874 LIGHT AND LIFE 



heme proteins in photosynthesis may be related to the resonance 

 properties ot the similar tetrapyrrole structures ot the chlorophyll 

 and cytochrome molecules. If these structures are juxtaposed so that 

 the magnesium of the chlorin lies close to the iron of the heme, the 

 excitation of chlorophyll by absorption of a photon, leading to an 

 excited singlet or triplet state, might be relieved by the transfer of an 

 electron from the heme. [In discussion, Linschitz described studies of 

 the quenching of triplet states, e.g., in porphyrin, by metal ions. 

 Generally, complexing of the metal ion effectively blocks such quench- 

 ing, but not when the metal is in a porphyrin complex. This work is 

 being extended to study the quenching of chlorophyll triplet states 

 by other porphyrins.] This electron transfer from heme to chloro- 

 phyll would produce a partially reduced chlorophyll radical and a 

 "partially oxidized" heme (Fe+ + + + ), most likely in their ground 

 states. If so, the oxidized form of heme would probably be at least 

 1.5 e.v. more oxidizing than the semi-reduced chlorinogen radical, 

 provided the energy difference is close to that of the original excitation 

 of chlorophyll. Kamen supposes that the heme in the higher oxidized 

 state might have a potential of +0.9, and hence the semichlorinogen 

 one of —0.6 or —0.7 e.v. So strong a reducing potential could readily 

 provide for the required electron transfer step to pyridine nucleotide 

 (£'o = —0.3) , and photophosphorylation might thus be coupled to 

 generation of TPNH from the photoreductant, here identified as the 

 semichlorinogen radical. If substrate amounts of the pyridine nucleo- 

 tide are present and are funneled off to provide for CO2 fixation, 

 the Fe+ + + + — heme component could withdraw electrons from other 

 components of the system, and so lead to oxygen evolution. One would 

 need to assume that in the photosynthetic bacteria the protein moiety 

 of the heme protein could permit only peroxidase reactions, so that 

 oxygen would not be evolved, but oxidizable substrate would be 

 needed. In any case, cytochrome oxidase would have to be absent 

 in the oxidation chain, as indeed seems to be the case in all such sys- 

 tems. Various other possibilities can be envisaged. As an alternative 

 to the model set forward by Calvin, this ingenious but speculative pro- 

 posal has the merit of suggesting certain kinds of experimentation that 

 might otherwise not be carried out. In particular, a study of photo- 

 chemical reactions in chlorophyll-heme systems of various kinds might 

 be profitably carried out with the newer techniques for studying very 

 fast reactions and detecting very short-lived intermediates and radicals. 

 Lipid function in the chloroplast is surprisingly unknown, consider- 

 ing that about 30 per cent of the dry weight of chloroplasts consists 



