BIOGEOCHEMICAL, HEALTH, AND ECOTOXICOLOGICAL PERSPECTIVES ON GOLD AND GOLD MINING - CHAPTER 4 pdf

4.1 PHYSICAL PROPERTIES Gold is a comparatively rare native metallic element, ranking fiftieth in abun-dance in the earth’s crust.. 4.2 CHEMICAL PROPERTIES The chemistry of gold is compl

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Properties

Gold is a complex and surprisingly reactive element, with unique physical, chemical, and biochemical properties Some of these properties are listed and dis-cussed below

4.1 PHYSICAL PROPERTIES

Gold is a comparatively rare native metallic element, ranking fiftieth in abun-dance in the earth’s crust The chemical symbol for gold is Au, from the Latin aurum

for gold Metallic gold is an exceptionally stable form of the element and most deposits occur in this form The main elements with which gold is admixed in nature include silver, tellurium, copper, nickel, iron, bismuth, mercury, palladium, platinum, indium, osmium, iridium, ruthenium, and rhodium The native gold–silver alloys have a color range from pale yellow to pure white, depending on the amount of silver present Finely divided gold is black, like most other metallic powders, while colloidally suspended gold varies in color from deep ruby red to purple Gold occurs

as metallic gold (Au0) and also as Au+ and Au+3, so that it occurs in combination with tellurium as calaverite (AuTe2) and sylvanite (AuAgTe4), and also with tellu-rium, lead, antimony, and sulfur as nagyagite, Pb5Au(Te,Sb)4S5-8 (Rose 1948; Ran-som 1975; Sadler 1976; Puddephatt 1978; Krause 1996)

Gold is characterized by an atomic weight of 196.967, atomic number of 79, a melting point of 1063°C, and a boiling point of about 2700°C In the massive form, gold is a soft yellow metal with the highest malleability and ductility of any element

A single troy ounce of gold can be drawn into a wire over 66 km in length without breaking, or beaten to a film covering approximately 100 m2 Traces of other metals interfere with gold’s malleability and ductility, especially lead, but also cadmium, tin, bismuth, antimony, arsenic, tellurium, and zinc It is extremely dense, being 19.32 times heavier than water at 20°C A cube of gold 30 cm (12 in.) on a side weighs about 544 kg (1197 lb) Gold has high thermal and electrical conductivity, properties that make it useful in electronics It is extremely resistant to the effects

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40 PERSPECTIVES ON GOLD AND GOLD MINING

of oxygen and will not corrode, tarnish, or rust Pure (100%) gold is 1.000 fine, equivalent to 24 carats Gold is usually measured in troy ounces, wherein 1 troy ounce equals 31.1 g vs 28.37 g in an ounce avoirdupois (Rose 1948; Ransom 1975; Sadler 1976; Puddephatt 1978; Elevatorski 1981; Gasparrini 1993; Cvancara 1995; Krause 1996; Petralia 1996; Merchant 1998)

Gold has 30 known isotopes, but only one, 197Au, is stable The nucleus of 197Au contains 79 protons and 118 neutrons Isotopes of mass numbers 177 to 183 are all

α emitters and all have a physical half-life of <1 min Isotopes of mass numbers

185 to 196 decay by electron capture accompanied by radiation and in some cases

by positron emission The only long-lived isotope is 195Au with a half life of 183 days The neutron-heavy isotopes of 198 to 204 all decay by emission accompanied by radiation The isotope 198Au is widely used in radiotherapy, in medical diagnosis, and for tracer studies (Puddephatt 1978; Windholz 1983)

The color of gold alloys depends on the metal mixture Red gold is comprised

of 95.41% Au and 4.59% copper (Cu); yellow gold of 80% gold and 20% silver (Ag); and white gold of 50% Au and 50% Ag The white gold commonly used in jewelry contains 75 to 85% Au, 8 to 10% nickel, and 2 to 9% zinc, while more expensive white alloys include palladium (90% Au to 10% Pd) and platinum (60%

