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Enzymatic production of gold?

S. Ashraf Ahmed
AS an enzymologist by training and profession, I always tried to glorify enzymes whenever I had a chance. But I was not prepared for this: making gold using enzymes! Enzymes are biocatalysts.
Chemical abbreviation for gold is Au. It is a heavy metal with an atomic mass of 197. Generally heavy metals are toxic to all life forms, including humans (please see Fishes we eat: Are they free from mercury poison? DS, April 27, 2010, http://archive.thedailystar.net/newDesign/news-details.php?nid= 136026).
There was a time when people believed that a metal such as iron could be converted into gold by a touchstone. This belief was so prevalent that even until late 1900s many literate people of British Bengal went bankrupt by investing their youth and money in experimenting with this method of getting rich. The enzymatic method described below will not make one rich but it promises a great number of potential applications.
Gold is ejected out of earth's interior during volcanic eruptions with various metals, especially iron and sulfur. Mineralogists knew that metallic gold found in mines was somehow converted by bacteria and cyanobacteria from gold compounds, notably gold chloride. One such organism is Delftia acidovorans.
An article in the April 14, 2013 issue of the journal Nature Chemical Biology reported involvement of a peptide synthetase gene. Enzymatic products of this gene cluster endow the organism with the ability to withstand high concentrations of the toxic gold chloride. The scientists also isolated an octapeptide called delftibactin from D. acidovorans and showed that it converted toxic gold chloride into metallic gold as Au-NP in just 10 minutes! The currently used gold extraction method employs toxic chemicals. What makes this discovery important is not the production of gold but a potential environmental friendly and easier gold extraction procedure.
The use of gold as a precious metal stems from the fact that it remains unchanged over the years because it is inert to most chemical reactions. With the introduction of an improved refining method, the first gold coin was minted almost 2600 years
ago by a king of Lydia, a place in modern day Turkey. Today, almost 50% of gold is used in jewelry, 40% in money, and the remaining 10% in technology. Of the 450 tons of gold used in technology, 320 tons are used in electronics and the rest in industry and dentistry.
In the past several years this apparently inert gold has been rapidly finding important applications in chemistry and medicine, and holds great potentials in microelectronics and optics, including computers. A form of gold is called gold nanoparticle or Au-NP. A short-lived glasswork technology in the 4,h century AD used Au-NP in producing a red-green dichroic effect in the now famous Lycurgus cup. The cup appears green in reflected light but red in transmitted light.
Au-NPs are 10-100 nanometers across. A nanometer is one millionth of a millimeter (one billionth of a meter). AuNP are great catalysts for innumerable chemical reactions, notably conversion of carbon monoxide in automobile exhaust fumes into carbon dioxide.
Visible and infrared light are absorbed and/or
reflected differently by Au-NP depending on their size. As a result, some Au-NP appear different in various coloured lights. Because of small size, Au-NP can easily penetrate blood brain barrier, and cellular layers. An Au-NP of a particular size or colour can be attached to a "molecular address" and a drug as well of, for example, a cancer tissue, in a patient. The address label drives the complex to the cancerous tissue that can then be visualised by the specific colour of the NP. At the same time, the drug can be released or heat produced by shining it with infrared laser beam to kill the cancer cells. IR has no health effect on the human body.
Although China is the top gold and gold jewelry producer of the world, the richest source of the metal is in the vast ocean water as low concentration gold chloride. Hopefully, scientists will try to find an application of the enzymes and bacteria in making Au-NP or gold from low concentrations of gold chloride.
The writer, a former teacher at Dhaka University, is a biomedical scientist working in the USA.