L.N.li. Duyeens 



the rate (or the quantum requirement) decreased at -220 C, Also 

 it was established that all cytochromes, which were at 20 C in 

 the reduced state, remained in the reduced state during cooling 

 in the dark; except for C 423.5, no photooxidation occurred at 

 -170°C. The rate of photooxidation of cytochrome C 422 strongly 

 decreased for temperatures below -90 C (see Pig. 2), 



5 sec 



CHROMATIUM 



Pig, 2. Time courses showingpg^^o'too^i*^^*^*^'^ 



Qix* 



of cyto- 

 chrome C 422 in Chromatium "• 'at various temperatures 

 but for the same intensities. The initial rate of oxi- 

 dation decreases below -90 C. 



The decrease in the rate upon lowering the temperature was re- 

 latively more pronounced at higher intensities of the actinic 

 light, which also indicated that the rate was limited by a tem- 

 perature dependent reaction. The photooxidation of C 422 (above 

 -90^C) was observed under conditions, in which C 423.5 ''as oxi- 

 dized, and was found to proceed with a quantum requirement of 

 1, if the specific absorbancy difference is assumed to be 100/ 

 (mWcm). These dark redox reaction may differ from other dark 

 reactions which stop at temperatures close to the freezing 

 point in that the participating molecules do not have to diffuse 

 before reacting. These observations suggest that C 423.5 ^^^ 

 C 422 are located on the same protein or cell constituent, and 

 that oxidation of C 423.5 causes a change in confirmation of 

 this protein, which brings the haem moiety of C 422 in the^right 

 position to be photooxidized even at a temperature of -100 C by 

 the probably adjacent primary photooxidant, P 890 (see below), 



Cy to chrome reactions in algae. The absorption difference 

 spectrum, light minus dark, of the red alga Porphyridium cruen^ 



