326 



LIGHT AND LIFE 



can then go further, into a tetrahydroporphyrin, which is a very 

 good reaction compared to the first one. This result was in keeping 

 with the notion that perhaps chlorophyll in the green plant was 

 functioning not between chlorophyll and protochlorophyll but be- 

 tween the chlorophyll and dihydro- or bacteriochlorophyll. The 

 latter reaction does not stop at the tetrahydroporphyrin but under 

 vigorous conditions it can be pushed to the hexahydroporphyrin. 



The reverse reaction, namely, tetrahydroporphyrin and quinone, 

 goes directly to porphin. The dihydro-form is not observed in be- 

 tween. The reaction of chlorin is much faster, so that the accimiula- 



benzoin or dihydroxyocetone, base, light; 



PORPHIN 



variable yield 



:hlorin 



Quinones; light 



Quinoncs; light; high yield 



benzoin, r^o bose; light 



low yield 



oxygen; dork; high yield 



high yield 

 Oxygen; dork; low yield 



TETRAHYDROPORPHIN 



benzoin or dihydroxyocetone 



light; high yield 



benzoin, benzene plus 1/2% piperidine; 

 light; variable yield 



HEXAHYDROPORPHIN <- 



Fig. 7. Redox relations among the zinc tetraphenylporphyiins. 



tion of chlorin is not seen. The hexahydro-form will auto-oxidize, 

 even in the dark, with oxygen to give the tetrahydroporphyrin very 

 readily. 



Thermodynamic Relationships 



The question arises as to what indeed are the various energy levels 

 of these porphyrins with respect to water, pyridine nucleotide, oxygen, 

 etc., the various molecides which are involved in the process of 

 photosynthesis itself. There is no direct and unequivocal information 

 about the energy of these various transformations, primarily because 

 the energy of hydrogenation of porj)hyrin to chlorin, or chlorin to 

 dihydrochlorin, is not known. Only indirect information is available 

 about this, and one must deduce, by indirection, what these energies 

 might be. It is interesting to note what is evolved if one makes the 



