PHOTOSYNTHETIC ,MK( IIANISM OF PURPLE BACTERIA 100 



intensity than the change at 420 m/i. Thus the peak at 420 and the 

 hump at 430 m/z must be caused by different pigments — a conchi- 

 sion consistent with the suggestion that the changes at 430 and 420 

 are mainly caused by cytochromes 428 and c, respectively. Since 

 thus cytochrome 428 seems to attain saturation at a lower intensity 

 than cytochrome c, it may further be concluded that in this experi- 

 ment oxidation of cytochrome c is presumably not caused by oxi- 

 dized cytochrome 428. 



CONCLUSIONS 



The experiments reported indicate that cytochrome 428 partici- 

 pates in photosynthesis, since its oxidation-reduction state is shifted 

 considerably towards the oxidized side upon onset of photosynthesis. 

 Also, cytochrome c can be oxidized by light, but this oxidation was 

 observed by us only under slightly unphysiological conditions. It is 

 possible, as Chance and Smith (6) concluded, that it occurs to a small 

 extent also in photosynthesizing cells. This conclusion was based on 

 the assumption that the minor band at 550 m^u in the difference 

 spectrum was caused by cytochrome c. It may, however, have been 

 caused by a band of cytochrome 428. A curve similar to that of Fig. 4 

 but measured at 550 mju would probably estabUsh this point. Vernon 

 and Kamen (3) extracted three cytochromes, one of which was 

 c, from Rhodospirilhim strain 1 and determined the absorption 

 spectra in the oxidized and reduced state. The difference spectrum 

 (oxidized minus reduced) of one of these cytochromes had a maximum 

 at 428 m/jL. However, the half-width of the band was 27 m/j., which is 

 different from the 12-mfj. half- width of cytochrome 428. The dif- 

 ference spectrum of the third cytochrome was also different from that 

 of cytochrome 428. 



The suggestion (4b) that cytochrome 428 was oxidized not only by 

 Ught, but also by bubbling air through an anaerobic suspension, was 

 confirmed by Chance and Smith (6). 



The changes, interpreted above as caused by oxidation of bac- 

 teriochlorophyll, were observed only under conditions in which cy- 

 tochrome 428 was in the oxidized state. It is possible, although not 

 yet proved, that the changes b and d occurring at high exciting in- 

 tensities in anaerobic peptone (Fig. 1, graphs III and IV) were also 

 caused by oxidation of bacteriochlorophyll. 



