ROBERT C. PLUMB
chemical principles exemplified
Worccltc. Polyterhnic Inslitul. Worcsst.r, M a s s a t h ~ ~ t 01609 ts
Mercury Poisoning lflustroting competitive equilibria and role of similarities in molecular structure
Contributim by Professor Laurence E . S t r a g , Earlham College Both mercury(I1) ions and trivalent arsenicals are poisonous for us humans with a number of similar symptoms. Poisoning in either case can be alleviated by treatment with 2,3-dimercaptopropanol, also known as British Anti-Lewisite and commonly referred to as BAL.
phate and is then transferred on to coenzyme A. However, the spacing of the two sulfhydryl groups permits ready combination with an arsenite group or with a mercury(I1) ion to form a quite stable lipoylarsenite complex
or lipoyl-Hg complex
li
:::g In many ways the poison symptoms mimic the symptoms of thiamine (vitamin B,) deficiency. The chemistry of the situation illustrates the role of similarities in molecular structure and competition among complexes controlled by equilibria. In the body's metabolism of glucose, a key step is the formation of acetyl-coenzyme A by the transfer of an acetyl group to coenzyme A. The linkage between coenzyme A and the acetyl group is through a sulfur atom that, in coenzyme A is present as a sulfhydryl group. This relation can be represented as CHZ-44-CoA
II
0
Linking up the acetyl group and coenzyme A is done through the intervention of a lipoic acid derivative. In its free form lipoic acid is
In animal cells lipoic acid is present as a lipoyl group attached to a lysine residue which in turn forms part of the enzyme dihydrolipoyl transacetylase. Here the lipoyl group has been reduced to give two sulfhydryl groups.
The sulfhydryl groups are supposed to become linked to an acetyl group that comes from thiamine diphos28
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Journal of Chemical Education
Since the arsenite or mercury complex prevents the acetyl group transfer to form acetyl-coenzyme A, this bloclcs an essential step in the metabolic reactions for glucose. The average concentration of lipoyl residues in the human body is estimated to be less than M which, in a 70 kg human, means that less than 1 millimole of Hg(I1) is capable of disrupting lipoic acid's role. In a different way, but a t the same point, the absence of thiamine blocks the transfer of the acetyl group. Hence thiamine deficiency diseases resemble poisoning by a r s e ~ t eor mercury. The stability of the mercury complex is very great, so that mercury acts as an accumulative poison, only released by the body quite slowly. An antidote for these two poisons has been constructed as a molecule that also has two sulfhydryl groups separated by two carbon atoms. Thus BAL, with its structural similarity to the sulfhydryl end of dihydrolipoic acid, serves as an effective antidote. It forms a stable complex with arsenite or mercury so that a moderate concentration of BAL can shift the equilibrium in a direction that leaves the lipoyl residue free to play its proper role in acetyl transfer. Complexed with BAL, the arsenite or the mercury is then excreted by the body. The part of the body most dependent on glucose metabolism is the brain. Nervous disorders are among the early symptoms of poisoning. Glucose metabolism is a vital part of nearly every cell so there are a variety of other symptoms as well. Wherever sulfhydryl groups are abundant, arsenite and mercury are likely to become attached. One such place is in the keratin of hair which is rich in sulfhydryl groups. Victims of poisoning can often be detected by
analysis of hair cuttings. The test can be used even long after t,hevictim is dead. General References LEIININQBR. A. L.. TJiaehemistry:' Worth Publishers. 1970, p. 344. MCGILYERY, R. I., "T%ioehemistly," W. n. Saunders Co., 1970, p. 259.
place where the oxygen pressure is highest, i.e., on the exposed area rather than a t the edge of the'pit where partial protection is provided by the paint. The anodic reaction, dissolution of iron, takes place in the remaining area-that is, under the paint.
When Your Car Rusts Out
The Effervescence of Ocean Surf
Illustrating principles of electrochemirtry
lllustroting the Gibbs absorption theory
C m t r i b u t i m by Professor W a r d Knoclcemus, T h e Pennsylvania State University
I n f o m a t i o n and suggestions provided by Professor Duncan C . Blanchard, Atmospheric Sciences Research C a t e r , S U N Y , Albany, and Howard S . Bilofsky and Professor Wilbur B. Bridgman, Worcester Polytechnic Institute
When your car begins to 'Lrusbout"-that. insidious process which forecasts an expensive series of visits to car dealers and banks and probably a restriction on the family budget for months to come-take a careful look at the corroded area. You will find that the metal under the paint has rusted. In fact, the bulk of the problem is corrosion of metal which one would have expected to be protected by the paint finish. Doesn't paint protect iron and steel from corrosion? Yes and no. Yes, if the metal is completely sealed; but. no, if the protective coating is scratched, pitted, or dented to expose even a microscopic area of bare metal. If this happens the metal under the paint rusts!
The sparkling liveliness of ocean surf enchants people. What makes sea water sparkle like champagne while fresh water is flat like apple juice? It's not just the presence of large waves at the seashore; there are really differences in the water, as a few simple laboratory observations can show. Bubbles produced in sea water behave differently from bubbles in fresh water. Shake up one-half lit,er quantities in one liter bottles and the sea water foams; sodium chloride solutions do too, but not as ell as sea water; pure water not a t all. newnc
water
foam (
