The Relationship between Spectral Shifts and Structural Changes in Uric Acids and Related Compounds

The absorption maxima of various uric acids have been determined in the non-ionized state and as function of pH, and the apparent dissociation constants have been derived. The sequence of ionization is established as 9,3,1. Both alkylation and ionization produce corresponding spectral shifts, but of different magnitude. Alkylation or dissociation at N⁹ or N³ are accompanied by a bathochromic shift, and at N¹ by a hypsochromic effect. An analogous behavior is observed with uracils, but is missing in the xanthine series and in hypoxanthine. The relationship between these spectral changes and the possible resonance forms at various stages of ionization is discussed.