EPR and ENDOR Studies of 1H and 14N Hyperfine and Quadrupolar Couplings in Crystals of l-O-Serine Phosphate after X-Irradiation at 295 K

Abstract

Single crystals of l-O-serine phosphate, (HOOC)CH(NH3+)CH2OPO3-H, were X-irradiated at 295 K and studied using EPR, ENDOR, and FSE techniques. At this temperature, three carbon-centered radicals were identified. Radical I, the deamination product (HOOC)ĊHCH2OPO3-H, is shown to have undergone a major molecular reorientation upon formation. Radical II is identified to be the product (HOOC)CH(NH3+)ĊHOPO3-H. This species exhibits a nonplanar site for the lone electron density. This deviation from planarity is ascribed to electrostatic interaction between the lone electron orbital and lone pairs centered on the neighboring oxygen atom. Hyperfine interaction with the β-nitrogen of the amino group is observed. Both the 14N hyperfine and quadrupole coupling tensors are determined and the signs of the hyperfine coupling tensor principal values are deduced from interpretation of the quadrupole tensor employing the Townes−Dailey approach. The β14N isotropic interaction is well described by a Heller and McConnell type cos2 θ rule. The results found in the present work, together with other recent observations, yield estimated values of the constants B0 = 1.0 MHz and B2 = 34.5 MHz. Radical III exhibits the structure (HOOC)CH(NH3+)ĊH2, formed by scission of the phosphate−ester bond at the carbon side. No β14N coupling is observed for this radical due to a dihedral angle close to 90°. Possible mechanistic routes for the formation of these radicals are discussed in comparison with previously published data on serine and other alkyl phosphate derivatives.