CHEMTLUMTNESCEXCE OF ALGAE 139 



detail. It is perhaps sufficient to quote from an abstract composed by 

 Professor Ftanck. He prepared tiiese remarks before his illness made 

 it impossible for him to be here. Professor Franck wrote as follows: 



"Our theory of the photochemical part of photosynthesis presup- 

 poses a main process of photosynthesis in which the energy of two 

 absorption acts is utilized for the transfer of one hydrogen atom to the 

 photosynthetic oxidant PGA and the simultaneous transfer of a 

 hydroxvl to an enzyme. There is a minor process in which DPGA is 

 reduced to -PGAH and phosphate whereby the energy of only one 

 absorption act is used.* The early products of this process are 

 the -PGAH radical and the chlorophyll radical which has lost one 

 H-atom in ring Y and is connected with two hydroxyls bound to 

 Cg. The latter radical is a potential OH donor as long as the 

 OH-carrying enzyme has not removed one of the hydroxyls. This 

 reaction is slow because a relatively high heat of activation is needed . 

 Thus the potential OH donor has a much longer lifetime than the 

 corresponding products of the main process. Back reactions between 

 the potential OH donor of the second process and a -PGAH radical 

 will therefore occur relatively often even if enough active enzyme for 

 the removal of OH is available. The particular back reaction as- 

 sumed is a removal of the H which the PGA has gained in the for- 

 ward process by a reaction with one of the hydroxyls of the chloro- 

 phyll radical. This back reaction results in the re-formation of PGA 

 and of chlorophyll in the enol state. The energy released by the re- 

 action is quite sufficient to excite the chlorophyll. Other factors very 

 favorable for the occurrence of chemiluminescence are that the 

 back reaction must occur in direct contact with chlorophyll and that 

 most probably the enol chlorophyll has a higher fluorescence yield 

 than the keto chlorophyll. 



"Certainly back reactions will also occur between the two radicals 

 PGAH- and HO-Enz- of process I, especially, if by addition of in- 

 hibitors, their lifetimes are prolonged. How^ever, these back reactions 

 have little chance to excite the chlorophyll because they will occur 

 everywhere in the solution and not just in contact with chlorophyll. 

 This interpretation can explain the differences of the chemilumines- 

 cence observed directly after irradiation and after a dark pause of 

 two-tenths of a second. It is further in agreement with observations 



* Compare addendum to "A Theorj- of the Photochemical Part of Photoaj'ii- 

 thesis" on page 142. 