Au, 40% Pt) Colloidally suspended gold varies in color from deep ruby red to purple, and is used in the manufacture of ruby glass Gold–silver–copper alloys are frequently used in coinage and gold wares A purple alloy results with 80% Au and 20% aluminum, but this compound is too brittle to be made into jewelry Gold forms alloys with many other metals, but most of these are also brittle As little as 0.02%

of tellurium, bismuth, or lead makes gold brittle (Rose 1948; Ransom 1975; Puddephatt 1978)

Analytical methodologies to measure gold in biological samples and abiotic materials rely heavily on its physical properties These methodologies include x-ray fluorescence (Borjesson et al 1993; Messerschmidt et al 2000), adsorptive stripping voltammetry (Lack et al 1999), bacteria-modified carbon paste electrodes (Hu et al 1999), inductively-coupled plasma mass spectrometry [ICP-MS] (Higashiura et al 1995; Perry et al 1995; Barefoot and Van Loon 1996; Christodoulou et al 1996; Barefoot 1998; Barbante et al 1999), atomic absorption spectrometry [AAS] (Brown and Smith 1980; Kehoe et al 1988; Niskavaara and Kontas 1990; Ohta et al 1995; Begerow et al 1997), fire assay (Gasparrini 1993), and neutron activation [NA] and spectrometry (Shiskina et al 1990) Analyses of gold based upon gravimetric, volumetric, and UV/visible spectrophotometric techniques have been largely dis-placed by instrumental methods, such as NA, AAS, and more recently ICP-MS and ICP-AAS In ICP-MS, for example, detection limits of gold after preconcentration

of samples were as low as 0.04 ng/g ash in vegetation, 0.1 to 0.8 ng/L in water and urine, and 0.1 ng/g in soils and sediments (Perry et al 1995; Barefoot and Van Loon 1996; Barefoot 1998) It is noteworthy that the fire assay method to analyze ore samples for gold content is the most convenient and least expensive method used throughout the world, despite interferences from copper, nickel, lead, bismuth, and especially tellurium and selenium (Gasparrini 1993) The fire assay, known to met-alworkers for at least 3000 years, involves a weighed sample of the pulverized rock

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PROPERTIES 41

melted at 1000°C in a flux containing lead oxide, a measured amount of silver, soda, borax, silica, and potassium nitrate (Kirkemo et al 2001) The lead fraction contains the gold and added silver and settles to cool as a button, which is subsequently remelted, oxidized to remove the lead oxide, leaving behind a bead consisting of precious metals The bead is dissolved in acid and usually analyzed by AAS

4.2 CHEMICAL PROPERTIES

The chemistry of gold is complex Gold can exist in seven oxidation states: –1,

0, +1, +2, +3, +4, and +5 Apart from Au0 in the colloidal and elemental forms, only

Au+ and Au+3 are known to form compounds that are stable in aqueous media and important in medical applications (Table 4.1; Puddephatt 1978; Shaw 1999a, 1999b) The remaining oxidation states of -1, +2, +4, and +5 are not presently known to play a role in biochemical processes related to therapeutic uses of gold (Shaw 1999b) Neither Au+ or Au+3 forms a stable aquated ion ([Au(OH2)2-4+] or [Au(OH2)43+], respectively) analogous to those found for many transition metal and main group cations Both are thermodynamically unstable with respect to elemental gold and can be readily reduced The gold-based anti-arthritic agents are considered pro-drugs that undergo rapid metabolism to form new metabolites (Shaw 1999a), a phenom-enon that will be discussed in detail later In complexes containing a single gold atom, the oxidation states +1, +2, +3, and +5 are well established (Puddephatt 1978) Divalent gold (Au+2) is rare, usually being formed as a transient intermediate in redox reactions between the stable oxidation states Au+ and Au+3 The first Au+5 complex containing the ion AuF6 was reported in 1972 The compound AuF5 can also be prepared Both are powerful oxidizing agents Gold also forms many com-plexes with metal–metal bonds in which it is difficult to assign formal oxidation states Additional information on stereochemistry, stability of complexes, oxidation– reduction potentials, current theories, and other aspects of gold chemistry is pre-sented in detail by Sadler (1976), Puddephatt (1978), Merchant (1998), and Schmid-baur (1999)

