150 LIGHT AND LIFE 



o o. 



:c= s. 



^O O. 



^C = S::V-H--B 



Fig. 6. The effect of hydrogen bonding on the thione absorption band. 



B, an anion. 



tion has disappeared. The formation of carbonyl compounds result- 

 ing from illumination of the corresponding sulfur analogues in the 

 presence of oxygen (17) suggested that DPNH may have been formed 

 from illumination of its thionicotinamide analogue. However, the 

 spectral changes resulting from exposure of the reduced coenzyme 

 analogue to illumination are also observed when the analogue is 

 contained in an anaerobic cuvette flushed with nitrogen. The follow- 

 ing properties of the illinnination product were also observed: 



(a) absence of enzymatic activity; 



(b) regeneration of the material absorbing at 398 m/i after the 

 solution was allowed to remain in the dark; and 



(c) enhancement of fluorescent intensity upon addition to beef 

 heart lactic dehydrogenase. 



The reversibility of the reaction suggested that illumination re- 

 sults in the shift toward one side of an equilibrium. The thioamides 

 may exist in an equilibrium involving the following tautomers (4, 

 14): 



S SH 



II I 



/?— C— NHo ^ /?— C=NH 



If illumination caused a shift toward the thiol form, complexation 

 of the thione grouping should prevent formation of the illuminated 

 product. One of the earliest reactions noted with thiones was the 

 formation of complex addition compounds with the salts of heavy 



