FLUORESCENCE OF CHLOROPHYLL ill vUrO 1835 



rate of photobleaching of chlorophyll in solutions containing various ad- 

 mixtures does not parallel the quenching effect of these admixtures on 

 chlorophyll fluorescence. The strongest quenchers, such as nitrophenol, 

 dinitrophenol, quinone and nitrosobenzene, do not enhance bleaching, while 

 ascorbic acid, which causes strong reversible bleaching, does not ([uench 

 fluorescence. 



{d) Activation 



The equilibrium between fluorescent and nonfluorescent forms of chloro- 

 phyll in mixed solvents {cf. Vol. II, Part 1, pp. 767-770) was farther 

 studied by Livingston and Weil (1952). They confirmed the observation 

 of Evstigneev, Gavrilova and Krasnovsky (p. 771) that pheophytin 

 fluoresces equally strongly in dry and in wet toluene, but not their obser- 

 vation that the fluorescence of magnesium phthalocyanine is activated 

 by moisture similarly to that of chlorophyll. However, the first experi- 

 ment alone is sufficient to justify ascribing the activating complex forma- 

 tion to the magnesium atom, rather than to the keto group in ring V 

 (Livingston's original hypothesis). 



Absorption measurements (described in section 2(c) above) confirmed 

 this surmise by showing approximately equal complexing tendency for 

 several metal porphyrins and chlorins. Strangely, the magnesium-chlorin 

 complex (chlorophyll) seems to be the only compound that requires com- 

 plexing with a base to be able to fluoresce; with Mg and Zn complexes of 

 porphyrin, there was no effect of base at all, while Zn-chlorin showed only 

 an increase of 20%. (In chlorophyll a or b, complexing with bases increases 

 the yield at least by a factor of 25!) 



The phosphorescence of chlorophyll solutions in isoamylamine (cf. 

 p. 754), not observed in other solvents, must be associated, according to 

 the experiments of Weller (chapter 37B, section 4) with the products of 

 aminolysis, rather than with chlorophyll itself. 



(e) Sensitization 



The sensitization of chlorophyll a fluorescence by chlorophyll b in 

 solution was observed by Watson and Livingston (1950) and Duysens 

 (1952). (For description of the same phenomenon in vivo, see next section.) 



Watson and Livingston (1950) used excitation with a band at X 475 m/x, 

 absorbed mainly by chlorophyll b, and observed fluorescence through a 

 filter (Wratten 88) that transmits the fluorescence of chlorophyll a much 

 better than that of chlorophyll b. The total transmitted fluorescence in 

 equimolar mixtures of a and b solutions proved to be markedly in excess 

 of that calculated under the assumption of independent absorption and 



