324 



LIGHT AND LIFE 



have accounted for the kinetic results. In this case the hydrogen ac- 

 ceptor is a very good oxidizing agent and the transfer does not entail 

 any energy storage. If the transfer of hydrogen could be demonstrated 

 from such a dihydroporphyrin to a more powerful reducing agent, 

 that is, a molecule (viz., pyridine nucleotide) which in its reduced 

 form was more nearly like molecular hydrogen, perhaps something 

 of more direct interest to photosynthesis could be shown. 



The next question to be answered involved the possibility of doing 

 the reverse reaction, i.e., a transfer of hydrogen from something which 



EXCITATION 



C_ +hv 

 Zn 



^ C 



S" 

 Zn 



DEACTIVATION 

 OF SINGLET ■• 



DEACTIVATION < 

 OF triplet: 



K4 



,S' 

 "Zn 



"Zn 



4n * Q 



-» C7 (hv orkT) 



-* C 



'Zn 



R,_ + HoQ 



-* ^Zn ^ "2' 



-» C-, + Q + kl 

 Zn 



RATE LAW RESULTING FROM THESE STEPS •• 

 (C2^)+ yL_ log d-iQ-^'^Zn^'^) =rKa lot + n 



WHERE QUANTUM YIELD, r, EQUALS '• 



kg+k^ kj+kg 



KINETIC ANALYSIS OF PHOTOXIDATION OF Zn TETRAPHENYLCHLORIN 

 Fig. 5. Kinetic analysis of pholooxidation of zinc tetiaplicnylchloiin. 



clearly was not as good a redu( ing agent as the dihydroporphyrin to the 

 porphyrin to make the dihydroj^orphyrin; this actually would involve 

 a storage of energy (58) . Fig. () shows the results of such an experi- 

 ment. Here zinc tetraphenylporphyrin is being reduced by benzoin, 

 which is an ene-diol resembling ascorbic acid in some respect. The 

 solid line in Fig. 6, ujiper, is the spectrum of the porphyrin, and after 

 7 minutes of illumination the porphyrin is dropping and the chlorin 

 is coming in. After one and one-half hours of illumination, most of 

 the dye is in the form of the chlorin and most of the porphyrin is 

 gone. The quantum yield of this reaction was extremely small, much 



