Nitrogen(II) oxide

nitrogen(II) oxide, nitric oxide or nitrogen monoxide (NO) is a colorless, water-soluble gas, present in small amounts in mammals. It is also spread through the air being produced in automobiles and power plants.

Not to be confused with nitrous oxide (N₂O), nitrogen dioxide (NO₂) or any of the other existing nitrogen oxides.

It is a highly unstable molecule in the air since it oxidizes rapidly in the presence of oxygen, becoming nitrogen dioxide. For this reason it is also considered a free radical.

Production and environmental effects

At high temperatures, molecular nitrogen (N₂) and oxygen (O₂) can combine to form nitric oxide; For this reason, human activities have greatly increased the presence of this vicious gas. in the atmosphere.

This gas in the air can later be converted to nitric acid, producing acid rain. Also NO and NO₂ are partly responsible for the hole in the ozone layer.

Its effect with solar radiation is double. While in the lower atmosphere they contribute to global warming, in the upper one they contribute to global dimming.

It is a highly toxic gas.

Technical applications

Nitrous monoxide is the primary product of the catalytic combustion of ammonia by the Ostwald method and therefore an important intermediate in the production of nitric acid (HNO₃). In the laboratory it is most conveniently generated by reaction of dilute nitric acid with copper, if the other possible reaction products such as nitrogen dioxide do not bother or can be eliminated (eg by absorption in water).

Used to detect radicals on the surface of polymers.

Reactivities

With the halogens, except iodine, it reacts to form nitrosyl halides (Hal-N=O). With nitrogen dioxide it can form the oxide N₂O₃ which is in equilibrium with its starting products and can be seen as nitrous acid anhydride HNO₂ . This mixture is also used to obtain nitrites.

Sulfur dioxide reduces nitrogen monoxide to form sulfur trioxide and dinitrogen oxide.

It is also used as an engine enhancer, giving it greater acceleration performance and final speed due to the expansion of oxygen to the combustion.

Biological functions

In the 1970s, pharmacologist Ferid Murad discovered that nitrates used in the treatment of chest pain and some cardiovascular indications release nitrogen monoxide under physiological conditions. This in turn has dilating effects for blood vessels. He found that this induces a relaxation of the muscle layer in the endothelium.

In 1987 it was discovered that the human body produces small amounts of NO from the amino acid arginine. This helped to understand the mechanism of action of various organic nitrates used as drugs in angina pectoris attacks that also release NO in the human body. Even the treatment of patients with atherosclerosis with arginine seems to have the same principle. In the brain, nitrogen monoxide can play the role of a neurotransmitter.

All these discoveries culminated in the awarding of the Nobel Prize to Robert Furchgott, Ferid Murad and Louis J. Ignarro in 1998. Although there is controversy over the awarding of this prize, since the Honduran scientist Salvador Moncada had previously identified the nitric oxide as a factor of cardiovascular relaxation since the results of the investigation were published six months before any other work in 1987 in the journal Nature. Salvador Moncada has mentioned: "The first demonstration of the presence of nitric oxide in the cardiovascular system was made in England in my laboratory. The rest is politics"

NO synthesis is carried out by the action of an enzyme, nitric oxide synthase (NOS), from the amino acid L-arginine that produces NO and L-citrulline, requiring the presence of a cofactor (specifically, a coenzyme) — reduced nicotinamide adenine dinucleotide phosphate or reduced NAD-phosphate (NADPH) — in the presence of oxygen.

NO is produced by a wide variety of cell types including epithelial, neural, endothelial, and inflammatory cells. There are three forms of NOS, 2 called constitutive and calcium-dependent (cNOS), which are endothelial and neuronal, which synthesize NO under normal conditions, and an inducible and calcium-independent (iNOS), which are not expressed or are they do very weakly under physiological conditions.

It is synthesized by endothelial cells, macrophages and a certain group of neurons in the brain. In neurons it can function as a neurotransmitter, easily crossing cell membranes due to its lipophilic nature. When it is produced in the endothelial cells of the blood vessels, it functions as a paracrine regulator; it diffuses into smooth muscle cells, where it induces the production of cGMP (cyclic guanosine monophosphate) by the enzyme guanylate cyclase, which in turn produces a relaxing effect on the muscles with vasculature and bronchodilation. Macrophages synthesize NO to destroy microorganisms that have been engulfed.

All of this has led to great direct applications in medicine and other health sciences.

Finally, NO also serves as a preservative. It is freed from the nitrite that is used in the preservation of meat. In fact, some viruses and microorganisms release NO to kill cells.

Cells in the corpus cavernosum of the penis produce nitric oxide during sexual arousal in response to noncholinergic, nonadrenergic neurotransmission. Nitric oxide stimulates the formation of cyclic GMP, which leads to relaxation of the smooth muscle of the corpora cavernosa and penile arteries, engorgement of the corpora cavernosa, and erection. It is possible to increase the accumulation of cyclic GMP by inhibiting the cyclic GMP-specific phosphodiesterase family 5 (PDE5). Sildenafil and its congeners inhibit phosphodiesterase 5 and have been shown to improve erectile function in patients with erectile dysfunction.