406 PHOTOMECHANICAL CONSIDERATIONS 



presence of Mg"*" , this site should occur somewhere in the conjugated 

 ring structure or its additives. 



We thus see that some of the necessary conditions for bacterio- 

 chlorophyll to function as photochemically active molecules in photo- 

 synthesis, namely, the capacity to act as electron donor and electron 

 acceptor at different sites in the molecule, may be deduced from the 

 experiments with bacteriochlorophyll in organic solvents. 



A highly schematic comparison of chemioxidation, photooxidation 

 and photo reduction and their possible function in vivo is suggested in 

 Fig, 8. In this figure an electron is assumed to be removed from the 

 ground state of bacteriochlorophyll either by strong oxidants (Fig. 8: 1) 

 or by light absorption (Fig. 8: 2-5), The ground state is filled by "high 

 energy" electrons under emission of luminescence in Fig. 8: 2, 3 and 

 4. 



In the natural state, the electron expelled by chlorophyll is used to 

 reduce some compound of relatively high redox potential (e.g., cyt. b, 

 DPN or other compound), while the ground state is filled with an elec- 

 tron from cytochrome c. A close connection is assumed to exist be- 

 tween bacteriochlorophyll and the electron donor at one site— Chance 

 and Nishimura (13) found cytochrome c to be oxidised by bacterio- 

 chlorophyll even at liquid nitrogen temperature— and bacteriochloro- 

 phyll and the electron acceptor at the other site. If cytochrome c is in 

 the oxidised state, it cannot provide electrons to the ground state of 

 bacteriochlorophyll and during illumination a stationary concentration 

 of BChl+ will exist. After illumination, back reactions will restore the 

 spectrum under emission of luminescence. 



If reduced cytochrome c is present, it will donate electrons to the 

 ground state of bacteriochlorophyll, resulting in a very low stationary 

 concentration of BChl"*" during illumination, while back reactions are 

 low in number; hence, afterglow is low. Emission of luminescence of 

 photosynthetic bacteria indeed was found to be manyfold enhanced by 

 the addition of quinone, ferricyanide or hydrogen peroxide, which 

 addition is assumed to result in oxidation of cytochrome c. 



A continuous conversion of radiation energy into electron energy 

 by oxidation- reduction is unlikely in organic solvents. To obtain such 

 a storage, a solvent should be chosen which facilitates both the leaving 

 of an electron from the excited state at the Mg-site and an entering of 

 the electron from an electron donor of lower energy into the ground 

 state at the siteof the ring structure. However, even in such a hypothe- 

 tical solvent the random movement of molecules in solution results 

 in a reentry by predilection of the electrons of higher energy, and 

 thus in a "short-circuit" of the pigment molecule. This "short- 

 circuit" will be prevented only if the pigment is attached to some 

 structure of highly specialised biological organisation. There are var- 

 ious indications that the bacteriochlorophyll molecules in vivo are 

 attached to proteins (cf. e.g., Bril , 14). In the bacteria, nearly all 



