to smell through the structure of the molecules

It is difficult to understand the relationship between the physical properties of a substance and the feeling it evokes, but it is possible to conclude from the physical-chemical structure of a molecule whether the smell will be more or less pleasant than other substances

Certain smells are able to evoke positive feelings and emotions in us - the smell of the earth after the rain, the smell of cut grass, the smell of citrus fruits - or negative: the smell of a rotten potato, mold or old urine. However, we do not know what defines the experience of smell.
By sight or hearing, it is relatively easy to define the relationship between physical characteristics and the sensation received by the senses: what is the wavelength of red light and what is its brightness, or what is the frequency and intensity of the sounds emanating from a saxophone when playing jazz. The odorants absorbed by our nose are made up of molecules - odorants - which have hundreds of properties: size, shape, types of atoms, types of chemical bonds between atoms, types of chemical bonds between molecules and other molecules, and more.

How do you decipher smells?Therefore, it is much more difficult to understand the relationship between the physical properties of the scent and the feeling it evokes. Until now, the "odor code" has not been decoded: the code that connects the properties of the molecule to the smell it evokes - similar to the already decoded code that connects color and wavelength. An article published in the Journal of Neuroscience describes a first step in this direction. The research described was conducted in laboratories at the Weizmann Institute of Science and the University of California at Berkeley, led by Prof. Noam Sobel from the Weizmann Institute.

In order to decipher the code, the researchers tried to determine what is the feature that can be used to characterize most of the diversity of molecules in nature, just as a large part of the diversity of light can be characterized using an axis that describes a wavelength. The eye, which is able to perceive and decipher these characteristics, can use them to produce a lot of information about the visible physical world. It is likely that the sense of smell also notices characteristics that allow it to extract from the scents as much information as possible about the chemical world around us. With the help of statistical software, the researchers found an axis that describes most of the variation in the chemical and physical properties of the fragrances.

Odor axis: pleasant without pleasantThe next step was to find a similar axis that explains the variation in the smell experience. There are many ways to describe a smell: sweet, spicy, musty, etc. The researchers used an existing scent database, which contains 160 scents that were rated by 150 experts - smell researchers and perfumers - according to 146 characteristics, such as sweet, floral, as well as "dry smell" and "burning candle smell". It turned out that the axis that describes most of the variation in the sensations evoked by the smell is the axis of pleasantness - that is, the axis on one side of which are unpleasant odors and on the other side of pleasant odors.
When comparing these two axes, the one that describes the physical and chemical properties of the scents and the one that describes the feelings that those scents evoke in humans, it turns out that they are compatible with each other. In other words, according to the model developed in the research, if we build an axis that explains most of the chemical variation in the world around us, and arrange smells along it, the smells will be arranged more or less from pleasant to unpleasant.
According to Sobel, it makes a lot of sense that the main feature with which we experience the smell corresponds to the features that best describe the physico-chemical variation of the scents. The sense of smell evolved to give us as much information as possible about the chemical world, just as the eye evolved to give us as much information as possible about the visible world. The eye uses receptors that detect the maximum variation in light (wavelength and degree of brightness), therefore we would expect the smell receptors to detect the features that contribute to the maximum variation in scents, as the model suggests.

How do different populations relate to smell?In order to test the model, the researchers conducted several experiments, in which subjects were asked to rate the smells of 52 scents, which were not part of the database according to which the contractions were developed, according to their degree of pleasantness. The researchers used the model to predict the pleasantness of each scent based on its physical-chemical properties, and compared the prediction to the subjects' rating. It turned out that the model was indeed able to successfully predict the degree of pleasantness of the scents, according to the subjects' reports.
In subsequent experiments, subjects were selected from three different populations - Americans from California, Jews from Israel, and Muslim Arabs from Israel - in order to test whether there is an acquired cultural factor in the degree of pleasantness that people attribute to smells, or whether it is a universal trait. It became clear from the experiments that the different populations rated the smells in a very similar way, and in all three cases the results largely coincided with the model's prediction.
When the ratings of the three groups were checked regarding the degree of "edibility" (edibility) of the smell, i.e. how appetizing it is, greater differences were found. It should be noted that a pleasant smell is not necessarily appetizing, and on the contrary - Sobel cites as an example the scent of the French ripened cheeses which arouse the appetite of many people, but most of them would not claim that the smell is pleasant, and as evidence, no one would think of marketing a perfume or soap with the scent of a ripe cheese. It turns out that the question of the "edibility" of the smell depends on cultural circumstances more than its pleasantness.

The smells help in the fight for survivalDespite the predictive ability of the current model, there is still room for improvement. According to Sobel, the system is not able to differentiate between very similar molecules, such as chiral molecules - molecules that are similar to each other in every way except that they are mirrored and do not overlap each other, similar to the relationship between a right hand and a left hand. A "left" odorant has a different smell than a "right" odorant, but the model is unable to predict this difference. An example of another problem is the lack of reference of the model to the strength of the smell.
The system refers only to the molecule and not to the concentration of the molecules when it predicts the pleasantness of the substance, but anyone who has stood next to a person who has been perfumed with a large amount of perfume can testify that in excess, even a pleasant smell can turn into a repulsive smell. To this must be added the fact that the experience of smell is partly a subjective thing, and even tends to change during life and be affected by different experiences and events.
However, the fact that there is indeed a connection between the structure of the scent and the feeling of pleasantness it evokes in most people, even in people from different cultures, indicates that there is a relatively rigid structured mechanism that controls at least some of the feelings of smell. Apparently, humans are born with an attraction to certain smells and an aversion to other smells, which can be of great help in the struggle for survival - an automatic aversion to rotten meat, for example, may prevent serious diseases.

150,000 samples for further researchThe new discovery is the first step in deciphering the "odor code" - now it is possible to conclude based on the physical-chemical structure of a molecule whether its smell will be more or less pleasant than that of another molecule.
In the future, the researchers hope to discover more connections between the physical structure and the sense of smell, and they will do so with the help of a new and much larger odor database, which is being built these days. Organizations engaged in smell research from the United States, Europe and Japan intend to collect about 150,000 samples in the next four years, and in this way they may be able to better understand the sense of smell, the sense that is so complex that receives relatively little attention in everyday life.

* Thanks to Noam Sobel and his lab team.

Written by: Yonat Ashchar and Noam Levithan. Originally published in "Galileo" magazine