EPR study of X-irradiated hydroxyalkyl phosphate esters. Phosphate radical formation in polycrystalline glucose phosphate, ribose phosphate and glycerol phosphate salts at 77 and 295 K
Polycrystalline samples of the barium, monosodium and disodium salts of D-glucose-6-phosphate, the disodium and dipotassium salts of α-D-glucose-1-phosphate, the barium and disodium salts of D-ribose-5-phosphate, and the disodium salts of α-DL- and β-glycerol phosphate have been X-irradiated at 77 and 280 K and studied using X-band EPR spectroscopy. In the monosodium salt of D-glucose-6-phosphate a phosphoranyl electron gain centre, (RO)˙P(OH)O22–, was observed at 77 K, transforming into phosphoryl radicals ˙HPO3– and ˙RPO3– at elevated temperatures. In a previous report by Nelson et al. (J. Chem. Soc., Faraday Trans., 1993, 89, 1955) this was erronously stated to occur in the disodium salt of D-glucose-6-phosphate, rather than in the monosodium salt. In all the salts of α-D-glucose-1-phosphate, and in the barium salt of D-glucose-6-phosphate and the disodium salt of β-glycerol phosphate, phosphoryl radicals ˙PO32– were observed at room temperature without any precursor phosphoranyl radicals being detected. In α-DL-glycerol phosphate an alkoxyl radical is trapped at 77 K. Three different mechanisms for the formation of phosphoryl radicals in hydroxyalkyl phosphates are suggested on the basis of these results and previously published data. Two of these originate from the electron-gain centre, the phosphoranyl radical, whereas the third proceeds from a precursor hydroxyalkyl radical by electronic reorganization. The net charge of the phosphate group seems to be decisive for directing the primary electron trapping to the hydroxyalkyl or to the phosphate part of the molecule.