ROBERT C. PLUMB
chemical principles exemplified
Worcester Polytechnic Institute Worcester, Massachusetts 01609
Mercury Poisoning
Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on January 29, 2019 at 05:06:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Illustrating competitive equilibria and role of similarities in molecular structure
Contribution by Professor Laurence E. Strong, Earlham College
Both mercury(II) 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(II) ion to form a quite stable lipoylarsenite complex H2
H2
o
Linking up the acetyl group and coenzyme A is done through the intervention of a lipoie acid derivative. In its free form lipoie acid is H2
H/C\
h2 h2
jc—c—c—c—c—c
W
ho^
ch2
In animal cells lipoie 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. H2
H r
—C—C-C—C—c—c" 1
s
11
n 0
H
CH2 1
s
H
The sulfhydryl groups are supposed to become linked to an acetyl group that comes from thiamine diphos28
/
A
As'S
lipoyl-Hg complex
II
CH3—C—S—CoA
H2
CH,
R
or
0
In many ways the poison symptoms mimic the symptoms of thiamine (vitamin Bi) 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
H2
S
H2
H2
H2
H2
H e.
—c—c—c—c—c—vch2
HO—CHS
H2
He!
„
I
H,
H2
II
H2
HS—d)Hs
h2 h2
H2
-C—C—C—C—C—C''
HS—CH2
°%
H2
Journal of Chemical Education
A
s
%g-S
Since the arsenite or mercury complex prevents the acetyl group transfer to form acetyl-coenzyme A, this blocks 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 10~6 M which, in a 70 kg human, means that less than 1 millimole of Hg(II) is capable of disrupting lipoie acid’s role. In a different way, but at the same point, the absence of thiamine blocks the transfer of the acetyl group. Hence thiamine deficiency diseases resemble poisoning by arsenite or 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. Compiexed 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 long after the victim is dead.
can
be used even
place where the oxygen pressure is highest, i.e., on the exposed area rather than at the edge of the pit where
partial protection is provided by the paint.
General References Lenninger, A. L., "Biochemistry,” Worth Publishers, 1970, p. 344. McGiLVERY, R. I., "Biochemistry,” W. B. Saunders Co., 1970, p. 259.
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 electrochemistry
Illustrating the Gibbs absorption theory
Contribution by Professor Ward Knockemus, The Pennsylvania State University
Information and suggestions provided by
When your car begins to “rust-out”—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 You will find that the metal at the corroded area. 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!
Professor Duncan C. Blanchard, Atmospheric Sciences Research Center, SUNY, Albany, and Howard S. Bilofsky and Professor Wilbur B. Bridgman, Worcester Polytechnic Institute
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 liter quantities in one liter bottles and the sea water foams; sodium chloride solutions do too, but not as well as sea water; pure water not at all. water sea
water
(GayHead, Martha’s Vineyard) NaCl solution (23.5 g/liter) distilled water
This unexpected result is readily understood by considering the electrochemical corrosion reaction and using the Nernst equation to determine the tendency for different areas of metal to be anodic or cathodic. When iron is in contact with oxygen and water in an acidic environment, the following oxidation-reduction reaction 77.0 kcal/mole) is energetically favorable (A(?° =
Fe
+
+ 2H +
V2O2
=
—
Fe2+
+ H20
Iron(II) will be further oxidized by 02 to form rust, iron (III) oxide. The electrochemical oxidation-reduction reaction may be divided into the anodic and cathodic half reactions V2O2
+ 2H+ +
Fe
=
Fe2+
2e~
=
II20
+
2e~
anodic
cathodic
In the vicinity of a pit in the paint finish, there can be substantial variations of the concentrations of the chemical species involved in these two half cells. In order to determine what region will be anodic and what region cathodic it is useful to write the Nernst equations for each of the half cells. Fo>.d
h":d
=
0.44
123
log [Fe2+]
-
2
og
F0.V2IH ' ]2
One sees qualitatively, by interpreting the cathodic chemical reaction with the help of Le Chatelier’s principle, or quantitatively from the fact, that Erea increases as Pq2 increases, that the cathodic reaction will take
height of foam (cm) 2.0
of foam (sec)
1.5
18
0
0
persistence 23
As is the case in so many (or, dare we say, all) of the phenomena which we observe in the world around us— the effect is produced by atoms and molecules busily directed by the laws of chemical science. A pure liquid will not foam but all true solutions will foam, the extent being determined by the specific substances involved. This remarkable generality follows from the fundamental concepts of surface thermodynamics enunciated by Gibbs in 1878. The theory of foam formation was developed by Foulk. [Ind. Eng. Chem., 21,815 (1929); 23, 1283 (1931); 33, 10S6 (1941).] The thermodynamically stable state of a liquid in the absence of external fields is one in which the surface energy is a minimum. The surface energy can be minimized by minimizing the surface area, but if the liquid is a solution it can also be minimized by concentration or depletion of the solute in a surface layer. A surface of a solution will become depleted in solute if lowering the concentration will decrease the surface tension or will become enriched in solute if increasing the concentration will decrease the surface tension. The concentration differential between the surface layer and the bulk is opposed by the entropy of mixing; for every solution there will be characteristic surface layer thickness and composition profde determined by the balance between the lowering of surface tension and the tendency to distribute solute randomly between bulk solution and the surface layer. Consider what happens as a bubble in a solution approaches a surface or another bubble. In the diagram below, the solute is assumed to concentrate in the interfacial layer, as indicated by shading. In order for two bubbles to join or a bubble to break at a surface, the liquid layer between the two surfaces Volume 49, Number 1, January 1972
/
29
unstable state is of higher Gibbs' free energy, as shown the chart above, i.e., an activated state controlling the rate of coalescence. Thus with solutions, froth forms and bubbles stay dispersed, while in pure liquids bubbles coalesce and do not persist at the surface. The solutes in sea water give it its tendency to froth and enhance bubble formation. Enhanced bubble formation leads to a corresponding increase in number of bubbles breaking. As has been elegantly shown by high speed photography, each breaking bubble introduces a jet of droplets of liquid into the air—hence even the saltiness of sea air is enhanced by the Gibbs adsorption effect. on
When this happens, the two surfaces with their non-bulk concentrations must join, producing a thermodynamically unstable state in which the thickness of the layer is not. the proper balance between surface tension and entropy of mixing. Presumably this must drain.
30
/
Journo/ of Chemical Education
General Reference Blanchard, Duncan 1967.
C., “From
Raindrops to Volcanoes," Doubleday,
