418 PHOTOMECHANICAL CONSIDERATIONS 



The oxidation of cytochrome can be induced with light absorbed by 

 either chlorobium chlorophyll or chlorophyll- 770. No significant dif- 

 ferences in the absorption spectrum changes or the kinetics of the 

 light reaction have been noted when the wavelength of the actinic light 

 is varied from 0.68/i to0.85/i. At the wavelength of maximum absorp- 

 tion in the red for Cps. ethylicum (-0.75 /U) the intensity for half- 

 maximum cytochrome oxidation in the steady- state at room tempera- 

 ture is about 10"^ einstein cm"2 sec"l (6 x lOl^ quanta cm-2 sec-1). 



Rate of cytochrome oxidation. 



Initial rates of cytochrome C-553 (8) oxidation in Cps. ethylicum 

 were measured with known quantum intensities of monochromatic red 

 light in order to determine the quantum efficiency and action spectrum 

 of cytochrome oxidation. The measurements were carried out at 2°C 

 to lower the rate of cytochrome reduction in the dark, (At this tem- 

 perature very little 6-type cytochrome is oxidized in the light, and the 

 troughs in the light-minus-dark difference spectrum appear at 422 m^ 

 and 553 m^.) The results for light of O.Slju are given in Fig. 5, where 

 initial rates of absorbancy change are plotted versus intensity of 

 actinic light. It appears that the initial rate of reaction is not strictly 

 proportional to light intensity, but that the slope actually increases 

 slightly as the intensity increases. This phenomenon is much more pro- 

 nounced at room temperature (24°C), The apparent quantum efficiency 

 (0) may be calculated for any intensity from Fig. 5 and the percentage 

 of actinic light absorbed by the bacteria (14). This calculation has been 

 made for intensities approaching zero and for 10"^ einstein cm"^ sec"-^ 

 on the assumption that for cytochrome C-553 ^6422 - ^^470 = 6. 8 

 X 104 M~l cm-1. Table 1 gives the apparent quantum requirements 



TABLE 1. 



Apparcjit qiiaiititiii rcqidrciiicnt of cytodironic 

 oxidation in Cps. ethylicum 



Figures within parentheses are calculated on the 

 basis of no additional correction for light-scattering 

 beyond the use of opal glass (see text). 



Intensity range 

 1 - T 



low high 



.22 3.2 2.4 



(.32) (4.8) (3.6) 



.12 2.6 2.0 



(.17) (3.5) (2.9) 