and Stability in Water Oxidation

–1 CsAu, ammoniacal Au – No

0 Metallic and colloidal gold Yes +1 Au(CN)2, aurothiomalate Yes +2 Au2(CH2PMe2CH2)2Cl2 No

Source: Data from Puddephatt 1978; Shaw 1999a, 1999b.

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Metallic gold (Au0) is comparatively inert chemically Gold is resistant to tarnishing and corrosion during lengthy underground storage or immersion in sea-water It does not oxidize or burn in air even when heated However, gold reacts with tellurium at high temperatures to yield AuTe2 and reacts with all the halogens Bromine is the most reactive halogen and, at room temperatures, reacts with gold powder to produce Au2Br6 At temperatures below 130°C, chlorine is adsorbed onto the gold, forming surface compounds; at 130 to 200°C, further reactions occur but the rate is limited by the diffusion rate of chlorine through the surface layer of gold chlorides; at >200°C, a high reaction rate occurs as the gold chlorides sublime, continually exposing a gold surface Atomic gold is considerably more reactive than the massive metal (Puddephatt 1978) Evaporation of gold at high temperatures under vacuum followed by cocondensation of the vapor with a suitable reagent onto an inert noble-gas matrix at liquid helium temperature produces Au(O2), Au(C2H4), Au(CO), and Au(CO2) Cocondensation of atomic gold with carbon monoxide and dioxygen gives the complex Au(CO)2O2; all of these gold compounds decompose

on warming the matrix (Puddephatt 1978) When auric oxide is treated with strong ammonia, a black powder is formed called fulminating gold (AuN2H3, 3H2O) Dried,

it is a powerful explosive as it detonates by either friction or on heating to about

145°C Caution is advised when handling this compound (Rose 1948)

Halogen compounds of gold are well known, especially aurous chloride (AuCl) and auric chloride (AuCl3; Rose 1948) Aurous chloride is a yellowish-white solid that is insoluble in cold water, but it undergoes slow decomposition into Au0 and AuCl3 Auric chloride takes the form of a reddish brown powder or ruby red crystals The auric chloride of commerce is aurichloric or chloroauric acid (HAuCl4·3H2O),

a brown deliquescent substance that is soluble in water or ether Aurichloric acid forms a series of salts called aurichlorides or chloroaurates Aurichlorides of Li, K, and Na are very soluble in water, and those of Rb and Cs much less soluble The sodium salt, NaAuCl4·2H2O, is sold as sodio-gold chloride and, unlike aurichloric acid, is not deliquescent Two gold bromides are known, AuBr and AuBr3, corre-sponding to their chlorine counterparts Auric iodide (AuI3) is unstable and decom-poses into aurous iodide (AuI) and free iodine Iodine in aqueous-alcoholic solutions combines with metallic gold to form aurous iodide, a white or lemon-yellow powder that is insoluble in water (Rose 1948)

Gold is inert to strong alkalis and virtually all acids, except aqua regia — a mixture of concentrated nitric acid (1 part) and hydrochloric acid (3 parts) The nitric and hydrochloric acids interact forming nitrosylchloride (NOCl) together with free chlorine, which reacts with gold In aqua regia, gold forms tetrachloroauric acid, HAuCl4, which is the source of gold chloride Gold is also soluble in hot selenic acid forming gold selenate, and in aqueous solutions of alkaline sulfides and thio-sulfates (Rose 1948; Krause 1996; Merchant 1998) Gold will dissolve in hydro-chloric acid in the presence of hypochlorite or ferric iron (Fe+3) as oxidant The dissolution of gold in cyanide solutions with air or hydrogen peroxide as oxidant is another example of this effect (Ransom 1975; Puddephatt 1978) The reaction with oxygen as oxidizing agent apparently takes place by adsorption of oxygen onto the gold surface, followed by reaction of this surface layer to yield AuCN, followed by the complex Au(CN) , which passes into solution (Rose 1948; Puddephatt 1978)

