INTRODUCTORY COMMENTS 

 James Franck 



Dcl'tii ttniiit oj Clifinistry, Uniiiersity of Chicago (Fels Fund), 



Chicago, Illinois 



Apparently it is expected that the moderator makes a few general 

 introductory remarks, connected with the topics of the session. My 

 own experience on excited states of molecides of biological interest 

 concerns only those molecules which are important in the process of 

 photosynthesis, i.e., the pigments of the chloroplasts, which, together 

 with certain prosthetic groups, are adsorbed at proteins. According to 

 views often expressed, the property of being a semi-conductor is as- 

 cribed to protein, and is assumed to be utilized in photosynthesis. Elec- 

 trons liberated by photo-ionization of chlorophyll molecules are sup- 

 ])osed to migrate through the protein to certain reaction centers. Ac- 

 cording to my own opinion, protein molecules are not semiconductors. 

 All physical evidence available indicates that energetically low-lying 

 conductivity bands are absent in protein. We may refer, e.g., to the 

 survey on this subject matter written by Michael Kasha in the Review 

 of Modern Physics, 1959. Observations on migration of energy through 

 protein can be explained better as excitation energy transfer. Buecher 

 observed that in the hemoglobin-CO complex, energy absorbed any- 

 where in the protein can be utilized for photochemical splitting of 

 the complex. For many years this was regarded by biochemists as 

 convincing proof for the occurrence of electron migration through 

 protein. Actually, it has all the earmarks of migration of excitation 

 energy. The two ways of excitation energy transfer, 1) the phenome- 

 non of sensitized fluorescence, and 2) that of exciton movement in 

 crystalline and quasi-crystalline arrays of molecules, are known for 

 nearly 40, resp., nearly 30 years. It needed time and many newer 

 observations with complex organic molecules before the biochemists 

 began to consider that these processes may be of importance for an 

 understanding of certain biological problems. According to my view, 

 their role in photosynthesis is still under-estimated. First, they pro- 

 vide the mechanism by which light energy absorbed by all plant pig- 

 ments is transferred to the one \vhose absorption spectrum extends 

 farthest to long wavelengths. Secondly, the same mechanism is utilized 

 for the transfer in the particular dye itself, from the region of its own 

 absorption act to the few places where the excitation energy is utilized 

 for photochemical reactions. 



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