Radiation-induced radical formation in beta-glycerophosphate single crystals studied by electron paramagnetic resonance and electron nuclear double resonance spectroscopy: formation of an allylic-type radical.

Single crystals of disodium beta-glycerophosphate pentahemihydrate (betaGP), Na2x(HO)CH2CH(PO(4)2-)CH2(OH)x5 1/2H2O, were X-irradiated at 77 and 280 K. EPR, ENDOR and FSE techniques were used to study the formation of free radicals in these irradiated crystals to characterize possible reaction mechanisms leading to dephosphorylation. Irradiation at 77 K reveals the presence of four different radicals: two alkoxy radical conformations, AR1 and AR2, (HO)CH2CH(PO(4)2-)CH2O. and two carbon-centered hydroxyalkyl radicals, LTR1, (HO)CH2CH(PO(4)2-) C.HOH, and LTR2, with a tentative structure (HO)CH2C.H(PO(4)2-)CH2OH. AR1 and AR2 were determined to be formed on each of the two independent molecules A and B of the asymmetrical unit of betaGP, each on the 3-end of the molecule. Irradiation at 280 K reveals the presence of three hydroxyalkyl allyl radicals, R1, R2 and R3, with the common chemical structure (HO)C.H-H=CH(OH). Radicals R1 and R2 are determined to form on molecules B and A of the asymmetrical unit, respectively. The site of radical formation for R3 could not be ascertained absolutely from the available data, and there is also evidence that suggests the possibility that R3 has a hydroxyphospho-allyl radical structure. Possible reaction mechanisms for the formation of both the 77 K radicals and the 280 K radicals are suggested and discussed. The formation of an allyl-type radical in such a small-molecule single-crystal model system is an interesting and surprising result which may be relevant to the formation of the so-called 3alphaH radical species commonly observed in irradiated solid cytosine nucleosides and nucleotides. The result is also important because of the formation of the hydroxyalkyl allyl radical in solid beta-glycerophosphate is at variance with mechanisms and products shown to dominate for this and similar systems in solution radiolysis.