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PROPERTIES 43

Gold is also soluble in liquid mercury and in dilute solutions of sodium or calcium cyanide The cyanide solvent was used in Australia in 1897 where it was used to remove finely disseminated gold from pulverized rock The cyanide process is the only known method of profitably treating massive low-grade gold ores Using the cyanide process, auriferous rocks containing as little as 1 part gold in 300,000 parts

of worthless materials can be treated successfully (Ransom 1975; Cvancara 1995) Gold is readily dissolved by halide or sulfide ions in the presence of oxidizing agents to yield Au+3 or Au+ complexes (Puddephatt 1978) It is probably in this way that gold is dissolved when hot volcanic rock is buried, or when a hot granite intrusion rises near the surface of the earth’s crust As the solution cools to 300 to

400°C, concentrations of oxygen and hydrochloric acid decrease sharply, and gold

is redeposited Hydrothermal transfer of gold as the complex ion [Au(SH)2]– may occur in some cases Dissolution and redeposition of gold in stream beds may also

be responsible for the formation of large crystals of alluvial gold (Puddephatt 1978) Solutions containing gold complexes, such as AuCl4, are easily reduced to Au0 and under controlled conditions colloidal gold may be formed Colloids of gold — first reported in the 18th century — may be red, blue, or violet depending on the mean particle size and shape (Puddephatt 1978) Various reducing agents can be used for preparing colloidal gold including tannin, phosphorus, formaldehyde, and hydrazine hydrate The “purple of Cassius” is a mixed colloid of hydrated Sn+4 oxide and gold formed by reducing AuCl4 with Sn+2 chloride, A purple or ruby-red precipitate is formed on heating the solution A sensitive test for gold is based on this process, that is, a purple color is formed if a 10–8M solution of AuCl4 is added

to a saturated solution of SnCl2 (Puddephatt 1978)

The chlorination process, introduced in 1867, remains one of the most important refining processes for raw gold (Dahne 1999) Chlorination makes use of the fact that silver, copper, and base metals in raw gold react with chlorine at about 1100°C

to form stable chlorides while gold and platinum chlorides are unstable at >400°C

At 1100°C, silver chloride and copper chloride are molten, and base metal chlorides are volatile The silver and copper chlorides are removed by skimming Chlorination — which is usually completed within a few hours — is usually stopped at 99% gold so that gold losses by vaporization are avoided Other refining processes for gold include electrolysis, and wet-chemical separation of gold from silver and base metals In electrolysis, the anode plates are a mixture of tetrachloroauric acid, hydrochloric acid, and raw gold, and the cathodes are thin titanium (Dahne 1999)

4.3 BIOCHEMICAL PROPERTIES

Gold is not an essential element for living systems (Brown and Smith 1980) Indeed, the administration of gold to patients has been more similar to that of toxic elements, such as mercury, than to that of biologically utilized transition elements such as copper and iron Gold distributes widely in the body and the number of possible reactions and reaction sites is large Most of the in vivo gold chemistry is concerned with the reaction of gold species with thiols Within mammalian systems subjected to Au0, Au+, or Au+3, gold metabolism resulted in both monomeric and

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polymeric species Most gold complexes administered orally or parenterally were

absorbed, but rate and extent of accumulation were highly variable among gold

compounds Gold circulated in blood mainly by way of the serum proteins, especially

albumin Gold was deposited in many tissues and was dependent on dose and

compound administered Likely storage forms included colloidal Au0, insoluble Au+

deposits, and possibly Au+3 polymers Accumulated gold containing sulfur was

documented There is no suitable animal model available for testing mechanisms of

action of gold compounds used in human medicine (Brown and Smith 1980)

