Smelling, as the New York Times put it in 1895, “like the blending of new-mown hay, the damp woodsy fragrance of a fern-copse, and the faintest possible perfume of the violet”, the aromatic allure of ambergris is not difficult to understand. In the Middle East it is an aphrodisiac, in China a culinary delicacy. King Charles II is said to have delighted in dining on it mixed with eggs. Around the world it has been a rare and precious substance, a medicine and, most of all, a component of musky perfumes.
You’d never think it started as whale faeces, and smelling like it too. As Herman Melville said in that compendium of all things cetacean Moby Dick, it is ironic that “fine ladies and gentlemen should regale themselves with an essence found in the inglorious bowels of a sick whale”.
But vats of genetically modified bacteria could one day be producing the expensive aromatic chemical craved by the perfume industry, if research reported by biochemists at the Swiss fragrance and flavourings company Firmenich in Geneva comes to fruition. Their results are another demonstration that rare and valuable complex chemicals, including drugs and fuels, can be produced by sophisticated genetic engineering methods that convert bacteria into microscopic manufacturing plants.
Made from the indigestible parts of squid eaten by sperm whales, and usually released only when the poor whale dies from a blocked and ruptured intestine and has been picked apart by the sea’s scavengers, ambergris matures as it floats in the brine from a tarry black dung to a dense, pungent grey substance with the texture of soft, waxy stone.
Because ambergris needs this period of maturation in the open air, it couldn’t be harvested from live sperm whales even in the days when hunting was sanctioned. It could be found occasionally in whale carcasses – in Moby Dick the Pequod’s crew trick a French whaler into abandoning a whale corpse so that they can capture its ambergris. But most finds are fortuitous, and as was reported recently large pieces of ambergris washed ashore can be worth many thousands of dollars.
The perfume industry has long accepted that it can’t rely on such a scarce, sporadic resource, and so it has found alternatives to ambergris that smell similar. One of the most successful is a chemical compound under the trademarked name of Ambrox, which was devised by Firmenich’s fragrance chemists in the 1950s and featured, I am told, in Dolce & Gabbana’s perfume Light Blue. One perfume website describes it, with characteristically baffling hyperbole, as follows: “You're hit with something that smells warm, oddly mineral and sweetly inviting, yet it doesn't exactly smell like a perfumery or even culinary material. It's perfectly abstract, approximating a person's aura rather than a specific component”.
To create Ambrox, chemists start with a compound called sclareol, named after the southern European herb Salvia sclarea (Clary sage) from which it is extracted. In other words, to mimic a sperm whale’s musky ambergris, you start with an extract of sage.
This is par for the course in the baffling world of the human sense of smell, or as scientists call it, olfaction. In this case Ambrox has a very similar structure to the main smelly molecules in ambergris, but this doesn’t always have to be so: two odorant molecules can smell almost identical while having very different molecular structures (they are all generally based on frameworks of carbon atoms linked into rings and chains). Equally, two molecules that are almost identical, even mirror images of one another, can have very different odours. Quite how such molecules elicit a smell when they bind to the proteins in the olfactory membrane of the nasal cavity is still not understood.
Back to ambergris – clary sage is easier to get hold of than whale faeces, but even so the herb contains only tiny amounts of sclareol, and it is laborious to extract and purify. That’s why Firmenich’s Michel Schalk and his colleagues wanted to see if they could take the sclareol-producing genes from the herb and put them in the gut bacterium Escherichia coli, the ubiquitous single-celled workhorse used by the biotechnology industry for cloning genes and expressing therapeutically useful products.
Sclareol belongs to a class of organic compounds called terpenes, many of which are strong-smelling and are key components of the essential-oil extracts of plants. Sclareol contains two rings of six carbon atoms each, formed when enzymes called diterpene synthases stitch together parts of a long chain of carbon atoms. The Firmenich researchers show that the formation of sclareol is catalysed in two successive steps by two different enzymes.
Schalk and colleagues extracted and identified the genes that encode these enzymes, and transplanted them into E. coli. That alone, however, doesn’t necessarily make the bacteria capable of producing lots of sclareol. For one thing, it also has to be able to make the long-chained starting compound, which can be achieved by adding yet another gene from a different species of bacteria that happens to produce the stuff naturally.
More challengingly, all of the enzymes have to work in sync, which means giving them genetic switches to regulate their activity. This approach – making sure that the components of a genetic circuit work together like the parts of a machine to produce the desired chemical product – is known as metabolic engineering. This is one level up from genetic engineering, tailoring microorganisms to carry out much more demanding tasks than those possible by simply adding a single gene. It has already been used to make bacteria produce other important natural compounds, such as the anti-malarial drug artemisinin.
With this approach, the Firmenich team was able to create an E. coli strain that could turn cheap, abundant glycerol into significant quantities (about 1.5 grams per litre) of sclareol. So far this has just been done at a small scale in the lab. If it can be scaled up, you might get to smell expensively musky without the expense. Or at least, you would if price did not, in the perfume business, stand for an awful lot more than mere production costs.
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