Long-distance spin-spin interactions with iron-sulphur clusters as observed by e.p.r. spectroscopy (original) (raw)
Related papers
Spin lattice relaxation and exchange interaction in a 2-iron, 2-sulphur protein
Biochimica et Biophysica Acta (BBA) - Protein Structure, 1976
A two-iron-two-sulphur non-haem iron protein, the ferredoxin from Spirulina maxima, has been studied by means of electron paramagnetic resonance (EPR) in the range where the spectrum loses resolution with increasing temperature. The spinlattice relaxation times were deduced from linewidths measured by spectral simulation and their variation as a function of temperature is interpreted in terms of an Orbach mechanism. On this basis, the exchange integral between the two iron atoms, assuming an antiferromagnetic interaction between them, is estimated to be --83 cm -~.
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.
Spin-Orbit Coupling in Enzymatic Reactions and the Role of Spin in Biochemistry
Handbook of Computational Chemistry, 2012
We review the general concept of nonadiabatic quantum spin transitions in biochemistry. A few important examples are highlighted to illustrate the concept: the role of spin effects in oxidases, cytochromes, in dioxygen binding to heme, in photosynthesis, and in tentative models of consciousness. The most thoroughly studied of these effects are connected with dioxygen activation by enzymes. Discussion on the mechanisms of overcoming spin prohibitions in dioxygen reactions with flavin-dependent oxygenases and with hemoglobin and myoglobin is presented in some detail. We consider spin-orbit coupling (SOC) between the starting triplet state from the entrance channel of the O binding to glucose oxidase, to ferrous heme, and the final singlet open-shell state in these intermediates. Both triplet (T) and singlet (S) states in these examples are dominated by the radical-pair structures D + −O − induced by charge transfer; the peculiarities of their orbital configurations are essential for the SOC analysis. An account of specific SOC in the open π g -shell of dioxygen helps to explain the probability of T-S transitions in the active site near the transition state. Simulated potential energy surface cross-sections along the reaction coordinates for these multiplets, calculated by density functional theory, agree with the notion of a relatively strong SOC induced inside the oxygen moiety by an orbital angular momentum change in the π g -shell during the T-S transition. The SOC model explains well the efficient spin inversion during the O binding with heme and glucose oxidase, which constitutes a key mechanism for understanding metabolism. Other examples of nontrivial roles of spin effects in biochemistry are briefly discussed.
Chemical Physics, 2009
The biradical recombination probability and its field dependence in the presence of a paramagnetic particle was calculated in balance approximation taking into account the extended character of the exchange interaction in the biradical. Previously, to describe the effect of the third spin on the spin evolution in the biradical, we considered the constant spin exchange interaction both inside the biradical and between the added spin and one of the paramagnetic biradical centers. In the present work we considered the long-distance character of the exchange interaction of the biradical with the third spin. It is shown that the field dependence retains the main characteristic peculiarities of the three-spin system in the approximation of the constancy of the exchange interaction, i.e., the existence of several extrema, their shift along the field depending on the values of the exchange integrals, and the existence of a field-independent, nonzero recombination probability, known as spin catalysis.
Electron-electron spin-spin interaction in spin-labeled low-spin methemoglobin
Biophysical Journal, 1995
Nitroxyl free radical electron spin relaxation times for spin-labeled low-spin methemoglobins were measured between 6 and 120 K by two-pulse electron spin echo spectroscopy and by saturation recovery electron paramagnetic resonance (EPR). Spin-lattice relaxation times for cyano-methemoglobin and imidazole-methemoglobin were measured between 8 and 25 K by saturation recovery and between 4.2 and 20 K by electron spin echo. At low temperature the iron electron spin relaxation rates are slow relative to the iron-nitroxyl electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(lIl) become comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl electron spin echo decay rate. Throughout the temperature range examined, interaction with the Fe(lIl) increases the spin lattice relaxation rate (1/T1) for the nitroxyl. The measured relaxation times for the Fe(lIl) were used to analyze the temperature-dependent changes in the spin echo decays and in the saturation recovery (T1) data for the interacting nitroxyl and to determine the interspin distance, r. The values of r for three spin-labeled methemoglobins were between 15 and 15.5 A, with good agreement between values obtained by electron spin echo and saturation recovery. Analysis of the nitroxyl spin echo and saturation recovery data also provides values of the iron relaxation rates at temperatures where the iron relaxation rates are too fast to measure directly by saturation recovery or electron spin echo spectroscopy. These results demonstrate the power of using time-domain EPR measurements to probe the distance between a slowly relaxing spin and a relatively rapidly relaxing metal in a protein.
The Journal of Physical Chemistry, 1979
An analysis is given of the general spin-spin dipolar interaction in the presence of a general bilinear superexchange interaction, and its effects on a rigid lattice EPR line shape are calculated. The general magic angle effect is considered, numerically, for specific cases of both single crystal spectra and randomly oriented microcrystallite samples. Analysis of the number of determinable spin-spin coupling coefficients suggests that a component analysis can be developed which can yield useful information about the geometrical arrangement of two interacting spin = molecules, as well as covalency information about the interaction. This analysis is applied to a Mo(V)-iron sulfur interaction in xanthine oxidase and xanthine dehydrogenase, which has been described by Lowe and Bray. Line shape calculations show that the observed small amount of dipolar anisotropy is consistent with a separation of the MOW) and Fe/S groups of 14 8, or less. Considerations of the long-range nature of superexchange, from existing experimental data on superexchange, are consistent with this estimate.
