PHOTOOXIDATION AND PHOTOREDUCTION REACTIONS 247 



sorption by Chromatium chromatophores. When all these data are con- 

 sidered, it appears likely that the photooxidation of DPIPH2 and TMPD 

 is linked to the photo reduction of ubiquinone contained within the 

 chromatophore itself. 



The response of the R. nibnim chromatophores to varying concen- 

 trations of NAD and fumarate have allowed the calculation of a 

 Michaelis constant for these secondary slow reactions. In the case of 

 NAD, a Km of 2.5 x 10"^ was calculated, while the corresponding Km 

 for fumarate was 1 x 10"'* molar. These values are in the range ex- 

 pected of ordinary enzymatic reactions, and agree fairly well with the 



3 



2 5 



'2 



I 5 



o 



^ 101- 

 < 



Fast Reaction 

 No Oxidant 



\ Reactionx20 



6 5 



9 



Fig. 10. pH optima for the three reactions involving DPIPH2 

 photooxidation with R. nibnim chromatophores. The ex- 

 perimental conditions given for Fig. 1 were employed 

 utilizing 0,20 mg of BChl in the reaction system. Tris 

 buffer was employed for the pH range from 7,0 to 9,0 and 

 phosphate buffer was employed for the lower pH values. 

 The rates for the slow reactions were multiplied by 20 to 

 place them on the same scale as the fast reactions. 



