not only evidence of primary inputs from bacterial phototrophs (nickel porphyrins derived from bacteriochlorophylls are present and are depleted in carbon-13 by 2 L relative to porphyrins derived from oxygenic phototrophs), but also abundant evidence of bacterial reworking of the primary material, especially through production and recycling of methane.17 As a result of these processes, the kerogen is depleted in carbon-13 by 7% relative to porphyrins for which derivation from oxygenic algae was inferred.17 Multiple sources for 2, from chlorophyll c and an unknown precursor chlorophyll or porphyrin of possible bacterial affinity further depleted in carbon-13, may explain the apparent disparity between the Messel shale and the two sediments studied here. From the present study it is suggested that 2 is mainly derived from chlorophyll c and is isotopically heavy. In order to explain the overall isotopically light value for 2 in the Messel sediment, an approximately equal amount of 2 would need to be derived from this unknown precursor assuming its isotopic value is similar to the isotopically light bacteriochlorophylls. In the Condor oil shale, the isotopic value for the kerogen, -20.2L, is only 0.6L lower than that of Ni-DPEP (8, column 3, Table I). In this case, chlorophylls originating from oxygenic algae are most likely the sole source of the nickel porphyrins. When it is taken into account that the chlorophyllide portion of the chlorophyll is commonly enriched in carbon-13 relative to total primary biomass by about O.5%o,I7 it follows that the isotopic composition of the Condor kerogen is very close to that of primary material in that ancient environment. In the Julia Creek sediment, the kerogen (-28.2%) is over 3% lighter than estimated primary biomass. Therefore, the further depletion in the kerogen is attributed to organic inputs from other sources having a light carbon-13 signature. This is probably due to terrestrial and nonphotosynthetic bacterial inputs.18 Any additional chlorophylls arising from these sources should be very minor and thus would not significantly influence the isotopic composition of the petroporphyrins other than certain etio s t r u ~ t u r e s , ' ~e.g., J ~ 9. Chlorophyll c is therefore considered to be the dominant source for 1 and 2 in both Condor and Julia Creek oil shales. Since 5 and 6 have the same isotopic compositions, they also must be related through a common precursor. The speculative pathway presented in Scheme I is an attempt to link the four structures to one common precursor. It has been suggested that the double bond in the acrylic side chain of chlorophyll c condenses with the adjacent five-membered ring at the 13(2)-positionto ultimately yield 2375(Scheme I, path A). At the same time, the acrylic double bond could cyclize with the methoxycarbonyl group attached to the 13(2)-carbonto give a fused seven/five ring intermediate-the common intermediate for 1 and 5 (Scheme I, path B); however, 6 is not readily explained in this way. Although the 13(2)-methoxycarbonylgroup of chlorophyll is known to be readily lost during diagenesis, a small fraction almost certainly survives intact and is preserved as a 13(2)-methyl group in the tentatively identified structure 49 (the absolute arrangement of the two ethyl and two methyl groups on rings A and B is (18) Boreham, C. J.; Powell, T. G.Org. Geochem. 1987, 11,433-449.
Boreham et al.
660 Energy & Fuels, Vol. 4, No. 6,1990
Scheme I. Speculative Scheme for the Formation of Various Petroporphyrin Structures from Chlorophyll Precursors
.I
MU+
I
OMs
Decompositibn
C02H
AMs
Decomposition
COlH
(I)
trona-cia isomerisation
Decarboxylation
- 1
-
(isotope effect?)
