SinXFY I'. ]ELICK 



115 



Iciciuially in non-eii/yinaiic oxidations and reductions. The dif- 

 erences in reaction rates are significant alter full cognizance is taken 

 of the possible isotopic rate effects. It may be argued that the two 

 hydrogen atoms at C-4 are not exactly equivalent because ol the 

 asymmetric influence of the ribosidic bond, but in view of the dis- 

 tances involved such an effect would be expected to be rather weak. 

 The probable explanation of the effect is based upon the properties 

 of the folded inner complex of the dinucleotide. It may be seen in 

 Fig. 1 that the two hydrogen atoms at pyridine-C-4 are in different 

 chemical environments. As long as a specific folded conformation 

 occurs, the chemical difference between the two reactive hydrogens 

 is real even though the details of the inner complex may not be 

 exactly as depicted. The two steric positions at pyridine C-4 also 

 react differentially when DPN is reduced by hydrosulfite. One may 

 therefore infer that DPN, like DPNH, also occurs in a folded con- 

 formation, although fluorescence of DPN is not available to test 

 the hypothesis. 



Isomerism at the pyridine ring nitrogen. Conformations of the 



Ha 



Hb 



Fig. 6. Skeletal models of the nicotinamide riboside moiety of DPN (I) and 

 DPXH (Ila and lib). A pyiimidal rino; nitrogen in TIa nrakes it possiijle for the 

 pyridine and ribose rings to lie in nearly parallel planes. A planar ring nitrogen 

 in lib puts the N-ribosidic bond in the plane of the pyridine ring but makes its 

 angles with the other two N C bonds unecpial. The actual strnclmc is probably 

 closer to lib than to Ila. 



