The chemist has a nose
In the article below, we will look at the problem of smell through the eyes of a chemist - after all, his nose will come in handy in his laboratory on a daily basis.
1. Innervation of the human nose - the thickening above the nasal cavity is the olfactory bulb (author: Wikimedia/Opt1cs).
We can share feelings physical (sight, hearing, touch) and their primary chemicali.e. taste and smell. For the former, artificial analogues have already been created (light-sensitive elements, microphones, touch sensors), but the latter have not yet surrendered to the “glass and eye” of scientists. They were created billions of years ago when the first cells began to receive chemical signals from the environment.
Smell eventually separated from taste, although this does not occur in all organisms. Animals and plants constantly sniff their surroundings, and the information obtained in this way is much more important than it seems at first glance. Also for visual and auditory learners, including humans.
Olfactory Secrets
When you inhale, the air stream rushes into the nose and, before moving on, enters a specialized tissue - the olfactory epithelium several centimeters in size.2. Here are the endings of nerve cells that capture odor stimuli. The signal received from the receptors travels to the olfactory bulb in the brain, and from there to other parts of the brain (1). The fingertip contains scent patterns specific to each species. A human can recognize about 10 of them, and trained professionals in the perfume industry can recognize many more.
Smells cause reactions in the body, both conscious (for example, you startle at a bad smell) and subconscious. Marketers use the directory of perfume associations. Their idea is to flavor the air in stores with the scent of Christmas trees and gingerbread during the pre-New Year period, which causes positive emotions in everyone and increases the desire to buy gifts. Similarly, the smell of fresh bread in the food section will make your saliva drip into your mouth, and you'll put more in the basket.
2. Camphor is often used in warming ointments. Three compounds with different structures have their own smell.
But what causes a given substance to cause this, and not another, olfactory sensation?
For the olfactory taste, five basic tastes have been established: salty, sweet, bitter, sour, oun (meat) and the same number of receptor types on the tongue. In the case of smell, it is not even known how many basic aromas exist, or whether they exist at all. The structure of the molecules certainly determines the smell, but why is it that compounds with a similar structure smell completely different (2), and completely dissimilar - the same (3)?
3. The compound on the left smells like musk (perfume ingredient), and on the right - almost identical in structure - has no smell.
Why do most esters smell pleasant, but sulfur compounds unpleasant (this fact can probably be explained)? Some are completely insensitive to certain smells, and statistically women have a more sensitive nose than men. This suggests genetic conditions, i.e. the presence of specific proteins in the receptors.
In any case, there are more questions than answers, and several theories have been developed to explain the mysteries of the fragrance.
Key and lock
The first is based on a proven enzymatic mechanism, when the reagent molecule enters the cavity of the enzyme molecule (active site), like a key to a lock. Thus, they smell because the shape of their molecules corresponds to the cavities on the surface of the receptors, and certain groups of atoms bind to its parts (in the same way enzymes bind reagents).
Briefly, this is a theory of smell developed by a British biochemist. John E. Amurea. He singled out seven main aromas: camphor-musky, floral, minty, ethereal, spicy and putrid (the rest are combinations of them). Molecules of compounds with a similar smell also have a similar structure, for example, those with a spherical shape smell like camphor, and compounds with an unpleasant odor include sulfur.
Structural theory has been successful - for example, it explained why we stop smelling after a while. This is due to the blocking of all receptors by molecules carrying a given odor (just as in the case of enzymes occupied by an excess of substrates). However, this theory was not always able to establish a connection between the chemical structure of a compound and its smell. She was unable to predict the smell of the substance with sufficient probability prior to obtaining it. She also failed to explain the intense smell of small molecules such as ammonia and hydrogen sulfide. The amendments made by Amur and his successors (including an increase in the number of base flavors) did not eliminate all the shortcomings of the structural theory.
vibrating molecules
The atoms in molecules constantly vibrate, stretching and bending the bonds between themselves, and the movement does not stop even at absolute zero temperatures. Molecules absorb vibrational energy, which lies mainly in the infrared range of radiation. This fact was used in IR spectroscopy, which is one of the main methods for determining the structure of molecules - there are no two different compounds with the same IR spectrum (except for the so-called optical isomers).
