Fig. 3. Chemical interactions between TEMPOL radical and amino acids mapped by DFT calculations.

The structural and energy landscapes of H-bonded glycine (a), alanine (b), serine (c), proline (d)-TEMPOL complexes are presented. The relative Gibbs free energies (ΔΔG) of the complexes involving different binding sites are referenced to the amino-binding state. The SOMO orbitals and the corresponding spin densities are presented along with the energy. The TEMPOL-substrate interaction sites are indicated with * symbol. The magnitude of spin densities on various carbons is presented in gray scale as shown in (d).