Spin coupling in iron proteins and model complexes

Transition metal ions are essential for the specific function of many biomolecules, like transport and activation of small molecules, electron transfer, etc. Their active sites contain (i) single paramagnetic or diamagnctic metal ions, or (ii) clusters of spin-coupled paramagnetic centers. Examples are for (i) hemoglobin or rubredoxin with one Fe, and for (ii) ferredoxins with [2Fe-2S] and [4Fe-4S] clusters, cytochrome c oxidasc containing Fe and Cu, or peroxidases with metal-radical pairs. Here, we present the results of our Mössbauer and EPR investigation of irondioxygen interaction in horse liver alcohol dehydrogenase (HLADH), of ironradical interaction in synthetic analogues of heme peroxidases and of iron-iron double exchange interaction in a synthetic ferric/ferrous mixed valence complex. We hope to demonstrate that the understanding of spin coupling can yield valuable insight in the relation between electronic structure and function of biological iron complexes.

     

Spin coupling in iron-containing proteins and model complexes

In the field of transition metals in biology we are concerned with a variety of homo- and hetero-polynuclear metal clusters which exhibit spin coupling. A further example for spin coupling in this field is provided by mononuclear transition metal complexes, with the paramagnetic metal site and an additional nonmetallic site forming a spin-coupled system. For both types of spin-coupled systems we present examples here, and we discuss the influence of localized and delocalized mixed-valence states, and the strength of superexchange, spin-dipolar and double-exchange contributions to spin coupling.     

Spin polarized Auger spectroscopy from Fe(100)

The spin polarization of the M₂₃M₄₅M₄₅ Auger line of Fe(100) is shown to reveal two gain satellites at 4 and 17 eV, respectively, above the M₂₃ threshold. The lower one was previously identified as autoionization emission, whereas the higher one is a newly observed feature with a strikingly large positive spin polarization of > 80%. The spin polarization of the M₁M₄₅M₄₅ line is also reported.     

Spin glass-like phenomena in antiferromagnetic (Fe0.77Mg0.23)0.92O

An investigation on magnesiumwüstite, (Fe_0.77Mg_0.23)_0.92O, has been carried out using neutron diffraction and susceptibility measurements. The diffraction patterns obtained both at room and liquid helium temperature are similar to the ones found on wüstite (Fe_yO). Plots of the susceptibility vs. temperature in the temperature range 5–300 K show evidence of spin glass-like phenomena in the long range antiferromagnetic order with a freezing temperature of 76 K and Néel temperature of 128 K. It is postulated that a similar defect structure with the one found in wüstite could also be expected here, with Mg²⁺ and Fe²⁺ ions, and vacancies found on octahedral sites of an fcc lattice, and Fe³⁺ ions on tetrahedral sites. It is thought that both Mg²⁺ substitutional defects and Fe³⁺ interstitial defects could play a role in the formation of a spin glass.     

Intercalation of dimethyltin(IV) cationic complexes in montmorillonite

Sn¹¹⁹ Moessbauer spectroscopy was used to study the dynamical aspects of intercalated Sn complexes in Montmorillonite. A sudden change of the recoil free fraction at 230K concommitant with an enhanced Goldanskii-Karyagin effect above this temperature is interpreted as arising from the melting of interlayer water.     

Origin of biquadratic coupling in Fe/Cr(100) superlattices

We investigate the magnetic properties of a (100) oriented [Fe(1.7 nm)/Cr(8.4 nm)](10) superlattice by means of perturbed angular correlation spectroscopy. The magnetic ordering in the Cr layers is obtained by measuring the magnetic hyperfine interaction at implanted 111Cd nuclear probes. We identify dynamic antiferromagnetic spin fluctuations in the Cr layers and show that it gives rise to the biquadratic interlayer coupling.     

Novel near-infrared photoluminescence from platinum(II)-porphyrin (PtOEP) aggregates

We investigated photoluminescence of the phosphorescent molecule platinum(II)-2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin. In neat films prepared by organic molecular beam deposition or precipitation from solution, we observe a near infrared emission feature around 1.65 eV, caused by aggregation effects.     

Spin splitting of image-potential states on fcc Fe/Cu(100)

Received: 19 October 1998     

Magnetic anisotropies and coupling mechanisms in Fe/Mo(110) nanostripes

Using low-temperature (5 K) spin-polarized scanning tunneling microscopy, we have studied the morphology and magnetic properties of monolayer (ML) and double layer (DL) thick Fe nanowires grown by step flow on a Mo(110) single crystal. Magnetic contrast has been obtained using tungsten tips covered by Au/Co thin films. We find that the DL Fe nanowires, similarly to ML Fe nanowires, are perpendicularly magnetized. Because of the dipolar coupling, separated DL Fe nanowires are antiferromagnetically coupled. DL wires that are touching at step edges are ferromagnetically ordered due to direct exchange coupling. We measured the widths of the magnetic domain walls in the ML and DL Fe nanowires. The domain wall width increases with the thickness of Fe.     

Spin scattering and spin-orbit coupling in metals

An identity between ε, the probability of a spin disorientation at a metal surface with the ratio of the phonon resistivity scattering time, τ_ph, to the phonon spin scattering time, τ_2ph, in the bulk metal is established. Also it is shown that, for diffuse resistivity scattering, ε at a metal-metal interface is given by the $\text{τ}{}_{\mathrm{ph}}\text{τ}{}_{\mathrm{2ph}}\text{>}$ ratio in the bulk of the higher Z metal. Finally from an approximate fit of ε and the $\text{τ}{}_{\mathrm{ph}}\text{τ}{}_{\mathrm{2ph}}$ ratios to α⁴Z⁴ it is proposed that CESR may be readily observed in titanium and molybdenum.