Creators vibrational theory of smell (J. M. Dyson, R. H. Wright) found links between the frequency of vibrations and the perceived smell. Vibrations by resonance cause vibrations of receptor molecules in the olfactory epithelium, which changes their structure and sends a nerve impulse to the brain. It was assumed that there were about twenty types of receptors and, therefore, the same number of basic aromas.
In the 70s, the proponents of both theories (vibrational and structural) competed fiercely with each other.
Vibrionists explained the problem of the smell of small molecules by the fact that their spectra are similar to fragments of the spectra of larger molecules that have a similar smell. However, they were unable to explain why some optical isomers with the same spectra have completely different odors (4).
4. Optical isomers of carvone: grade S smells like cumin, grade R smells like mint.
Structuralists easily explained this fact - receptors, acting like enzymes, recognize even such subtle differences between molecules. The vibrational theory also could not predict the strength of the smell, which the followers of Cupid's theory explained by the strength of the binding of odor carriers to receptors.
He tried to save the situation L. Torinosuggesting that the olfactory epithelium acts like a scanning tunneling microscope (!). According to Turin, electrons flow between parts of the receptor when there is a fragment of an aroma molecule between them with a certain frequency of vibrational vibrations. The resulting changes in the structure of the receptor cause the transmission of the nerve impulse. However, the modification of Turin seems to many scientists too extravagant.
Traps
Molecular biology has also tried to unravel the mysteries of smells, and this discovery has been awarded the Nobel Prize several times. Human odor receptors are a family of about a thousand different proteins, and the genes responsible for their synthesis are active only in the olfactory epithelium (i.e., where it is needed). Receptor proteins consist of a helical chain of amino acids. In the stitch stitch image, a chain of proteins pierces the cell membrane seven times, hence the name: seven-helix transmembrane cell receptors ().
Fragments protruding outside the cell create a trap into which molecules with the corresponding structure can fall (5). A specific G-type protein is attached to the site of the receptor, immersed inside the cell. When the odor molecule is captured in the trap, the G-protein is activated and released, and another G-protein is attached in its place, which is activated and released again, etc. Cycle repeats until the bound aroma molecule is released or destroyed by enzymes that constantly clean the surface of the olfactory epithelium. The receptor can activate even several hundred G-protein molecules, and such a high signal amplification factor allows it to respond to even trace amounts of flavors (6). The activated G-protein starts a cycle of chemical reactions that lead to the sending of a nerve impulse.
5. This is what the odor receptor looks like - protein 7TM.
The above description of the functioning of olfactory receptors is similar to that presented in the structural theory. Since the binding of molecules occurs, it can be argued that the vibrational theory was also partly correct. This is not the first time in the history of science that earlier theories were not completely wrong, but simply approached reality.
6. Human nose as a detector of compounds in the analysis of their chromatographically separated mixtures.
Why does something smell?
There are many more odors than there are types of olfactory receptors, which means that odor molecules activate several different proteins at the same time. based on the entire sequence of signals coming from certain places in the olfactory bulb. Since natural fragrances contain even more than a hundred compounds, one can imagine the complexity of the process of creating an olfactory sensation.
Okay, but why does something smell good, something disgusting, and something not at all?
The question is half philosophical, but partially answered. The brain is responsible for the perception of smell, which controls the behavior of humans and animals, directing their interest to pleasant smells and warning against bad-smelling objects. Enticing odors are found, among other things, the esters mentioned at the beginning of the article are released by ripe fruits (therefore they are worth eating), and sulfur compounds are released from decaying residues (best to stay away from them).
The air does not smell because it is the background against which odors spread: however, trace amounts of NH3 or H2S, and our sense of smell will sound the alarm. Thus, the perception of smell is a signal of the impact of a certain factor. relation to species.
What do the upcoming holidays smell like? The answer is shown in the picture (7).
7. The smell of Christmas: on the left, gingerbread flavors (zingerone and gingerol), on the right, Christmas trees (bornyl acetate and two varieties of pinene).
