ROBERT C. PLUMB
chemical principles exemplified
The Lesser of Two Evils Illustrating principles of transition metal chemistry Contribution by Raymond Chang, Williams College, and Larry Vichery, Lawrence Berkeley Laboratory A common cause of accidental death is carbon monoxide poisoning, as a result of prolonged inhalation of auto exhaust. or. in the old davs. of domestic pas for cookina. The rensm for this is well known. Carhon monoxide formsi r o w DIVX with hemorlohin (in which it is hound t o the I.'r~Ill as the sixth ligandof an octahedral complex), called carboxyhemoglohin, with a binding constant about 200 times stronger than that for the corresponding oxygen complex. If a sufficient amount of CO is inhaled, the oxygen trausport efficiency from the lungs to the tissues for metabolic processes can he greatly reduced. The actual toxicity level depends on a number of parqmeters, but fatal cases often occur when carhoxyhemogloh~nis present a t or above 50% of the saturation level, which can occur with partial pressures of CO near 0.1 mm Hg. Another ligand which is capable of this type of interaction is the cyanide ion. We might therefore conclude that inhaling some hydrogen cyanide gas is equivalent to inhaling some auto exhaust. However, this is not true, concentrations of a few parts per million of HCN in air are sufficient to cause death, and therein is the interesting chemistry. According to ligand field theory, the strong honding hetween Fe(I1) in hemoglohin and CO is the result of a sigma bond formed by the donation of a lone-pair from the carbon to an empty metal d-orbital, plus a pi bond which is formed by the back donation of a filled metal dorhital to the empty antibonding molecular orhital of CO. Cyanide, however, has a very low affinity for Fe(II), or ferrous hemoproteins. This is probably due to a repulsion between the negatively charged cyanide ion and the high charge density a t the iron arising from electron donation from the porphyrin and the fifth axial ligand, the imidazole side chain of a protein histidine residue. HCN is lethal because of the high affinity of CN-, formed and transported in the blood a t neutral pH, for Fe(III), or ferric hemoproteins in which the iron now bears a net positive charge. Cyanide will bind almost irreversibly to the oxidized, Fe(II1) form of cytochrome oxidase, the hemoenzyme which serves as the terminal electron acceptor of the mitochondria1 respiratory chain. This binding blocks the reduction of cytochrome oxidase by other components of the chain and leads to a buildup of cellular reductants while preventing chemical energy storage. The interaction of O2 and CO with Fe(II1) hemoproteins is much weaker, since the net positive charge on the iron weakens metalligand pi honding. The reduced form of cytochrome oxidase normally reacts with oxygen to produce water. This reaction is not so sensitive to carbon monoxide poisoning for two reasons: First, cytochrome oxidase has an oxygen affinity about tenfold greater than hemoglobin since it must accept 0 2 800
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Journal of Chemical Education
from the latter directly or via myoglohin. Second, reaction of oxygen with Fe(I1) cytochrome oxidase oxidizes the heme back to the Fe(III) form which can then accept further electrons. Thus the enzyme can still turn over in the presence of CO and 0 2 . Reversal of CO poisoning can he effected by increased breathing rates and/or oxygen tension. If cyanide poisoning is detected early enough, attack of cyanide on cytochrome oxidase can be prevented by the rapid administration of an oxidizing agent such as amyl nitrite to concert some Fe(I1) hemoglohin to the Fe(EI) form. The latter can complex the cyanide and then he eliminated. Practically speaking, being caught in traffic may only make you yawn or cause your head to ache, hut if someone should spill acid over potassium cyanide in the laboratory, it is time to try the 100-yard dash.
Better (?) Photo Albums-Through
Chemistry
Illustrating intermolecular and intramolecular forces Information provided by R. C. Brou,n Director, Chemlcal Laboratory. U. S. Envelope Co and J. W Bird, J. W. Bird Photo Supply Store, Worcester, MA. The concepts of intermolecular and intramolecular forces are essential, but abstract, components of the theoretical framework of modern chemistry. One of the most clear cut illustrations of these concepts is only as far away as your photography shop or department store, or perhaps you have one already-the "magnetic" photo albums. With such an alhum you can actually "feel" intermolecular forces. In a "magnetic" alhum you place the photo under a sheet of transparent plastic; the plastic adheres to a paper page and holds the picture in place. You may re-lift the plastic and adjust or change the picture and reseal it again-without the use of paste, tape or gummed photo corners. The plastic appears to be held down to the page by magnets; in reality of course there is no magnetiasm involved-it is a clever application of pressure sensitive adhesives. The operation of pressure sensitive adhesives depends on a judicious balance of intermolecular and intramolecular forces. If you take a thin plastic sheet and try to apply it to an adhesives-coated flat surface you will prohahly get a mess-wrinkles, air pockets and light and dark blotches. The clever idea (patented) which made these albums saleable was to arrange the adhesive in a pattern of dots with paths for the air to he squeezed out so that even a child can make a neat seal. Unfortunately, a t least some of the albums on the market have an adhesive which does not retain its power very long and after a few months the pictures fall out a s though they had been placed loosely between the pages of a hook! The discussion here applies to an album in good working condition. Adhesion is not a simple phenomenon; although many of the general principles of surface chemistry are fairly well established there is much yet to he learned in surface
