Radiation chemistry of orotic acid as studied by pulse radiolysis

Abstract

The optical absorption spectra of the adduct species formed by H, OH or e^–_aq addition to orotic acid peak at 345, 340 and 330 nm, and their molar extinction coefficients are 1100, 1700 and 8500 M^–1cm^–1 respectively. The pK_a for protonation of the electron-adduct lies between 3.3 and 4. The protonated form absorbs at shorter wavelengths (∼320 nm) and has an extinction of several 1000 M^–1 cm^–1. These properties are quite different from those of the H-adduct formed by H atom attack at the 5,6-double bond. At neutral pH, the same electron-adduct species is formed, either by direct electron attachment to orotic acid, or by electron transfer from the ethanol radical formed in N₂O saturated solutions. No such transfer occurs from the corresponding t-butanol radical.

The absolute rate constants for OH and e^–_aq reactions have been measured by following the build-up of these adducts, and for OH attack, by the bleaching of the 5,6-double bond chromophore. The values so obtained (5.2 × 10⁹ and 1.4 × 10¹⁰ M^–1 s^–1 respectively) agree well with those obtained by competition kinetics or by e^–_aq decay. The electron reactivity shows a 2-fold decrease at the pK_a for ionization of the carbonyl groups as expected from diffusion theory. No such effect is observed for OH reaction rates, but there is an increased reactivity at low pH which is not understood.