SIDNEY SHJI'RIN AND NATHAN O. KAPLAN \o6 



-I- (CH3)2S0, 



NH 

 II 



X — SCH. 



N + CH3HSO4 



The absorption of the methylated coenzyme analogue is maximal 

 at 341 m/x and has a millimolar extinction coefficient of 4.8. These 

 values are comparable to those observed for the illumination product 

 of the reduced thionicotinamide analogue (Amax 342 m/i,, 10-^ c 4.4) . 



The virtual transparency of a methyl group when attached to nitro- 

 gen, oxygen, or sulfur has permitted a study of tautomerism in the 

 mercapto derivatives of some heterocyclic compounds (19, 16, 10, 1) . 

 A comparison of the absorption data of the thionicotinamide analogue 

 of DPNH, of its illumination product, of pyrid-2-thione, and of 2- 

 benzylthiopyridine are given in Table 2. 



The similarities in the spectrophotometric changes accompanying 

 illumination and alkylation suggest that the light-induced product 



SH 



I 

 has the structure of a thiolimide, — C=NH. Although there are re- 

 ports of a great many photochemical rearrangements (3) and reac- 

 tions of non-enolizable ketones in sunlight (18), there are very few 

 well-described examples of photo-induced tautomerism (9) . It has 

 not been possible to isolate the product of the illumination reaction 

 l^ecause of its rapid equilibration with the thione form. The struc- 

 tural evidence is based entirely on its spectral properties, which are 

 compared with those of known thioamide systems. 



While evidence for interaction between the components of pyridine 

 dinucleotides in the reduced form has been presented, there has 

 thus far been little indication that adenine interacts with the pyri- 

 dinium portion of DPN. The 3-aminopyridine analogue of DPN has 

 been prepared (2) , and its ultraviolet absorption spectrum has been 

 shown to have a maximum at 332 m^ in water. The spectral shifts 

 observed upon changing the solvent to propylene glycol are compared 

 for the analogue and 3-aminopyridine methiodide in Table 3. 



The "blue shift" which is observed on increasing the dielectric 

 constant of the solvent is in accord with the suggestion made by 

 Kasha (13) that the absorjnion results from an n -^ tt* transition. 



