10 RADIATION BIOLOGY 



oxidutioii of allyltliiouicii pliotosensitizod \>y cliloiopliyll ((lulTroii, 1927). 

 It seems safe to assume that the reaction involves an interaction between 

 an energy-rich chlorophyll molecule and either an oxygon or an allyl- 

 thiourea molecule. It iiotinal chloropiiyll is represented by G, its fluo- 

 rescent state by G*, some long-lived excited state by G', and the reacting 

 molecule, either oxygen or the reducing agent. I)y B, a generalized (and 

 simplified) mechanism for the reaction may be written as follows: 



(1) G -\- hu ^ G* ' (absorption), 



(2) G* -^G -\- hvf (fluorescence), 



(3) G* —> G' (internal conversion), 



(4) G'-^G (degradation), 



(5) B + G* — >-^ G + products (chemical reactions). 



Expressed in appropriate units (einsteins per liter per second), the rate of 

 step (1) is i'l = lab.- The rates of the four subsequent steps are, respec- 

 tively, V, = /.-,[. I *],/•:, - A-4.l*],r4 = k,[A'], and V, = k,[B][G*]. As long 

 as the measurements of the fluorescence and of the chemical reaction are 

 made under the same conditions, it is immaterial whether ks is a function 

 of the nature of the solvent, i.e., whether the solvent quenches the fluo- 

 rescence. Under steady-state conditions, 



djA*) „ , d{A') „ 



-^^0 and ^^-0, 



and thus 



Vi = i>2 + Vs + /'o, and v^ = Vi 



may be written. Substituting for the several Vi, the following values for 

 the fluorescence yield <pfi and for the photochemical yield are obtained: 



^ la ^ ^2 



^'' Us k2 + k, + kABy 



V, k,[B] 



f = 



labx ki + A:3 + k^{B] 



In a dilute solution and in the absence of quenchers the fluorescence yield 

 iPfi for chlorophyll (Prins, 1 934) is approximately 0. 1. It is not detectal)ly 

 quenched by allylthiourea, and it recjuires about 2 X 10^- mole of oxygen 

 to reduce the fluorescence to half its maximum value. A quantum yield 

 ip of practically unity was obtained in a solution containing about 0.1 mole 

 of allylthiourea and 0.002 mole of oxygen. The natural mean life of the 

 excited state must be of the order of 10~^ sec; therefore k-i =10^ sec~'. 

 Since in the absence of quenchers the fluorescence yield is approximately 

 0.1, 



^h = 0.1 = f:^; 



»o 