Hydrogenation
Hydrogenation
-
c=oo OMe
Ring closure
unknown at presentg). Another mechanism, which is deemed the most likely, for the production of 1,5, and 6 from chlorophyll c is a cyclization involving the acrylic side chain carboxylate group and the 13(2)-carbon (Scheme I, path C; first requires a trans-cis transformation). Once the fused ring system has formed, reduction of the 13(1)-oxo to a hydroxo followed by protonation a t carbon 13(2)and water loss can generate a carbonium ion at this position. Subsequent carbon migration from 13(2)to 13(1) and proton abstraction from the 15(2)-carbon can lead to 7, reduction of which would give 6. Prior reduction of the double bond to a propionate moiety followed by a similar carboxylate cyclization (Scheme I, path D) could provide an additional pathway.' In this latter case precursor chlorophylls that possess the propionate side chain (chlorophyll a and b) cannot be excluded. Recently, it has been shown that the chlorin with structure 12 (Figure 1) could be derived from a dihydroporphyrin precursor, such as chlorophyll a but not chlorophyll c since the latter already has a fully conjugated porphyrin ring.l9 Dehydrogenation of 12 could then yield 5 (Scheme I, path E). The carbon isotope data presented here however do not support a major contribution from chlorophyll a or b, implying that paths D and E have made, at most, a minor contribution in these two sediments. A similar conclusion can be drawn based on reactivities; a propionate carboxylate should be less amenable to nucleophilic attack than an acrylic carboxylate group. The reasons for the heavy carbon isotopic signature of the two closely related pairs of porphyrins (1 and 2; 5 and 6) and the slight difference in isotopic composition between the two pairs are unclear at present. Again we offer some speculative mechanisms that might account for the isotopic variabilities. Assuming that the different precursor chlorophylls (a, b and c ) are isotopically similar, the heavy values might be a result of kinetic isotope effects as noted (19) Keely, B. J.; Maxwell, J. R. Proceedings of the 14th Internationul Meeting on Organic Geochemistry, Paris, September 18-22, 1989, European Association of Organic Geochemists; Abstract 179.
in Scheme I. Decomposition pathways (Scheme I) would be expected to remove isotopically light carbon, resulting in a progressive enrichment in the novel-ringed porphyrin end products. Alternatively, the original chlorophyll c may be inherently enriched in carbon-13 relative to chlorophylls a and b. Indirect evidence in support of this is from the heavy carbon-13 isotopic value of -18% found for chlorophyll c1 + c2 isolated from the algae Sargassum sp. compared with an isotopic value of -24% for chlorophyll a isolated from the same organism.m If the chlorophyllide nucleus of chlorophyll a is assumed to have the same carbon-13 isotopic composition as chlorophyll c1 c2, then isotopic mass balance requires a carbon-13 isotopic value of -34% for the esterifying phytol side chain of chlorophyll a . Clearly, an isotopic difference of 16% between both groups is unlikely," necessitating an isotopically light chlorophyllide a compared with chlorophyll c1 c2. Another possibility is that different source organisms may have isotopically distinct chlorophylls. In the Condor oil shale, for example, the dominant source of the organic matter is from green algae21with a moderate contribution from dinofhgellates.21~In modern analogues, the former organisms did not contain chlorophyll c while the latter contain both chlorophyll a and c.= It is therefore likely that for the ubiquitous DPEP, 8, the carbon isotopic composition is a weighted average value from all preserved chlorophyll inputs as is the isotopic composition of 13(2)-methyl-substituted DPEP, 4. The presumed chlorophyll b derived porphyrin 11 from Julia Creek has an isotopic value very similar to 8, supporting this relationship. However, it is slightly heavier, which may suggest an additional minor contribution from
+
+
(20) Bidigare, R. R. Texas A&M University, private communication. (21) Hutton, A. C. Lacustrine Petroleum Source Rocks; Fleet, A. J., Kelts, K., Talbot, M. R., Eds.; Geological Society Special Publication 40; Blackwell: Melbourne, Australia, 1988; pp 329-339. (22) Boreham, C. J. Unpublished results. 4Methyl-24-ethylcholeatane has been identified by GC-MS in the sediment extracts and is an indicator for freshwater dinoflagellates. (23) Jeffrey, S. W. Primary M u c t i o i t y in the Sea;Falkoviaki, P. G., Ed.; Plenum: New York, NY, 1980; pp 33-58.
