Enzyme activation of molecular oxygen

The oxygen molecule possesses a triplet ground state but the connections between the paramagnetic nature of the O$_2$ molecule, its sluggish reactivity and activation by enzymes are still not completely understood. The higher energy and spin-allowed reactivity of singlet dioxygen than the triplet O$_2$ is a major factor of radical balance in living cells. The involvement of O$_2$ in respiration is accompanied by dioxygen transport systems and O$_2$-carring proteins. The interaction of dioxygen with human hemoglobin is studied now by numerous physical methods, including impuls photolysis and EPR techniques. DFT calculations of paramagnetic intermediates in such cooperative processes with account of EPR and NMR spectral modeling would be very helpful for understanding the dioxygen transport systems.

Another important issue in this respect is O$_2$ activation by cytochromes and by other reductive enzymes (oxygenases). A concerted insertion of the ground state dioxygen into organic (diamagnetic) molecules is a spin-forbidden process. Such a limited reactivity is essential for biosynthetic processes, otherwise dioxygen would burn the substrates rather than supplying energy in a controlled way. The biological systems activate triplet dioxygen for controlled chemical synthesis via electron-transfer and proton transfer reduction. Dioxygen generally then undergoes reactions in a stepwise manner via formation of free radical intermediates with unpaired electrons. It is clear that modern high-field EPR and paramagnetic NMR technologies can give clues to the understanding of dioxygen activation in $in$ $vivo$, provided the measured quantities could be related to the presence and character of these intermediate radicals through simulations of the spin-Hamiltonian parameters. Paramagnetic protein complexes of transition metal ions play a crucial role in O$_2$ activation and usually EPR studies of such reactions have mostly relied on the symmetry of the g-tensor and on the magnitude of HFC constants in the analysis of the reaction center, while the quantitative structure of the g-tensor, which provides additional information on the ligands is difficult to analyse in term of structural characteristics. It has mostly been used for analytical identification of the active radical center. With our new DFT methodology for calculations of EPR g-tensors and NMR parameters it is possible to make an accurate quantitative identification of the radical structures.