Understanding the EPR Parameters of Glycine-Derived Radicals: The Case of N-Acetylglycyl in the N-Acetylglycine Single-Crystal Environment

Development and Validation of the B3LYP/N07D Computational Model for Structural Parameter and Magnetic Tensors of Large Free Radicals

Extensive calculations on a large set of free radicals containing atoms of the second and third row show that the B3LYP/N07D computational model provides remarkably accurate structural parameters and magnetic tensors at reasonable computational costs. The key of this success is the optimization of core-valence s functions for hyperfine coupling constants, while retaining (and even improving) the good performances of the parent 6-31+G(d,p) basis set for valence properties through reoptimization of polarization and diffuse p functions.

Halogen bonds between 2,2,6,6-tetramethylpiperidine-N-oxyl radical and CxHyFzI species: DFT calculations of physicochemical properties and comparison with hydrogen bonded adducts

Structural, thermodynamic, and magnetic properties of adducts between the 2,2,6,6-tetramethylpiperidine-N-oxyl radical and representative hydrogen and halogen bond donors in solution have been investigated by an integrated computational tool including hybrid density functionals and discrete-continuum solvent models. From a quantitative point of view, the computed values show a fair agreement with experiment when environmental effects are taken into the proper account. From a more general point of view, our analysis points out a number of analogies, but also some difference, between hydrogen and halogen bond, which have been interpreted in terms of the various effects tuning thermodynamic and spectroscopic parameters.

Understanding the electon paramagnetic resonance parameters of protein-bound glycyl radicals

The number of enzymes that require a glycyl-based radical for their function is growing. Here, we provide systematic quantum-chemical studies of spin-density distributions, electronic g-tensors, and hyperfine couplings of various models of protein-bound glycyl radicals. Similarly to what is found in a companion paper on N-acetylglycyl, the small g-anisotropy for this delocalized, unsymmetrical system presents appreciable challenges to state-of-the-art computational methodology. This pertains to the quality of structure optimization, as well as to the choice of the spin-orbit Hamiltonian and the gauge origin of the magnetic vector potential. Environmental effects due to hydrogen bonding are complicated and depend in a subtle fashion on the different intramolecular hydrogen bonding for different conformations of the radical. Indeed, the conformation has the largest overall effect on the computed g-tensors (less so on the hyperfine tensors). This is discussed in the context of different g-tensors obtained by recent high-field electron paramagnetic resonance (EPR) measurements for three different enzymes. On the basis of results of a parallel calibration study for N-acetylglycyl, it is suggested that the glycyl radical observed for E. coli anaerobic RNR may have a fully extended conformation, which differs from those of the corresponding radicals in pyruvate formate-lyase or benzylsuccinate synthase.