Ever put a rosemary leaf in your mouth? Ever wondered where that intense flavour and scent comes from? We hope today’s post will pique your interest, and go some way towards answering your questions.
(This post has been prepared by students from the Theory and Modelling in the Chemical Sciences Centre for Doctoral Training, who are currently doing their PhD research in Oxford, Southampton and Bristol.)
1. The main image shows sprigs and leaves of rosemary, as well as a round-bottomed flask and a beaker, two items of glassware used in many chemical procedures. The green molecular structure is the carbon atom skeleton of α-pinene (alpha-pinene), one of the key components of the familiar scent of rosemary.
Rosemary (Rosmarinus officinalis) is native to the coastal Mediterranean, but as a hardy perennial herb it can easily weather cooler climates, surviving winter temperatures down to -10 °C. Intensely aromatic, one often smells it before one can see it and so provides much of the olfactory backdrop to hot Provençal evenings. The rich aroma of this herb makes it a valuable ingredient in a variety of cuisines, often used to add a fresh, ethereal quality to meat dishes and a layer of savouriness and complexity to desserts. Commonly accompanying roast lamb and pork or used as an ingredient in stuffing for roast pheasant, the essential oils dissolve in hot fat and infuse deep into the meat. The aromatic intensity allows to prepare desserts with less sugar, while avoiding blandness; this also makes for a successful pairing between dessert and wine, a famously difficult task with most desserts. The dark, herbal, woody character of rosemary complements the flavours of ripe, baked stone fruit like apricots particularly well, and a few pinches of chopped-up leaves instil a depth of flavour that turns even a simple sponge into haute cuisine.
The aromatic properties of rosemary stem from essential oils, similar to those in eucalyptus: they actually share the same principal essential oil – eucalyptol/cineole – but combination with a range of other oils makes their scents different, and distinctive. The oils can be extracted in a number of ways, something we have explored previously in our post on roses, for example, and extraction can be as simple as passing steam through crushed leaves or boiling them in water and then distilling.
But it’s not just the taste and smell of rosemary that humans have made use of. Rosemary has long been thought to have medicinal uses. Traditionally, it is associated with memory (in Hamlet, Shakespeare wrote “There’s rosemary, that’s for remembrance” (iv. 5.)). Although this link is mainly traditional and in myths, recent research (summarised here) has suggested long-term memory can in fact be improved by exposure to rosemary oil. This may be due to a chemical in rosemary called ursolic acid, which prevents the breakdown of a neurotransmitter (brain chemical), a process which has been blamed for memory loss in sufferers of Alzheimer’s disease. As well as memory loss, rosemary has been suggested to be helpful for treating: migraines; rheumatic pains, muscle aches, and arthritis; digestion problems, including intestinal cramps; and exhaustion. However, these effects have not been clearly demonstrated or understood. If you’d like to read a bit more, have a look at this link on the BBC, and this study.
Aside from health benefits, rosemary has been used as an antioxidant (a molecule that prevents oxidation, normally a reaction with oxygen, of other molecules) and microbiocidal agent (destructive to microbes), both of which are useful for preserving food. This can be rationalised through the presence of several compounds, including rosemarinic acid and carnosic acid (antioxidants) and camphor and eucalyptol (microbiocidal agents).
2. The woody scent you can smell in a rosemary leaf is due to a molecule called pinene (pronounced pine-een). Pinene is an example of a terpene (pronounced ter-peen) molecule, with the molecular formula C10H16. Terpene molecules are common in nature and can be identified quite easily, as they generally have a number of carbon atoms which is a multiple of five. This happens, because they are biosynthetically derived from the isoprene molecule (also featured in our post on sunflowers), which is a five carbon molecule. α-Pinene is the most abundant terpene found in nature.
The double bond means that α-pinene belongs to the family of alkene molecules, and it contains both a 6 carbon and a 4 carbon ring in its structure. 4 carbon rings are relatively unusual in chemistry, as the structure is very strained – it is hard to accommodate the structural preferences of carbon (sp3 hybridised, so angles of around 109.5 °) in such a small ring. This means that terpene is very reactive, and its reactions usually involve breaking open this ring; in computational chemistry, we would talk about this as a high energy ring, because it is favourable to break it open as more stable structures generally result.
There are two structural isomer forms of pinene, the alpha (α) and the beta (β) form. Illustrated in Scheme 1, the α isomer has a carbon-carbon double bond inside the six atom carbon ring, while in the β isomer, the double bond is outside the ring, where previously there was a methane functional group. For both the α- and the β-form, two enantiomers exist.
Enantiomers have been mentioned in other posts, such spearmint and roses, where two molecules can have the same chemical structure, but they are non-superimposable mirror images. This can be seen in Figure 1 below, where the bridging carbon atom can either be above or below the ring; depending on the optical rotation observed from pure enantiomers, these are labelled in chemistry as (+) or (-). Both the (+) and (-) forms are present in pine trees, however, the (-) molecule is more common in European pines, while the (+) molecule is more common in the North American version of the trees!
Although their structures are very similar, pinene isomers smell distinctively different. The β isomer has been described as having a woody-green pine-like smell, while the α isomer smells like turpentine. Even a small difference in structure can give a completely different smell, related to where in the olfactor bulb the molecules bind!
3. α-pinenes and β-pinenes are important compounds in industry (see, for example, J. Am. Chem. Soc., 1976, 98 (5), pp. 1227–1231). As discussed above, these molecules are also highly reactive, due to the constrained four membered ring, making them useful precursors in synthesis. Hence, it’s important that such compounds can be manufactured efficiently and cheaply on an industrial scale.
When coming up with a feasible synthesis for a particular compound, chemists regularly take inspiration from nature. Within Rosemary, pinene molecules are formed by the cyclisation reaction of geranyl pyrophosphate, catalyzed by the enzyme pinene synthase. The observed selectivity has been analysed in greater detail in this article (J. Am. Chem. Soc., 1987, 109 (14), pp. 4399–4401). This process happens in two stages:
Firstly, the enzyme pinene synthase catalyses the conversion of geranyl pyrophosphate (1) into linaloyl pyrophosphate (2). An idea about the mechanism of this step is given in Scheme 2 and results in the phosphorylated alcohol functional group (OPP) migrating from a primary carbon to a tertiary carbon centre. As shown, the stereochemical selectivity of this step determines the stereochemistry of the product. In nature, both (+)- and (-) pinene synthase exist, which produce linaloyl pyrophosphate in the corresponding enantiomeric form with high stereoselectivity.
The second step involves an intramolecular cyclisation reaction of linaloyl pyrophosphate, via an intermediate carbocation. The mechanism of this reaction step is given in Scheme 3 – it corresponds to the one given J. Am. Chem. Soc., 1987, 109 (14), pp. 4399–4401, but is perhaps easier to see (adapted from this article). The resulting carbocation formed is highly stable, with the positive charge residing on a tertiary carbon and stabilised by hyperconjugation. The product is then formed by deprotonation of a carbon adjacent to the positive charge. The carbon at which the deprotonation event occurs thus determines which structural isomer is formed – i.e. α- or β-pinene.
So next time you dream of a hot and sunny afternoon as you brush past a rosemary bush, ponder the small differences in molecular structure that help to create a tempting bouquet of smell; it may be easier to remember this, too.
Contributors: Hannah Bruce-Macdonald, Timothy Wiles, Jonathan Mannouch, Daniel Tracey,Domagoj Fijan, Domen Presern (shared research, writing, images etc.), Natalie Fey (editing)