XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Spin Catalysis of Dioxygen Activation by Enzymes
Boris Minaev,1,2 Valentina Minaeva1 and Hans Agren2
1Department of Chemistry, Bogdan Khmelnitskij National University, Cherkasy, Ukraine
2Department of Theoretical Chemistry, Institute of Biotechnology, Royal Institute of Technology, Stockholm, Sweden
In stable organic substances all electron spins are paired and molecules have the singlet ground state. In order to activate chemical transformations and cleave the chemical bond one has to produce spin uncoupling. Interaction with collision partner in bimolecular reactions of diamagnetic species usually leads to activation barrier produced by avoided crossing of the closed shell reactant state and the doubly-triplet excited singlet state. In order to lower the barrier one can add new non-paired electrons to the reacting system or induce spin flip for enhancement of the inter-system crossing to the singly-excited triplet state. This can be done by exchange interaction with paramagnetic transition metal, or by internal (spin-orbit coupling) and external magnetic fields. This is the key idea behind spin-catalysis [1-7].
Activation barriers in chemical reactions are determined by the exchange repulsion between molecules in the singlet ground states. The barriers are often getting much lower when, at least, two spins are unpaired, thus when the singlet-triplet transition occurs. Chemical reactivity is often coded by the triplet excited state of the molecule, in which two spins are unpaired (because exchange interaction destabilizes chemical bond in this case). Enzymatic reactions in live matter are so efficient because they often involve strong spin uncoupling induced by transition metals. This is especially important for O2 (dioxygen) production by photosynthesis of green plants and for dioxygen consumption by respiration of mammals.
Molecular oxygen is a vital elixir beneath the sun. Lavoisier had established that dioxygen is essential for aerobic life and is the oxidant for combustion of organic fuels. But the main puzzle of molecular oxygen is still unclear in modern biochemistry: the O2 molecule has a triplet ground state and how it can react with stable organic substances at 36.7 C to produce water and CO2 if the processes are spin-forbidden? These transformations are equivalent to combustion in the net energetic and material balance. Combustion is a radical chain process. In order to burn a fuel one needs to produce a spark (to create first radical). The radical can interact with triplet O2 and create new radical (the brunch chain reaction). In mammals the organic food is not burned by radical chain process, since the temperature is lower than in the fire. Though the oxidation of food produces enough energy for life, the reactions are spin-forbidden and can be activated and controlled by oxidase enzymes. This should involve particular spin catalysis: the strictly controlled T-S transition in the enzyme active site with low activation energy, which does not produce any radicals, but only short-lived biradical intermediate. The T-S transition means the subsequent spin flip in the biradical.
We present DFT calculations of the electronic structure, zero-field splitting (ZFS) and the phosphorescence spectra of biopolymers that contain chromophores being excited to the triplet state (porphyrin systems, chlorophylls and bacteriochlorophylls, aromatic amino acids, polypeptides and proteins, riboflavin, FAD, FADH2, nicotinamide, NAD+, NADH and active centers of glucose oxidase, copper amine oxidase, hemoglobin, horse-reddish peroxidase and cytochrome P450). This is used for better understanding of the role of spectra in structural analysis of biopolymers and also for predictions of spin catalysis models in electron transfer and oxygen activation processes in these biosystems. The spin transitions are important for enzymatic reactions with and without paramagnetic transition metal involvement. Both types of spin catalysis are studied by DFT calculations of reaction models.
Our experience in fine structure calculations and spin-selective photoprocesses in ZFS triplet states helps us to consider the influence of external magnetic field on the T-S transitions in dark enzymatic reactions. Analysis of possible magnetic field effects on bioprocesses, including birds navigation in the Earth field, is also presented.

1. B.F. Minaev. Electronic mechanisms of molecular oxygen activation. Rus. Chem. Rev., 76 (2007) 998-1023.
2. B.F. Minaev, V.A. Minaeva. Spin-dependent binding of dioxygen to heme. Ukrainica Bioorganica Acta,2 (2008) 56-64.
3. B.F. Minaev. Solvent induced emission of singlet oxygen. J. Mol. Struct. (Theochem), 183 (1989) 207-214.
4. B.F. Minaev. Intensities of spin-forbidden transitions in molecular oxygen and selective heavy-atom effects. Int. J. Quant. Chem., 17 (1980) 367-374.
5. R. Prabhakar, P. Siegbahn, B.F. Minaev, H. Agren. Activation of dioxygen by glucose oxidase. J. Phys. Chem., B, 106 (2002) 3742-3750.
6. B.F. Minaev. Spin effects in reductive activation of O2 by oxidases. RIKEN Rev., 44 (2002) 147-149.
7. B.F. Minaev, H. Agren. Spin catalysis phenomena. Int. J. Quant. Chem., 57 (1996) 510-525.

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