LUCILE SMITH 



437 



+ .012 n 



- +.0066 



E 

 u 



I +.0033 



c 

 o 



o -.0033 

 u 



a. 

 O 



-.0066-'r 

 500 



^(m^) 



Fig. 1. Difference spectra of RhodospiriJlum rubrum grown under illumination. 

 Measurements were made in the double-beam s|3cctrophotometer at the wa\e- 

 lengths indicated, a, Anaerobic minus aerobic bacteria in the dark, b, Dark minus 

 illuminated anaerobic bacteria, c, Dark minus illuminated aerobic bacteria. Dif- 

 ferent samples of bacteria were used in the two wavelength regions. 



longer wa\elengths (11). Figs. 1 and 2 also plot the difference in 

 absorption spectrnm between anaerobic and aerobic cells in the dark. 

 These difference spectra show that the same carotenoid difference 

 spectra result from either illumination of the anaerobic cells or from 

 addition of oxygen in the dark. Since we are discussing here the re- 

 actions of cytochromes, the changes of absorption spectrum related to 

 the carotenoid pigments Avill be only briefly summarized (12) . (1) 

 The changes of carotenoid absorption spectrum take place very 

 rapidly on illumination, more rapidly than the oxidation of the 

 cytochromes. (2) The carotenoid changes can be seen, imder some 

 conditions, when there are very small or no cytochrome changes. 

 (3) The rapid shift in carotenoid absorption bands to longer wave- 

 lengths appears to follow any procedure which initiates electron 

 transport processes in the bacteria, either in light or dark. 



The changes in absorption spectrum in the region between 400 and 

 450 m/A shown in the difference spectra of Figs. 1 and 2 result from 

 the oxidation of one or more cytochromes. This is the region of the 

 absorption spectrum where the large "Soret" or y - absorption bands 

 of tlie cytochromes are seen. In R. rubrum the changes in the a and 



