66 J. E. Falk and D. D. Perrin 



Ligands such as ROH (including HOH), RS", RCOO-, HQ- and others, by 

 their dipolar or electrostatic interactions with the metal ion, should facilitate 

 the movement of the mobile porphyrin electrons away from the metal, and 

 shift absorption to longer wavelengths. 



Among the haemoproteins, one clear example of these two effects is seen 

 when Fe++ cytochrome c and Fe++ peroxidase are compared. The former is 

 low-spin (diamagnetic) and the latter is high-spin (4 unpaired electrons found). 

 The visible absorption maxima lie at 520, 550 m/^ and 558, 594 m/< respec- 

 tively (Lemberg and Legge, 1949). In cytochrome c one, and possibly both, 

 protein groups bound to the haem iron are histidine nitrogens, and in 

 peroxidase the groups are a — COO on one side of the haem and probably 

 a water molecule on the other (Chance, 1952; Theorell and Paul, 1944). 



Among Fe++ haemoglobin derivatives a similar change from high- to 

 low-spin (para to diamagnetism) is found on substitution of water (Hb itself) 

 by the double-bonding ligands O2 or CO. Similarly, when the 6th position 

 on the haem of Fe+++ haemoglobin is occupied by HgO, F~, 0H~, EtOH, 

 high-spin complexes, and by the double-bonding ligands CN~, — SH, Ng" or 

 imidazole, low-spin complexes are found (for references see Falk and Nyholm, 

 1958). Similar examples are to be found among peroxidase derivatives (Lem- 

 berg and Legge, 1949). Complexes of the haems with non-protein ligands 

 have been studied extensively, but many of the data which may be very 

 relevant to understanding of the haemoproteins are lacking. For comprehen- 

 sive reviews of this subject, see Lemberg and Legge (1949) and Martell and 

 Calvin (1953). 



As might be expected, with ligand atoms of high field strength and also 

 capable of double bonding, such as =N — (in pyridine, nicotine, a-picoline, 

 imidazole, etc.), low-spin complexes are formed. Thus all the complexes of 

 protohaem listed in Table 5 are diamagnetic. Though there must be variations 

 in complex-forming ability between these ligands, as indicated by their pA!' 

 and Eq values (Table 5), the spectra of the complexes are very similar; this 

 is probably because of comparable back-double bonding ability. In these 

 hexaco-ordinate complexes, the wavelengths would be expected to be modified 

 by the ligands on the 5th and 6th co-ordination positions only if these hgands 

 alter the spatial distribution of the electrons round the central metal ion in 

 such a way as to affect electronic transitions in the plane of the porphyrin 

 molecule. Such effects would be expected if spin-free and spin-paired com- 

 plexes were compared, or even in complexes with very different amounts of 

 back-double-bonding, spin-paired and least back-bonded having maxima 

 displaced towards longer wavelengths. 



However, especially with unrelated ligands, there is no reason why the 

 factors that govern their complex-forming ability with two different valence 

 states of a metal should bear any relation to the effective component of 

 the metal's c?-electrons at right angles to the direction of bond formation, 



