78 



Britton Chance axid Walter D. Bonner, Jr. 



that the electron transfer reaction "betveen cytochrome and chlor- 

 ophyll is very unusual; its temperature insensltivity underlines 

 the jxrxtaposition of these tvo metalloporphyrins in a vay so that 

 collision reactions are not required for electron transfer. In 

 this paper, ve have not yet determined the quantum requirement 

 for cytochrome f oxidation at the lov temperature, but it can be 

 inferred from the kinetic data of Fig. k vhere the low tempera- 

 ture rate is considerably faster than the room temperature rate, 

 that the quantum efficiency is probably high. 



It is of interest that the oxidation of cytochrome f at low 

 temperatures is rapid in aged spinach chloroplasts as foiind by 

 Witt and extended by us to fresh spinach chloroplasts (Fig. 9). 

 This is not true of Chromatiijm chromatophores which have been 

 found by us (l5), to be more temperatxire sensitive than the reac- 

 tions in the intact cells; the reaction comes to a halt at 77 K. 

 It has further been noted by Duysens (l6) that some algae show a 

 rate limitation in cytochrome oxidation at low temperatures. 

 Whether or not this is due to a basic difference in the mechanism 

 or whether the essential orientation of cytochrome and chlorophyll 

 is deranged during the freezing of some materials and not others 

 cannot be stated at the present time. 



In Dreliminary experiments we have examined the effectiveness 

 of red-light in cytochrome f oxidation and find thot the rate of 

 oxidation'falls to half the maximum value at spproximaLoly 695mn. 

 In other words, the quantum efficiency falls to half maximal val- 

 at 695 mn. This result gives evidence for the intimate identifi- 

 cation of cytochrome f with system I in the leaf. 



Kinetics of cytochrome f and Pyoo 



The possibility of measuring absorbancy changes corresponding 

 to these two components in the frozen leaf would appear to pre- 

 sent optimal conditions for a critical evaluation of the possi- 

 bilities of their interaction (l^). Under these conditions, 

 thermal reactions in which the two might be involved would be 

 negligible and any possibilities for their direct interaction 

 might be observable. 



In the intact leaves, we routinely observe absorbancy changes 

 at 555 and 705 m|i typified by the data of Table II, approximately 

 double the absorbancy change at 705 as compared with 555 m^. In 

 fresh chloroplasts the absorbancy change at 555 m|j. is observed in 

 roughly the same relationship to the chlorophyll content as in 

 the intact leaves (see Fig. 9). At 700 m^, less than one fifth 

 the absorbancy change is observed and this is partly reversible 

 on cessation of actinic illumination. This result at 700 m^ may 



