1586 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



spectroscopic, observations of the Hill reaction). Mehler reported that 

 Tolmach (working in the same laboratory) could observe photochemical 

 oxygen liberation from the cytochrome-chloroplast mixture by using azide 

 instead of cyanide as poison for the cytochrome oxidase. 



In chapter 37A we will describe the finding, in chlorophyll bearing plant 

 tissues, of a new iron-porphyrin-protein compound, designated as ''cyto- 

 chrome/" (Davenport and Hill 1952). It has a normal redox potential of 

 — 0.37 volt (at pU 6-8; Eo' increases at the higher pH values by 0.06 volt 

 per pH unit). Compared with the normal redox potential of cytochrome 

 c (Eo' = —0.26 volt), cytochrome / is a stronger oxidant. Hill (1951) and 

 Davenport and Hill (1953) pointed out that the difference between the 

 normal potentials of cytochrome / and of the oxygen electrode at pH 7 

 (_0.81 volt) is 0.44 volt, or 1 1.2 kcal/mole. The free energy of the reac- 

 tion: 



(35.34B) 2H2O +4Cyt/Fe + + + > 4 H+ + 4Cyt/Fe + + + O2 



is therefore 44.8 kcal— or close to the energy of a red quantum. Daven- 

 port and Hill suggested that this reaction may represent the primary photo- 

 chemical process in photosynthesis (with further increase in reduction po- 

 tential achieved by an energy dismutation mechanism, as repeatedly dis- 

 cussed before). However, the transfer of four electrons by one quantum is 

 an implausible mechanism. Furthermore, what matters for the possibility 

 of a photochemical electron transfer is not the free energy change, AF, re- 

 sulting from a similar transfer by the "slow" thermal mechanism, but the 

 total energy change, AH*, required for an instantaneous transfer (with all 

 nuclei held more or less rigidly in position, in approximate accordance with 

 the Franck-Condon principle). Since these reactions involve a change in 

 ionic charges, the two energy values, AF and AH*, may be quite different. 

 (The AF of such reactions includes changes in the ion-dipole interaction 

 with the medium— which are different for sudden and gradual transfer— 

 and large entropy changes produced by liberation — or immobilization — of 

 dipole molecules around the ions whose charges had been increased or de- 

 creased.) Not much significance could therefore be attached to the simi- 

 larity between the standard free energy of the thermal reaction (35.34B), 

 and the energy content of a red quantum, even if it were not for the improb- 

 ability of an elementary photochemical process involving simultaneous 

 transfer of 4 electrons. We conclude that if cytochrome / molecules do 

 serve as intermediates in photosynthesis, they must do so by accepting 

 single electrons transferred from single H2O molecules by single quanta 

 (which implies the loss of much more than one-half of the quantum energ\') ; 

 alternatively, they could play a role in the thermal back reaction (which 



