292 MARTIN D. KAMEN 



not excluded. In the reactions studied by George and Irvine movement of 

 two protons is required and, most likely, a "ferryl" (FeO + +) state is 

 formed. This is not required in a photo-induced electron transfer reaction 

 of the type shown in Fig. i. It may be, therefore, that the only way to 

 reach the Fe^ + state in the chloroplast cytochrome, or other haem protein, 

 is by a photochemical oxidation which proceeds by electron transfer 

 unaccompanied by proton transfer. 



The search for new haem proteins in photosynthetic tissues and 

 intensive study of their structures should be intensified. At the same time, 

 experiments designed to reveal photochemical capacities of haem proteins 

 should be pursued. The present status of knowledge about haem proteins 

 seems well suited to application of Charles Darwin's admonition that 

 "without speculation there is no good and original observation". 



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