Understanding the EPR parameters of glycine-derived radicals: the case of N-acetylglycyl in the N-acetylglycine single-crystal environment.
As a step toward an in-depth understanding of the electron paramagnetic resonance parameters of glycyl radicals in proteins, the hyperfine tensors and, particularly, the g-tensor of N-acetylglcyl in the environment of a single crystal of N-acetylglycine have been studied by systematic state-of-the-art quantum chemical calculations on various suitable model systems. The quantitative computation of the g-tensors for such glycyl-derived radicals is a veritable challenge, mainly because of the very small g-anisotropy combined with a nonsymmetrical, delocalized spin-density distribution and several atoms with comparable spin-orbit contributions to the g-tensors. The choice of gauge origin of the magnetic vector potential, and of approximate spin-orbit operators, both turn out to be more critical than found in previous studies of g-tensors for organic radicals. Environmental effects, included by supermolecular hydrogen-bonded models, were found to be moderate, because of a partial compensation between the influences from intramolecular and intermolecular hydrogen bonds. The largest effects on the g-tensor are caused by the conformation of the radical. The density functional theory methods employed systematically overestimate both the Delta gx and Delta gy components of the g-tensor. This is important for parallel investigations on the protein-glycyl radicals. The 1H alpha and 13C alpha hyperfine couplings depend only slightly on the supermolecular model chosen and appear less sensitive probes of detailed structure and environment.
Kacprzak
Sylwia
S
Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D 97074 Würzburg, Germany.
Reviakine
Roman
R
Kaupp
Martin
M
D000596:Amino Acids / Q000737:chemistry; D002244:Carbon / Q000737:chemistry; D002627:Chemistry, Physical / Q000379:methods*; D003460:Crystallization; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D008968:Molecular Conformation; D012984:Software; D013816:Thermodynamics; D014965:X-Rays
D000596:Amino Acids; D005609:Free Radicals; D002244:Carbon; D006859:Hydrogen; D005998:Glycine; C006368:aceturic acid