ALKYL KETONES

Ketones with various chain lengths are widely found in nature where they contribute to the flavors and odors in animals as well as in plants.
As a first example, several 2-methyl ketones, with a 9 to 15 carbon-chain length, were identified in milk and milk products. Some of them are also present in brandies, where they are produced through b-oxidation and decarboxylation of fatty acids by yeast. One of 2-methyl ketones, 2-tridecanone, is secreted by the glandular trichomes in the tomato leaves and as an insecticide makes the plant resistant to a variety of insects.

Symmetrical ketones.

They are commonly found in epicuticular wax at the surface of many angiosperms. They have the common formula :

[CH₃(CH₂)_n]₂C=O     with n = 3-20

One of the most common of these symmetrical ketones is palmitone (with n = 14).

Cyclic ketones.

 These compounds are known to have sex pheromone activities in mammals, such as civetone (1-cycloheptadecen-10-one) and muscone (1-methyl-cyclopentadecan-3-one).

Furthermore, it was shown that the odor of cyclic ketones in related to the ring size.

Long-chain ketones

Formation of long-chain ketones containing from 29 to 35 carbon atoms are formed by pyrolysis of free fatty acids or triacylglycerols. 
Experimentation involving heating of oleic acid and palmitic or stearic acid at temperature higher than 300°C provided evidence of the formation of mixtures of ketones with 33 or 35 carbon atoms (Evershed RP et al., Tetrahedron Lett 1995, 36, 8875). 

n = 14 or 16

These compounds were studied to determine the origin of organic residues preserved in archaeological pottery but they may originate from both animal tissues or higher plant leaf waxes.

Alkenones.

These long-chain unsaturated methyl or ethyl ketones are very important compounds which are produced by a specific class of phytoplankton represented notably by the coccolithophorid Emiliana huxleyi (Volkman JK et al., Phytochemistry 1980, 19, 2619), the most abundant unicellular phytoplankton which plays a fundamental role in the total primary production in the oceans. Satellite images clearly demonstrate that point. The Emiliana blooms have a major impact on the biological carbon cycle in the ocean and on ocean/atmosphere fluxes of carbon dioxide (Westbroek et al., Global Planetary Change 1993, 8, 27). 

Emiliana huxleyi

Alkenones possess several unusual characteristics, including their very long chain-length (C35-C40) and the spacing (C7) and configuration (trans) of their positions of unsaturation (Marlowe IT et al., Chem Geol 1990, 88, 349) and they were shown to be membrane-unbound lipids (Sawada K et al., Phytochemistry 2004, 65, 1299).
The two most abundant alkenones (C37:2 and C37:3) have so far unknown physiological function but their characteristic is that they remain partially intact in oceanic sediments after the cellular death and thus may be used as biomarkers.

Until now they were reported exclusively from the oceanic haptophytes Emiliana huxleyi and Gephyrocapsa oceanica and from coastal species of Chrysotila (Rontani JF et al., Phytochemistry 2004, 65, 117).
As for the phospholipid fatty acids in all living cells, the proportion of the more unsaturated alkenone is increased when the growth temperatures get colder and vice versa. Thus, a simple index ([37:2]/{[37:2]+[37:3]}) was formulated to quantify the degree of unsaturation in a given alkenone series and shown to be linear versus the growth temperature (Prahl FG et al., Nature 1987, 330, 367). Consequently, stratigraphic measurement of that index in dated sediments is done by paleoceanographers to assess climate changes on timescales ranging from interannual (El Nino) to millenial (glacial/interglacial). Thus, the variations in sea-surface of the eastern equatorial Atlantic over the past 500,000 years were inferred from the distribution of alkenones in sediments (Brassell SC et al., Nature 1986, 320, 129).
Similarly, the dissolved carbon dioxide concentration of surface waters in which E. huxleyi grew can be calculated using the proportion of the two different isotopic forms, ¹²C and ¹³C.
Thus, for their peculiarities, alkenones are not only the most abundant extractable lipids in Quaternary marine sediments but also the most precious molecules for the knowledge of past sea-surface temperature.