The
Chemical Compound is a
Gas with
Chemical Formula N O . It is an important
Signaling Molecule in the body of
Mammal s including
Human s, one of the few gaseous signaling molecules known. It is also a
Toxic Air Pollutant produced by
Automobile Engine s and
Power Plant s.
Nitric oxide (NO) should not be confused with
Nitrous Oxide (N
2O), a
General Anaesthetic , or with
Nitrogen Dioxide (NO
2) which is another poisonous air pollutant.
The nitric oxide molecule is a
Free Radical , which makes it very reactive and unstable. In air, it quickly reacts with
Oxygen to form the poisonous nitrogen dioxide.
At high temperatures, molecular nitrogen and oxygen can combine to form nitric oxide. A major natural source is lightning. Human activity has drastically increased the production of nitric oxide in combustion chambers. One purpose of
Catalytic Converter s in cars is to partially reverse this reaction.
Nitric oxide in the air may later convert to
Nitric Acid , which has been implicated in
Acid Rain . Furthermore, both NO and NO
2 participate in
Ozone Layer Depletion .
Nitric oxide has some industrial uses. As a raw material it is used in the
Semiconductor industry for various processes. In one of its applications it is used along with
Nitrous Oxide to form oxynitride gates in
CMOS devices. It is an intermediate of the
Ostwald Process , which converts
Ammonia into
Nitric Acid .
Nitric oxide can be used for detecting surface radicals on polymers. Quenching of surface
Radical s with nitric oxide results in incorporation of nitrogen, which can be quantified by means of
X-ray Photoelectron Spectroscopy .
Because of its production in allergic reactions, there is research on using levels of
Exhaled Nitric Oxide to optimize treatment of
Asthma .
It has also recently been seeing use as a supplement in
Bodybuilding . It is believed that it's dialation of the blood vessels (vasodilation) increases blood flow to the muscles, which in turn leads to increased strength, endurance and muscle size. As it is a fairly new product on the market, there is as of yet little research to conclusivley prove or disprove these claims. Nitric oxide supplements do not actually contain nitric oxide, but rather arginine and other precursors such as citrulline, Pycnogenol, L-aspartic acid, and ginsenosides which the body synthesizes into nitric oxide.
''See also:
Endothelium-derived Relaxing Factor (EDRF) and
Signal Transduction ''
In the body, nitric oxide serves several roles, mainly involving small blood vessels. Nitric oxide is synthesized from L- (blood vessels around the heart), thereby increasing its blood supply. A chemical known as
Asymmetric Dimethylarginine can interfere with the production of nitric oxide and is considered a marker of cardiovascular disease.
) causes excess production of nitric oxide by macrophages, leading to
Vasodilatation (widening of blood vessels), probably one of the main causes of
Hypotension (low
Blood Pressure ) in sepsis.
Nitric oxide also serves as a
Neurotransmitter between
Nerve Cell s. Unlike most other neurotransmitters that only transmit information from a
Presynaptic to a postsynaptic neuron, the small nitric oxide molecule can
Diffuse all over and can thereby act on several nearby
Neuron s, even on those not connected by a synapse. It is conjectured that this process may be involved in
Memory through the maintenance of
Long-term Potentiation . Nitric oxide is an important non-adrenergic, non-cholinergic (NANC) neurotransmitter in various parts of the gastrointestinal tract. It causes relaxation of the gastrointestinal smooth muscle. In the stomach it increases the capacity of the fundus to store food/fluids.
Production of NO also plays a role in development and maintenance of
Erection by stimulating
PDE5 -related intracellular
CGMP in the
Smooth Muscle cells surrounding the blood vessels supplying the
Corpus Cavernosum ; through relaxation of these muscles, more blood can flow in. This is the biological basis of
Sildenafil (Viagra)or tadalafil (Cialis), which works to turn off the enzyme that converts cGMP to GMP. The high levels of cGMP that result lead to vasodilation and hence erection. The effects of the
Recreational Drug s known as ''
Poppers '' are also thought to be due to nitric oxide.
The discovery of the biological functions of nitric oxide in the
1980s came as a complete surprise and caused quite a stir. Nitric oxide was named "Molecule of the Year" in
1992 by the journal
Science , a Nitric Oxide Society was founded, and a scientific journal devoted entirely to nitric oxide was created. The
Nobel Prize In Physiology Or Medicine in
1998 was awarded to
Ferid Murad ,
Robert F. Furchgott , and
Louis Ignarro for the discovery of the signalling properties of nitric oxide. It is estimated that yearly about 3,000 scientific articles about the biological roles of nitric oxide are published.
