Phytane

Phytane is a 20–carbon atom branched-chain alkane. Formally known as 2,6,10,14-tetramethylhexadecane, it has three chiral centers and therefore eight possible stereoisomers. It is normally found as a mixture of most if not all of its isomers.

Unlike lower alkanes that are flammable and present breathing hazards, phytane is essentially nonhazardous. As a chemical, it has no practical uses or commercial value.

Why is phytane a Molecule of the Week? It turns out to be quite useful to geoscientists as a marker of carbon history and, in particular, as a proxy for long-ago carbon dioxide levels in Earth’s atmosphere.

Phytane is a degradation product of chlorophyll, which, of course, is related to CO₂ consumption. Jaap Sinninghe Damsté and her co-workers at the Royal Netherlands Institute for Sea Research obtained more than 300 ocean sediments and oils from different locations and geological periods, extracted phytane from them, and determined the ¹²C/¹³C ratios in the phytane.

Photosynthesis, and therefore chlorophyll, consumes ¹²CO₂ preferentially to ¹³CO₂. So when the atmosphere is rich in CO₂, phytane from chlorophyll has a high ¹²C/¹³C ratio. Thus, the researchers could estimate atmospheric CO₂ back to the Cambrian period, ≈500 million years ago. Their results matched well with those from existing CO₂ proxies that covered shorter time periods.

Chlorophyll is known to have existed as long ago as 1.7 billion years. Geoscientists therefore have the challenge of using proxies to determine atmospheric CO₂ levels much farther back than the Cambrian.