General Remarks on Chlorophyll- Sensitized 

 Photochemical Reactions in Vitro 



JAMES FRAXCK, University of Chicago, Chicago, Illinois, as pre- 

 sented by R. Livingston /rom an abstract 



The discussion of photochemical reactions sensitized by chlorophyll 

 in vitro shows that the only reactions which proceed with high quantum 

 yields are exothermic or slightly endothermic. More endothermic re- 

 actions have very small yields because these recjuire that the bulk of 

 the excitation energy of the chlorophyll be stored as chemical energy. 

 In the process of sensitization, the excitation energy must be greater 

 than the sum of the heat of activation and the chemically stored 

 energy. Keeping this in mind, one finds it astonishing that the latter 

 type of reaction takes place. However, the difficulty vanishes if one 

 assumes that, for the first photochemical step, the energy of two 

 quanta rather than of one quantum is utilized. The transfer of a 

 hydrogen atom from the chlorophyll to a weak oxidant is an example. 

 We suppose that the hydrogen to be transferred is the one bound to 

 Cio in ring V of chlorophyll. The excitation energy of 41 kcal. (first 

 excited singlet state) permits the transfer only to a strong (not to a 

 weak) oxidant. When chlorophyll is in the lowest triplet state (corre- 

 sponding to approximately 30 kcal.), even the transfer to oxidants as 

 strong as molecular oxygen or quinone in one act becomes energetically 

 impossible. (If one considers an electron transfer instead of an H-atom 

 transfer, the energy situation becomes still more unfavorable.) There 

 is a possibility that one hydrogen atom may be transferred in two 

 chemical steps. However, the assumption which best fits the kinetic 

 data for reactions in vivo is that two quanta are utihzed to excite one 

 chlorophyll molecule into an excited triplet state, whereby between 60 

 and 70 kcal. become available for one photochemical act — in our case 

 the transfer of one hydrogen atom. 



That can be achieved by the following steps: (1) Excitation of a 

 chlorophyll molecule by light absorption into the first excited singlet 

 state. (2) Internal transition into the lowest metastable, long-lived 

 triplet state. (3) Excitation of the chlorophyll in its metastable triplet 



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