486 LIGHT AND LIFE 



George and Irvine (10) have presented evidence for this change as 

 a production of "ferryl iron" (FeO++) or, alternatively, as in the 

 formulation of Fig. 1. At present it is not necessary to postulate pro- 

 duction of a ferryl complex, which requires movement of two pro- 

 tons off the ligand water. This is indicated in Fig. 1 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 

 (4) , begins with loss of an electron from excited chlorophyll, con- 

 comitant with generation of a positive hole in the chlorophyll com- 

 plex. This postulate necessitates a delayed oxidation of the cyto- 

 chrome, or at least reduction of some acceptor, such as pyridine 

 nucleotide, before oxidation of haem iron occurs. There is no con- 

 clusive evidence at present to refute this notion, although the low- 

 temperature measurements of Chance and Nishimura (5) on the 

 photo-induced oxidation of the Chromatiiim cytochrome system, to- 

 gether with the quantum yield data of Olson and Chance (17, 6) , 

 seem to favor 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 behavior of a higher oxidation state 

 such as that postulated in Fig. 1. Fe^+, which is isoelectronic with 

 Mn3+, would contain four unpaired electrons distributed in the five 

 3d orbitals of the metal ion. Upon combination with the ligand 

 groups, at least two could pair, leaving two impaired electrons and 

 the two free 5d orbitals, so that the Fe^+ orbitals could hybridize 

 as usual to give the octahedral complexes found for ¥e^+ and Fe-+. 

 There is evidence from the studies on magnetic susceptibility of 

 metmyoglobin-peroxide complexes that this occurs (20, 2) . 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 H^O, would favor 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 sil- 

 ver, bisnuith, cobalt, etc. Winfield and King have emphasized this 

 possibility (15). Dwyer (7) 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 