The chemistry of nitric oxide is very rich. The following is a brief overview of its preparation and reactivity.
As stated above, nitric oxide can be produced from the elements at high temperatures. It can be produced from nitric acid,
8HNO
3 + 3Cu → 3Cu(NO
3)
2 + 4H
2O + 2NO
or from the following aqueous reactions,
2NaNO
2 + 2NaI + 2H
2SO
4 → I
2 + 4NaHSO
4 + 2NO
2NaNO
2 + 2FeSO
4 + 3H
2SO
4 → Fe
2(SO
4)
3 + 2NaHSO
4 + 2H
2O + 2NO
The iron (II) sulfate reaction is a simple method that has been used in undergraduate labs for gas experiments. NO can be produced from the following non-aqueous reagents,
3KNO
2(l) + KNO
3 (l) + Cr
2O
3(s) → 2K
2CrO
4 (s) + 4NO
Commercially, NO is produced by the oxidation of ammonia at 750 to 900 °C in the presence of platinum. The combination of the elements at high temperatures has not been developed into a practical commercial synthesis.
When exposed to oxygen, NO is converted into NO
2. This has been speculated as occurring via the ONOONO intermediate. In water NO will react with oxygen and water to form HNO
2. The reaction is thought to be the following:
4NO + O
2 + 2H
2O → 4HNO
2
NO will react with
Fluorine ,
Chlorine , and
Bromine to from the XNO species, known as the nitrosyl halides. Nitrosyl iodide can form but is an extremely short lived species and tends to reform I
2.
NO will react with CF
3I to form CF
3NO and I
2. CF
3NO is one of the few known blue gases in contrast to NO and CF
3I which are both colorless.
At 25 °C and 1 atm NO is thermodynamically unstable. In the 30 to 50 °C range, NO rapidly decomposes to N
2O and NO
2.
NO reacts with
Superoxide in a diffusion limited reaction to form
Peroxynitrite (ONOO-), a potent oxidizing and nitrating agent.
NO can also behave as a ligand bonded to transition metal complexes. The most common bonding mode of NO is the terminal linear type (M-NO). These groups can have a bond angle of 160-180° but are still termed as linear. In this case the NO group is formally consider a 2-electron donor but actually will donate the free electron to the metal center to become a 3-electron donor of the type NO
+. This is what leads to the slightly bent linear geometry.
Nitric oxide can also donate only one electron to the metal. This leads to a bent MNO group where the bond angle is between 120-140°
The NO group can also bridge between metal centers through the nitrogen. The μ
2-symmetric or unsymmetric, μ
3 and μ
4 bonding modes are possible.
The reactive chemistry of coordinated NO is extensive and will only be mentioned briefly. Some reactions that can occur include:
Cp
2NbMe
2 + NO → Cp
2(Me)Nb(O)NMe + heat → Cp
2Nb(O)Me + ½MeN=NMe
(In the second part of this reaction the O is bonded both to the Nb atom and N atom, the N atom is bonded to Nb, O, and Me. This would be easier to understand if it was drawn out.)
(Ph
3P)
2(CO)ClOsNO + HCl → (Ph
3P)
2(CO)ClOsN(H)O
Generic oxidation can also occur
L
’’n’’MNO + ½O
2 → L
’’n’’MNO
2
That is just a taste.
Terminal NO groups can be recognized by their strong IR signals at 1610cm
-1. Bent dervatives have frequencies below 1610cm
-1.
Nitric oxide can be measured using a simple
Chemiluminescent Reaction involving
Ozone .
A sample containing nitric oxide is mixed with a large quantity of ozone. The nitric oxide reacts with the ozone to produce
Oxygen and
Nitrogen Dioxide . This reaction also produces
Light (chemiluminescence), which can be measured using a
Photodetector . The amount of light produced is proportional to the amount of nitric oxide in the sample.
: NO + O
3 → NO
2 + O
2 + light
There are other methods of testing, including electrochemical methods, where nitric oxide in a test sample reacts to produce a current or voltage difference on a surface.
Carl Djerassi 's
1998 novel ''NO'' (for nitric oxide) chronicles the life of grad student Renu Krishnan when she is sent to
Israel to do research about nitric oxide and its role in
Penile Tumescence . The book deals with the — sometimes unhappy — symbiotic relationship between academic research and business interests.
- F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann; ''Advanced Inorganic Chemistry'', 6th ed. Wiley-Interscience, New York, 1999.
- R.B. King; ''Inorganic Chemistry of Main Group Elements'', VCH Publishers, New York, 1995.
