Some quantitative aspects of hydroxyl radical induced reactions in γ-irradiated aqueous solutions of pectins

The reactions of radiolytically generated hydroxyl radicals (OHradical dot) with galacturonic acid residues of pectins in an 0.1% aqueous solution were studied in the absence of oxygen (samples saturated with N2O) and in the presence of oxygen (samples saturated with N2O/O2). Two high-methoxyl pectin preparations, from citrus fruit peels (CU 401) and apples (AU 202), were used for comparison. In γ-irradiated pectin solutions (dose range between 0.1 and 1 kGy), the content of unaltered galacturonic acid residues is rapidly decreased. The G-values of galacturonic acid decomposition were equal to (4.3±0.2)×10−7 mol J−1 for CU 401 and (4.5±0.2)×10−7 mol J−1 for AU 202 preparations irradiated in N2O saturated solutions and (8.8±0.3)×10−7 mol J−1 for CU 401 and (8.2±0.3)×10−7 mol J−1 for AU 202 both irradiated after saturation with N2O/O2. These values indicate that in pectins, irradiated in the presence of oxygen, one OHradical dot radical induced alteration of more than one molecule of galacturonic acid. As the result of alteration of galacturonic acid moiety, the formation of free carboxylic acids, aliphatic aldehydes and deoxy-compounds was confirmed. Radical reactions did not induce the release of methanol from esterified carboxylic groups of galacturonic acid residues.