Analysis and Interpretation of Absorption Spectra of Haemin Chromoproteins 159 



susceptibility corresponding with 5 unpaired electrons, to MHbOH with 3 

 unpaired electrons; Coryell, Still and Pauling, 1937) could be correlated with 

 the spectra of the transition of one spectroscopic species to the other. 

 (In ferricytochrome c, appreciably more pH-stable than ferrihaemoglobin, 

 four such spectroscopically operative acid groups have been uncovered 

 (Theorell and Akesson, 1941).) 



The pA'3 for human MHbOH is 8-15 at /i = 0-1, nearly the same as that of 

 dog MHbOH. This value is derived from the automatically recorded spec- 

 trum curves in Fig. 12 (Drabkin and E. Thorogood, unpublished). That 

 only two species participate in the spectroscopic transition was evident both 

 in the visible and near infra-red spectral regions and was reflected by the 

 presence of 5 isosbestic points (see legend to Fig. 12). However, it may be 

 seen in Fig. 13 that there is a departure from this behaviour in the near ultra- 

 violet region, since band number 6 is appreciably higher and located at a 

 shorter wavelength for MHb than the corresponding band for MHbOH (cf. 

 also Hicks and Holden, 1929). In the present connection, the main point 

 which may be stressed is that bands 4, a, /5 and 5 are evident in the spectra of 

 both MHb and MHbOH, but in the latter a and ^ dominate, whereas in the 

 former a and ^ are only weakly expressed, while 4, particularly, and 5 are 

 relatively dominant (see Fig. 12). 



Using the spectra of MHb and MHbOH as models, the relation between 

 the quantum mechanical deductions drawn from measurements in Linus 

 Pauling's laboratory of the molal paramagnetic susceptibilities of haemin 

 and its derivatives and the spectra of these compounds may be placed upon a 

 somewhat broader base by a suggestive correlation of the spectroscopic 

 pattern with the corresponding electronic character. In general, all essentially 

 covalent structures have prominent a and ^5 bands and weak number 4 bands, 

 whereas essentially ionic structures may have relatively weak a and /5 bands, 

 but are mainly characterized by strong number 4 bands or marked absorption 

 in the spectral region of the 4 band. This is brought out in Table 3. The 

 spectral patterns in the visible region are not as diverse as may have 

 been supposed (Drabkin, 1942a and b, and Figs. 15 to 17). However, 

 oxy-, carbonyl, cyanide and pyridine derivatives of ferrohaem are spectro- 

 scopically distinguishable from each other, whereas their electronic configura- 

 tion is the same (Table 3). It seems reasonable to infer that both the electronic 

 structure of the haemin iron and the nature of the co-ordinating ligand contri- 

 bute to the spectrum (cf. also Williams, 1956). 



The co-ordination of haemin iron with OH ion is a general reaction, 

 exhibited also by the ferrihaemochromes (cf. Haurowitz, 1927; Davies, 1940). 

 In Fig. 14, the writer's spectrophotometric measurements of the equilibrium 

 pyridine ferriprotoporphyrin ^ pyridine ferriprotoporphyrin hydroxide are 

 supplied. From these data a pAT value of 9-64 may be derived (see Insert to 

 Fig. 14). However, there is a most interesting difference between these 



