80 



Britton Chance and Walter D. Bonner, Jr. 



/Actually there -tvcwld appear to he no need for a spectroscopi- 

 cally distinct forra of chlorophyll to be formed simultaneously 

 \i±th the oxidation of cytochrome f , presumably the metastahle 

 state of chlorophyll -which acts as the energy trap for the ini- 

 tial light reaction could accept an electron from cytochrome and 

 transfer it to the electron donor in a temperature insensitive re- 

 action, \7ithout the need for the accumulation of a measurable 

 amount of a chlorophyll intermediate. 



A second mechanism that fits the needs of the experimental 

 data and which tak;es into account the certainty vith vhlch the 

 disappearance of the 555 niia band of cytochrome f indicates an 

 oxidation of ferro-cytochrome f is that the Pyoo absorption is 

 actually that of a reduced chlorophyll intermediate . Under these 

 conditions, an exact correspondence of the molar rates of change 

 •would be expected. Before this hypothesis can be considered 

 seriously, the apparently sound basis upon which it has been con- 

 cluded that P700 i^ ^^ oxidized forra of chlorophyll must be crit- 

 ically reexamined (l^). 



SUMMARY 



1. The demonstration of the temperature insensitive oxidation 

 of cytochrome f in leaves of three types of plants is reported; 

 difference spectra are provided and the kinetics of the change 

 are measured. 



2. The g^u8Ji"tum efficiency for cytochrome f oxidation falls 

 rapidly in the red region, a half maximal efficiency is obtained 

 at approximately 695 it^I^* 



3. The rates of light-induced absorbancy changes due to cyto- 

 chrome f and to Pvoo have been compared at temperatures of liquid 

 nitrogen. Wiile ihe times for completion of the two reactions 

 are approximately the same, the TOO mia change corresponds to a 

 larger absorbancy and hence, has a larger optical rate. However, 

 conversion to a molar basis (assuming that the effect of tempera- 

 ture upon the extinction coefficient of chlorophyll is the same 

 as that upon cytochrorae f ) brings the two rates approximately in- 

 to agreement at the two values of light intensity employed here. 



k. Two reaction mechanisms fit the experimental data: a) that 

 cytochrome f and P700 ^^^^"^ ^^ different chlorophyll molecules, 

 cytochrome f presumably reacting at the photosynthetically reac- 

 tive center, and P700 acting at a chlorophyll molecule involved 

 in energy transfer; and b) that cytochrome f and P^qq react at 

 the same active center and that cytochrome f is an electron donor 

 and P7QQ an electron acceptor. The first mechanism is consistent 

 with che data of other workers; the second is not in agreement 

 with studies of the oxidation-reduction reactions of PyqO' 



