Absorption spectra and structure of 2‐thiouracil derivatives as a function of pH

Abstract

AbstractSummary ‐ The ultraviolet absorption spectra of several 2‐thiouracil derivatives were measured at various pH values and apparent dissociation constants determined from these spectra. General comparisons were made with corresponding uracil analogues. The structure of 2‐thiouracil is shown to be in the diketo form in acid and near‐neutral aqueous solutions. With further increase in pH two equilibria are demonstrated spectrophotometrically, referring to tautomerism and concomitant ionization at the 2 and 4 positions respectively. The spectra and structure of other 2‐thiouracil derivatives are discussed.A comparison of the degrees of dissociation of 2‐thiouracil and its 6‐methyl analogue at blood pH values as determined from their spectra places in question the necessity of assuming appreciable dissociation of these compounds for iodine absorption in anti‐thyroid activity.