77 



Britton Chance and Walter D. Bonner, Jr. 



and with a variety of measuring wavelengths, the difference spec- 

 trum for the TOO mp. pigment can be determined, and it is fo\md to 

 lie between TOO and T05 mu in accordance with the results of Witt 

 (T) and Kok (ik). At low temperatures, the half width of the 

 band is roughly 10 mjj.. 



Data calculated from Fig. S are summarized in Table II. The 

 maximum absorbancy change in the long wave region is about twice 

 that at the shorter i/ave region and the rate in the longer wave 

 region three times that in the shorter wave region. We maJce the 

 usual assumption that the extinction coefficient of the TOO mu 

 compound is the same as that of chlorophyll a at 680 m|i, and 

 further assume that the extinction coefficients of cytochroane f 

 and Pyoo change in proportion at low temperatures. On this basis, 

 the ratio of the concentrations of tte two substances is about 2 

 to 1. The ratio of the rates is of more interest; the molar rates 

 of absorbancy change are about equal (act-ually, the cytochrome f 

 rate is 1.^4- times the Ptoo rate). It should be emphasized, how- 

 ever, that these comparisons are only approximate and may be re- 

 vised when more acciorate data are obtained. 



DISCUSSION 



The observation of cytochrome f oxidation isolated from other 

 reactions of the complex matrix of photochemistry and biochem- 

 istry allows a detailed study of the mechanism of the electron 

 transfer reaction between cytochrome f and chlorophyll. While 

 it is not the purpose of this preliminary note to discuss this 

 in detail, it is apparent that the accurate recordings of the 

 kinetics in the illuminated frozen leaf will be of great advan- 

 tage in further experimentation. 



The present discussion will be limited to a consideration of 

 the low temperature oxidation of cytochrome f in relation to the 

 oxidation of cytochrome cg in photo synthetic bacteria and to the 

 absorbancy change at TOO m^i. 



Properties of the lovr tonperature oxidations of cytochrome f and 

 cytochrome c^' 



Our previous observation of the kinetics of oxidation of cyto- 

 chrome C2 in Chromatium at liquid nitrogen temperatures (U) is 

 supported by the observation of the cytochrome f kinetics in 

 leaves. These results greatly extend those obtained on aged chlo- 

 roplast suspensions by Witt (T). It is clear from these results 



