370 PHOTOMECHANICAL CONSIDERATIONS 



X is not unambiguously identified at present although ubiquinone has 

 been proposed as the primary electron acceptor, Flavoprotein has 

 been isolated by Horio (4) and identified to be functional by Atebrin 

 (quinacrine) inhibition of electron transport (5), Both quinone and 

 flavoprotein may participate in one electron transfer reaction, but two 

 electron reactions could easily be accommodated, Rhp participation 

 in electron transport is suggested by Horio (4), This hemeprotein may 

 react with oxygen as well. Cytochrome b and c^ function in light- 

 activated electron transfer was indicated by Smith and Ramirez (6), 

 The reaction velocity constants kj , . . k6 areof the second order. The 

 mechanism by which chlorophyll is activated directly by a photon or 

 indirectly by resonance energy transfer is schematically represented: 



k 

 Chi — -^ Chi* (1) 



where ko = £01; € is the extinction coefficient; the quantum effi- 

 ciency; and I the light intensity. 



The excited state of chlorophyll (Chi*) can accept an electron from 

 cytochrome Co or donate an electron to X. Here we have chosen to 

 represent the primary reaction as the acceptance of an electron from 

 cytochrome for the simple reason that this reaction is found to occur 

 at the same rate at 300 and 77° K (7). 



^1 

 c" + Chi* — -^ c'" + Chi (2) 



- ^2 - 

 X + Chi -— -^ X + Chi (3) 



Thus, activated chlorophyll is returned to the ground state and can 

 be reactivated, as in Equation 1, The remainder of the electron trans- 

 port reactions occur as for members of the cytochrome chain, in 

 which one electron transfer reaction is assumed. The mechanisms 

 described here are, of course, applicable to two electron transfer re- 

 actions as well, 



^3 - 

 X + fp — "^ fp + X (4) 



^4 

 fp + rhp ■* rhp + fp (5) 



rhp + b"' — ^^ b" + rhp (6) 



