PHOTOCHEMICAL PART OF PHOTOSYNTHESIS 143 



4. Transition of the chlorophyll from its metastable triplet state 

 to the first excited triplet state via the process of sensitized fluores- 

 cence. A molecule of chlorophyll in the first excited singlet state is 

 degraded to the ground state, while another molecule in the lowest 

 metastable state is raised to the first excited metastable state. 



5. Utilization of the stored excitation energy for the transfer 

 of the H atom bound to Cio to the PGA and of one of the 

 hydroxyls bound to Cg to the enzyme. The formation of a 

 double bond between Cg and Cio is part of this process and 

 leaves the chlorophyll in the ground state in its enol form. 



A number of enzymatic dark reactions follow the production of 

 the two radicals. We enumerate: {T) The chlorophyll must revert to 

 its hydrated keto form. (^) Several enzymatic reactions are con- 

 nected with the oxygen evolution. (5) The PGAH radicals are, in this 

 theory, supposed to dismute enzymatically into triose and PGA. 

 Considerable energy will be released in this process and can be used 

 for ATP production. The energy stored in ATP is supposed to be 

 utilized in the Calvin cycle, by which the carbon dioxide acceptor, 

 ribulose diphosphate, is made fro trimose. 



Practically the same process is postulated for the main course of 

 the reduction of Hill reagents. Since they are stronger oxidants, a 

 second process can contribute to their reduction to some degree. 

 The stronger oxidants, like quinone or oxygen, are quenchers of the 

 chlorophyll fluorescence. Since the concentrations of the Hill reagents 

 used in these experiments are low, the quenching impacts are not 

 frequent enough to reduce the fluorescence intensity by more than 

 20% to 30%. The quenching is due to the utihzation of the excitation 

 energy for an H transfer to the oxidant. The energy available in one 

 chlorophyll molecule in the singlet state is sufficient for this reaction. 

 Thus only two steps occur in this process: excitation of the chloro- 

 phyll to the first excited singlet state and transfer of an H atom to 

 the oxidant, for instance, to the quinone. The result will be the for- 

 mation of a semiquinone and of a chlorophyll radical, which still re- 

 tains both its hydroxyls. The energy needed to remove one of the 

 hydroxyls from this radical is small because this removal permits the 

 double bond between Cg and Cio to close. However, a heat of activation 

 is required for the transfer of one of the OH's to the enzyme. In photo- 

 synthesis, too, the second process may play a minor but significant 

 role. The energy relations will permit a hydrogen transfer in one act 



