MEW IN CALVIN 327 



best estimates one can about the energies involved in these 

 transformations. 



In the conversion of a porphyrin to a chlorin, a double bond is 

 hydrogenated to give the dihydro compound. In doing so, the con- 

 jugated macrocycle is not destroyed. One such double bond can be 

 removed Avithout destroying the conjugated macrocycle, and in bac- 

 teriochlorophyll, two such double bonds can be removed and a con- 

 jugated macrocycle still exists. Therefore, as a first approximation, 

 I woidd suggest that we use, in attempting the thermodynamic esti- 

 mate, a A^ for this reaction of about — 30 kcal, which is approxi- 

 mately that of a substituted olefin. ^ 



porphin + H2 — > chlorin A// ^^ —30 kcal (7) 



In order to estimate the energy for the hydrogenation of the pyridine 

 nucleotide, one must remember that the free energy of this reaction 

 is very nearly zero, that is, the reduction j^otential of TPN is very 

 nearly the same as that of molecular hydrogen. If the free energy 

 of this reaction is near zero, the heat will be equal to the entropy 

 loss, which is 9 kcal. 



pyridine nucleotide + H. -^- PNH2 ^H ~ -9 kcal {2) 



The reason lor this figure being so small is that a very large aromatic 

 resonance is destroyed, and that is why there is 20 kcal less energy 

 evolved in the hydrogenation of this material (TPN) than in the 

 hydrogenation of an ordinary olefin. 



Combining these two reactions, one can write for the hydrogen 

 transfer from chlorin to pyridine nucleotide, to give reduced pyridine 

 nucleotide: 



chlorin + TPN -^ 



or 

 (dihydrochlorin) 



porphin + TPNHo ^H ~ 20 kcal 

 or 

 (chlorin) {3) 



This is an "uphill" reaction of the order of 20 kcal. To complete the 

 cycle, the dihydroporphyrin has to be recovered. What is available 

 now is a porphyrin (protochlorophyll, if the reaction is rimning 



-'The relation of protochlorophyll to chlorophyll might vcrv well be materially 

 different due to the steric requirements of the isocyclic (C-,) ring. Here one might 

 expect somewhat less steric interference in the chlorophyll and thus the ^H might 

 he more negative. 