Gold has a unique biochemical behavior (Sadler 1976) Biochemical behaviors

of heavy metal ions show some similarities, particularly in their affinity for

polar-izable ligands But they also show important differences Gold, for example, has a

comparatively low affinity for amino and carboxylate groups, a stable higher

oxi-dation state in water, and proven anti-inflammatory activity of selected Au+ organic

salts (Sadler 1976) The biochemistry of gold has developed mainly in response to

prolonged use of gold compounds in treating rheumatoid arthritis and in response

to efforts to develop complexes with anti-tumor and anti-HIV activity (Shaw 1999b)

Chemical reactions of gold drugs exposed to body fluids and proteins are mainly

ligand exchange reactions that preserve the Au+ oxidation state (Shaw 1999a)

Aurosomes (lysosomes that accumulate large amounts of gold and undergo

mor-phological changes) taken from gold-treated rats contain mainly Au+, even when

Au+3 has been administered However, the potential for oxidizing Au+ to Au+3in vivo

exists Monovalent gold drugs can be activated in vivo to an Au+3 metabolite that is

responsible for some of the immunological side effects observed in chrysotherapy

For example, treatment of rodents and humans with anti-arthritic monovalent gold

drugs generates T-cells that react to Au+3 but not to the parent compound (Shaw

1999a)

Although metallic gold (Au0) is arguably the least corrosive and most biologically

inert of all metals, it can be gradually dissolved by thiol-containing molecules such

as cysteine, penicillamine, and glutathione to yield Au+ complexes (Merchant 1998)

Metallic gold reacted with cysteine in aqueous or saline solution in the presence of

oxygen to produce an Au+–cysteine complex; Au+ and cysteine formed a 1:1 Au+–

cysteine complex; L-cysteine reduced most Au+3 compounds in solution to produce

the Au+–L-cysteine complex (Brown and Smith 1980) With D-penicillamine, Au0

formed a Au0–penicillamine complex; Au+ under a nitrogen environment formed a

R3PAu+–penicillamine complex; and Au+3 formed a bis complex with penicillamine

With glutathione, Au+ formed a stable 1:1 complex in solution; Au+3 oxidized

glutathione to sulfoxide, the gold being reduced to Au+, which was stabilized by

complexing with unreacted glutathione (Brown and Smith 1980) These processes

were amplified at alkaline pH, significantly at pH 7.2, and perceptibly in acidic

environments having pH values as low as 1.2 (Merchant 1998) The rate of the

reaction was controlled by the concentrations of thiol-containing molecules and by

the pH; reactions might take place within cells and inside lysosomes Under favorable

conditions, reactions occurred at low rates on skin surfaces Skin samples taken from

beneath gold wedding bands of normal individuals averaged 0.8 mg/kg dry weight

skin In vitro studies designed to simulate conditions inside phagocytic lysosomes

[See Chapter 9 for additional details.]

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PROPERTIES 45

showed substantial dissolution of Au0 in the presence of hydrogen peroxide and

amino acids such as histidine and glycine There are reported instances of rheumatoid

arthritis patients who, on initiation of gold drug treatment (chrysotherapy), have

promptly produced rashes in the skin areas that have had regular contact with gold

jewelry Gold jewelry, if in close contact with skin, could be slowly dissolved by

sweat Thus, the thinning of gold rings over time, thought to be due mainly to abrasion,

could also be due, in part, to dissolution (Merchant 1998)

Colloidal gold is readily accumulated by macrophages (Sadler 1976) The gold

particles are taken into small vesicles, which form by surface invagination, and into

vesicles fusing to form vacuoles with subsequent transport to the centrosomic region

