284 MARTIN D. KAMEN 



there is a cyclic process involving first photo-oxidation of haem, then 

 thermal reduction by reducing equivalents supplied from the H-donor 

 through a chain of intermediates. 



The bases for these suggestions may be reviewed briefly. We know 

 from a voluminous literature on electron transfer processes in systems 

 containing organic metal conjugates or chelates, that the presence of a 

 macrocyclic resonating system can induce rapid electron exchange between 

 ions otherwise shielded by solvent [39, 40]. In all photoactive systems a 

 situation exists in which an efficient resonating macrocyclic system — 

 porphyrin or a derivative reduced porphyrin ring — is chelated to mag- 

 nesium or iron as the central metal ion. If we suppose that the magnesium 

 chelate (chlorin) is close to the iron chelate (haem), then excitation of the 



C /e C, ,Fe JZ ,Fe 



— C N OH' ^-^ -C— N OH- ^°'^ — C N OH" 



I / I / I / 



C Mq''"' -C Mg' 



-C N 



/ I / 



Fig. I. Electron transfer reaction proposed as part of the primary photo- 

 chemical process in photosynthesis. 



magnesium chelate by a photon which gives rise to the characteristic red 

 absorption band will result in an excited chlorophyll system with energy 

 equivalent ~ i-8 to 2-0 e.V. above the ground state. De-excitation can 

 occur immediately by electron transfer from the neighbouring haem 

 system. If the iron complex is one which is originally in the formal valence 

 state of Fe" +, it will be oxidized to a formal valence of Fe" +^. Likewise, 

 the chlorophyll acquires an excess negative charge which makes it equiva- 

 lent to a "semichlorinogen " (see Fig. i). 



This process, which most probably leaves both products in their ground 

 states, results in two systems separated in energy content by an amount 

 close to the original energy of excitation of chlorophyll, the "mid- 

 point" potential of the semichlorinogen system is more reducing than 

 that of the oxidized haem system by ~ i -8 e.V. Stabilization against back 

 reaction may require 0-2-0-3 e.V., so that we may assume safely a maxi- 

 mum of ~ I • 5 e.V. available for the spread in potential. 



