PHOTOCHEMISTRY OF BACTERIOCHLOROPHYLL 



399 



of chlorophyU a, no slow restoration of the long wavelength band 

 (770 mfj) occurs with "oxy"bacteriochlorophyll during prolonged stor- 

 age in the dark. Instead, a different change in the absorption spectrum 

 occurs (Fig, 2), The rate of change of the spectrum of the "oxy"form 

 as given in Fig, 1 into the one as given in Fig. 2 depends upon the 

 concentration of the oxidant (in about 30 min, with 10-5M, within two 

 minutes with 10-4M). After the occurrence of this change the re- 

 versibility caused by addition of ferrous ions is lost for the greater 

 part. 



log lo/ 

 1.0 



0.5 



400 500 



600 



Fig. 2. Reversible chemical bleaching of bacteriochlorophyll in methanol after 

 prolonged storage in the dark (25 minutes). The spectrum of the "bleached" 

 compound is formed from the spectrum given in Fig. 1. The insert shows that 

 addition of ferrous ions results in only a weak, immediate, restoration of the 

 bacteriochlorophyll spectrum, while such a recovery is much higher and con- 

 sists of a slow and an "immediate" reaction upon addition of ascorbic acid. 



Also in contrast to the behaviour of chlorophyll a, reversible chem- 

 ical oxidation of bacteriochlorophyll was found not only to occur in 

 methanol, but also in acetone or ether. The difference in behaviour be- 

 tween the two pigments is most probably due to the lower redox poten- 

 tial of bacteriochlorophyll. Measured under the same experimental 

 conditions, the redox value of the bacterial pigments (bacteriochloro- 

 phyll and chlorobium chlorophyll) is about 150 mV lower than that of 

 chlorophyll a (cf. Goedheer, de Haas and Schullev, 3), In agreement 

 with this it was found that also chlorophyll a dissolved in acetone can 

 be oxidised with eerie ions (more strongly oxidising than ferric and 



