MELVIN CALVIN 



319 



ship can also be seen in Fig. 1. There are two more "extra" hydro- 

 gens in bacteriochlorophyll, presumably on a pyrrole ring on a 

 diagonal from the hydrogen-bearing one in chlorophyll; and on the 

 No. 1 pyrrole ring the vinyl group has been transformed into an 

 acetyl group. The protochlorophyll (Fig. 1) , which is the material 

 formed in plants when they are grown in the absence of light, can 

 be recognized as the dehydrochlorophyll (a porphin) . The No. 4 

 pyrrole ring in protochlorophyll has a double bond in it, and it 

 has been sho^vn by a number of workers that the first thing that 

 happens in etiolated plants (which have no chlorophyll in them) 

 when the light is turned on, is the addition of the two hydrogens 

 to the double bond in ring No. 4 to generate chlorophyll. 



CH2 CHj 



CH H CHj 6h 



HC M, CH 



I — n' n — I 



9^ H-C C^O 



9^ iOjCHj 



COgCzoHsg 



PROTOCHLOROPHYLL 



^^'H^ H^ ^- 



-C=0 



(J;h2 cozCHj 

 CO2C20H39 



chlorophyll a (b) 



CHz-CHj 



9^ CO2CH3 

 COzCzoHss 



BACTERIOCHLOROPHYLL 



Fig. I. Stnictiiial formulas of chlorophyll, bacteriochlorophyll, and 



protochlorophyll. 



An examination of these three formulas shows this relationship of 

 chlorophyll to protochlorophyll and bacteriochlorophyll very neatly. 

 It shows that the macrocycle of chlorophyll lies midway in the oxida- 

 tion level between that of protochlorophyll and that of bacteriochloro- 

 phyll. With the diffusion of the idea of Van Niel (72) that the primary 

 photochemical reaction of green plants involved the fission, or split- 

 ting, of the water molecule to give hydrogen and oxygen, or to give 

 a reducing agent and an oxidizing agent, and that this reducing 

 agent was used to reduce carbon dioxide while the oxidizing agent 

 actually generated oxygen, it occurred to me that the function of 

 the chlorophyll might be as a hydrogen carrier from the water toward 

 the ultimate reducing agent which is used to reduce carbon dioxide. 

 Today we believe one of these ultimate reducing agents to be pyri- 

 dine nucleotide. It seemed likely that the chlorophyll might be 

 functioning between the stage of chlorophyll and protochlorophyll. 



