Antioxidant Properties of Kynurenines: Density Functional Theory Calculations
Fig 2
The highest occupied molecular orbitals (HOMOs) of kynurenines and phenolic antioxidants.
Color scheme, atoms: H–white, C–grey, O–red, N–blue. Isosurface value: 0.05. The lowest unoccupied molecular orbital (LUMO) has multiple nodes in the bonding region, mainly localized outside of the OH group (S1 Fig). For the ionized compounds, LUMO is moved from the charged group to the aromatic system; for L-3HOKNH3+, its form is nearly the same as that of L-3HOK HOMO. The geometry of HOMOs closely resembles the geometry of spin-orbits of cation radicals (S2 Fig). Hence HOMO correctly reproduces the geometry of the electron density in phenolic cations calculated at the DFT level. The main differences are between anion HOMOs and the corresponding spin-orbits after electron abstraction: spin-orbit is localized mainly on aromatic atoms and partly on α-carboxylic group. Spin-orbit of radicals after H-atom abstraction is localized on O*-atom and π-conjugated moiety, including the aromatic system and unsaturated side chains (S3 Fig). Delocalization is low in L-KYN, KYNAENOL, and phenol radicals. This may explain their lower capability to donate H compared with hydroxykynurenines and substituted phenols.