776 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



1921) found, for example, that chlorophyll-liiiide emulsions in water are 

 fluorescent, while pure aqueous chlorophyll sols are not. The fluorescence 

 of chlorophyll colloids in the presence of lecithin was confirmed by Bakker 

 (1934). This may be either a true case of protection, or merely a dilution 

 effect, since the concentration of chlorophyll molecules in the lipide drops 

 may be lower than in the particles of the hydrosol. 



Seybold and E^lc (1940) found that chlorophyll on filter i)aper is 

 fluorescent only if the solution from which it was adsorbed contained lipides, 

 waxes or other lipophilic organic materials. (They even used the ab- 

 sence of a fluorescent rim on a filter paper strip dipped into a chlorophyll 

 solution as a control of the purity of both solution and paper.) Nonfluores- 

 cent chlorophyll adsorbates on starch "light up" if they are wetted by or- 

 ganic solvents, or if ether vapor is blown at them. 



Wakkie (1935) prepared a series of colloidal chlorophyll solutions (b}' 

 diluting alcoholic solutions with increasing amounts of water), with and 

 without a lipide (sodium oleate) . If no oleate was added, the shift of the 

 absorption band from 660 to 672 m/x (completed within a narrow concen- 

 tration range, and considered indicative of the transition from a monomole- 

 cularly dispersed to an aggregated state) was accompanied by complete 

 disappearance of fluorescence. In the presence of oleate, the absorption 

 band began to shift at the same dilution, but the displacement ceased at 

 670 instead of 672 m/x; and, at the same time, fluorescence reappeared. 

 By salting out, fluorescent, birefringent chlorophyll oleate "coacervates" 

 could be precipitated from these fluorescent colloidal solutions. 



From these and similar observations, it appears that lipophilic mole- 

 cules can protect the fluorescence of chlorophyll from self-quenching even 

 without diluting the pigment, and without disrupting the chlorophjdl- 

 protein or chlorophyll-cellulose bond. One can visualize the protecting 

 molecules as enveloping the lipophilic parts of the adsorbed pigment mole- 

 cules (e. g., in the case of chlorophyll, the phytol "tails"), and thus inter- 

 rupting their mutual interaction. The "wrapping up" of flexible parts of 

 the molecule may stiffen the latter and interfere with the internal conver- 

 sion of electronic into vibrational energ3^ This stabilization effect may 

 become manifest in a single pigment molecule, as well as within a complex 

 of several such molecules. In the light of the above-described, more recent 

 experiments by Livingston, one has also to consider the possibihty that 

 tautomeric transformations from a nonfluorescent into a fluorescent form 

 of chlorophyll could be responsible (or coresponsible) for effects of this type. 



It does not seem that the association of chlorophyll with proteins can 

 in itself protect fluorescence. True, natural "chloroplastin" preparations 

 apparently are fluorescent. Although Smith (1938) called aqueous chloro- 

 phyll-protein extracts from spinach leaves "nonfluorescent," Noack 