The part played by the surface of the Au0 particle may be due to Au+ ions on the

surface, which promote uptake A soluble gold-uptake stimulating factor of MW

<100,000 is reportedly secreted by lymphocytes and acts upon the macrophages

(Sadler 1976)

Gold+ drugs were metabolized rapidly in vivo (Shaw 1999a) The half life for

gold excretion in dogs was 20 days, but major metabolites had half-life times of 8

to 16 hours Within 20 minutes of administration, gold was protein-bound mainly

in the serum Injectable gold+ drugs were not readily taken up by most cells, but

bound to cell surface thiols where they affected cell metabolism The high affinity

of Au+ for sulfur and selenium ligands suggested that proteins, including enzymes

and transport proteins, were critical in vivo targets It was clear that extracellular

gold in the blood was primarily protein bound, suggesting protein-mediated transport

of gold during therapy (Shaw 1999a) Metallothioneins play an important role in

metal homeostasis and in protection against metal poisoning in animals (Eisler 2000)

Metallothioneins are cysteine-rich (>20%), low-molecular-weight proteins with a

comparatively high affinity for gold, copper, silver, zinc, cadmium, and mercury

These heat-stable metal-binding proteins were found in all vertebrate tissues and

were readily induced by a variety of agents — including gold — to which they bind

through thiolate linkages The role of metallothioneins in maintaining low

intracel-lular gold concentrations needs to be resolved

Following a chrysotherapy-type regimen with gold disodium thiomalate in mice,

Au+3 generation was analyzed with a lymph node assay system using T-lymphocytes

sensitized to Au+3 (Merchant 1998) The findings were consistent with three separate

anti-inflammatory mechanisms:

1 Generation of Au +3 from Au + scavenges reactive oxygen species, such as

hypochlo-ric acid

2 Au +3 is a highly reactive chemical that irreversibly denatures proteins, including

those lysosomal enzymes that nonspecifically enhance inflammation when they

are released from cells at an inflammatory focus.

3 Au +3 may interfere with lysosomal enzymes involved in antigen processing or

may directly alter molecules along the lysosomal–endosomal pathway, resulting

in reduced production of arthritogenic peptides (Merchant 1998).

If all of these activities occurred within a redox system in phagocytic cells, then

the anti-inflammatory actions of Au+/Au+3 could be effective for protracted periods,

and explain, in part, both the anti-inflammatory and the adverse effects of antirheumatic

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Au+ drugs Deviation of proteins could also contribute to the rare instances of auto-immunity reported in association with chrysotherapy (Merchant 1998)

Knowledge of Au+ binding sites on large molecules, such as proteins, is limited to

a few studies using Au(CN)2 (Sadler 1976) Although Au(CN)2 is one of the most stable gold ions in solution, it is considered too toxic for clinical use The simple Au+ cation does not appear to exist in solution, and most Au+ compounds are insoluble

or unstable in water Mercaptides stabilize Au+ in water, and sodium gold thiomalate

is now in widespread use as an anti-inflammatory drug Ionic Au+ seems to enter many cells but localize within the lysosomes of the phagocytic cells called macrophages Here they may inhibit enzymes important in inflammation Studies with sodium gold thiomalate suggest that anti-tumor mechanisms, like inflammation, are also macro-phage-mediated (Sadler 1976)

Canumalla et al (2001) report on two recent advances in understanding gold

metabolism in vivo In one finding, gold+ drugs and their metabolites react in vivo

with cyanide, forming dicyanoaurate+, (Au+(CN)2)–; this ion has been identified as

a common metabolite of Au+ drugs in blood and urine of chrysotherapy patients Second, Au+ is the primary oxidation state found in vivo although there is increasing

evidence for the generation of Au+3 metabolites Biomimetic studies indicate that the oxidation of sodium gold+ thiomalate and sodium gold+ thioglucose by hypochlo-rite ion (OCl)–, released when cells are induced to undergo the oxidative burst at inflamed sites, is rapid and thermodynamically feasible in the formation of Au+3 species The OCl– ion is involved in both the generation of Au(CN)2 and the formation of Au+3 species in vivo (Canumalla et al 2001).

