1068 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



of the primary photochemical process. As described previously (c/. Vol. 

 I, page 546, chapter 23. A8, and chapter 24, the three processes— energy 

 dissipation (rate constant, h; quantum yield, 5), chemical transformation 

 (rate constant, kt] quantum yield, 7) and fluorescence (rate constant, 

 kf-, quantum yield, <p) — compete for the absorbed light energy. If all 

 three competing processes obey the law of monomolecular reactions, their 

 quantum yields are determined by equations of the type : 



(28.49) 'P = A-/ /(A-/ + ki + ki) 



{cf. equation 19.8). If the primary photochemical process requires en- 

 counters with a kinetically independent reaction partner, A, the quantum 

 yield equation becomes: 



(28.50) f = kj/(kf + ^'1[A] + ki) 



where fcl is a bimolecular rate constant {cf. equation 23.18, p. 797). In 

 photosynthesis, we assumed the primary photochemical process to be a 



tautomerization (such as X • Chi • HZ ^ HX • Chi • Z, or ACO2 • Chi • A'HaO 

 ^ AHC02-Chl-A'H0, or AC02-Chl-A'H20 ^ AC02-ChlH-A'H0 

 ^^ AHC02-Chl-A'H0). Since tautomerization is a monomolecular 

 process, equation (28.49) can be used; changes in tp are thus indicative of 

 variations in the composition or structure of the photosensitive complex, 

 which affect the rates kt and ki. (The fluorescence rate constant, kf, itself 

 remains practically unchanged as long as the intensity of the absorption 

 band is not changed significantly, since both are determined by the transi- 

 tion probability between the ground state and the excited state.) One 

 could, of course, also consider the possibility of fluorescence quenching by 

 collisions with alien molecules (i. e., the addition of "bimolecular" terms in 

 the denominator of eq. 28.49), since this is often observed in fluorescent 

 gases and solutions; however, it seems plausible that changes in fluorescence 

 associated with photosynthesis are due to changes within the chlorophyll 

 complex, rather than to the formation or disappearance of new kinetically 

 independent quenching substances. The "natural" life-time of the excited 

 state of the chlorophyll molecule has been estimated (cf. page 534) as of the 

 order of 8 X lO^^ sec; and the low yield of fluorescence in vivo (order 

 0.1%) indicates that the actual life-time is about one hundred times shorter, 

 or ~8 X 10-^'' sec. To produce, under these conditions, a marked effect 

 on the intensity of fluorescence by kinetic encounters the quenching mole- 

 cules must occur in concentrations high enough for the encounter intervals 

 to be not much longer than 10"'" sec; and this requires concentrations 

 of the order of at least 0.01 and more prol)ably 0.1 mole per liter. It seems 

 unlikely that such high concentrations of freely moving molecules of reac- 

 tion products should actually arise and disappear during photosynthesis. 



