186 B. CHANCE 



and through the same solution but differs sufficiently in wavelength 

 to give an adequate response to the change in the magnitude of a 

 sharp absorption band (see Fig. 1). We prefer this method to those 

 in which the compensating beam follows a different path from the 

 measuring beam. Concrete evidence of the superior performance 

 of this method is afforded by a comparison of the published records 

 of the effects of illumination upon the cytochromes of R. rubrum 

 (1,2), We also show that our signal-to-noise ratio is sufficient to 

 permit a recording of the kinetics of the light reaction and to identify 

 those components that react rapidly. These spectroscopic changes are 

 initiated sufficiently rapidly for the purpose of these studies by a 

 light flash obtained with an electromagnetically operated shutter. 



RESULTS 



A typical record of the fast and slow phases of the light effect is 

 shown in Fig. 2. The rise of the trace indicates a decrease of light 



light off 

 4- 



'^30-440m;j 

 lbglo/l=0.002 



20 40 



Time (sec) 



Rhodospinllum rubrum 35° 

 Oxidation t 



F'ig. 2. A typical example of the kinetics of the anaerobic light effect measured 

 in R. rubrum at 35 °C. The abrupt rise of the trace upon illumination with a low 

 intensity of infrared light, proceeds at a rate that is measurable in recordings at a 

 faster time scale. The optical density changes due to the rapid and slow phases 

 of the effect are plotted as a function of wavelength in Fig. 3. One centimeter optical 

 path. 



absorption at 430 m/i with respect to that at 440 m/x. Following the 

 abrupt rise is a nearly stationary state, followed by a much slower 

 rise to a plateau. On turning off the light, the kinetics of the fast and 

 slow effects merge into one another. The slow phase is not com- 

 pleted at the end of the record. 