Energy & Fuels 1990,4,661-664 an isotopically heavier source. In the Condor sediment, 11 has an isotopic value 3.3% heavier than 8 and 0.9% lighter than the average of 1 and 2. It would appear from what has been outlined above that chlorophyll c derived petroporphyrins are isotopically heavy in these two sediments and that chlorophyll c3, which contains a 7carboxymethyl group,24may be inferred as an alternative source for 11. Previously it had been suggested that the two 13(1)methyl-substituted porphyrins, 3 and 7, might be related through a common precursor of unknown origin.8 Since (24) Fookes, C. J. R.; Jeffries, S. W. J. Chem. Soc., Chem. Commun. 1989, 1827-1828.
661
6 is most likely the hydrogenated derivative of 7, it too should be related. Although 3 and 6 have not been isolated together from the same sediment in this study, their relative carbon isotopic values (3 from Condor oil shale and 6 from Julia Creek oil shale; Table I) have the same relationship; they lie almost at the midpoint between 8 and 1 or 2. Thus the isotopic evidence also supports the notion of a common precursor, possibly chlorophyll c. Acknowledgment. We thank CSR Limited and Southern Pacific Petroleum/Central Pacific Minerals for providing the Julia Creek oil shale and the Condor oil shale, respectively. Mr. A. Juodvalkis is gratefully acknowledged for his technical assistance with the HPLC porphyrin isolations.
Diastereoisomers in Sedimentary Vanadyl Porphyrins Christopher J. Boreham* Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 378, Canberra, 2601, Australia
Peter S. Clezy Department of Organic Chemistry, University of New South Wales, P.O. Box 1, Kensington, NSW,2033, Australia
Glen B. Robertson Research School of Chemistry, Australian National University, P.O. Box 4, Canberra, 2601, Australia Received April 27, 1990. Revised Manuscript Received August 6, 1990
The observation that the product of vanadyl insertion into the free-base porphyrin Id-methyl15,17-propanoporphyrin separated into two near equal area peaks on HPLC analysis precipitated an investigation into the structure of both vanadyl porphyrins. Semipreparative HPLC and TLC were used to isolate pure components, and an X-ray structure determination on the more polar compound revealed a structure wherein the out-of-plane vanadyl and 15I-methylgroups were opposed, referred to as up-axial, relative to the plane of the porphyrin ring. The other complex is tentatively assigned to the diastereoisomer in which both groups are juxtaposed, down-axial. Metalloporphyrins in which the metal is coplanar with the ligand plane cannot have this diastereoisomerism. Sedimentary vanadyl porphyrins from the immature Serpiano oil shale contain both diastereoisomers but in a different ratio compared to the synthetic mixture. Artificial maturation experiments on the diastereoisomers suggest isomer interconversion can occur and the up-axial configuration is the more stable.
Introduction During the past few decades our understanding of the processes that lead to the formation of petroporphyrins has increased dramatically. The early hypothesis of a direct pathway from chlorophyll or heme to porphyrin' has had to be drastically modified as investigations continue.2$ ~
~~~
~~
~
(1) Treibs, A. Ann. Chem. 1934,509, 103-114. (2) Baker, E. W.; Louda, J. W. Biological Markers in the Sedimentary Record; Johns, R. B., Ed.; Elaevier: New York, 1986; pp 124-225. (3) Filby, R.H.;Van Berkel, G. J. Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 2-39.
Recognition of a much greater complexity of the interconversion has resulted, principally, from increased ability to characterize individual petroporphyrins through the use of new or improved instrumentation. Thus, methods used to characterise petroporphyrins have evolved from UV-vis and mass spectral analysis4of bulk porphyrins or of simple chromatographic concentrates, to structural identification of single porphyrins by high-field NMR,"12 by X-ray (4) Baker, E. W.;Palmer, S. E. The Porphyrins, Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. lA, pp 485-552. (5) Fookes, C. J. R. J. Chem. SOC.,Chem. Commun.1983,1472-1473. (6) Fookes, C.J. R. J. Chem. Soc., Chem. Commun.1983,1474-1476.
0887-0624/90/2504-0661$02.50/0 Published 1990 by the American Chemical Society
