title: Direct radiation effects to the amino acid side chain: EMR and periodic DFT of X-irradiated L-asparagine at 6 K.
abstract: Radical formation in single crystals of L-asparagine monohydrate following X-irradiation at 6 K has been investigated at 6 K and at elevated temperatures using various electron magnetic resonance (EMR) techniques such as electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) spectroscopy. Molecular structures of the three free radicals stable at 6 K were assessed by detailed analysis of the experimental data and density functional theory (DFT) calculations in a periodic approach. Radical LI is assumed to result from one-electron reduction at the amide functional group in the asparagine side chain followed by protonation at the amide carbonyl oxygen by proton transfer from a neighboring molecule across a hydrogen bond. Radical LII is assigned to a one-electron reduction of the carboxyl group in the amino acid backbone, followed by proton transfer across a hydrogen bond between a carboxylic oxygen and a neighboring asparagine molecule. Radical LIII is suggested to be formed by a net CO2 abstraction from an initial one-electron oxidized amino acid backbone. For the DFT modeling of LIII at 6 K, it was chosen to include the CO2 group stably embedded in the crystalline lattice. The assignments made are discussed in relation to previous work on L-asparagine. The relevance of these results to possible charge transfer processes in protein:DNA complexes is discussed.
authors.affiliation: Department of Physics, University of Oslo , P.O. Box 1048, Blindern, N-0316 Oslo, Norway.
authors.forename: Live F
authors.initials: LF
authors.lastname: Øyen
authors.forename: Siv G
authors.initials: SG
authors.lastname: Aalbergsjø
authors.forename: Ingerid S
authors.initials: IS
authors.lastname: Knudtsen
authors.forename: Eli O
authors.initials: EO
authors.lastname: Hole
authors.forename: Einar
authors.initials: E
authors.lastname: Sagstuen
mesh_terms: D001216:Asparagine / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D013696:Temperature; D014965:X-Rays
chemical_list: D005609:Free Radicals; D001216:Asparagine