79 



Britton Chance and Walter D. Bonner, Jr. 



be compared vlth that of Witt (3) who states that he was unable 

 to observe any changes at 700 m[i in fresh chloroplasts . It is 

 possible that the reversible light response at TOO m\x is associ- 

 ated vith some damage to the leaves. 



One possible explanation is based upon the idea that tvo 

 light-induced oxidations occur at low temperatures . The disap- 

 pearance of absorption at TOO m\x is concluded to represent an 

 oxidation state of chlorophyll but its chemical configuration is 

 quite unknown (l^). The evidence for the oxidation of cytochrome 

 f is fiiroly based upon the disappearance of the characteristic 

 absorption band of ferrocytochrome f . It becomes, therefore, of 

 considerable interest to determine vhich is oxidized more rapidly. 

 As Table II indicates, the absorbancy decrease is relatively more 

 rapid at T05 mn than at 555 m|i. Hovever, a comparison of Figs. 

 8 to 9 indicates that the reactions come to completion at about 

 the same time. In Table II, ve have attempted to make the com- 

 parison more meaningful by converting the rates of absorbancy 

 change to molar rates, making assumptions vhich need, hovever, a 

 detailed study and critical evaluation. Hovever, the simple 

 assumption that the extinction coefficient of cytochrome and 

 chlorophyll bear the same relation to each other at lov tempera- 

 t\ires as they do at room temperatures brings the rates of the 

 light-induced reactions of cytochrome f and '^^qq closely to the 

 same range, cytochrome f being slightly faster than PyoO* "^^ ^^® 

 basis of any of a number of simple mechanisms, it appears that 

 neither cytochrome f nor Pyoo ^^ ^ ^^'^® limiting intermediate in 

 the oxidation of the other. 



Even qualitative aspects are useful in this respect. First, 

 there is no induction period in the light -induced oxidation of 

 either of these pigments vhich vould suggest a sequential reac- 

 tion, i.e., a delay in the oxidation of cytochrome f prior to 

 the oxidation of 'P'jqq and vice versa. This lack of induction 

 period is also observed at the measuring light and actinic light 

 intensities. One mechanism, vhich appears to meet the needs of 

 the kinetic data is that cytochrome f and P^^qq are interacting 

 vith different chlorophyll molecules, vith cytochrome f being at 

 the active center of the photosynthetic unit and Pyoo ^^i^S at a 

 chlorophyll molecule vhich is on the energy transfer pathvay from 

 the initial receptor to the active center. It is apparent that 

 a very detai led examination of the quantim requirements for these 

 tvo light -induced oxidations vould be of great importance in this 

 respect. Since both oxidation reactions occur simultaneously, 

 their quantum requirements should be additive . 



