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Priestley, Joseph -  , A STRANGE BIOCHEMICAL: NITRIC OXIDE

gases science studied molecule

(1733–1804) British chemist: discoverer of gases, including oxygen.

After a difficult Yorkshire childhood during which he was orphaned and often ill, Priestley began training as a non-conformist minister and 3 years later (in 1755) was appointed Unitarian minister in a Suffolk village. Later, he taught in schools and was librarian-companion to Lord Shelburne, afterwards Prime Minister. His stutter and his radical views on theology, and in particular on politics, made him unpopular as a preacher. As a vociferous supporter of the French Revolutionary idea he became very unpopular and, after his house had been burned in 1791 by a Birmingham mob, he felt forced to seek refuge in the more liberal USA in 1794.

He had a simple character, much personal charm and exceptional intelligence, and wrote on theology, education, history, philosophy, politics, physics, chemistry and physiology and knew at least nine languages. He was an amateur in science, with little use for theory, but he was the greatest English-speaking experimental chemist of the 18th-c, even though his interpretation of his results was usually unsound.

He met in London in 1766 and suggested that he would write a History of Electricity if Franklin would lend him some books; he had earlier done some reading in science and had shown his school pupils in Nantwich, in Cheshire, some experiments in electricity and optics. It was a good book, published in 1767 and included new experiments; Priestley became a Fellow of the Royal Society in 1766. After this his interest in science turned to chemistry; but theology was always more important to him than science.

His work as a minister in Leeds in 1767 was near a brewery, and he became interested in the ‘fixed air’ (CO 2 ) generated by fermentation, and then in other gases, although only three were then known: air, CO2 (studied by ) and H2 (studied by ). All these formulae and modern names came much later, of course. Priestley used the pneumatic

 

A STRANGE BIOCHEMICAL: NITRIC OXIDE

The prestigious US journal Science chose nitric oxide (NO) as its Molecule of the Year in 1992, citing it as ‘a molecule of versatility and importance that has burst on to the scene in many guises. In the atmosphere it is a noxious chemical, but in the body in small controlled doses it is extraordinarily beneficial’.

The first point about NO is not to confuse it with nitrous oxide (N2O, ‘laughing gas’), one of the other six nitrogen oxides and a long-used and popular anaesthetic for brief surgical procedures.

Nitric oxide, NO, was discovered by in 1774. Pure NO is a colourless gas, acrid and very toxic, used in the bulk production of nitric acid for industry and nitrate fertilizers for agriculture. It is the simplest stable molecule known to have an odd number of electrons, which contributes to its intense chemical reactivity. It reacts readily with air or oxygen to form the brown gas nitrogen dioxide NO2 , and it also reacts with metals, one of its aspects which have been much studied by inorganic chemists. Until recently it had no interest for biochemists or physiologists, except perhaps as a component of the nitrogen oxide mixture (‘NO x ’) forming part of the atmospheric pollution from petrol engines and adding to the troubles of asthmatics.

Until the later 1980s it was never expected that a molecule that is small, light, gaseous and reactive would have a previously undiscovered and subtle role in physiology: but since then it has been found to be essential in digestion, blood-pressure regulation and antimicrobial defence.

In the body, it is made from an amino acid (arginine) by an enzyme, NO synthase. When released by cells in the wall of blood vessels, it relaxes nearby muscle cells, the vessel dilates and blood pressure falls. The effect has been used (without understanding its origin) since the discovery in the 1860s that nitroglycerin has a dramatic effect in providing relief to victims of coronary artery narrowing. However, too much NO, in response to a bacterial infection, causes septic shock, a major cause of death in intensive care wards; from 1992 NO inhibitors have saved such cases. In the body’s defence system NO acts as an antitumour agent. It also combats bacteria, a reminder that nitrates and nitrites that release NO have been used for centuries in curing meat. In the brain, NO acts as a neurotransmitter, usually desirably, but in stroke cases its release in excess is toxic and can be fatal. Also in the brain, there is some evidence that it has a key place in learning and memory. In the digestive system, the relaxation component in peristalsis (the wave-like movement of the gut that propels the food) depends upon NO. Lack of it is the cause of infantile pyloric stenosis, which can be fatal.

The intensive studies on NO by neuroscientists in the 1990s have shown conclusively that in male mammals this is the molecule that converts sexual excitement into potency. The brain causes NO to be released in penile blood vessels, and erection is the result; with some 10% of human males suffering from impotence, the possible clinical use of this knowledge makes the discovery of this sexual neurotransmitter the subject of intensive research. The ‘discovery phase’ for NO in physiology clearly will run for some years to come.

In 1998, three American pharmacologists, R F Furchgott (1916– ), L Ignarro (1941– ) and F Murad (1936– ) shared a Nobel Prize for their discovery that NO can transmit signals in the cardiovascular system.

IM

trough invented by to collect gases, filling it with mercury if the gas was water-soluble. He used a large lens and the Sun to provide a clean heat. Within a few years he had discovered and examined the gases we now know as HCl, NO, N2O, NO2 , NH3 , N2 , CO, SO2 , SiF4 , and O2 ; as said, ‘no single person ever discovered so many new and curious substances’. His results were described in papers and books, notably Experiments and Observations on Different Kinds of Air and other Branches of Natural Philosophy (1790).

His most famous experiment was carried out on 1 August 1774 at Bowood (Shelburne’s house near Calne, Wiltshire). He had been given a large (12 in/30 cm) lens, and used it to try to make gases by heating various chemicals given by his friend J Warltire (1739–1810). When he heated mercury oxide (HgO) he was surprised to find that it gave a colourless gas that was not very soluble in water and in which a candle burned with a dazzling light. A few months later he wrote that ‘two mice and myself have had the privilege of breathing it’ and recommended its use in medicine. In October 1774 in Paris he talked with about this; later Lavoisier repeated and improved the experiment, and saw (as Priestley had not) the full significance of the discovery. Had in fact made oxygen (O2 ) earlier, in this and other ways, but did not publish until 1777, after Priestley. However, Priestley showed that O2 is given off by plants, and that it is essential for animals. He also studied hydrogen and used it to reduce metal oxides, noticing that water is formed in this reaction; and that water is also formed by exploding hydrogen with oxygen.

Priestley had exceptional energy and skill but he remained always a firm believer in the erroneous phlogiston theory in chemistry, although his own results did much to refute that theory.

He also studied the densities of gases, their thermal conductivity and electrical discharges in gases. After he joined his sons in the USA he continued to work in chemistry until 1803, although he declined the professorship of chemistry at Philadelphia. </

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