HAEM PROTEIN CONTENT AND FUNCTION IN RELATION TO STRUCTURE 287 



In terms of the chromatophore structure, we may visualize an aggregate 

 of bacteriochlorophyll molecules [15] together with the accessory pigments 

 such as carotenoids, which for the most part are not attached to molecules 

 with which thev can undergo irreversible electron transfer reactions upon 

 excitation. Most of these chlorophylls upon excitation merely transfer 

 energy by some obligatory mechanism, such as inductive resonance. 

 Migration of the energy quantum proceeds through the pigment aggregate 

 until a particular chlorophyll molecule is reached which can be de- 

 excited by electron transfer in such a way as to produce the two electro- 

 chemical svstems postulated above.* 



Of all the molecules mentioned as analogues of the respiratory chain 

 previously, the most plausible reactants which can produce both highly 

 positive electrochemical systems while affording the possibility of stabiliza- 

 tion are the haem proteins. They possess the necessary electron source — 

 the metal atom — the necessary protein component for close coupling to the 

 chlorophyll and the porphyrin ring for stabilization. Hence, we may 

 assume it is the haem protein that reacts with the excited chlorophyll, 

 rather than a quinone, a flavin, or a pyridine nucleotide. 



The rest of the reaction sequence requires that back reaction between 

 the reduced chlorophvU and the oxidized haem be slow relative to the 

 reduction by reduced chlorophyll of pyridine nucleotide or some other 

 H-acceptor. This, as mentioned above, may be the role of the "photo- 

 nucleotide reductase" discovered by San Pietro in chloroplasts. A similar 

 enzyme may exist in bacterial chromatophores, but so far has not been 

 found. It may be that the H-acceptor in the bacteria is not a pyridine 

 nucleotide, but rather a SH-compound. The presence of large quantities 

 of the yellow flavin enzyme which can not only function as a haem protein 

 reductase, but also can show very great diaphorase activity [27] suggests 

 that some SH-compound may be involved; on the basis of the recent 

 demonstrations by Massey [50] and by Sanadi and Searls [51] regarding 

 the possible coupling of SH-groups to flavin in diaphorase, it seems 



* A possibility is that such a reactive site is chlorophyll dimer. S. S. Brody (see 

 Science 128, 835 (1958), also Brody, S. S. and Brody, M., Arch. Biochem. Biophys. 

 82, 161 (1959)) have shown that in many plant systems an appreciable fraction of 

 the chlorophyll is in the form of a non-fluorescent dimer. While it is not clear how 

 such a complex could react to give two systems sufficiently stable and separated 

 by a sufficient equivalent voltage, participation of such dimers in photochemistry 

 certainly is not excluded. 



It is also of interest that on the basis of an approach based wholly on analysis 

 of fluorescence depolarization, G. Weber (see ref. [37], p. 408) has arrived at a 

 scheme for the energy conversion mechanism in photosynthesis which is similar 

 to the one proposed in this report in requiring resonance transfer to bring an 

 excited electron in a chlorophyll singlet in contact with an electron donor. In later 

 steps, he postulates separation and transfer of an electron from the chlorophyll- 

 donor complex to an electron acceptor. 



