Isoxanthopterin, tautomerism

UV spectra play a very important role in the characterization of pteridines and in the determination of tautomeric equilibria as well as pKa values. The shape of the spectra, their dependence on the pH and the typical regions of absorbance are characteristic for structurally related pteridine derivatives and are therefore often used for structural assignments and proofs. A comparative study of xanthopterin (4) and isoxanthopterin (5), respectively, with their corresponding 6- and 7-thioxo derivatives indicates that the thioxo function causes a strong bathochromic shift of almost 70 nm in the neutral species (92HCA2317). Other striking features are the increased acidity of the thioamide ionization and the hypsochromic shift of the absorption on anion formation. 

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