The potential anti-tumor activity of gold complexes is driven by three rationales: (1) analogy to immunomodulatory properties underlying the benefit from Au+ com-plexes in treating rheumatoid arthritis; (2) the structural analogy of square-planar

Au+3 to platinum+2 complexes, which are potent anti-tumor agents; and (3) complex-ation of Au+ or Au+3 with other active anti-tumor agents in order to enhance the activity and alter the biological distribution of Au+3 (Shaw 1999a) For example, the rate of hydrolysis of AuCl4 in water is 375 times greater than that of PtCl4 (Sadler 1976) There is potential for developing new cytotoxic gold complexes that have anti-tumor properties, and this requires robust, new ligand structures that can move gold through cell membranes and into the cytoplasm, and perhaps into the cell nucleus (Shaw 1999a) Trivalent gold (Au+3) compounds are potential anticancer agents (Calamai et al 1997) These compounds are soluble in organic solvents, such

as methanol or DMSO, but poorly soluble in water In water, AuCl3 undergoes hydrolysis of the bound chloride without loss of the heterocycle ligand When Au+3 compounds react with proteins, like albumin or transferrin, Au+3 is easily reduced

to Au+ Cytotoxicity studies with tumorous cells showed marked anticancer activity

of Au+3 complexes, probably mediated by a direct interaction with DNA However, rapid hydrolysis of Au+3 to Au+ under physiological conditions may severely restrict their use More studies are needed to understand the biological mechanisms of gold complexes, including extent of cell penetration and biodistribution (Calamai et al 1997)

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PROPERTIES 47

Anti-HIV activity of monovalent gold compounds were associated with inhibi-tion of reverse transcriptase (RT), an enzyme that converts RNA into DNA in the host cell (Shaw 1999a) Other reports indicate that Au+ inhibits the infection of cells

by HIV strains without inhibiting the RT activity, with the critical target site tenta-tively identified as a glycoprotein of the viral envelope Other reports show that Au(CN)2 at concentrations as low as 20 µg/L is incorporated into a T-cell line susceptible to HIV infection, and retards the proliferation of HIV in these cells This concentration is well tolerated in patients with rheumatoid arthritis, suggesting that Au(CN)2 may have promise for existing HIV patients (Shaw 1999a)

When Au+3 compounds were used as labels for crystalline proteins, the nature

of the bound species was uncertain (Sadler 1976) Labelling with AuI4 has been claimed, but this ion appears unstable in aqueous solution In addition, Au+3 com-pounds often have strong oxidizing properties With a careful choice of ligands for

Au+3, a range of antitumor drugs may emerge because Au+3 has a high affinity for polynucleotides and may interfere with cell division properties (Sadler 1976)

Elemental gold is a soft yellow metal with the highest malleability and ductility

of any known element It is dense, being 19.32 times heavier than water at 20°C; a cube of gold 30 cm on a side weighs about 544 kg Metallic gold is inert to strong alkalis and virtually all acids; however, solubility is documented for aqua regia, hot selenic acid, aqueous solutions of alkaline sulfides and thiosulfates, cyanide solu-tions, and liquid mercury Sensitive analytical methodologies developed to measure gold in biological samples and abiotic materials relied heavily on its physical prop-erties Gold has 30 known isotopes and exists in seven oxidation states Apart from

Au0 in the colloidal and elemental forms, only Au+ and Au+3 are known to form compounds that are stable in aqueous media and important in medical applications The remaining oxidation states of –1, +2, +4, and +5 are not presently known to play a role in biochemical processes related to the therapeutic uses of gold Gold has a unique biochemical behavior, characterized by a comparatively low affinity for amino and carboxylate groups, a stable higher oxidation state in water, and proven anti-inflammatory activity of selected Au+ organic salts The biochemistry

of gold has developed mainly in response to prolonged use of gold compounds in treating rheumatoid arthritis and in response to efforts to develop complexes with anti-tumor and anti-HIV activity

