602 B. Chance 



the dark reaction proceeds extremely slowly, being inhibited about 10-fold 

 by the change of temperature. This result is of considerable significance with 

 respect to our ideas on energy transfer processes, since the very small tempera- 

 ture effect suggests that a chlorophyll-cytochrome transfer does not involve 

 a 'thermal reaction'. When the small effect of temperature upon the cy to- 

 chrome-chlorophyll reaction is compared with the large effect of temperature 



423-460m>j ± 

 log I./I = 0.004 

 on T 



Fig. 1 . A double-beam spectrophotometric recording of the effect of temperature 

 on the light-induced oxidation of 'cytochrome 423' of the purple sulphur 

 bacterium Chromatium. On the left the temperature is -|-28°C, and on the right 

 the temperature is — 22°C. The rates of oxidation of the cytochrome are com- 

 puted on the basis of a molecular extinction coefficient of 100 cm"^ x mM~^ 

 (Olson, 1958). The cuvette of the double-beam spectrophotometer is contained 

 in a Dewar flask and infra-red illumination is reflected onto the sample from 

 above (Expt. 3). 



on the cytochrome-cytochrome interactions of the respiratory chain (Chance 

 and Spencer, 1959), the fundamental difference between the two types of 

 energy transfer processes is underlined. 



Another interesting aspect of this result is that it affords direct evidence 

 for rapid biological processes in frozen cells as discussed by Keilin (1959). 



Intercytochrome Carriers 



The high velocity of the direct interaction of soluble cytochrome c and 

 purified cytochrome oxidase suggests that a direct interaction of cytochromes 

 can occur in the particles, but nevertheless intercytochrome carriers (Green, 

 1959) or by-passes (Martins, 1956) such as quinones have been postulated. 

 However, titration of the inhibited respiratory chain with reduced diphospho- 

 pyridine nucleotide (DPNH) has shown that the amount of such components 

 involved in transfers between the cytochromes (Chance, 1958b; Estabrook 

 and Mackler, 1956) does not allow for the participation of an appreciable 

 portion of total quinone content (Green, 1959; Redfearn, 1959). More 

 recent studies of the sites of oxidation and reduction of ubiquinone (co- 

 enzyme Q) in the non-phosphorylating Keilin and Hartree preparation show 

 this material to be exclusively on the substrate side of the antimycin-A 



