Antimicrobials: ammonium salts


Quaternary ammonium salts are able to attach to the walls of bacterial cells and destroy them from the inside.

In order to create a substance capable of killing bacteria, it is necessary to endow it with three properties: the first is the ability to attach to the bacterial wall; the second is the ability to penetrate through the wall; and third, the ability to destroy vital components of the bacterial cell or prevent the reproduction of the bacterium. The destructive effect of an antibacterial substance can also be directed at the bacterial cell membrane itself, leading to its rupture and leakage of the bacterial content into the environment. So far, there are no perfect disinfectants, but quaternary ammonium salts are very close to them in their properties.

Quaternary ammonium salts satisfy the first of the above properties of effective bactericidal due to the positive charge that their molecules carry, due to which they are attracted to the outer surface of the negatively charged bacterial membrane. And although the effect of substitution of other positively charged ions by quaternary ammonium salts is not so great, it turns out to be sufficient to destabilize the microbe. At the point where the ammonium salt binds to the bacterium, a membrane rupture occurs, through which the fatal leakage of cellular contents occurs. To cause such damage, the hydrocarbon chain length of the ammonium salt must be at least 30 nanometers. The molecule acts like a blade plunged into the membrane, creating a tear that cannot be repaired.

The German biochemist Gerhard Domagk (1895-1964) began an era of research on the bactericidal properties of quaternary ammonium salts. He was engaged in the study of sulfonamides, one of the first known antibacterial drugs containing a nitrogen atom in its composition, which under certain conditions easily acquires a positive charge. In addition to the charge of the molecule, Domagk was also interested in the question of the length of the hydrocarbon chain, he found that molecules containing from 8 to 18 carbon atoms bonded to nitrogen have good antibacterial properties. Many substances with a similar structure have been investigated, but only a small part of them has become commercially successful.

The best quaternary ammonium salts that have stood the test of time are benzalkonium chloride and cetavlon, both of which appeared in the 1960s. Benzalkonium chloride has had many commercial names, the two most famous being «Sefiran» and «Germinol». Its molecule consists of a nitrogen atom, to which two methyl (CH3) and one benzyl groups are bonded (this is a methyl radical in which one hydrogen atom is replaced by a benzene ring). Due to the fourth valency, a hydrocarbon chain consisting of 8-18 carbon atoms is attached to the nitrogen atom. And, as the name implies, the positive charge of the nitrogen is neutralized by the negative charge of the chloride ion, C1-. Benzalkonium chloride is highly soluble in water, and this is important for its practical use.

Cetrimide is a mixture of quaternary ammonium salts in which the nitrogen atom is bonded to three methyl groups and one long hydrocarbon chain, which may be 12, 14 or 16 carbon atoms. It was first obtained in 1946, it is still used to create skin antiseptic ointments, which have an excellent healing effect.

In case of epidemics, it would be great to make sure that the surfaces of objects that a large number of people come into contact with, such as doorknobs, switches, chairs, restaurant menus, and the like, cannot serve as a means of spreading infection. It may someday be possible to chemically bond quaternary ammonium salts to such surfaces, making them lethal to bacteria. And now work is underway to create such materials.

Bacteria that are safe for a normal healthy person can threaten the health of those who have immune system diseases or who have suffered severe burns. Usually, they try to isolate such people in intensive care units, where active measures are taken to prevent the development of a secondary infection. But despite the best efforts of doctors and nurses, these patients often become victims of antibiotic-resistant bacteria. In 1996, the journal Annals of Therapy reported that such bacteria can be found on the uniforms of healthcare workers for a long time. Since textiles can be modified by incorporating chemicals such as dyes, UV filters, flame retardants into their fibers, there is no reason why textiles cannot be made germicidal by binding substances with antibacterial activity to their fibers. Previously, fabrics were given antimicrobial properties by treating them with antiseptic agents, however, such protection was temporary, since the agent was completely washed out after several washes. Now it is possible to create textiles with antibacterial properties that are resistant to numerous washings.

Quaternary ammonium salt-embedded fabrics were first produced in the US in the 1970s by Dow. This textile gained wide popularity in Japan, China and other Asian countries, as clothes made from it made it possible to get rid of the unpleasant odor caused by skin bacteria. Later, MIT chemists found a way to incorporate quaternary ammonium salts into a wide variety of materials, such as glass, polyethylene, polypropylene, nylon, and polyester, all of which proved to be effective against bacteria.

Meanwhile, other American chemists have found a way to bind quaternary ammonium salts with carbohydrate-containing materials such as cotton, wool and paper. In their technology, they used ammonium salts with a hydrocarbon blade formed by sixteen carbon atoms. Such materials are supposed to be used for sewing surgical suits that can be washed without losing their bactericidal properties. Studies have shown that such fabrics inhibit the growth of most types of bacteria.

The German company Degussa has developed a polyamine polymer that is effective against many bacteria, including E. coli and Staphylococcus aureus, as well as against certain fungi and algae. An interesting fact is that the monomer that makes up this substance does not have bactericidal properties. This feature is due to the fact that the negative charge of monomeric molecules is too weak to harm bacteria. But in the polymer, the total charge of individual monomers is sufficient for interaction with microorganisms and the manifestation of a biocidal effect. Perhaps the new substance will be used for wood stains, boat painting, for the protection of historical values, as well as in the food industry and in water treatment plants.

Source: based on the book by John Emsley «On the benefits and harms of the products we love to buy»

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