158 



David L. Drabkin 



Table 3. Correlation of spectroscopic pattern with electronic structure 

 S = strong band; W = weak band; M = moderately strong band; N = negligible band 



* The permanent magnetic dipole moment in these complexes is due almost entirely to 

 Hsy the spin moment of the unpaired electrons, /is is derived from the measured molal 

 paramagnetic susceptibility. Theory for //s for 1 to 5 unpaired electrons is 1-73, 2-83, 

 3-83, 4-90 and 5-92 Bohr magnetons. 



t The terms ionic and covalent should be prefaced by 'essentially' to indicate partial 

 ionic and covalent character. Thus, ferrihaemoglobin is more ionic than ferrihaemoglobin 

 hydroxide (Pauling, 1940, 1948, 1949). References: 1, Coryell, Stitt and Pauling, 1937; 

 2, Theorell, 1941 ; 3, PauUng and Coryell, 1936b; 4, Pauling and Coryell, 1936a. 



t Deduced from the graphic analysis of the spectrum. 



series. The relative prominence of these four components resuUs in the 

 spectral differences. This is well illustrated in supplementary Figures 12 and 

 13, which show the transition with change in pH of the spectra of human met- 

 er ferrihaemoglobin to ferrihaemoglobin hydroxide. The pH dependency of 

 the spectrum of ferrihaemoglobin was originally studied by Hartridge, 1920 

 and Haurowitz, 1924. A detailed and very careful spectrophotometric study 

 by Austin and Drabkin (1935) of dog ferrihaemoglobin, MHb, with reference 

 to the equilibrium MHb ^ MHbOH revealed a reaction of the first order 

 with OH ion, and pi^g (as it is now usually designated) was accurately 

 established as 8- 12 ± 0-01 at /< (ionic strength) = 0-1 and a = 0-6. This value 

 for p^3 has been confiimed by the independent techniques of magnetometric 

 titration (Coryell, Stitt and Pauling, 1937) and differential acid-base titration 

 (Wyman and Ingalls, 1941). Furthermore, the demonstration of the effect 

 of ionic strength, regarded in that day as unusual for such complex com- 

 pounds, was also apparent in their magnetic behaviour. Thus, the haemin- 

 linked group, responsible for ^K^ (i.e. the release of a proton from a molecule 

 of water co-ordinated with the iron in acid ferrihaemoglobin, MHb • HOH+, 

 to form MHb -OH -H H+) was spectroscopically, magnetometrically and 

 titrimetrically operative. It appeared that at least in this case the electronic 

 structural change involved in MHb ^ MHbOH (MHb, with a magnetic 



