370 4. ALLOXAN 



should be represented mainly in the keto form. Nevertheless, the C — C 

 and C — N bond lengths indicate at least 50% double bond character, so 

 some aromatic resonance must occur. In any event, even though the py- 

 rimidines may generally be in the keto form, alloxan presents additional 

 problems, and the lower pK^ points to more extensive enolization in al- 

 loxan. Alloxan has two H atoms and they are probably not locahzed but 

 migrate between the N and atoms. There is a total of 8 tautomeric forms 

 for alloxan: 1 all-keto form, 4 monohydroxy forms, and 3 dihydroxy forms. 

 The relative importance of these forms remains to be determined. 



Ionization of alloxan makes it more difficult to distinguish between 

 keto and enol forms since the anion can resonate between the structures: 



— C— N-— ^ _C=N— 



There are thus 16 structures which can be written for the alloxan anion, 

 there being 5 resonance hybrids. Inasmuch as alloxan is bilaterally sym- 

 metrical and it is unlikely that any one structure is markedly dominant, 

 it might be better to represent the alloxan anion as: 



providing the r5-charges are not assumed to be equal. The negativity is 

 probably more associated with the atoms than the N atoms. Although 

 the anion is definitely more unstable than the neutral molecule, which spe- 

 cies is more important for enzyme inhibition or ^-cell damage is not known. 



Oxidation-Reduction Reactions 



Alloxan is readily reduced to dialurate* and this was studied in detail 

 by Richardson and Cannan (1929), who determined the values shown in 

 the following 



pH E,' (mv) 



6.56 + 78.9 



7.03 + 60.2 



7.50 + 36.0 



* Dialuric acid was found to have a -gKa^ of 2.83by EichardsonandCannan (1929), so 

 it presumably exists at physiological pH as the dialurate ion; no other ionizing groups 

 were detected. This pA'^ is surprisingly low for the structure depicted for dialuric acid. 



