MARTIN 1). KAMEN 485 



peiniaiigaiiale. (hloriridatc) gives a prodiut spectroscopically identi- 

 cal with the intciincdiate "complex 11" lornied when nietmyoglobin 

 acts as a peroxidase. They have established the E'o as ~+().9 v. 

 This value is ~0.1 v. more oxidizing than that for the standard 

 oxygen electrode. Hence, the oxidizing equivalents present in this 

 complex can extract an electron from HoO. If it is assumed that 

 the haem chelate-protein complex acquires a similar E'^. then the E'<, 

 for the seniichlorinogen formed would be ~— 0.6 to — 0,7 e.v., 

 assuming 1.5 e.v. as the value of a£'o between the reducing and 

 oxidizing comjjonents. Such a strong reducing potential would be 

 more than sufficient to provide an electron transfer step to pyridine 

 nucleotide (£'„ = —0.3) which could be coupled to formation of 

 ATP from ADP and inorganic phosphate ("photophosphorylation") 

 (1, 9). On this basis, the "photoreductase" of San Pietro and Lang 

 (18) woiUd have as its substrate the seniichlorinogen which is identi- 

 fied as the photoreductant generated by the light reaction. 



The reactions initiated by the presence of the Fe^^-haem complex 

 depend on the environment presented. A simple combination of 

 chlorophyll and haem protein would have only the possibility of 

 back-reaction, or reversal of the process shown in Fig. 1. However, 

 if an enzymic pathway (such as through the photoreductase to pyri- 

 dine nucleotide) is available to remove electrons from the senii- 

 chlorinogen, then it can be expected there will be a preferential flow 

 of electrons to the enzyme substrate. If a source of electrons is 

 present in the haem complex, either in the protein or as a simple 

 ligand (water) , then reduction of the Fei^'-haem to its original 

 state would occur with the production of a free radical, probably 

 in the protein moiety. 



The evidence available from paramagnetic spin resonance studies 

 of the nietmyoglobin oxidation complex, while somewhat ambiguous, 

 appears to be consistent with this postulated sequence of events. 

 Gibson, Ingram, and Nichols (11) have shown that the complex, 

 studied by George and Irvine and produced by peroxidation of 

 nietmyoglobin, exhibits an ESR signal with a g-value close to that 

 for the free electron. The precise value for g is somewhat smaller 

 than expected for a TT-electron localized at a methine bridge carbon. 

 It is more consistent with the presence of a delocalized electron in 

 an orbital spread over the whole macrocyclic structure, or of a 

 substrate free radical, such as OH. At the same time, the signal at 

 g =: 6, corresponding to the unpaired electrons at the Fe site, is 

 quenched, indicating a change in the bonding at the metal ion site. 



