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 B 0 = 1.0 MHz and B 2 = 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.

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“…However, it has been stated that the given experimental value is associated with a degree of uncertainty. (Sanderud & Sagstuen 1996) In the optimized structure of II the C 3 centre is not completely planar where an angle between the hydrogen on C 3 and the plane defined by C 3 -C 2 -O was found to be 22.0º. (Lipfert et al 2004) This is in good agreement with the corresponding experimentally determined value of 19.5º.…”

“…In addition, it is noted that near planarity was observed for C 1 and C 2 and their substituents, in agreement with experiment. (Sanderud & Sagstuen 1996) With respect to its calculated HFCCs, as can be seen in Table 8 the values calculated for the α-hydrogen are in good agreement with experiment. The slight deviation in A iso from experiment is possibly due to the fact that the calculations are performed in the gas-phase on an isolated molecule and hence effects due to crystal packing are ignored.…”

“…It was found that this occurred for dihedral angles of -1.7º and 117.2º. (Sanderud & Sagstuen 1996) isotropic and anisotropic components (MHz) of the hyperfine coupling tensor of radicals I, II, and III.…”

“…The C 2 -centered radical I and C 3 -centered radicals II and III proposed to be formed upon the irradiation of non-and partially-deuterated single crystals of L-O-serine phosphate. (Sanderud & Sagstuen 1996) In particular, evidence has been obtained suggesting the formation of several different radical species. Specifically, the irradiation of non-and partially-deuterated single crystals of L-O-serine phosphate has been suggested to produce the three radicals I, II and III shown in Figure 7.…”

“…Specifically, the irradiation of non-and partially-deuterated single crystals of L-O-serine phosphate has been suggested to produce the three radicals I, II and III shown in Figure 7. (Sanderud & Sagstuen 1996) In particular, it is thought that upon irradiation serine phosphate can take up a now free electron to form a C 1 -centered radical a n i o n . T h i s m a y t h e n u n d e r g o deamination to form the C 2 -centered radical anion I. Alternatively, a serine phosphate may lose an electron to form a neutral C 1 -centered radical which then undergoes decarboxylation to give a C 2 -centered radical.…”