NEW FLASHING LIGHT EXPERIMENTS 



1471 



tion, measurements of the maximum flash yield become titrations of Eb, 

 and permit a separation of the factors k^ and Eb (whose product can only 

 be derived from the maximum rate in constant hght, cf. equation 28.47, or 

 Tamiya's equation IV-3). Conversely, to explain the dependence of the 

 maximum flash yield on temperature (which he has observed) Tamiya as- 

 sumed (in essence) that the back reaction is not fast enough to prevent some 

 enzyme molecules from operating twice (or more) in the wake of a single 

 flash. (He retained Franck's first assumption, k' <C keE%, as needed to 

 explain effective utilization of all absorbed quanta in weak light.) With 

 the stabilization reaction now effectively competing with the back reac- 

 tion, the flash yield becomes a function of their relative rate constants, 



Experimental 



Theoretical 



J I I L 



800 



0.10 0.20 0.30 ^ 200 400 600 



t^, sec. f, lux sec. 



Fig. 34.24. Interpretation of Tamiya and Chiba's results by a first-order back reaction 

 (after Taniiya 1949): (a) P = /(ta) curves; (b) P = /(I) curves. 



and, since the two reactions are Hkely to have a different dependence on 

 temperature, also a function of the latter. The possibihty of determining 

 the total amount of the limiting enzyme. Eg, is lost in this scheme, since 

 only the products k^E^ and k'j^^ occur in the kinetic equations. 



By using the measured yields in constant and flashing hght as param- 

 eters, Tamiya calculated the rate constants /c', A^eEs and kj^^ (designated 

 by him as k^, ki and ko, respectively) at the three temperatures used (/CsEb 

 had to drop faster with decreasing temperature than keE^B to account for 

 the decrease in the flash yield). The rate constant, k', of the back reac- 

 tion was estimated as about 200 sec.~^ at 25° C, and 100 sec.~^ at 7° C. — 

 i. e., not higher than Emerson and Arnold's values for the rate constant of 

 the stabihzing reaction, k'^ (in contradiction to Franck's assumption). 



Inserting the calculated values of the rate constants into the kinetic ex- 

 pressions derived for the flash yield as function of dark time, and for the 

 flash yield as function of flash energy, Tamiya was able to reproduce with 

 fair approximation the experimental course of the first-named function 

 (fig. 34.24a), but not that of the second one (fig. 34.24b). The theoretical 



