REACTIONS OF CYTOCHROME PIGMENTS IN 

 PHOTOSYNTHETIC BACTERIA 



LuciLE Smith 



Department of Biochemistry 

 Dartmouth Medical School 

 Hanover, New Hampshire 



We have made some observations of changes in the absorption spec- 

 tra of pigments in photosynthetic bacteria upon illumination or 

 upon reaction with oxygen. So far, it has proved to be easier to 

 observe the reactions of cytochromes in these bacteria than in green 

 plant tissues. 



The photosynthetic bacteria investigated were Rhodospirillum 

 rubrum and Rhodopseudomonas spheroides, grown under illumina- 

 tion. Measurements of changes in the absorption spectrum on illumi- 

 nation or oxygenation of anaerobic suspension of intact bacteria or of 

 cell-free extracts were made in spectrophotometers specially designed 

 to measure small changes in optical density of the pigments present, 

 with minimal interference from non-spectral changes in signal (I, 

 15, 2) . 



Illumination of anaerobic suspensions of R. riibrum or R. spheroides 

 with light absorbed by the bacteriochlorophyll (light of wavelength 

 greater than 720 niyu.) results in changes of absorption spectrum of 

 two kinds of pigments in the wavelength region between 400 and 

 630 m/^: (1) there is oxidation of cytochrome pigments; and (2) 

 there are changes in the absorption spectrum of carotenoid pigments 

 (2, 11) . These changes are illustrated in Figs. 1 and 2, for R. rubrinn 

 and R. spheroides, respectively. Curve b in Fig. 1 and the dashed 

 curve of Fig. 2 plot the difference in absorption spectrum between 

 illuminated and dark anaerobic cells. 



The absorption peaks at 550 and 515 m^x in the R. rubrum dif- 

 ference spectrum and at 508, 471, and 445 irifi in that with R. 

 spheroides result from a decrease of the absorption peaks of the 

 carotenoid pigments of these organisms on illumination, while the 

 troughs at 575 m^j, in R. rubrum and at 525, 490, and 460 ni/* in 

 R. spheroides result from the aj)])carancc of absorption peaks at 



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