Pristanic structure

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

Pristane is an alkane that is present to the extent of about 14% in shark liver oil. Its lUPAC name is 2,6,10,14-tetramethylpentadecane. Write its structural formula. 

Identification becomes more difficult in the case of isoparaffins since many structural isomers are possible with increasing carbon number. Nevertheless, all possible isoalkanes from through have been found in crude oils, along with several isomers of and some of Many isoprenoids (e.g., pristane and phytane) which serve as biomarkers to the genesis of petroleum have been detected in significant... 

Figure 9.8 Chemical structures of the isoprenoid hydrocarbons pristane (C19 isoprenoid hydrocarbon) and phytane (C20 isoprenoid hydrocarbon).

Very recently it was demonstrated that tocopherol moieties in kerogen are likely precursors of prist-l-ene (Figure 7) (30). This idea was supported by the fact that tocopherols are widely distributed in photosynthetic tissues and that they also occur as such in several recent sediments (31). It is tempting to conclude that during "natural pyrolysis" the generated pristene will be transformed to the well known component, pristane, in ancient sediments and oils. This example nicely illustrates that we have to be very careful when we conclude that acyclic isoprenoid hydrocarbons such as pristane originate from the chlorophyll side chain, phytol, based solely on structural similarities. 

We have also examined the composition of the hydrocarbons above C9 in the products of the semi-corona discharge. The gas chromatogram is very unresolved (Figure 2). No normals or branched-chain isopropre-noid hydrocarbons were identified. The preponderance of normal hydrocarbons and the presence of the isoprenoids, pristane, and pythane are considered to be indicative of biological origin. Analysis of the mixture by mass spectrometry shows that the compounds are possibly cyclic in structure (9). 

Only a few major compound series can be recognized at the level of their molecular structures based on relative retention times and distribution patterns by gas chromatography alone. This applies to n-alkanes in the nonaromatic hydrocarbon fraction, n-fatty acids in the carboxylic acid fraction and in some cases n-alkanols in the neutral polar fraction. High abundance of a few single compounds (pristane, phytane, long-chain alkenones) sometimes also allow their direct identification from gas chromatograms. 

Isoprenoid hydrocarbons such as norpristane (2,6,10-trimethylpentade-cane),pristane (2,6,10,14-tetramethylpentadecane) and phytane (2,6,10,14-tetramethylhexadecane) (Structures I-III). 

Recent studies have led to a full resolution of the structure of the phytanic acid a-oxidation pathway. Indeed independent studies by Croes et aC and Verhoeven revealed that 2-hydrox5 hytanoyl-CoA undergoes cleavage to produce formyl-CoA and pristanal respectively, which is than oxidized to pristanic acid (Fig. 3B). The pristanic acid is now ready for p-oxidation after its activation to its CoA-ester. 

Since phytane (C20H42) has at one end an iso and at the other an ante-iso structure, its fragmentation pattern is different from pristane (C19H40). Below mass (m/z) 113 both fragmentation patterns seem very similar. 

Figure 1 Fatty-acid structure and nomenclature. (A) Chemical formula and carbon atom numbering system for a 16-carbon saturated fatty acid (16 0). (B) Schematic representation of 16 0. (C) A monounsaturated fatty add, 18 1n-9, showing the double bond nine carbon atoms from the methyl end (carbon 18). (D) The essential n-6 fatty acid 18 2n-6, where the first double bond is found six carbon atoms from the methyl end. The two double bonds are separated by a methylene (-CH2-) group. (E) The essential n-3 fatty acid 18 3n-3, where the first double bond is found three carbon atoms from the methyl end. (F) Phytanic acid, a dietary / -methyl-branched-chain fatty acid (3,7,11,15-tetramethyl 16 0). The melhyl group on carbon 3 prevents this fatty acid from degradation by /3-oxidation. (G) Pristanic acid (2,6,10,14-tetramethyl 15 0) is the product of phytanic acid o-oxidation, in which a single carbon (carbon 1) is lost. The methyl group on carbon 2 does not preclude subsequent degradation by /3-oxidation.

---