OXIDATION AND REDUCTION 



than two molecules of cysteine for one iron atom.* The two re- 

 maining coordination places may be filled in by other atoms or atom 

 groups of small size having an unused electron pair. One example 

 of such an atom group is carbon monoxide, CO. In fact, the coordi- 

 nation compound, Fe^^(RS)2(CO)2, can be readily prepared by the 

 interaction of Fe++, cysteine, and carbon monoxide in the form of its 

 well crystallizable alkali salt.f Just like CO, O2 also has (at least) 

 one unused electron pair, and may combine instead of CO, as in the 

 case of hemoglobin which combines either with O2 or with CO. In 

 hemoglobin, four coordination places of iron are occupied by the four 

 nitrogen atoms of the porphyrin ring, a fifth by protein, and the sixth 

 can combine with O2 or with CO. So we arrive at hypothetical 

 iron-cysteine-oxygen complexes, analogous to the well-known carbon 

 monoxide complex, Fe"(RS)2(CO)2. One cannot tell whether one 

 oxygen molecule is attached at one or at two coordination places, or 

 whether even two oxygen molecules can be attached. Suffice it to 

 imagine the complex Fe"(RS)202. This complex is said to be hypo- 

 thetical because it cannot be prepared, and undergoes a redistribution 

 of electrons, or, in other words, intramolecular oxidation-reduction, 

 which may be symbolized in this way: The original, oxygen-con- 

 taining complex may be imagined to consist of the following con- 

 stituents: Fe++ + 2 RS~ + O2. The electron redistribution will 

 occur thus: 



Fe3+ + 2 RS- + O2" (O2 withdraws one electron from Fe + +) (d) 



Fe3+ + RS- + RS + 02~ (O2- reduced to 02~, /. e., H2O2, by 



withdrawing one electron also from one RS ") (e) 



Fe++ + RS + RS + O2 — (Fe3+ withdraws one electron from RS") (f) 

 An alternative scheme, probably of equal probability, is: 



* It is interesting to compare cysteine complexes of cobalt with those of 

 iron. For the cobaltous state, no complex with more than two cysteine molecules 

 can be obtained. For the cobaltic state, both a complex with two and another 

 with three molecules of cysteine can be prepared. The cobaltous complex, then, 

 is quite analogous to the ferrous complex. The cobaltic complex stands no com- 

 parison, because the ferric-cysteine complex is too unstable, due to the rapid 

 intramolecular rearrangements to be described presently. 



t The formula in the first footnote on page 222 shows that atom group R 

 contains a carboxyl group. It is by means of the two carboxyl groups in the com- 

 plex that alkali salts can be formed. 



223 



