General magnetic dipolar interaction of spin-spin coupled molecular dimers. Application to an EPR spectrum of xanthine oxidase (original) (raw)
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Journal of the American Chemical Society, 1994
The structural organization of paramagnetic centers in biomolecules can be predicted on the basis of a quantitative study of their magnetic interactions. These studies are usually carried out within the framework of the so-called point dipole approximation, in which the delocalization of the magnetic moments over the centers is ignored. In this paper, we examine how this delocalization can be taken into account in the spin Hamiltonian describing the magnetic interactions between polynuclear paramagnetic centers. A local spin model is described and applied first to a system made of a dinuclear center interacting with a mononuclear center and then to a system comprising two dinuclear centers. In both cases, the EPR spectrum calculated from the local spin model is compared to that given by the point dipole model for different geometrical configurations. The model is illustrated by a detailed quantitative study of the magnetic interactions between the molybdenum center and one [2Fe-2S] center (center 1) in the enzyme xanthine oxidase. These studies emphasize the effective character of some important structural parameters given by numerical simulations of EPR spectra based on the point dipole approximation.
Size effect on local magnetic moments in ferrimagnetic molecular complexes: an XMCD investigation
Monatshefte für Chemie/ …, 2003
Molecular chemistry allows to synthesize new magnetic systems with controlled properties such as size, magnetization or anisotropy. The theoretical study of the magnetic properties of small molecules (from 2 to 10 metallic cations per molecule) predicts that the magnetization at saturation of each ion does not reach the expected value for uncoupled ions when the magnetic interaction is antiferromagnetic. The quantum origin of this effect is due to the linear combination of several spin states building the wave function of the ground state and clusters of finite size and of finite spin value exhibit this property. When single crystals are available, spin densities on each atom can be experimentally given by Polarized Neutron Diffraction (PND) experiments. In the case of bimetallic MnCu powdered samples, we will show that X-ray Magnetic Circular Dichroism (XMCD) spectroscopy can be used to follow the evolution of the spin distribution on the Mn II and Cu II sites when passing from a dinuclear MnCu unit to a one dimensional (MnCu) n compound.
Electron spin echo envelope modulation spectroscopy of the molybdenum center of xanthine oxidase
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1994
The applications of electron spin echo envelope modulation (ESEEM) spectroscopy to study paramagnetic centers in photosystem I (PSI) are reviewed with special attention to the novel spectroscopic techniques applied and the structural information obtained. We briefly summarize the physical principles and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. In PSI, ESEEM spectroscopy has been used to the study of the cation radical form of the primary electron donor chlorophyll species, P 700 , and the phyllosemiquinone anion radical, A 3 1 , that acts as a lowpotential electron carrier. For P 700 , ESEEM has contributed to a debate concerning whether the cation is localized on a one or two chlorophyll molecules. This debate is treated in detail and relevant data from other methods, particularly electron nuclear double resonance (ENDOR), are also discussed. It is concluded that the ESEEM and ENDOR data can be explained in terms of five distinct nitrogen couplings, four from the tetrapyrrole ring and a fifth from an axial ligand. Thus the ENDOR and ESEEM data can be fully accounted for based on the spin density being localized on a single chlorophyll molecule. This does not eliminate the possibility that some of the unpaired spin is shared with the other chlorophyll of P 700 ; so far, however, no unambiguous evidence has been obtained from these electron paramagnetic resonance methods. The ESEEM of the phyllosemiquinone radical A 3 1 provided the first evidence for a tryptophan molecule Z-stacked over the semiquinone and for a weaker interaction from an additional nitrogen nucleus. Recent site-directed mutagenesis studies verified the presence of the tryptophan close to A 1 , while the recent crystal structure showed that the tryptophan was indeed Z-stacked and that a weak potential H-bond from an amide backbone to one of the (semi)quinone carbonyls is probably the origin of the to the second nitrogen coupling seen in the ESEEM. ESEEM has already played an important role in the structural charaterization on PSI and since it specifically probes the radical forms of the chromophores and their protein environment, the information obtained is complimentary to the crystallography. ESEEM then will continue to provide structural information that is often unavailable using other methods. ß 2001 Published by Elsevier Science B.V.
Inorganic Chemistry, 2007
The careful validation of modern density functional methods for the computation of electron paramagnetic resonance (EPR) parameters in molybdenum complexes has been extended to a number of low-symmetry Mo V systems that model molybdoenzyme active sites. Both g and hyperfine tensors tend to be reproduced best by hybrid density functionals with about 30−40% exact-exchange admixture, with no particular spin contamination problems encountered. Spin−orbit corrections to hyperfine tensors are mandatory for quantitative and, in some cases, even for qualitative agreement. The g 11 (g | ) component of the g tensor tends to come out too positive when spin−orbit coupling is included only to leading order in perturbation theory. Compared to single-crystal experiments, the calculations reproduce both gand hyperfine-tensor orientations well, both relative to each other and to the molecular framework. This is significant, as simulations of the EPR spectra of natural-abundance frozen-solution samples frequently do not allow a reliable determination of the hyperfine tensors. These may now be extracted based on the quantumchemically calculated parameters. In a number of cases, revised simulations of the experimental spectra have brought theory and experiment into substantially improved agreement. Systems with two terminal oxo ligands, and to some extent with an oxo and a sulfido ligand, have been confirmed to exhibit particularly large negative ∆g 33 shifts and thus large g anisotropies. This is dicussed in the context of the experimental data for xanthine oxidase.
