PHOTOCHEMISTRY OF BACTERIOCHLOROPHYLL 405 



A chemiluminescence also occurs if chemicaHy bleached BChl+ is 

 reduced to chlorophyll by addition of excess ferrous ions or other re- 

 ductants. Such a chemiluminescence may be thought to be brought about 

 as follows. Removal of an electron from a chlorophyll molecule in its 

 ground state due to the electron attracting force of a ferric ion occurs 

 via the excited states. By a feeding of electrons back into BChl from 

 ferrous ions or ascorbic acid there is a certain probability that light 

 emission from the first excited state will occur with the spectral dis- 

 tribution of bacteriochlorophyll fluorescence. The emitted light energy 

 then is assumed to originate from the entropy difference existing be- 

 tween ferric and ferrous ions separated and mixed. The reversible 

 chemical oxidation then might be written as 



chemiluminescence 

 BChl + Fe^^"^ ' BChl' + Fe^"^^ ^^=^ BChl^ + Fe"^^ 



An electron from bacteriochlorophyll may reach its first excited 

 state by light absorption as well. If in this case an electron acceptor 

 is present which does not possess sufficient electron affinity to oxidise 

 bacteriochlorophyll in its ground state (and oxygen is rigourously 

 excluded), bacteriochlorophyll will be oxidised as a result of light 

 absorption and the electron acceptor reduced. Such experiments have 

 not yet been done with bacteriochloiophyll, but results of Linschitz 

 and Rennert (10) and Tollin and Green (11) with chlorophyll a-quinone 

 mixtures at low temperatures yield evidence for the occurrence of 

 such a mechanism in organic solution, 



Chemiluminescence and reversible absorption changes due to oxida- 

 tion of bacteriochlorophyll were not observed with the pheophytins. 

 This indicates that most probably electrons do not leave excited bac- 

 teriochlorophyll and chlorophyll a— whether excited by light absorption 

 or by the ferric ion— if the Mg ion is absent. 



REVERSIBLE PHOTOREDUCTION 



Reversible photo reduction, however, can be measured with the 

 chlorophyllous pigments and as well as, or even better, with the 

 pheophytins. Krasnovskii and Vojnovskaja (7) determined reversible 

 absorption changes due to illumination of these pigments dissolved in 

 air-free pyridine in the presence of ascorbic acid as an electron donor 

 (cf. also Krasnovskii, 12), Such a reduction might be explained by 

 the hypothesis that, as long as the excited electron lingers in some 

 excited state (singlet or triplet), there is a certain probability for an 

 "external" electron to enter the ground state at some site in the molec- 

 ular configuration. As reversible reduction is independent of the 



