HAEM PROTEIN CONTENT AND FUNCTION IN RELATION TO STRUCTURE 283 



supporting photophosphorylation when incubated with certain external 

 factors [3]), there are 40 bacteriochlorophylls, 17 carotenoids, 2-8 haem 

 proteins, o • s flavins, and i pyridine nucleotide. Data on the quinone 

 contents of the two preparations are not available. We may generalize 

 these observations to the statement that similar molecules are present as 

 major components in all photoactive structures. 



Now, we may ask what mechanism we can assume for energy storage 

 and which molecules of those mentioned as major constituents are likely 

 reactants for production of molecular species sufficiently stable to couple 

 to the biochemical phase of photosynthesis. Of course, there is little doubt 

 that one reactant will be excited chlorophyll. The reactions it may undergo 

 upon excitation are many but a most likely type of reaction is one involving 

 electron transfer. It is not possible that electron ejection (photo-ionization) 

 will occur because the quantum energy in actinic light is insufficient for 

 such a process. However, electron donation, or acceptance, from a neigh- 

 bouring molecule is possible. Some theories [32] are built on the notion 

 that chlorophyll loses an electron to some acceptor and so becomes a strong 

 oxidizing agent. An alternative notion is that it gains an electron and 

 becomes a strong reducing agent. There is no way at present of deciding 

 between these two alternatives. 



On the basis of some arguments based on comparative biochemistry 

 and the physical chemistry of the haem proteins (see later discussion in 

 this paper) and results obtained by Duysens, Chance and others, using an 

 approach based on differential spectrophotometry of fast reactions in 

 suspensions of cells and extracts [33, 34, 35, 36], I have suggested [37, 38] 

 that the primary electron transfer act involves reduction of chlorophyll by 

 the iron haem protein complex, resulting in a reduced chlorophyll- 

 chlorophyll couple on the one hand (£",; ~ — i-o V.) and an oxidized- 

 reduced haem protein couple on the other (Fig. i). The potential developed 

 depends on whether the oxidation of the central iron atom proceeds to a 

 formal valence state of three positive, or whether it goes to a higher 

 effective valence ( + 4 or +5, as in catalytic processes catalyzed by haem 

 protein). In the former case, E^ will vary from ~ o to + o • 3 V. In the latter, 

 it may rise as high as + i -o V. There is insufficient energy in the infrared 

 quanta (~ 1-3 V.) effective in bacterial photosynthesis to provide the gap 

 created by the reduced chlorophyll and oxidized Fe^ ^ or Fe' + systems, 

 which are separated by ~ i -7 to 2 -o V., depending on what potentials are 

 assumed for the reduced chlorophyll. Hence, it is not expected that the 

 haem protein in the purple photosynthetic bacteria will be oxidized to a 

 valence state higher than 3 + , so that the high positive potential required 

 to liberate oxygen ( + o-8 V.) is not reached. In this way, we may account 

 for the absence of oxygen as a product in bacterial photosynthesis and for 

 the requirement of an added H-donor, other than water. On this view 