Magnetic interactions and spin densities in molecular compounds: an example
The discovery of ferromagnetism in the charge transfer salt [Fe(Cp*2)]•+[TCNE]•+ has raised a lot of interest. The nature of this magnetic coupling was controversial. The group of Miller invoked a McConnell II mechanism, with con"guration interactions. Another view was given by Kahn and coworkers who proposed a coupling between spin carriers (McConnell I mechanism), implying a positive exchange due to an overlap between spin densities of opposed signs. In the "rst interpretation, there would be a positive spin density located on the carbon rings of the ferrocene, but in the second interpretation, this spin density should be negative. To clarify this mechanism, it was decided to investigate separately, by polarized neutron di!raction, the spin density of [TCNE]z•+, associated with a nonmagnetic donor and the magnetiz- ation density of [Fe(Cp*2)]•+, associated with a nonmagnetic acceptor. For [TCNE]•+, the measurement was straightfor- ward: it was found that most of the spin density was located on the central carbon while a noticeable amount was delocalized on the terminal nitrogens. Several attempts to measure the magnetization density of [Fe(Cp*2)]•+ were unsuccessful due to a loss of symmetry on cooling. To overcome this difficulty, we have "nally measured the magnetization density of [Fe(Cp*2)]•+ in a crystal of space group P1, where four of these ions are associated with the polyoxotungstate [SiW12O40]4-. We have found that the Fe atoms carry a moment of 2.0 µB and that the carbons of the rings carry -0.005±0.001 µB. The signs of these carbon spin populations are consistent with the McConnell I mechanism, but their magnitudes are too small to account for the experimental interactions.
2012
The function of a living cell, independent of we are talking about a prokaryotic singlecellular organism or a cell in the context of an complex organism like a human, depends on intricate and balanced interaction between its components. Proteins are playing a central role in this complex cellular interaction network: Proteins interact with nucleic acids, with membranes of all cellular compartments, and, what will be in the focus of this article, with other proteins. Proteins interact to form functional units, to transmit signals for example perceived at the surface of the cell to cytoplasmic or nuclear components, or to target them to specific locations. Thus, the study of protein-protein interactions on the molecular level provides insights into the basic functional concepts of living cells and emerged as a wide field of intense research, steadily developing with the introduction of new and refined biochemical and biophysical methods.
Influence of Noncovalent Cation/Anion−π Interactions on the Magnetic Exchange Phenomenon
The Journal of Physical Chemistry Letters, 2011
b S Supporting Information M agnetic materials based on pure organic substances are of fundamental importance. 1,2 The envisaged technological applications of such magnetic materials require that these exhibit strong ferromagnetic exchange interactions. In organic molecular magnetism, the m-phenylene coupler is found to be a promising spacer in convening a substantial ferromagnetic exchange interaction between the two attached spin sources. There have already been a number of efforts devoted to increasing the strength of the ferromagnetic exchange interaction on the basis of m-phenylene couplers. 5À8 Mostly these are based on substitution on the different positions of the spacer using various electron withdrawing or electron donating groups, constraining the rotation of the spin containing groups, and by cross conjugation of various spin-sources. 9,7,10 The m-phenylene coupler mediates ferromagnetic exchange interactions; however, the nonmagnetic singlet state could be the ground state if the dihedral angles between the plane of the phenyl ring and the spin-containing groups become significantly large to prevent the π-conjugation. 11 In this work, we have adopted a different approach to enhance the ferromagnetic exchange coupling interaction mediated by m-phenylene couplers, namely, by deploying an additional external ion near to the coupler. The presence of such ions in the vicinity of the coupler's π-electron cloud can sustain the ionÀπ interactions. These interactions driven by noncovalent forces have already attracted considerable attention in recent years. 12À15 The cationÀπ and anionÀπ interactions originate from the electrostatic and ion-induced polarization terms, which can be rationalized by means of the permanent quadrupole moment in aryl systems. Mecozzi et al. have demonstrated that the calculated electrostatic potential (ESP) at the vertical single point from the center of the benzene ring can be used to predict the strength of cationÀπ interactions. 17 Wheeler and Houk have observed that the noncovalent binding strength solely depends on the direct through-space interaction, the π-polarization effect has very little impact. Alternatively, the anionÀπ interactions have been explained by the π-acidic nature of the aryl substrates. 20,21 Recently such interaction has experimentally been captured as the functional relevance of Cl À ion recognition by anionÀπ interactions. G€ uell et al. have observed the influence of ion-π interactions on the local aromaticity of the aryl species upon a small amount of charge transfer between them. 23 However, the aromaticity of the aryl couplers contributes to a large extent to control the intramolecular magnetic exchange interactions. Ali and Datta have observed a proportionality relationship between the magnetic exchange coupling and difference of aromaticity index (ΔNICS) between the coupled and uncoupled aryl systems. 24 Using configuration interaction (CI) calculations, Barone et al. 25 investigated the contribution of various molecular fragments to the magnetic exchange coupling. They also noticed that the aromatic bridges play a significant role in the magnetic exchange coupling. Bhattacharya et al. and Latif et al. independently observed this connection for a series of biverdazyle radicals. 26, The present work is based on seeking to exploit such fundamental understanding of the relationship between the aromaticity and the strength of the magnetic exchange coupling to tailor the desired magnetic properties.