Most of the in vivo gold chemistry is concerned with the reaction of gold species

with thiols, especially Au+ Gold is not considered essential to life, although it distributes widely in the body and the number of possible reactions and reaction

sites is large Monovalent organogold drugs were metabolized rapidly in vivo, usually

within 20 minutes of administration; however, half-time excretion rates ranged between 8 hours and 20 days, depending on the metabolite

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LITERATURE CITED

Barbante, C., G Cozzi, G Capodaglio, K van de Velde, C Ferrari, C Boutron, and P Cescon.

1999 Trace element determination in alpine snow and ice by double focusing

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Anal Atomic Spectr., 14, 1433–1438

Barefoot, R.R 1998 Determination of the precious metals in geological materials by

induc-tively coupled plasma mass spectrometry, Jour Anal Atom Spectrom., 13, 1077–1084.

Barefoot, R.R and J.C Van Loon 1996 Determination of platinum and gold in anticancer

and antiarthritic drugs and metabolites, Anal Chim Acta, 334, 5–14.

Begerow, J., M Turfeld, and L Dunemann 1997 Determination of physiological noble metals

in human urine using liquid-liquid extraction and Zeeman electrothermal atomic

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Borjesson, J., M Alpstein, S Huang, R Jonson, S Mattsson, and C Thornberg 1993 In vivo

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Com-position, K.J Ellis and J.D Eastman, (Eds.), Plenum, New York, 275–280.

Brown, D.H and W.E Smith 1980 The chemistry of the gold drugs used in the treatment

of rheumatoid arthritis, Chem Soc Rev., 9, 217–240.

Calamai, P., S Carotti A Guerri, L Messori, E Mini, P Orioli, and G.P Speroni 1997 Biological properties of two gold(III) complexes: AuCl3 (Hpm) and AuCl2 (pm), Jour.

Inorg Biochem., 66, 103–109.

Canumalla, A.J., N Al-Zamil, M Phillips, A.A Isab, and C.F Shaw III 2001 Redox and ligand exchange reactions of potential gold(I) and gold (III)-cyanide metabolites

under biomimetic conditions, Jour Inorg Biochem., 85, 67–76.

Christodoulou, J., M Kashani, B.M Keohane, and P.J Sadler 1996 Determination of gold and platinum in the presence of blood plasma proteins using inductively coupled

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Cvancara, A.M 1995 A Field Manual for the Amateur Geologist John Wiley & Sons, New

York, 335 pp

Dahne, W 1999 Gold refining and recycling, in Gold: Progress in Chemistry, Biochemistry

and Technology, H Schmidbaur, (Ed.), John Wiley & Sons, New York, 120–141.

Eisler, R 2000 Zinc, in Handbook of Chemical Risk Assessment: Health Hazards to Humans,

Plants, and Animals, Volume 1 Metals Lewis Publishers, Boca Raton, FL, 605–714.

Elevatorski, E.A 1981 Gold Mines of the World Minobras, Dana Point, CA, 107 pp Gasparrini, C 1993 Gold and Other Precious Metals From Ore to Market Springer-Verlag,

Berlin, 336 pp

Higashiura, M., H Uchida, T Uchida, and H Wada 1995 Inductively coupled plasma mass spectrometric determination of gold in serum: comparison with flame and furnace

atomic absorption spectrometry, Anal Chim Acta, 304, 317–321.

Hu, R., W Zhang, Y Liu, and J Fu 1999 Determination of trace amounts of gold (III) by cathodic stripping voltammetry using a bacteria-modified carbon paste electrode,

Anal Commun (Roy Soc Chem.), 36, 147–148.

Kehoe, D.F., D.M Sullivan, and R.L Smith 1988 Determination of gold in animal tissue

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