CHEMILUMINESCENCE OF CHLOROPHYLL til vHvO AND in VIVO 1839 



oxides (at / > 100° C). The kinetics of luminescence decay (studied with 

 Zn-tetraphenyl porphin and tetralin hydroperoxide) indicated that the over- 

 all process includes: (1) a second-order reaction of peroxide, P, with the 

 dye, DH2; (2) an approximately second-order catalytic decomposition of 

 the peroxide; and (3) a slow first-order, noncatalyzed decomposition of 

 the peroxide. Between 20 and 60 peroxide molecules are decomposed for 

 each permanently destroyed dye molecule. The decay of chemilumines- 

 cence follows the second-order law, / ^ [PlfDHo]. The following free- 

 radical reaction mechanism (of the type first proposed by J. Weiss for the 

 chemiluminescence of luminol) is suggested for the main catalytic reaction, 

 and the emission of light : 



H, 



+ H2O 



H.> H 



(37C.6) 



H2 

 R — C — — and DH — are free radicals, and their reaction, (b), should lib- 

 erate enough energy to produce an electronically excited dye molecule, 

 DH2*. It will be noted that reading reactions (c), (b), and (a) backwards, 

 one gets the over-all reaction : 



H2 +DH2 H2 



(37C.7) RC=0 + H,0 > R— CO— OH 



+ hi' 



i. e., a dye-sensitized photochemical peroxide formation. This illustrates 

 the inverse relation between photosensitization and chemiluminescence. 



The chemiluminescence of photosynthesizing cells was discovered by 

 Strehler and Arnold (1951), using sensitive methods of fight detection. 

 It can be observed in higher plants as well as in green and red algae. In 

 its low yield, it resembles the weak luminescences which Audubert and co- 

 workers have found to accompany many common chemical reactions. An 

 important difference is that the emission occurs in the visible, not in the 

 ultraviolet as in Audubert's experiments. Its spectrum could be deter- 

 mined only rather crudely, but appeared to be identical with that of chloro- 

 phyll fluorescence. The emission could be followed for several minutes 

 after the cessation of illumination; about 0.1 sec. after the exciting light 

 had been turned off, the intensity of the afterglow was about 0.1% of the 

 intensity fluorescence has had during the illumination (in which less than 

 saturating light had been used). The intensity of emission decays in the 

 dark, following an approximately bimolecular law at 6.5° C. and a lower 

 order law at higher temperatures. 



That the emission is due to chemiluminescence (and is not a fluores- 



