FLUORESCENCE OF CHLOROPHYLL in VltW 1833 



where ^o is the maximum yield of fluorescence at high dihition, <p the yield 

 in concentrated solution, and [Chi] the concentration in mole/1. 



The temperature effect on the yield of fluorescence appears to be some- 

 what stronger in the quenched state than in dilute solution (d<p/dt = 0.83, 

 13-49° C, in 1.2 X 10"- M solution of chlorophyll b in acetone, where 

 <p/,po = 0.64 at 30° C). 



The absorption spectrum of chlorophyll a (in diethyl ether) was found 

 to be the same in 0.02 M as in 0.001 M solution, despite an over 50% effec- 

 tive quenching of fluorescence in the first-named solution. This, and the 

 form of the concentration dependence of <p, argue against dimerization as 

 the cause of self-quenching. Neither does quenching appear to be due to 

 encounters of excited with normal dye molecules, because it cannot be 

 represented by the Stern-Volmer ecjuation {(po/<p = 1 + Const. [Chi]). 

 The experimental results, although they can be expressed approximately 

 by the one-constant eciuation (37C.3), are not inconsistent with Vavilov's 

 three-constant equation, which is based on the assumption of a constant 

 probability of dissipation in every energy transfer between two molecules: 



(37C.4) Wv = (l + a + - ^e-sfo[Chl])eno[Chl] 



The data are, however, insufficient to calculate the three constants a, l3 and 

 Oo separately. If the assumption is made that a « %, the two constants 

 fio and /3 can be derived from the empirical data, and the results (f2o = 

 8.3 X 10~-^ and /3 = 1.32 X 10"^^ cc. per molecule) are of the same order 

 of magnitude as the constants derived by Vavilov and co-workers for other 

 dyes in glycerol-water mixtures. 



According to p. 760 {cf. also chapter 32, section 5), the probability of 

 energy dissipation by resonance transfer may depend on concentration 

 more strongly than is postulated in Vavilov's model (e. g., if dissipation is 

 caused by dimers encountered in the resonance chain, as suggested by 

 Forster) ; it may also depend on temperature (if dissipation occurs in "hot" 

 chain links as suggested by Franck) . 



The study of the quenching of chlorophyll fluorescence by different 

 admixtures, described in chapter 23 (section 6) was extended by Li\angston, 

 Thompson and Ramarao (1952) to mesoporphyrin and protoporphyrin. 

 Fourteen admixtures were tested, and the only significant difference be- 

 tween the porphyrins and chlorophyll was found in sensitivity to oxygen — 

 the quenching constant (A'l in the equation <po/(p = I + Ki[Q] -f- KilQ]-, 

 cf. p. 785) was ten times higher (A'l = 335 l./mole) for the oxygen effect on 

 porphyrins than for its effect on chlorophyll {Ki = 35). The porphyrin 

 value is similar to that found for many aromatic hydrocarbons; chlorophyll 

 fluorescence is thus exceptionally insensitive to oxygen — a fact that reminds 



V^ 



