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Nuclear spin relaxation in paramagnetic complexes of S=1: Electron spin relaxation effects (1999)

Bertini, Ivano

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 111 (1999), S. 5795-5807

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Electron spin relaxation for an S=1 system and its field dependence in the presence of static zero-field splitting (ZFS) has been described and incorporated in a model for nuclear spin-lattice relaxation in paramagnetic complexes in solution, proposed earlier by the group in Florence. Slow reorientation is assumed and the electron spin energy level structure (at any orientation of the molecule with respect to the laboratory frame) is described in terms of the Zeeman interaction and of the static ZFS. The electron spin relaxation is assumed to be caused by a transient ZFS modulated by the deformation of the complex described as a distortional (or pseudorotational) motion and the Redfield theory is used to derive the electron spin relaxation matrices. In the description of the electron spin relaxation we neglect any contribution from mechanisms involving modulation by reorientation, such as those of the static ZFS and the less important Zeeman interaction, as we limit ourselves to the slow-rotation limit (i.e., τR(very-much-greater-than)τS). This in general covers the behavior of proteins and macromolecules. The decomposition (DC) approximation is used, which means that the reorientational motion and electron spin dynamics are assumed to be uncorrelated. This is not a serious problem, due to the slow-rotation condition, since reorientational and distortional motions are time-scale separated. The resulting nuclear magnetic relaxation dispersion (NMRD) profiles obtained using the Florence model are calculated and compared with the calculations of the Swedish approach, which can be considered essentially exact within the given set of assumed interactions and dynamic processes. That theory is not restricted by the Redfield limit and can thus handle electron spin relaxation in the slow-motion regime, which is a consequence of not explicitly defining any electron spin relaxation times. Furthermore, the DC approximation is not invoked, and in addition, the electron spin relaxation is described by reorientationally modulated static ZFS and Zeeman interaction besides the distortionally modulated transient ZFS. The curves computed with the Florence model show a satisfactory agreement with these more accurate calculations of the Swedish approach, in particular for the axially symmetric static ZFS tensor, providing confidence in the adequacy of the electron spin relaxation model under the condition of slow rotation. The comparison is also quite instructive as far as the physical meaning of the electron spin relaxation and of its interplay with the nuclear spin system are concerned. © 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.479876

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Simultaneous interpretation of Mössbauer, EPR and 57Fe ENDOR spectra of the [Fe4S4] cluster in the high-potential iron protein I Ectothiorhodospira halophila (1999)

Dilg, A. W. E. ; Mincione, Giovanna ; Achterhold, Klaus ; [et al.]

Springer

JBIC 4 (1999), S. 727-741

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ISSN: 1432-1327

Keywords: High-potential iron protein IEctothiorhodospira halophila ; Mössbauer ; Electron paramagnetic resonance ; 57Fe electron-nuclear double resonance

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology

Notes: 4 S4]3 + and the reduced [Fe4S4]2 + clusters in the high-potential iron protein I from Ectothiorhodospira halophila were measured in a temperature range from 5 K to 240 K. EPR measurements and 57Fe electron-nuclear double resonance (ENDOR) experiments were carried out with the oxidized protein. In the oxidized state the cluster has a net spin S = 1/2 and is paramagnetic. As common in [Fe4S4]3 + clusters, the Mössbauer spectrum was simulated with two species contributing equally to the absorption area: two Fe3 + atoms couple to the “ferric-ferric” pair, and one Fe2 + and one Fe3 + atom give the “ferric-ferrous pair”. For the simulation of the Mössbauer spectrum, g-values were taken from EPR measurements. A-tensor components were determined by 57Fe ENDOR experiments that turned out to be a necessary source of estimating parameters independently. In order to obtain a detailed agreement of Mössbauer and ENDOR data, electronic relaxation has to be taken into account. Relaxing the symmetry condition in a way that the electric field gradient tensor does not coincide with g- and A-tensors yielded an even better agreement of experimental and theoretical Mössbauer spectra. Spin-spin and spin-lattice relaxation times were estimated by pulsed EPR; the former turned out to be the dominating mechanism at T = 5 K. Relaxation times measured by pulsed EPR and obtained from the Mössbauer fit were compared and yield nearly identical values. The reduced cluster has one additional electron and has a diamagnetic (S = 0) ground state. All the four irons are indistinguishable in the Mössbauer spectrum, indicating a mixed-valence state of Fe2.5 + for each.