Evolution of Photosynthetic Mechanisms 41 



find a way of removing the electron from the iron, not with hydrogen 

 peroxide but with light, in order to couple the photochemical reac- 

 tion to what we now know to be nonphotochemical processes. 



I think this event happened very late in the evolutionary scheme, 

 and the evidence for it lies in the fact that the chlorophyll molecule 

 is today manufactured by a sequence of reactions almost identical 

 with the sequence of reactions used to manufacture the heme, but just 

 before the iron is put into the heme, a branching occurs, leading not 

 to heme but to the chlorophyll molecule in which magnesium is situ- 

 ated (Fig. 14 ) . I think the reason for that reaction is, first, that the 



CH=CH 2 CH 3 



CH 2 H CH 2 



CH 2 CH 2 



C0 2 H C0 2 H 



HEME (os in hemoglobin and 

 cytochrome) 



Fe -PROTOPORPHYRIN NO. 9 



-> HC 



CH 2 CO2CH3 



C0 2 C 2 oH39 



CHLOROPHYLL a 



FIG. 14. Structural relations between heme and chlorophyll. 



light-absorbing ability of the heme itself is very poor. Although heme 

 is red, it does not have anywhere near the light-absorbing capacity 

 of chlorophyll, and one of the reasons for the evolutionary selection 

 of magnesium chlorophyll (magnesium chlorin) is the fact that the 

 absorption of light by a magnesium chlorin is several thousand times 

 greater than that of the iron porphyrin. Secondly, something very 

 special about the electronic structure of the magnesium and of the 

 packing together of the chlorophyll molecules in a crystal lattice, 

 leading to the separation of electrons from the chlorophyll, 11 is 

 better achieved by the chlorin than it is by the porphyrin. Finally, if 

 the dehydration-phosphate activation idea (by the 9-10 enol of 



