NOMENCLATURE 163 



3. Ferrous compounds with essentially ionic bonds. Purple-red com- 

 pounds with a broad band at about 560 m^. Examples: heme; hemoglobin; 

 ferroperoxidase. 



4. Ferrous compounds with essentially covalent bonds. Brilliant red 

 compounds with two well-defined bands in the green. Examples: hemo- 

 chromes; oxy- and carbon monoxide hemoglobin; cytochrome c; carbon 

 monoxide ferroperoxidase. 



Although it is clear that this scheme is certainly an over-simplifi- 

 cation (c/. Chapter VI), the spectra being influenced by factors other 

 than bond type, it provides a useful guide for classification of 

 hematin compounds. 



2. NOMENCLATURE 



We shall apply the name "hematin compounds" to the iron com- 

 plexes of porphyrins and of similarly constituted tetrapyrrolic 

 substances,* when we refer to them irrespective of the nature and 

 arrangement of side chains, the valency of the iron atom, and the 

 presence or absence of additional nitrogenous or other groups linked 

 to the iron atom. A more specific nomenclature is required to differ- 

 entiate between ferrous and ferric iron porphyrin compounds, and 

 between these compounds and those which have other groups (e.g., 

 two molecules of a nitrogenous compound) bound to the iron atom, 

 or which, as in the case of ferric hematin compounds, are bases which 

 can form salt-like derivatives. 



Our knowledge of the exact structure of many of these substances 

 is still too incomplete to attempt a systematic nomenclature of the 

 hematin compounds according to the rules usually adopted in complex 

 chemistry. Even if this were possible the names would be too long 

 for general use. Conventional names, or names which give an 

 indication of structure without being fully descriptive, must therefore 

 be used. Unfortunately a variety of such nomenclatures is used by 

 various authors. The nearest approach to a systematic nomenclature 

 is that proposed independently by Clark {Jt.52,J^53) and by Drabkin 

 {617,620) (Table I). Its use is strongly recommended for substances 

 the constitution of which is exactly known, and where precision is 

 required. Nevertheless shorter names are still needed as generic 

 terms, and in some cases for medical and other general purposes. 



* It has been suggested that this name he restricted to the iron porphyrins {2307), 

 but this raises difficult problems in the nomenclature of the parallel series of tetra- 

 pyrrolic substances with altered porphyrin nucleus ('■/. Section 8, and Chapter X) 



