Iron energy relaxation

In subunit R2 of ribonucleotide reductase there is a tyrosyl radical (Y ) in close proximity to a di-iron cluster.100 In the protein from E. coli the EPR signal from Y can be observed up to room temperature. However, in the protein from yeast the Y signal broadens above 15 K and is not observable above about 60 K. Saturation recovery measurements at 140 GHz showed that at 60 K the spin-lattice relaxation rates for the Y signal in the yeast protein were about 2 orders of magnitude faster than for the E. coli protein. The temperature dependence of the relaxation enhancement was consistent with the activation energy for the first excited state of the di-iron cluster, so the relaxation enhancement was attributed to interaction with the di-iron cluster. Relaxation enhancements measured at 140 GHz showed little orientation dependence so the enhancement was assigned to isotropic exchange, which is different from the orientation-dependent dipolar interaction observed for the E. coli protein.100... 

Zhang, Y Straub, J. E., Direct evidence for mode-specific vibrational energy relaxation from quantum time-dependent perturbation theory. 11. The and v, modes of iron... 

For all known cases of iron-sulfur proteins, J > 0, meaning that the system is antiferromagnetically coupled through the Fe-S-Fe moiety. Equation (4) produces a series of levels, each characterized by a total spin S, with an associated energy, which are populated according to the Boltzmann distribution. Note that for each S level there is in principle an electron relaxation time. For most purposes it is convenient to refer to an effective relaxation time for the whole cluster. 

With trinuclear clusters, we are now dealing with systems whose electronic structure depends on multiple intersite interactions that may differ from one iron pair to another. As a result, the separation between adjacent energy levels depends, not on the magnitude of these interactions, but on their difference. This may give rise to low-lying excited levels, which may have far-reaching effects on both the EPR spectrum and the relaxation properties. 

Fe(acac2trien)]N03. For the iron(III) complex of the hexadentate Schiff-base ligand acac2trien, the barrier heights have been determined from ultrasonic relaxation [94] as AG[h = 6-28 kcalmoD = 2196 cm and AGJil = 5.85 kcalmol= 2046 cm The difference of zero-point energies has been obtained from equilibrium studies as AG° = 0.43 kcal mol =... 

The physical origin of this structural flexibility of the FeO overlayer is still unclear, the more so since no clear trend is observable in the sequence of lattice parameters of the coincidence structures. The FeO(l 11) phase forming up to coverages of 2-3 ML is clearly stabilized by the interactions with the Pt substrate since FeO is thermodynamically metastable with respect to the higher iron oxides [106,114], FeO has the rock salt structure and the (111) plane yields a polar surface with a high surface energy [115], which requires stabilization by internal reconstruction or external compensation. The structural relaxation observed in the form of the reduced Fe—O... 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

The Mossbauer spectra do, however, indicate that the anisotropy energy barrier for the particle magnetization flipping is quite large, 10-20 J (215), for the 1.5-nm iron particles. As discussed in Section III, A, 3, this barrier was estimated from Mossbauer spectra at various temperatures by measuring the fraction of the spectral area that appears paramagnetic. At the temperature for which this ratio is 0.5, the relaxation time can be estimated... 

High spin iron(II) complexes are obtained with weak or medium strength ligands. The electron relaxation times are rather short (r = 10 12 s) as the electron configuration probably is as shown in Fig. 5.22 and the excited levels are close in energy for the same reasons as in the case of low spin iron(III). Consistently, the NMRD profile of Fe(OH2) +, obtained from Mohr salt ((NH4)2Fe(S04)2 6H2O), reported in Fig. 5.23, does not exhibit any dispersion below 50 MHz of proton Larmor frequency. 

---