Studies of the substrate binding to xanthine oxidase using a spin-labeled analog
Journal of Inorganic Biochemistry, 1994
A spin-labeled adenine derivative [N6-(2,2,6,6-tetramethyl-1-o~iperidin-4-yl)adenine; SLADI is found to be a very slow substrate of xanthine oxidase based on the observed reduction of enzyme by SLAD under anaerobic conditions. A room-temperature EPR spectrum of SLAD in the presence of oxidized xanthine oxidase shows the appearance of "wings" on the three-line spectrum of the free spin-label, indicating formation of an E*SLAD complex. This spectrum can be obtained on a timescale that is short compared to catalysis. Using this spectral change as an experimental probe, the room-temperature K,'s of SLAD binding to oxidized xanthine oxidase at various pH's have been determined. Obtained K, values are 1.5 f 0.3 mM, 1.6 f 0.3 mM, and 1.5 f 0.3 mM at pH 10.0, 8.5, and 7.0, respectively, indicating no significant difference in the equilibrium dissociation of SLAD from enzyme upon pH change. These results are consistent with the calculated equilibrium dissociation constant for substrate binding to oxidized molybdenum center based on K, to reduced enzyme and the perturbation of Movr/Mov and Mov/Morv reduction potentials by product and substrate analogs.
The Biochemical journal, 1980
Rapid type 2 molybdenum(V) e.p.r. signals from reduced functional xanthine oxidase have been further investigated. These signals, which show strong coupling of two protons to molybdenum, have been obtained under a variety of new conditions: specifically either at pH 8.2 in the presence of borate ions, or at pH 10.1--10.7 with or without various other additions. Parameters of the signals were obtained with the help of computer simulations. In at least some of these signals, the coupled protons must be located on the enzyme rather than on bound species. The relationship between type 1 and type 2 Rapid signals is discussed. They may represent geometrical isomers, or alternatively, hydroxyl uptake as a ligand of molybdenum may be involved in formation of type 2 species.
Interpretation and Quantification of Magnetic Interaction through Spin Topology
This work develops a formalism to quantify the interaction among unpaired spins from the ground state spin topology. Magnetic systems where the spins are coupled through direct exchange and superexchange are chosen as references. Starting from a general Hamiltonian, an effective Hamiltonian is obtained in terms of spin density which is utilized to compute exchange coupling constants in magnetic systems executing direct exchange. The high-spin−low-spin energy gap, required to extract the coupling constant, is obtained through the broken symmetry approach within the framework of density functional theory. On the other hand, a perturbative approach is adopted to address the superexchange process. Spin transfer in between the sites in the exchange pathway is found to govern the magnetic nature of a molecule executing superexchange. The metal−ligand magnetic interaction is estimated using the second order perturbation energy for ligand to metal charge transfer and spin densities on the concerned sites. Using the present formalism, the total coupling constant in a superexchange process is also partitioned into the contributions from metal−ligand and metal−metal interactions. Sign and magnitude of the exchange coupling constants, derived through the present formalism, are found to be in parity with those obtained using the well-known spin projection technique. Moreover, in all of the cases, the ground state spin topology is found to complement the sign of coupling constants. Thus, the spin topology turns into a simple and logical means to interpret the nature of exchange interaction. The spin density representation in the present case resembles McConnell's spin density Hamiltonian and in turn validates it.
Journal of Inorganic Biochemistry, 2009
Electron transfer proteins and redox enzymes containing paramagnetic redox centers with different relaxation rates are widespread in nature. Despite both the long distances and chemical paths connecting these centers, they can present weak magnetic couplings produced by spin-spin interactions such as dipolar and isotropic exchange. We present here a theoretical model based on the Bloch-Wangsness-Redfield theory to analyze the dependence with temperature of EPR spectra of interacting pairs of spin 1/2 centers having different relaxation rates, as is the case of the molybdenum-containing enzyme aldehyde oxidoreductase from Desulfovibrio gigas. We analyze the changes of the EPR spectra of the slow relaxing center (Mo(V)) induced by the faster relaxing center (FeS center). At high temperatures, when the relaxation time T 1 of the fast relaxing center is very short, the magnetic coupling between centers is averaged to zero. Conversely, at low temperatures when T 1 is longer, no modulation of the coupling between metal centers can be detected.
Shaping distinct magnetic interactions in molecular compounds
Journal of Magnetism and Magnetic Materials, 2011
Oxalates containing various 3d transitional elements and positive NH 4 or negative OH groups were newly synthesized. Each above-mentioned component has directly influenced the structure, the electronic or interaction properties, while some unexpected behaviors were revealed by various magnetic and Mössbauer measurements. The main magnetic parameters, the long-range anti-ferromagnetic couplings observed at very low temperature and, particularly the uncompensated moment are discussed in detail. The induced lower spin states for bivalent ions and especially the anti-parallel arrangement of the spins belonging to trivalent and bivalent iron inside the molecule are also emphasized.
Experimental and Theoretical Spin Density in a Ferromagnetic Molecular Complex
The association of phenylboronic acid (no unpaired electron) with the free radical phenyl nitronyl nitroxide (S = 1/2) constitutes an inter-heteromolecular hydrogen bonding system presenting ferromagnetic intermolecular interactions. We have investigated its spin density distribution in order to visualize the pathway of these magnetic interactions. The spin density of this complex was measured by polarized neutron diffraction. The data were treated using both direct and indirect methods. As in the isolated PNN, the main part of the spin density is located on the O-N-C-N-O fragment of the PNN radical. But, with the PNNB, the global spin density distribution give evidences that the phenylboronic acid constitutes a spin transmission path between PNN radicals via hydrogen bonds. The experimental results are compared to those obtained by density functional theory calculations.