286 MARTIN D. KAMEN 



of " ferryl iron" (FeO + +) or, alternatively, as in the formulation of Fig. i. 

 At present it is not necessary to postulate production of a ferryl complex 

 which requires movement of two protons off the ligand water. This is 

 indicated in Fig. i by leaving the iron in a formal valence state equivalent 

 to Fe^+, without alteration of the chemical nature of the ligand. 



An alternative reaction scheme, which has been discussed by Calvin 

 [46], begins with loss of an electron from excited chlorophyll, concomitant 

 with generation of a positive hole in the chlorophyll complex. This 

 postulate necessitates a delayed oxidation of the cytochrome, or at least 

 reduction of some acceptor, such as pyridine nucleotide before oxidation 

 of haem iron occurs. There is no conclusive evidence at present to refute 

 this notion, although the low-temperature measurements of Chance and 

 Nishimura [35] on the photo-induced oxidation of the Chromatium cyto- 

 chrome system, together with the quantum yield data of Olson and 

 Chance [36], seem to favour prior oxidation of haem iron as a primary 

 reaction following quantum absorption. 



A variety of interesting problems comes to mind when predictions are 

 attempted for the chemical behaviour of a higher oxidation state such as 

 postulated in Fig. i . Fe^ +, which is isoelectric with Mn^ + would contain 

 four unpaired electrons distributed in the five 3d orbitals of the metal ion. 

 Upon combinations with the ligand groups, at least two could pair leaving 

 two unpaired electrons and the two free 3d orbitals, so that the Fe'* + 

 orbitals could hybridize as usual to give the octahedral complexes found 

 for Fe^ + and Fe^ +. There is evidence from the studies on magnetic 

 susceptibility of metmyoglobin-peroxide complexes that this occurs [47, 

 48]. If all the electrons paired, then seven orbitals rather than six would 

 be available with the Fe* + in a diamagnetic state. A ligand such as OH ", 

 but not HoO, would favour such an arrangement, if analogy with the lower 

 valence forms holds. 



The stabilizing effect of both the porphyrin ring, and possibly the 

 protein moiety, in a higher valence form can be inferred from many well- 

 known examples such as the metal porphyrin complexes of silver, bismuth, 

 cobalt, etc. Winfield and King have emphasized this possibility [49]. 

 Dwyer [40] has discussed similar situations, especially the case of the 

 nitroprusside ion, and it is from his discussion that the suggestion of a 

 possible diamagnetic complex structure is drawn. 



One point which should be made is that until direct data can be 

 obtained on the chemistry of iron haem chelates when in a state of oxida- 

 tion formally higher than Fe^ +, it is unsafe to assume that a molecule such 

 as CO is specific for the Fe + + state. The criterion of a light sensitive CO- 

 binding has been used universally to establish the presence of ferrous iron, 

 but the possibility that Fe^ + could bind CO in a similar fashion is not 

 excluded. 



