844 LIGHT AND LIFE 



Microsomal cytochrome reductase, like many of the foregoing, occurs 

 in organized cell structures. It catalyzes reduction of cytochrome h-' 

 by DPNH. A pyridine nucleotide flavoprotein complex is formed in 

 which the flavin is fully bleached, i.e., reduced, and the pyridine 

 nucleotide is presumably oxidized. It possesses a sulfhydryl group es- 

 sential for the binding and oxidation of DPNH. In conclusion, it 

 may be stressed that "although each flavoprotein seems to exhibit 

 unique properties of its own, many of these are variations of a com- 

 mon theme." 



The Emission of Light by Chemical Reactions 



Two chemical reactions that emit light have features of special 

 interest bearing on the interpretation of bioluminescence and photo- 

 synthesis. One of these is the oxidation of luminol, one of the two 

 known reasonably efficient chemiluminescent reactions. The other 

 is the chemiluminescence of metallo-porphyrins such as magnesium 

 phthalocyanine and zinc tetraphenylporphine in the presence of per- 

 oxides, a photochemical reaction resembling the reversal of the split- 

 ting of water in photosynthesis. 



Henry Linschitz has summarized recent work with the porphyrin. 

 The catalyzed reaction is a decomposition of tetralin hydroperoxide 

 to yield a-tetralone, water, and the electronically excited porphyrin. 

 The luminescence intensity closely parallels the rate of decomposition 

 of the peroxide, both being second-order reactions; and the ratio of 

 initial rate to steady state is also the same for both. Up to the half- 

 life of the peroxide, about 30 molecules of peroxide are decomposed 

 for every mole of porphyrin which disappears. This ratio is very 

 constant, being independent of temperature over the range studied 

 and independent also of the initial concentration ratio of peroxide 

 and porphyrin. The chemiluminescence spectrum at 148°C corre- 

 sponds closely to the fluorescence of the porphyrin at the same tem- 

 perature in organic solvent, even to minor features of the curves. 

 The bound metal is essential to the catalytic action of the tetraphenyl- 

 jjorphine, copper being most effective, lead somewhat surpassing zinc, 

 and tin being weakest. 



The close agreement of the chemiluminescent and fluorescent spec- 

 tra strongly suggests to Linschitz that the excited, unchanged por- 

 l^hyrin is itself the luminescent molecule and that it is excited dur- 

 ing catalysis rather than in its subsequent breakdown. The metal re- 

 quirement makes it reasonable to suppose that the first step in the 

 reaction is the complexing of the peroxide at the central metal ion. 



