Time-resolved CIDNP and laser flash photolysis study of the photoreaction between triplet 2,2′-dipyridyl and guanosine-5′-monophosphate in water

Abstract

Laser flash photolysis and time-resolved CIDNP have been applied to the investigation of the kinetics and the mechanism of the photoreaction between triplet 2,2′-dipyridyl (DP) and guanosine-5′-monophosphate (GMP) over a wide pH range in aqueous solution. The pH dependence of the rate constant k_q of quenching the triplet dipyridyl by the nucleotide has been measured. Upon pH titration, four pairs of the reacting species contribute to the observed value of k_q: pH < 2.4, ^TDPH⁺ and GH⁺; 2.4 < pH < 5.8, ^TDPH⁺ and G; 5.8 < pH < 9.4, ^TDP and G; and pH > 9.4, ^TDP and G(−H)⁻, with the corresponding quenching rate constants k₁ = 1.3 × 10⁹ M⁻¹ s⁻¹, k₂ = 2.7 × 10⁹ M⁻¹ s⁻¹, k₃ = 1.6 × 10⁸ M⁻¹ s⁻¹, k₄ = 1.1 × 10⁹ M⁻¹ s⁻¹. Based on LFP and CIDNP data, the established mechanism of the quenching reaction is hydrogen atom transfer in neutral solution (5.8 < pH < 9.4), and electron transfer in all other pH regions. Kinetic CIDNP measurements reveal that in acidic and basic solutions the CIDNP kinetics for GMP is determined by the degenerate electron exchange between the GMP radical and its parent molecule with the rate constants 1.3 × 10⁸ M⁻¹ s⁻¹ (acidic conditions) and 4.0 × 10⁷ M⁻¹ s⁻¹ (basic conditions). The nuclear paramagnetic relaxation time for the proton H8 of GMP, T₁ = 20 ± 5 μs, obtained from the simulations of the CIDNP kinetics, is found to be independent of the protonation state of the radical.