PHOTOCHEMISTRY OF BACTERIOCHLOROPHYLL 



407 



10 Mg 



— •from Fe" to O. 

 or a.o. 



Organic solvents. 



via Mg vio Mg 



Photochemicol oxidati^ 



'o structure 



'la Mg 



vio ring structure 



- ■• — • from cytc,^jj (substr 



Photosynthesis 



Fig. 8. Hypothetical energy scheme of oxidations and reductions of bacterio- 

 chlorophyll in vitro and hi vivo. 



1. An electron is removed from the ground state of the pigment by a strongly 

 oxidising agent, leaving oxidised bacteriochlorophyll. The bacteriochlorophyll 

 spectrum is restored by a feeding of electrons into the groimd state from a 

 reductant, added in excess. This restoration coincides with emission of chemi- 

 luminescence. A similar picture holds for bacteriochlorophyll in chromato- 

 phores. As this reaction does not proceed with pheophytin, it is assimied to be 

 mediated by the Mg ion. 



2. An electron is removed from the ground state by light absorption. The ex- 

 cited bacteriochlorophyll molecule reacts withox>'gen. Addition of ascorbic acid 

 results in oxidation of thiscompoimd and reformation of excited bacteriochloro- 

 phyll, resulting in luminescence. 



3. The electron removed from the ground state by light absorption is assumed 

 not to be able to leave the bacteriochlorophyll molecule in the solvent used 

 (pyridine) in the absence of oxygen. It thus lingers some time in the first excited 

 triplet or singlet state. During this time an electron from ascorbic acid is as- 

 sumed to be able to enter the ground state, resulting in a reduced pigment 

 molecule. As this reaction occurs also with pheophytin, the site of entry is 

 assumed to be somewhere in the ring structure. 



4. An electron excited by light may leave the pigment molecule to an electron 

 acceptor situated close to the molecule, and from there to DPN. Oxidised bac- 

 teriochlorophyll is left over if the empty space in the groimd state is not im- 

 mediately filled by a closely adjacent cytochrome molecule, for example, if 

 cyt. c is oxidised. The spectrum is restored under emission of luminescence 

 by back reaction from the high energy compound. 



5. Under photosynthesising conditions, when cyt. c is kept reduced by the sub- 

 strate, the stationary concentration of BChl"*" is very low, coinciding with a very 

 low luminescence. 



