192 V. HEMATIN COMPOUNDS 



6. METALLOPORPHYRINS CONTAINING NICKEL, 

 COBALT, AND MANGANESE 



Nickel, cobalt, and manganese combine with various porphyrins giving 

 complexes which have points both of similarity and difference with hemes 

 and hematins. The relationships are best understood by reference to Chapter 

 II, Table II, Sect. 6.1., which shows the type of bonds possible with these 

 metals. No combination of porphyrins with chromium has been observed. 

 The nickel porphyrins, discussed in Section 1 of this chapter, are diamag- 

 netic, and are unable to combine with bases to form hemochrome-Iike com- 

 pounds. The electronic structure of nickel giv^es rise to four dsp"^ covalent 

 bonds, so that after entry into the porphyrin molecule, no bonds are available 

 for hemochrome formation. The two-banded spectrum which the nickel 

 porphyrins have in common with hemochromes thus indicates merely a dia- 

 magnetic covalent complex, regardless of whether this is of dsp"^ or d'^sp^ type. 

 Table II of Chapter II (Sect. 6.1.), indicates that manganous complexes 

 should be of d-sp^ covalent type, similar to those of ferric iron, and that 

 manganic complexes of a similar type should be expected to occur. The 

 manganese porphyrins have not yet been studied magnetochemically, but 

 Taylor {27It'J) has shown by spectrophotometric and potentiometi-ic methods 

 that both manganous and manganic porphyrins combine with two molecules 

 of pyridine. The absorption spectra of manganoporphyrin and of dipyridine 

 manganoporphyrin are of Theorell's type II and resemble somewhat those of 

 hem/chromes. They probably contain covalently linked manganese. The 

 spectra of manganic compounds have only one band in the green and probably 

 contain ionic linkages. 



The cobaltic complexes should be of d'^sp^ type, should be diamagnetic, 

 and should resemble hemochromes. A hemochrome-like absorption spectrum 

 has indeed been found for cobaltiporphyrins {lS13J7JfU). While Taylor did 

 not find any spectrophotometric evidence for combination of cobaltimeso- 

 porphyrin with nicotine, Holden {1313) reported spectroscopic differences of 

 pyridine, imidazole, and globin compounds in the position of their two 

 absorption bands. The type of spectrum is not altered by these combinations. 

 Cobaltoporphyrins have only one absorption band in the green part of 

 the spectrum, which is of Theorell's type III. They have thus essentially 

 ionic bonds, and from Table II, Chapter II, it can be seen that d'^sp^ coordi- 

 nation is impossible for cobaltous ion. Holden found no clear evidence of 

 combination of cobaltoporphyrin with nitrogenous compounds except with 

 native globin, which, however, also forms compounds with free porphyrins. 

 Taylor found a slight decrease of the absorption band of cobaltoporphyrin 

 in the presence of nicotine or cyanide, which in itself would not suffice to 

 establish that combination occurs. Oxidation-reduction potential measure- 

 ments, however, prove that it does so. On addition of nicotine to cobalto- 

 porphyrin the potential becomes more positive, showing that the affinity of 

 cobaltoporphyrin for nicotine is greater than that of cobaltiporphyrin. 



The potential of the cobalt and manganese base compounds does not 

 change with pH, so that the oxidants prol)al)ly remain in the ionic form, and 

 do not associate with hydroxyl ions. The potentials of the uncombined 



