7S0 FLUORESCENCE OF PlfiMENTS IN VITRO CHAP. 23 



by oxj^gen was suggested by Weiss (1935). He postulated an autoxidation 

 (dehydrogenation) of the dye: 



(23.15) D* + O2 > HO2 + oD 



(o signifies oxidized). The radical HO2 can diffuse across air gaps and 

 cause the oxidation effects ascribed by Kautsky to metastable oxygen 

 molecules. 



A reaction of the type (23.15) could be responsible not only for the 

 quenching of the fluorescence of adsorbed dyes, but also for that of the 

 fluorescence of dyestuffs dissolved in organic solvents. Since the quantum 

 jaeld of irreversible photoxidation of chlorophyll in pure organic solvents 

 is very low (Vol. I, p. 496), reaction (23.15), if it is responsible for quenching, 

 must be practically completely reversible (at least as far as the chemical 

 composition of chlorophyll is concerned). The restoration of oxidized 

 chlorophyll may be brought about either by direct reversal of (23.15), 

 or — if the HO2 radicals are partly consumed by dismutation or side reac- 

 tions — by interaction with the solvent. In the latter case, the net result 

 is sensitized autoxidation of the solvent S : 



(23.16a) Chl*+ O2 > oChl + HO. 



(23.16b) oChl + S > oS + Chi 



(23.16c) HO. > ^ H2O + f O2 



(23.16) S + i O2 > oS + m.O 



For the solvent, one may substitute an oxidizable substrate — e. g.' 

 benzidine, or potassium iodide — thus obtaining a mechanism of chloro" 

 phyll-sensitized autoxidation of such substrates. (This mechanism wa^ 

 discussed in Volume I, chapter 18, cf. equations 18.33, and chapter 19, cf. 

 scheme 19.11; there, tautomerization was added as a preliminary step.) 



An alternative interpretation of ser^sitized autoxidation, also discussed 

 in Volume I, chapter 18 (cf. equations 18.40), envisages a primary reaction 

 between excited chlorophyll inolecules and the oxidation substrate (rather 

 than oxygen). Whenever this mechanism operates, oxidation substrates 

 should quench the' fluorescence of chlorophyll more effectively than does 

 oxygen. Franck and Levi (1934) measured the quenching of chlorophyll 

 fluorescence by benzidine and potassium iodide. Their quenching curves 

 are not labeled and therefore do not permit reading off the half-quenching 

 concentration, but the authors state that, under the conditions of Noack 

 experiments on the chlorophyll-sensitized autoxidation of benzidine (cf. 

 page 528), quenching by benzidine must have been many times more ef- 

 ficient than that by oxygen. If this is true, then the mechanism of this 

 reaction must be different from that of chlorophyll-sensitized autoxidation 

 of substrates such as allylthiourea (cf. below). 



