Effect of Na+ and Ca2+ ions on the photochemical formation of hydrogen peroxide in frozen aqueous solutions of adenine derivatives

Abstract

The content of hydrogen peroxide is determined in NaCl-containing (0.001–0.1 M) aqueous solutions of adenosine-5'-diphosphate (ADP) and adenosine (Ado) irradiated with near-UV light at 77 K. The obtained data are compared to estimates of the integral intensity (Int) of the EPR signals from the irradiated solutions prior to their thawing and the contents of different components in these spectra (including peroxyl free radicals), calculated by analyzing these spectra from their modeling. It is found that at a low content of photosensitizers (2 × 10−4 M), the relationship between [H2O2] and Int differs for ADP and Ado (a positive linear correlation in the case of ADP and a decreasing nonlinear dependence for Ado). It is assumed that these differences are due to the combination of two main factors: (1) the difference between the self-association constants (Ksa) of ADP and Ado and (2) the effect of the freezing process, during which added NaCl exerts a disaggregating action of the formation of ADP and Ado associates. The relation between [H2O2] and the EPR signal characteristics differs for high (1 × 10−3 M) and low (2 × 10−4 M) concentrations of photosensitizers, suggesting there are two paths of hydrogen peroxide formation in the investigated systems. According to the first path, H2O2 is formed during thawing as a result of interaction between peroxyl radicals O2-· and HO2· that accumulate during irradiation. According to the second path, the formation of H2O2 is direct (without releasing peroxyl radicals into the medium). This probably occurs inside aggregates of adenine derivatives, which form upon freezing the solution. In both cases, adding CaCl2 to test solutions of Ado and ADP ([CaCl2] = 0.1–0.2 × [NaCl]) results in a substantial drop in the yield of H2O2.