QUENCHING BY ADMIXTURES 777 



(1927), StoU and Wiedemann (1938) and Fishman and Moyer (1942) 

 found that they fluoresce weakly, and that fluorescence is preserved also 

 in precipitates prepared from such extracts by salting out. Wassink, 

 Katz and Dorrestem (1942) observed that the yield of fluorescence was 

 about the same (~0.1%) in live purple bacteria and in aqueous, colloidal 

 bacteriochlorophyll-protein suspensions prepared from them. However, 

 chloroplastin fluorescence probably must be attributed to the presence of 

 as much as 30% hpophilic material. Artificial complexes containing only 

 chlorophyll and proteins do not fluoresce. Accordmg to Noack, adsorb- 

 ates of chlorophyll on globin sometimes fluoresce faintly; but Seybold and 

 Egle (1940) suggested — probably with justification — that this must be as- 

 cribed to the presence of impurities of a lipide nature. 



The bearing of these results on the problem of the state of chlorophyll 

 in the IW'mg cell was discussed in chapter 14 (Vol. I). It was argued there 

 that the nonfluorescence of pure chlorophyll adsorbates on proteins does 

 not prove Seybold's hypothesis that the chlorophyll fluorescence in vivo 

 is caused by a small fraction of chlorophyll dissolved in a lipide phase; 

 more probably, all chlorophyll in the plants is weakly fluorescent {despite 

 its high density and irrespective of its association with proteins) because of 

 its simultaneous association with protective substances such as fats or 

 phospholipides (Hubert, 1935). Livingston's experiments, (p. 766), in- 

 dicate association with "activating" groups (such as OH, NH, or SH) as 

 another possible explanation of fluorescence. 



6. Quenching and Activation of Chlorophyll Fluorescence by Admixtures 



While the limitation of chlorophyll fluorescence in pure solutions may 

 be caused equally well by physical dissipation or by photochemical reac- 

 tions (tautomerization or reaction with the solvent), strong quenching by 

 small amounts (<10~^ mole/1.) of foreign substances must be attributed 

 to chemical interactions, since the rate of physical energy dissipation is not 

 likely to be affected by the comparatively rare encounters of excited dyestuff 

 molecules with the molecules of the "quencher," or by changes in the aver- 

 age properties of the solvent caused by the presence of the latter. (An 

 exception may be the case of resonance — to be discussed below.) 



As mentioned on page 757, the two most likely mechanisms of chemical 

 quenching are oxidation-reduction (equations 23.1), and complex formation 

 (equation 23.2). The second one is particularly probable when the 

 ([uencher is another dyestuff with overlapjiing bands, so that self-quenching 

 conditions are closely ai)proximated. In this case, permanent association 

 of the ciucncher with the fluorescent molecule also becomes a likely possi- 



