358 DANIEL I. ARNON 



In illuminated cell-free preparations of R. rubrimi, Smith and Ramirez 

 [no] and Smith and Baltscheffsky [in] have observed changes in the 

 absorption spectrum of cytochrome f., that were associated with phos- 

 phorylation. Their conclusions [m], that in the facultative anaerobe R. 

 rubrum the " photosynthetic " cytochrome Co is not a part of the respiratory 

 chain, and that "two different enzyme systems mediate the oxidation of 

 substrates by oxygen and the phosphorylation of ADP by illumination", 



400 



500 600 

 A {mil) 



Fig. 6. Difference spectrum (reduced minus oxidized) of purified cytochrome 

 ^2 oi Chromatiimi (Nozaki, Ogata, and Arnon [114]). 



are concordant with our distinction between photosynthetic and oxidative 

 phosphorylation in green cells [14, 46, 112, 50]. 



A reversible light-induced oxidation of cytochrome r., i^i cell-free 

 preparations of Chromatium—z cytochrome that has been isolated and 

 purified by Bartsch and Kamen [113] — was measured by Nozaki et al. 

 [114]. The absorption spectrum of the reduced form of a purified cyto- 

 chrome ^2 from our preparations (Fig. 6) is the same as that described by 

 Bartsch and Kamen [113]. 



The effect of light on the absorption spectrum (difference spectrum, 

 light minus dark) of cytochromes in cell-free preparations of Chromatiutn 

 is shown in Fig. 7. On illumination, the absorption spectrum of cyto- 

 chromes shows oxidation followed by a reduction in the dark and re- 

 oxidation on repeated illumination. Under the experimental conditions in 

 which the C/iromatinm cell-free system was investigated, the light-dark 

 reversible oxidation-reduction reactions were sufficiently decelerated to be 

 conveniently measurable with a recording spectrophotometer, at room 

 temperature. 



