PROCESSES IN THE PHOTOSENSITIVE COMPLEX 1023 



tation on P/I^ must exist that is quite independent of the Umited quantity 

 of the carbon dioxide acceptor, A. One way in which this Hmitation can 

 arise is for fc* in equation (28.14) to become a function of [ACO2], such that 

 the product /c* X [ACO2] never exceeds a certain maximum vahie; another 

 is for /a to cease to be proportional to 7, i. e., for k* in equation (28.14) to be- 

 come a function of P. 



The first phenomenon is a common occurrence in photochemical proces- 

 ses in vitro, where the photochemical secondary reaction competes with the 

 deactivation of the light-activated molecules. (The latter can occur by 

 fluorescence, or by energy dissipation within the activated molecule, or by 

 energy transfer to other molecules.) The competition between a bi- 

 molecular photochemical reaction (rate constant k,) and one (or several) 

 monomolecular deactivating reaction (combined constant k') leads to a 

 yield equation (Stern- Volmer equation) : 



(28.17) y = kr[^]/{k' + Av[S]) 



according to which the quantum yield, 7, does not increase indefinitely 

 with the concentration [S] of the reactant, but approaches, at /bJS] ^ k' , 

 a maximum value (for the primary process !) : 



(28.18) 7max. = 1 



This maximum quantum yield is independent of the light intensity, /. 

 Comparison of equation (28.17) with (28.16) indicates that in this case — 

 assuming that ACO2 is the reactant S : 



(28.19) ank* = kr/{k' + kAkCO-z]) 



i. e., k* is in fact a function of the concentration of the reactant, decreasing 

 with increasing [ACO2] in such a way that the product ak*n[kC02\ can 

 approach, but never exceed, 1. 



In our schemes of photosynthesis (c/., for example, schemes 28. lA and 

 28. IB below), we do assume a competition of the secondary photochemical 

 reaction (such as 28.20b or 28.21b) with monomolecular deactivation, 

 (such as 28.20a'), but mean by the latter not the energy loss by fluorescence, 

 or by immediate energy dissipation in the light-absorbing complex, but 

 the slower back reactions that follow primary tautomerization. The 

 photochemically altered forms of the chlorophyll complex, HX.Chl.Z 

 (or HX.Chl.HZ), play the part of "long-lived activated states" discussed 

 in Volume I, p. 484. This changes the order of magnitude of the deac- 

 tivation constant, /c', from > 10^ or 10^ sec.~^ (the inverse of the life-time 

 of electronic excitation states in strongly light-absorbing molecules) 

 to perhaps as little as 10^ sec.~^; but formally, relation (28.17) remains 

 valid as long as the back reaction follows the monomolecular law. 



The possibility that the "activated" complex can be deactivated before 



