44 R. J. P. Williams 



this transition is naturally more difficult in ionic ferric than in ionic ferrous 

 complexes on account of the charge difference. The band is at shorter Amax 

 in the ferric complex. In the low-spin states both ferrous and ferric ions 

 become better cr-acceptors and on these grounds the Soret transitions should 

 both be more difficult in the promoted state (low-spin) complexes and a 

 shift of Amax to shorter wavelengths would be expected. Now the ferrous 

 ion also becomes a stronger tt donor in the low-spin state which again makes 

 the transition of the electrons concentrated on the nitrogen more difficult 

 (Note 1). Thus in the case of Fe""^ both a and tt interactions of the cations in 

 the ligand change so as to shift Amax (porphyrin) to shorter wavelengths 

 when the complex becomes of low spin. On the other hand, the ferric ion in 

 the promoted state is a tt electron acceptor with one hole in the de shell. 

 This property makes the Soret transition of the porphyrin easier. We conclude 

 that the increased stabilization of tt electrons in the excited state overrides 

 the increased stabilization of the electrons in the ground state on change 

 from the high- to the low-spin ferric state. Thus we can see that the difference 

 in TT-bonding characteristics of the cations can explain the opposed shifts. 

 A similar explanation has been advanced in discussing the phenanthroline 

 series of ferric and ferrous complexes where spectral changes in opposed 

 directions are observed in a series of ferrous and ferric complexes which we 

 have considered as models for the study of the physical properties of iron 

 porphyrins (Williams, 1955, 1956). 



A good illustration of the opposed shifts of the Soret band in the Fe++ and 

 Fe+++ states is that observed in the study of the effect of pH upon the spectra 

 of Rhodospirillum cytochrome (Morton, 1958). At pH 7 the band positions 

 are: Fe++, 424 m/t, and Fe+++, 390 m/<, with a band at 640 m/< in the ferric 

 spectrum indicating an ionic complex. At pH 11-8 the bands are: Fe++, 

 413 m/i, and Fe+++, 407 m/.<, with no band at longer wavelengths than 565 m/i 

 in the Fe+++ spectrum indicating a covalent complex. Thus at pH 7-0 this 

 cytochrome is very largely ionic but at pH 11-8 it is largely covalent and the 

 value of Am/t has changed from 34 m// to 6 m/i. The suggested change of 

 magnetic moment is in keeping with the observed fall in the ratio of the 

 y to a peak intensities of the reduced cytochrome on increasing pH. The 

 observations are also very suggestive with regard to the group of the protein 

 which is responsible for the pH dependence. As both the ferric and ferrous 

 complexes show changes it can not be the reaction HoO -> 0H~. Imidazole 

 groups are completely ionized at a pH just greater than 7-0 and we are left 

 with the strong impression that the change involved is — NH3+ -^ NHg (see 

 pp. 46, 49, 50). 



It will be observed that from the above analysis we consider that cytochrome 

 ^3 is largely ionic. This tentative conclusion is supported by the following 

 evidence. (1) The intensity ratio of the a to the y peak is 1:11. The usual 

 ratio in covalent complexes is 1 : 6 as in the pyridine complex of haemoglobin 



