860 LIGHT AND LIFE 



tetrahydro- and even a hexahydro-porphyrin state; and with more 

 readiness than the initial stage of reduction. This resuk strengthened 

 the tentative view that chlorophyll might lunction by shifting to and 

 from the bacteriochlorophyll condition. Thermodynamically, the shift 

 between chlorin and porphin (or more probably between the tetrahy- 

 dro and dihydro forms) in the reduction of TPN will require a AH 

 of about +20 kilocalories, and the photolysis of water to produce a 

 reduced porphyrin and molecular oxygen will require about +26 

 kilocalories. The fact is that light will bring about a transfer of hydro- 

 gen atoms to a pyridine nucleotide acceptor, but in the model systems 

 so far studied only a transfer to a very good acceptor, such as a quinone, 

 has been obtained, and not one to so relatively poor a hydrogen ac- 

 ceptor as TPN. Moreover, reduction has only been accomplished with 

 much better hydrogen donors than water. A model that will yield, 

 in solution, similar reactions involving large energy storage is still 

 lacking. Classic examples are the transfers of hydrogen from ascorbic 

 acid or hydrazine to acceptors such as azo dyes (methyl red). Krasnov- 

 skii has studied these reactions extensively in the post-war period. 

 He demonstrated that ascorbic acid reduces chlorophyll to some inter- 

 mediate form, detectable by a pink color, and this intermediate will 

 in turn reduce a hydrogen acceptor and revert to green chlorophyll. 

 But this type of reaction is thermodynamically "downhill" and does 

 not involve any storage of free energy. Nor, even in those cases where 

 the reactions are to some extent photodynamically reversible, is there 

 any extensive storage of free energy. 



These difficulties have directed many students of photosynthesis to 

 consider the possibility of an energy transfer like that in solid systems. 

 The structure of the chloroplast may be resembling that of molecular 

 crystals. The absorption spectrum of chlorophyll in vivo (with a peak 

 at 6800 A) is not that of chlorophyll after extraction (with a peak at 

 6600 A); and that fact, plus the orderly arrangement of structure 

 within the chloroplast to be seen in electron micrographs, led to the 

 thought of photoconductivity like that in solid lattices. This shift is 

 to be expected in molecules with vr-electrons when packed into crys- 

 tals, for the interaction between the TT-clouds brings about a shift in 

 the energy state toward longer wavelengths. In "crystalline" chloro- 

 phyll the peak moves even to 7200 A. 



Hunting for a convenient model of such a system to study, Calvin 

 and his group selected phthalocyanine, a tetrapyrrole connected by 

 nitrogen bridging atoms and bearing benzene rings on each pyrrole 

 ring (Fig. 6). Crystalline phthalocyanine laid down in a layer with a 



