328 LIGHT AND LIFE 



between chlorophyll and protochlorophyll) which has to be returned 

 to the chlorin stage in order for the reaction to continue. The hy- 

 drogen for this return must ultimately come from water, knowing 

 what we do about the stoichiometry of the photosynthetic reaction. 

 The energy of getting hydrogen from water is -(-56 kcal. This re- 

 action can now be combined with the hydrogenation of the porphyrin 

 to the dihydroporphyrin: 



H2O -> H2 + 1/2 O2 AH ~ +56 kcal (4) 



porphin -f H2O — > 



chlorin + V2 O. AH ^ +26 kcal (5) 



The photolysis of water has now been divided into two steps, (5) 

 and (5) , one of which is the transfer of hydrogen to the porphyrin, 

 if the reaction is running between porphyrin and chlorin (5) . The 

 calculations for the two reactions between chlorophyll and bacterio- 

 chlorophyll would be exactly the same as far as our precision is con- 

 cerned. The total energy required for the reaction of two electrons 

 — the generation of one-half mole of oxygen — has now been divided 

 into two approximately equal parts, with one reaction "uphill" 

 approximately 20 kcal and the second one approximately 26 kcal. 



In actual fact, this half mole of oxygen probably does not come 

 off directly as molecular oxygen but goes through some oxygen ac- 

 ceptor species, which then goes on to molecular oxygen. Reaction 

 (5) would thus be broken into several steps, the size of which would 

 depend on the nature of the unknown oxygen carrier. 



Our model reactions have indeed shown that hydrogen can be 

 transferred from dihydroporjihyrin to an acceptor. Unfortunately, 

 this particular hydrogen acceptor — quinone — is a very good one (not 

 as poor a hydrogen acceptor as pyridine nucleotide) , so it does not 

 really correspond to such a transformation as reaction (5) with a 

 -\-/\H. The fact remains, however, that light will produce an excited 

 state which induces the transfer of these hydrogen atoms to an ac- 

 ce[)tor. The reduction of the double bond has been achieved only 

 with nujch better hydrogen donors than water. They have always 

 been relatively good reducing agents, such as ascorbic acid and a 

 variety of other ene-diols or hydrazines which are the common 

 materials used for this kind of transformation. 



In liut, the j)hotochemistry of chlorophyll models, and chloroj)hyll 

 itself for that matter, has not yet prockued iu solution a model re- 

 action in which such reactions as those described above, which in- 

 volve relatively hirge energy storage acts, have been accomplished. 



