N. N. Breslavskaya

Magnesium isotope effects in enzymatic phosphorylation.

Recent discovery of magnesium isotope effect in the rate of enzymatic synthesis of adenosine triphosphate (ATP) offers a new insight into the mechanochemistry of enzymes as the molecular machines. The activity of phosphorylating enzymes (ATP-synthase, phosphocreatine, and phosphoglycerate kinases) in which Mg(2+) ion has a magnetic isotopic nucleus 25Mg was found to be 2-3 times higher than that of enzymes in which Mg(2+) ion has spinless, nonmagnetic isotopic nuclei 24Mg or 26Mg. This isotope effect demonstrates unambiguously that the ATP synthesis is a spin-dependent ion-radical process. The reaction schemes, suggested to explain the effect, imply a reversible electron transfer from the terminal phosphate anion of ADP to Mg(2+) ion as a first step, generating ion-radical pair with singlet and triplet spin states. The yields of ATP along the singlet and triplet channels are controlled by hyperfine coupling of unpaired electron in 25Mg+ ion with magnetic nucleus 25Mg. There is no difference in the ATP yield for enzymes with 24Mg and 26Mg; it gives evidence that in this reaction magnetic isotope effect (MIE) operates rather than classical, mass-dependent one. Similar effects have been also found for the pyruvate kinase. Magnetic field dependence of enzymatic phosphorylation is in agreement with suggested ion-radical mechanism.

Chemistry of Enzymatic ATP Synthesis: An Insight through the Isotope Window

Chemistry of Enzymatic ATP Synthesis: An Insight through the Isotope Window Anatoly L. Buchachenko,* Dmitry A. Kuznetsov, and Natalia N. Breslavskaya N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin Street, 119991 Moscow, Russian Federation and Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russian Federation Department of Medicinal Nanobiotechnologies, N. I. Pirogov Russian State Medical University, 1 Ostrovityanov Street, 117997 Moscow, Russian Federation N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation

Magnetic isotope and magnetic field effects on the DNA synthesis

Magnetic isotope and magnetic field effects on the rate of DNA synthesis catalysed by polymerases β with isotopic ions 24Mg2+, 25Mg2+ and 26Mg2+ in the catalytic sites were detected. No difference in enzymatic activity was found between polymerases β carrying 24Mg2+ and 26Mg2+ ions with spinless, non-magnetic nuclei 24Mg and 26Mg. However, 25Mg2+ ions with magnetic nucleus 25Mg were shown to suppress enzymatic activity by two to three times with respect to the enzymatic activity of polymerases β with 24Mg2+ and 26Mg2+ ions. Such an isotopic dependence directly indicates that in the DNA synthesis magnetic mass-independent isotope effect functions. Similar effect is exhibited by polymerases β with Zn2+ ions carrying magnetic 67Zn and non-magnetic 64Zn nuclei, respectively. A new, ion–radical mechanism of the DNA synthesis is suggested to explain these effects. Magnetic field dependence of the magnesium-catalysed DNA synthesis is in a perfect agreement with the proposed ion–radical mechanism. It is pointed out that the magnetic isotope and magnetic field effects may be used for medicinal purposes (trans-cranial magnetic treatment of cognitive deceases, cell proliferation, control of the cancer cells, etc).

Ion-Radical Mechanism of Enzymatic ATP Synthesis: DFT Calculations and Experimental Control

A new, ion-radical mechanism of enzymatic ATP synthesis was recently discovered by using magnesium isotopes. It functions at a high concentration of MgCl(2) and includes electron transfer from the Mg(H(2)O)(m)(2+)(ADP(3-)) complex (m = 0-4) to the Mg(H(2)O)(n)(2+) complex as a primary reaction of ATP synthesis in catalytic sites of ATP synthase and kinases. Here, the structures and electron transfer reaction energies of magnesium complexes related to ATP synthesis are calculated in terms of DFT. ADP is modeled by pyrophosphate anions, protonated (HP(2)O(7)H(2-), HP(2)O(7)CH(3)(2-)) and deprotonated (HP(2)O(7)(3-), CH(3)P(2)O(7)(3-)). The reaction generates an ion-radical pair, composed of Mg(H(2)O)(n)(+) ion and pyrophosphate anion-radical coordinated to Mg(2+) ion. The addition of the latter to the substrate P=O bond results in ATP formation. Populations of the singlet and triplet states and singlet-triplet spin conversion in the pair are controlled by hyperfine coupling of unpaired electrons with magnetic (25)Mg and (31)P nuclei and by Zeeman interaction. Due to these two interactions, the yield of ATP is a function of nuclear magnetic moment and magnetic field; both of these effects were experimentally detected. Electron transfer reaction does not depend on m but strongly depends on n. It is exoergic and energy allowed at 0 < or = n << infinity for the deprotonated pyrophosphate anions and at 0 < or = n < 4 for the protonated ones; for other values of n, the reaction is energy deficient and forbidden. The boundary between exoergic and endoergic regimes corresponds to the trigger magnitude n* (n* = 4 for protonated anions and 6 < n* << infinity for deprotonated ones). These results explain why ATP synthesis occurs only in special devices, molecular enzymatic machines, but not in water (n = infinity). Biomedical consequences of the ion-radical enzymatic ATP synthesis are also discussed.

