Crocetane, or 2,6,11,15-tetramethylhexadecane, is an

isoprenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes", ...

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

compound. Unlike its isomer

phytane Phytane is the isoprenoid alkane formed when phytol, a constituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol. Prista ...

, crocetane has a tail-to-tail linked isoprenoid skeleton. Crocetane has been detected in modern sediments and geological records as a

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...

, often associated with

anaerobic methane oxidation Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

Research

Crocetane was first studied in the late 1920s and early 1930s for the structural identification of

crocetin Crocetin is a natural apocarotenoid dicarboxylic acid that is found in the crocus flower together with its glycoside, crocin, and '' Gardenia jasminoides'' fruits. It forms brick red crystals with a melting point of 285°C. The chemical struct ...

, which is its polyunsaturated diacid analogue. The infrared spectrum was reported in 1950, the mass spectrum was described in 1968 and the ¹H and ¹³C NMR spectra was obtained in 1990s. In 1994, Liangqiao Bian first reported strong ¹³C depletion in crocetane from anoxic sediments in the

Kattegat The Kattegat (; sv, Kattegatt ) is a sea area bounded by the Jutlandic peninsula in the west, the Danish Straits islands of Denmark and the Baltic Sea to the south and the provinces of Bohuslän, Västergötland, Halland and Skåne in Sweden ...

. Such low ¹³C content is thought to originate from microbes harvesting biogenic

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

, which is always ¹³C depleted, as a carbon source. Years later several groups made similar observations in either modern or ancient sediments near methane seeps. Crocetane was found in environments with anaerobic methane oxidizing consortium, composed of

methanotroph Methanotrophs (sometimes called methanophiles) are prokaryotes that metabolize methane as their source of carbon and chemical energy. They are bacteria or archaea, can grow aerobically or anaerobically, and require single-carbon compounds to su ...

archea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeb ...

and

sulfate-reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...

. These work makes crocetane the first biomarker of anaerobic methanotrophy. In 2009, Ercin Maslen and her colleagues detected crocetane in highly-mature

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

sediments and crude oils of the

Western Canada Sedimentary Basin The Western Canadian Sedimentary Basin (WCSB) underlies of Western Canada including southwestern Manitoba, southern Saskatchewan, Alberta, northeastern British Columbia and the southwest corner of the Northwest Territories. This vast sedimentary ...

. They propose that natural product precursor for this crocetane is

green sulfur bacteria The green sulfur bacteria are a phylum of obligately anaerobic photoautotrophic bacteria that metabolize sulfur. Green sulfur bacteria are nonmotile (except ''Chloroherpeton thalassium'', which may glide) and capable of anoxygenic photosynthesi ...

derived isorenieratene and palaerernieratene, which means that crocetane can also be related to photic zone euxinia in highly matured samples.

Analysis

Due to structural similarities, crocetane often co-elutes with

and is hard to identify. People have been using specialized gas chromatographic methods to achieve partial separation. For example, Volker Thiel and his colleagues used a 25-m squalene capillary column with hydrogen as a carrier gas. For the same reason mass spectra of crocetane and phytane are very similar except for that crocetane does not have intense m/z=183 fragments. To identify crocetane, the mass spectrometer can be operated in selection ion monitoring(SIM) mode to monitor m/z 113, 169, 183, 197 and 282. Paul Greenwood and Roger Summons in 2003 reported using GC MS-MS instrument to measure the daughter ion of m/z 196→127/126 and 168→126 to distinguish crocetane from phytane.

References

{{Reflist, colwidth=30em Terpenes and terpenoids Alkanes Biomarkers