Cyberlipid
Search Cyberlipid

WAXES AND OTHER ESTERS

Waxes is a general term used to refer to the mixture of long-chain apolar lipids forming a protective coating (cutin in the cuticle) on plant leaves and fruits but also in animals (wax of honeybee, cuticular lipids of insects, spermaceti of the sperm whale, skin lipids, uropygial glands of birds, depot fat of planktonic crustacea), algae, fungi and bacteria. Some waxes are of mineral origin. Montan wax originates from mob or lignite, that fossilized compound representing a late step of the transformation of vegetal into hydrocarbons.

The various materials named waxes do not form a chemically homogeneous group. All waxes are water-resistant materials made up of various substances including hydrocarbons (normal or branched alkanes and alkenes), ketones, diketones, primary and secondary alcohols, aldehydes, sterol esters, alkanoic acids, terpenes (squalene) and monoesters (wax esters), all with long or very long carbon chains (from 12 up to about 38 carbon atoms) and solid in a large range of temperature (fusion point between 60 and 100°C).
More commonly, waxes are esters of an alcohol other than glycerol (long chain alcohol, sterol, hydroxycarotenoids, vitamin A) and a long chain acid (wax esters). Wax esters are saponified by hot alkaline solutions and give a fatty acid and an alcohol. They are soluble in aromatic solvents, chloroform, ethers, esters and ketones.

pict40.gif

R1 et R2 have commonly 10 to 20 carbon atoms or more. Example of wax esters are: dodecyl hexadecanoate (lauryl palmitate), octadecyl octadecanoate (stearyl stearate), etc. The acid or the alcohol chain can be unsaturated. In plants and some algae, phytol may be the alcohol component of was esters protecting leaves against dessication and parasites. Wax monoesters account for about 25% of sebum lipids in human, this wax being characterized by a high amount of an unusual fatty acid, 16:1n-10.
The physical characteristics of wax have maximized their usage in various industries, particularly in cosmetics. Wax are formulated in numerous personal care products due to their excellent emollient behavior. Wax esters are fine chemicals which are produced in low volume but are highly priced. Jojoba oil and sperm whale oil are natural waxes that fall within that fine chemicals group. Several attempts have been made to synthesize wax esters with cheap starting materials. Thus, waxes were synthesized from palm oil through enzymatic transesterification with oleyl alcohol using Lipozyme RM IM as the catalyst (Keng PS et al., Ind Crops Prod 2009, 29, 37).
In the uropygial gland of birds, the acids of wax esters may have a mono- or multi-branched chain (diester waxes). The physiological function of these waxy material is still a matter of debate but may contribute to protect birds against wetting, to make the feather flexible, to play a role as antiparasitic compounds or to provide UV protection. It has been shown that uropygial waxes shifted their structure during the mating season from a monoester type to one of two diester types.

wax diester

Waxes of type 1 are based on C8 to C16 b-hydroxy acids (R2) esterified with C6 to C16 fatty acid (R1) at the b-hydroxy position and with  C16 to C20 fatty alcohol (R3) at the carboxyl group. These C30-C50 diester waxes have been described in several shorebirds (Rijpstra WI et al., J Nat Prod 2007, 70, 1804) and in female mallards (Anas platyrhynchus) (Kolattukudy PE et al., J Lipid Res 1987, 28, 582). Waxes of type 2 are based on C12-C23 alkane-1,2-diols (or 2,3-diols, sometimes named uropygiols) esterified with C10-C20 acids at the two hydroxyl groups. These diester waxes have been described first in chicken (Haahti EO e al. J Lipid Res 1967, 8, 131) and then in turkey (Hansen IA et al. J Lipid Res 1969, 10, 267), in the pheasant (Phasianus colchicus) (Sa