Concentrated Form of Loss
Professor Richard Newcomb
The scent of violets is comprised of two major volatile compounds, alpha (α) and beta (β) ionone. Together they form a pleasant bouquet that many of us associate with the smell of these flowers. The two compounds act through receptors in our nose to produce signals that are interpreted by our brain. Receptors for β-ionone are different among people, resulting in a highly personal experience of the compound and of violets. Below is a detailed account of the perception of these compounds by our olfactory apparatus.
Ionones are a breakdown product of carotenoids. With a common molecular weight of 192.2973g/mol the two forms, α- and β -ionone only differ in the position of a double bond in the ring structure. Ionones are produced by many plants, synthesised in some flowers, fruit and leaves. They are the major component of the scent of violets, their scent being described as everything from ‘powdery’ and ‘woody’ through to ‘floral’ and ‘fragrant’.
Both compounds are highly volatile, with β-ionone having a vapour pressure of 0.009 hPa and α-ionone of 0.014 hPa at 25oC. The compounds enter the nose orthro- or retronasally (either through our nostrils or through the back of the throat when you are eating), where they pass over the olfactory epithelium. The odorants become trapped in a mucus layer containing binding proteins that transport the odours to the dendrites of specialised olfactory sensory neurons. Each sensory neuron expresses a single type of olfactory receptor on its surface that binds a subset of odorants, sometimes as few as one but generally a number.
On binding of the odorant, the receptor alters its shape or conformation which facilitates the hydrolysis of GTP to GDP inside the cell by a G protein complex. The complex then dissociates and interacts with other proteins eventually resulting in the opening of ion channels in the sensory neuron and the generation of an electrical current. The axons of all the sensory neurons expressing the same olfactory receptor each converge on one of approximately 1000 glomureli within the olfactory bulb sitting behind the olfactory epithelium. Mitral cells synapse with the glomureli and other olfactory neurons and transmit integrated signals to centres in the higher brain. Centres of the brain that are innervated by mitral cells and sensory neurons include the anterior olfactory nucleus, the olfactory tubercle, the amygdala, the piriform cortex, and the entorhinal cortex. Such signals are interpreted as ‘pleasing’ within the amygdala, one of the major pleasure centres of the brain.
This process all occurs within milliseconds and almost simultaneously the odorant is degraded, so the system is ready to receive the next signal. If too much signal is present the system attenuates and for a few seconds we cannot smell the compound.
Humans vary in their ability to detect α- and β-ionone. Variation among people to detect α-ionone is quite narrow, whereas for β-ionone there is a five order of magnitude range in threshold concentration between and best and the worst smellers. The response distribution for β-ionone is bimodal; that is there are two groups of people in the world, one that can and another that essentially cannot detect the compound. Distinct receptors are responsible for the detection of the compounds, with the olfactory receptor, OR5A1, responsible for detecting β-ionone. Within the OR5A1 gene a single DNA polymorphism accounts for the difference between those that can and those that cannot detect the odorant. This polymorphism encodes the single amino acid substitution N183D within the second external loop of the receptor. Those that carry one or two copies of the D183 allele are able to detect β-ionone at low concentrations, whereas those that carry two copies of the N183 allele cannot. This difference not only accounts for the ability to detect β-ionone, but to sense and share a common experience associated with smelling the compound.
Even at high concentrations of β-ionone, those that carrying two copies of the N183 allele only ever experience the compound as an unpleasant solvent. They never get to experience the pleasant floral note associated with the compound that others do. Their perception of violets will be entirely through the experience associated with smelling α-ionone, also given off by the flowers.