Fenihaemoprotein Hydroxides 1 39 



MoRiTA, Y. & Kameda, K. (1958). Mem. res. Ins fit. Food Science, Kyoto, Japan, No. 



14,61. 

 Orgel, L. E. (1955). J. chem. Phys. 23, 1819. 



ScHELER, W. & FiscHBACH, I. (1958). Acta Biol. Med. Germ. 1, 194, 

 ScHELER, W., SCHOFFA, G. & JuNG, F. (1957). Biocliem. Z. 329, 232. 

 Sternberg, H. & Virtanen, A. I. (1952). Acta. chem. Scand. 6, 1342. 

 Taube, H. (1952). Cliem. Rev. 50, 69. 

 Theorell, H. (1941). J. Amer. chem. Soc. 63, 1820. 

 Theorell, H. (1942). Ark. Kemi, Min. Geol. 16A, No. 3, 1. 

 Theorell, H. & Akesson, A. (1941). /. Amer. ciiem. Soc. 63, 1812. 

 Theorell, H. & Ehrenberg, A. (1951). Acta chem. Scand. 5, 823. 



DISCUSSION 



Spin States and Spectra of Haeryioproteins 



The Electronic Origins of the Spectra 



By P. George and J. S. Griffith (Philadelphia) 



George : It is perhaps desirable to say something about the electronic origins of the spectra 

 of the pure high-spin and low-spin compounds (ferrous and ferric), although we have 

 not yet made a detailed analysis of them. 



Considering first the iron-porphyrin group we may divide the possible electronic 

 transitions into three categories: porphyrin transitions, metal transitions and charge- 

 transfer transitions. Free porphyrin has strong absorption in the visible and also a 

 Soret peak and the intensity associated with these cannot be lost in the metal com- 

 pound. Therefore one naturally supposes the Soret band of the latter and some at 

 least of its visible absorption to be porphyrin transitions. These porphyrin transitions 

 have the characteristic that the electric vector of the light lies in the porphyrin plane 

 so that when it is at right-angles to it the light does not get absorbed. This is not 

 necessarily true of the other transitions discussed later. In thinking about the part of 

 the spectrum which arises from the porphyrin ring one should also remember that 

 the singlet-triplet transitions may enhance their intensity considerably through coupling 

 with the metal ion when the latter has non-zero spin. 



The metal transitions in the visible and infra-red are d-d transitions which would 

 be of low intensity and probably completely masked by the porphyrin bands. At least 

 they can hardly be responsible for the gross visible structure. The metal 3d-4p transi- 

 tions would probably be in the ultra-violet although it is possible that the 4pz orbital 

 might have its energy lowered sufficiently by interaction with a porphyrin tt orbital 

 to invalidate this view. If this were so, however, the transition would also involve 

 charge-transfer to the ring and so be partly included in our third category. 



Charge-transfer transitions are of two kinds — to and from the metal ion. Naturally 

 we expect the low energy ones to be to the metal ion for ferric compounds, and from 

 the metal ion for ferrous compounds. In each case, then, they would involve the iron 

 atom commuting between the ferrous and the ferric state. It is natural to suppose 

 that the infra-red bands characteristic of high-spin ferric compounds, oxy-haemoglobin 

 and myoglobin, and the single-equivalent higher oxidation states, are indeed charge- 

 transfer bands. Some components of such transitions are of course fully allowed for 

 electric dipole radiation, and can therefore account for the relatively high intensities. 



The ligands in the fifth and sixth positions can also have transitions and give charge 

 transfer to and from the iron, and so in a particular compound one or more bands 

 may arise which have no counterpart in other compounds. It would be quite feasible, 

 for example, that the infra-red band of oxyhaemoglobin might be of this type: it 

 could be a transition from a weakly bonding to a weakly antibonding orbital embracing 

 the ferrous ion and the oxygen molecule. 



We have for simplicity deliberately treated the system as if it can be broken up in 

 a unique and well-defined manner into a number of pieces. This is not strictly 



H.E. — VOL. I — L 



