chemical vignettes ROBERT C. BRASTED University of Minnesota Minneapolis, 55455
Hydrogen Sulfide Under Any Other Name Still Smells A Poisonous Story General Chemistry, Biochemistry, Inorganic, and Complex Chemistry
There is no single compound that turns off the uncommitted (hut potential clientele) of our chemistry programs more than does hydrogen sulfide. Here we surely have one area of descriptive chemistry that any student can appreciate if not understand. During our brief lecture encounters with simple molecules such as carbon monoxide, sulfur dioxide, and oxides of nitrogen, hydrogen sulfide also provides a beautiful opportunity to "cross" our disciplines with some hio and physiologicalchemistry. Why not expend a little extra effort, enlarge our horizons a hit and really justify the student's "distaste" for this molecule. As well, we can use the same chemical logic applied for H B and CO to give the student an appreciation for hydrogen cyanide (the arch villian of the chemical species for the mystery novel writer when death by poisoning is part of the plot), fluorides, azides and a number of barbiturate drugs. The carbon monoxide poisoning story is found in virtually all introductory texts but usually includes only the ideas of the displacement of oxygen by carbon monoxide in the iron(I1) porphyrin complex. I n actual fact we are dealing with a respiratory inhibitor, with such molecules or ions as HCN, CO, F-, H2S, N,affecting this iron(I1) containing enzyme (one of the cytochrome oxidases). One of the reactions specifically inhibited may he represented schematically as cytochrome oxidase
Cytoohrome c(Fea+)
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Confucius must have had a point in asking for pictures. A very general one may be helpful, showing heme which acts as a coenzyme involved in the oxygen transfer for the cytochrome (noted above).
When one of our persona non grata enters the scene as the inhibitor (such as the sulfide from HsS) the planar iron(I1) complex assumes an octahedral configuration with the pi-bonding ligands occupying the trans sites A and B above and below the plane. Now the coenzyme is finished in its useful role of providing for the oxygen transfer.' The major thrust of our Vignette is not so much a discussion of the specific enzyme which is attacked as the fact that the same hlocking or inhibiting action occurs with all of these ligands. The partial pressure ratio of hydrogen sulfide to oxygen needed to assume this blocking or paralysis is probably even lower than that of carbon monoxide to oxygen-1:200. This is a frighteningly small quotient to think about. I n other words, don't knock the smell of hydrogen sulfide. Most of us wouldn't be here without this warning mechanism. To carry our ecumenical movement in chemical disciplines further, for the purposes of this Vignette, modern biochemistry leans heavily both upon the inorganic theoretician and the descriptive chemist. The theory behind the role these apparent a-bonding ligands (S2-, CN-, F-, etc) play in the spin functions of the iron(I1) ion and the complex it forms with the heme can be explained. Even the O2 adduct has some pretty esoteric overtones since we need to invoke a change in spin multiplicity for Oz to account for the stability and magnetic properties of the metal ion complex. It is not immediately obvious why there is no Fe(II1) in the oxyhernoglobin. This oxidation state exists in the methemoglohin, hut the latter cannot combine with 02. However, therein lies another Vignette to be written by someone more astute in these matters than the author. However, we digress. What about other possible reasons for the action of our poisonous friends. It is tempting to attribute the inhibition of the sulfide to the formation of a highly insoluble iron sulfide, or perhaps to the reduction of the thio linkage between protein molecules. All of these involve good chemistry but probably contribute only a minute fraction of the real and very rapid effect, that of the enzyme oxidase inhibition. As will he very increasingly obvious, we are trying to provide ways of introducing important and relevant chemistry a t a level and to a student audience where it MAHLER, H. R., AND CORDES, E. H., lLBiologi~al Chemistry," Harper 8 Row Publishers, Inc., New York, 1966, Chap. 8; W. S., P T X .Nat. Acad. Sci., US., WANQ,J. H., AND BRINIGAR, 46,958 (1960).
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can be appreciated even though the niceties may not all be evident. In summary, the union of nonmetal-nonmetal interactions with transition metal chemistry is suggested. The physiological chemistry blends nicely with well known air pollutants and their effects. When some additional theoretical chemistry is added, the singlettriplet oxygen story can he added as well as the mag-
netism of the iron(11) and (111) complexes. Partial pressures become pretty important when we know that the HzSpressure is less than 1/200 of the 0% pressure to form a stable complex with the iron(I1) of the enzyme. Finally, for the student in the mechanism course(s~the Vignette may provide time for reflection on electron transport.
Volume 47, Number 8, August 1970
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