MARTIN D. KAMEN 187 



obtained on the clicniistry of iron hacni chelates, when in a state of 

 oxichition iornially 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. 



Returning to the photochemical system, it may be imagined that 

 in the green plant chloroplast, chlorophyll molecules are arranged 

 in an oriented, or aggregated fashion, so that exciton transfer, or a 

 transfer of energy by inductive resonance, is jiossible. Upon excita- 

 tion of a given chlorophyll, the quantum energy is transferred rapidly 

 to successive chlorophylls (r = ~10-i- to 10-^ sec). When a 

 chlorophyll adjacent to a haem protein is involved (~1 in 100-500) 

 the quenching process shown in Fig. 1 could occur. If enough com- 

 ponents of an electron-transfer chain were present (and they seem 

 to be from molecular analyses of the active photophosphorylating 

 systems studied so far — see, for example, 16) , a reaction chain would 

 result involving reduction of the pyridine nucleotides and thence 

 flavin, quinone, haem protein, etc., including the coupled phos- 

 phorylation observed in illuminated chloroj^lasts. If substrate amounts 

 of pyridine nucleotide are present and funnelled off to COo fixation, 

 the Fe^+haem component could initiate electron withdrawal from 

 a moiety, such as protein, leading to eventual formation of oxygen. It 

 can be supposed that reversal of an analogue of an oxidase system is 

 involved, depending on the protein moiety linked to the haem. In 

 bacterial systems, the protein moiety may be one which allows only 

 peroxidation reactions, so that oxygen is not evolved, but rather 

 substrate must be supplied to be oxidized. Another possibility is 

 that there is insufficient energy in the effective quantum for bac- 

 terial photosynthesis (~1.3 e.v.) to allow the formation of the 

 strongly oxidizing Fe^+-Fe2+ couple. 



Implicit in this formulation is the assumption that no cytochrome 

 oxidase is present in the oxidation chain. All experimental evidence 

 at present supports this assumption. A detailed discussion of this 

 evidence is not within the scope of this paper and in any case 

 is available in the literature (13). 



Many more consequences can be pictured, none of which are at 

 variance with what is known about plant and bacterial photosynthesis, 

 but these can be omitted in the interests of brevity. What is of im- 

 mediate interest is the experimentation indicated. It is evident that 

 chemical interaction initiated by light between model systems com- 



