Vanadium complexes chlorophyll

Other metalloporphyrins can also be found in Nature, though they do not seem to perform any vital physiological function. For example, the copper(II) complex of uroporphyrin-III (Table 1) occurs in high concentrations in the wing feathers of Turacus indicus, and is the source of most commercial samples of uroporphyrin-III. Chlorophyll degradation products, as the nickel and vanadium complexes, are found in oil shales and as a troublesome contaminant in crude petroleum oils. 

The amazing richness of vanadium complexes in crude oil and shale raises the obvious questions of where these vanadium complexes come from and what their biogenic source is. It is generally believed that the vanadium porphyrins result from the incorporation of vanadium in porphyrin structures originating from chlorophyll. (The reader is re-... 

Figure 10,1. Key types of vanadium complexes in petroleum (a) porphyrins, (b) mixed ligands such as N2O2 type, the -ketimines, (c) pseudoaromatic pheophorbides such as a bacterio-chlorophyll (the outer conjugation is interrupted, but still belong to the diaza-ld-annulene system), and (d) highly aromatic porphins such as the dehydrogenated product of m-a-naphylporphyrin, which was identified in Nonesuch shale. Type (a) is found in all petroleum. Types (b), (c) and (d) are commonly referred to as the nonporphyrin type of vanadium...

Vanadium complexes of porphins derived from chlorophyll have been found in oil-bearing shales of the Silurian and later geological periods but these complexes are probably of secondary origin (140). The fact that these compounds can be isolated indicates the relatively mild conditions under which petroleum is formed. 

In addition to hydrocarbons, petroleum also contains compounds that consist of nitrogen, oxygen, and sulfur (in the minority) as well as trace amounts of metals such as vanadium, nickel, iron, and copper. Porphyrins, the major organometaUic compounds present in petroleum, are large, complex cyclic carbon structures derived from chlorophyll and characterized by the ability to contain a central metal atom (trace metals are commonly found within these compounds). 

Chlorophylls and Iron porphyrins are prevalent In plant and animal matter whereas only nickel (as Nl(II)) and vanadium (as oxovanadlum V(IV), V 0) metalloporphyrlns are found In petroleum. To determine a plausible reaction sequence for these conversions, we are studying hydrolysis and metallatlon reactions of metal complexes of pheophytlns (the demetallated ligands of chlorophylls) and of porphyrins. The pheophytlns and metal pheophytlnates, Including the chlorophylls and the most abundant natural porphyrins, are highly llpophyllc and have very low solubilities In aqueous... 

The problem of the origin of the metal-porphyrins is closely related to that of the origin of petroleum and is oie of the most basic and interesting questions of petroleum geochemistry. The most probable conclusion seems to be that the nickel and vanadium porphyrin complexes are formed by metal exchange reactions from animal and/or plant metabolic pigments such as hemoglobin and chlorophyll. 

Several different sequences of reactions may be postulated for the conversion of the magnesium complexes of pheophytlns (chlorophylls) and Iron complexes of protoporphyrin IX and related porphyrins (hemes and hemlns) Into the nickel and vanadium porphyrins found In petroleum. One possible reason for the Isolation of only the nickel and oxovanadlum metalloporphyrlns Is that only they were resistant to degradation. While studies of Hodgson do Indicate that complexatlon of vanadium and nickel do Impart added thermal stability to porphyrins (12). Berezin has found that complexatlon of other metal Ions such as cobalt and copper also Imparts added thermal stability (13. 14). In addition, Hodgson s study Indicates that relatively little thermal degradation of the metalloporphyrlns has taken place In most crude oils (which would lead to unbound vanadium and nickel), One would expect that if little degradation of these metalloporphyrlns has occurred, complete disappearance of other metalloporphyrlns by thermal degradation Is an unreasonable assumption. 

The first two of these compounds occur at sufficiently high concentrations in some crudes to enable production of them from petroleum. Benzonitrile has also been detected [6]. Much of the trace metal content of petroleums, in particular vanadium and nickel, is present in association with petroporphy-rins, which are polycyclic pyrroles closely related in structure to the hemes and chlorophylls. These materials are examples of the more complex nitrogen heterocycles to be found in petroleum. These particular heterocycles with their complexed metal atoms contribute much to our present knowledge of the original biogenesis of the petroleum hydrocarbons [6]. 

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