Mechanism and energetics of 1,2-addition of dioxygen 1O2(1Δg) to ethylene

The optimal geometry and energy parameters for five electronic states of the {1O2 (1Δg) + C2H4} system that characterize the elementary reactions of two-step 1,2-addition giving the dioxetane molecule were calculated using various quantum chemical methods (RHF, B3LYP, MPn, n = 2–4, QCISD, and CCSD) and basis sets (from 6-31+G(d,p) to 6-311+G(3df,2p) and pVTZ). The first step of the reaction was found to pass through the ethylene perepoxide intermediate. Considering experimental and published calculated data, the dependence of the results on the calculation procedure was exampled. The higher-level methods (QCISD, CCSD, CASSCF) and the standard methods (DFT, MPn) were found to reliably lead to virtually the same description of the energetics of this two-step reaction corresponding to experimental estimates.

Comparative quantum-chemical study of complexes of cyclic ether with ethylene glycol

The electronic structure and geometry of cyclic ethers—tetrahydrofuran and 1,4-dioxane—and their 1: 1 and 1: 2 complexes with ethylene glycol, as well as their complexes with ethylene glycol dimers, have been studied by density functional theory method at the B3LYP/6-31+G** level. It has been shown that moderate hydrogen bonds are mainly responsible for complex formation. A great number of conformations of the H-bonded complexes have been found.

Structural phase transition in quasi-one-dimensional H-bonded ferroelectric PbHPO 4 (LHP) crystal: Quantum-chemical analysis

Thermodynamic features of the structural phase transition (SPT) in the H-bonded ferroelectric material PbHPO4 (LHP) have been considered using a pseudo-spin Ising model with inclusion of tunneling and long-range effects. To determine all pseudo-spin Hamiltonian (PSH) parameters necessary for analysis of the SPT—Slater parameters and tunneling integrals, a technique based on an independent quantum-chemical method of their finding was applied. A simplified scheme has been suggested for selecting a model cluster, which makes it possible to use higher-level methods (CCSD and QCISD with the 6-311+G** basis set) in calculations of double-well potential profiles and PSH parameters. The computation results have been discussed in the framework of two statistical models—in the molecular field approximation and using the Bethe cluster method. The critical temperature of the transition of LHP has been evaluated and it has been demonstrated that experimental data can be semiquantitatively reproduced only in the statistical cluster approximation with inclusion of tunneling and long-range effects.

Quantum-chemical analysis of the thermodynamic isotope effect in quasi-one-dimensional H-bonded Pb(H/D)PO4 ferroelectrics

The thermodynamics of the structural phase transition of H-bonded ferroelectric materials, Pb(H/D)PO4, were considered in terms of the pseudo-spin Ising model with inclusion of tunneling and longrange effects. The pseudo-spin Hamiltonian parameters needed for analysis of the transition were determined by a procedure based on an independent quantum chemical method. A simplified scheme for the selection of model clusters was proposed, which allows the application of various quantum chemical methods, including high-level methods (CCSD/6-311+G** and so on), in the calculations of double-well potential profiles and Slater parameters. The calculation results were discussed in terms of two statistic models: molecular field approximation (MFA) and Bethe cluster method (BCM). The theoretical estimates of critical transition temperature for both systems are discussed and it is shown that the (semi)quantitative reproduction of experimental data is possible only in terms of BCM taking into account the tunneling effects. The explanation is given for the observed isotope effect caused by very pronounced increase in the critical transition temperature upon deuteration (ΔTс ≈ 140 K). The crucial role belongs to the difference between tunneling effects in the ferroelectric crystals in question. It is emphasized that the observed differences between the crystal lattice and H/D bond geometries, including the mutual orientation of the bonds, must be accurately included in the calculations.

Quantum-chemical simulation of the elementary step of the oxidation reactions of styrene and its derivatives involving 1О2 (1Δg)

The results of simulation of the oxidation reaction of styrene and its methyl (two isomers) and phenyl derivatives with molecular oxygen in the excited singlet state (1Δg) have enabled the conclusion that the reaction can proceed through several mechanisms. For styrene and its phenyl derivative, three reaction channels are possible, and for the methyl derivative, there are four possible channels. For the first two substrates, the major channel is 1,2-addition to form dioxetane; for the methyl derivatives, an extra channel to give a hydroperoxide species is possible in addition to the above channel. The multichannel reaction character revealed by calculations makes it possible to qualitatively understand the reason behind the moderate selectivity (no more than 70%) of such reactions in the case of styrene and its derivatives.

Quantum-chemical simulation of unit process for the oxidation reactions of ethylene and its derivatives invoving 1O2 (1Δg)

The results of simulation of oxidation reactions of ethylene derivatives with different substituents (F atoms, CH3O and CH3 groups) and butadiene molecule with participation of 1O2 (1Δg) have shown the possibility to realize different routes for the majority of the considered reactions. The largest product variety is obtained for butadiene and CH3 derivatives of ethylene. For butadiene, along with 1,2-cycloaddition reactions resulting in four-membered dioxetane (which is realized in all cases), the possibility to form six-membered cyclic epidioxides (1,4-addition) and diepoxide products with two three-membered rings (epoxidation) has been found. The formation of hydroperoxide forms along with 1,2-addition reactions is also possible for all CH3 derivatives of ethylene. Formation conditions and relative stability of the noted products have been analyzed for each case and certain features of the revealed reaction pathways with the transfer of two oxygen atoms have been discussed.