MELVIN CALVIN 331 



constitute some energy storage. One of these is the reaction (1, 10, 

 54) 



thionine + Fe++ -^ leucothionine + Fe"^''"'' 



dark 



Here also the energy storage is small (~ 5 kcal per quantum) at best. 



Photophysical Effects in Model Systems 



Energy Transfer in Solid Systems 



The failure of all of these various types of solution model reac- 

 tions to provide a case where energy of the order of 20 to 40 kcal 

 per quantum is being stored is in itself significant. I think it may 

 be demonstrated that this is not the direction in which to look for 

 the energy-storing reaction in photosynthesis. Some years back, as 

 the structure of the chloroplast became somewhat clearer to us (pri- 

 marily through electron microscopy) (28, 66) and as our knowledge 

 of the photochemical or photophysical behavior of ordered systems 

 developed in the form of a body of theory and information on the 

 photoresponse of atomic crystals (31, 70), the notion that the photo- 

 synthetic apparatus might not be functioning as ordinary molecules 

 in solution, but rather as something approaching molecular crystal 

 behavior, became popular (13, 67). 



We undertook to seek possible models for such systems in the 

 laboratory. In addition to the electron microscopy on the chloro- 

 plasts, there was, of course, the very well-known fact that the absorp- 

 tion spectrum of chlorophyll in the living organism is not identical 

 with the absorption spectrum of chlorophyll in solution. This, to- 

 gether with the ordered structure that was seen in the electron micro- 

 scope, suggested that the chlorophyll in the living organism might 

 be in a physical form quite different from a true solution. The dif- 

 ference in the spectra is quite obvious. The solution spectrum of 

 chlorophyll has a peak at about 6600 A and in the living organism 

 chlorophyll has its peak somewhere near 6800 A, This shift toward 

 a longer wavelength, from 6600 to 6800 A, is exactly the kind of 

 shift observed in the spectra of all sorts of /7i-molecules when they 

 are packed in crystals. When the p/clouds of large, conjugated sys- 

 tems are brought close together, there is an interaction which shifts 

 the energy of the excited state (or the difference between the ground 

 and the excited state) . This is quite common in all pi-molecules, 

 and in chlorophyll it has been examined by Rabinowitch and by 



