Radiation effects on crystalline l-asparagine, revisited: Radical formation by EMR and periodic DFT after X-irradiation at 275 K

•Radicals in l-aspargine at 295 K investigated using EPR, ENDOR, EIE and DFT.•Radicals by net loss of H-atom from the backbone and sidechain are identified.•A possible successor from a primary side-chain amide anion has been observed.•Radical characterization has been supported by periodic boundary DFT calculations.•Deamination and decarboxylation radicals at room temperature were not observed.

Radical formation by X-irradiation of single crystals of l-asparagine monohydrate at 275 K has been investigated at 240 K and 295 K using Electron Paramagnetic Resonance (EPR), Electron Nuclear DOuble Resonance (ENDOR) and ENDOR-Induced EPR spectroscopic techniques. Free radical structures suggested by detailed analysis of the experimental data were assessed by Density Functional Theory calculations in a periodic approach. The combined experimental and computational evidence revealed three stable radical species at room temperature. Two of these could reliably be established as net H-abstracted species, one from the methylene group in the amino acid side chain and the second from the amino acid back-bone carbon atom. For the third room temperature radical species, a variety of deamination- and decarboxylation products were investigated and dismissed. Even if an unambiguous structure could not be established, this radical is tentatively suggested to descend from a side-chain O3-protonated amide anion by hydrogen atom transfer from the C3 to the C4 position.