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The Antiferromagnetism of Diiron-Oxo Proteins

Azidomet Hemerythrin is a member of the family of  non-heme diiron-oxo proteins. It contains a binuclear center with two high spin ions, Fe1,2 (III)(S1,2 = 5/2), antiferromagnetically coupled to a singlet (S1 - S2 = 0) ground state. One oxo and two carboxilato bridges act as superexchange pathways that allow the unpaired electron orbitals of one iron site to overlap with those of the other site. Such indirect overlap leads to the ant iferromagnetic state.

Oxo- and hydroxo-bridged diiron centers are ubiquitous in biology. The importance of these structural motifs is illustrated by the name given to an entire class of metalloproteins, namely the diiron-oxo proteins. The family of hemerythrins (Hr), in particular, are oxygen-transporting proteins that can be considered the best-characterized of the class. Combined crystallographic and spectroscopic data show that oxy-Hr and other derivatives, such as azidomet-Hr, contain strongly antiferromagnetically coupled oxo-bis(carboxylato)-bridged Fe3+-Fe3+ centers. By contrast, deoxy-Hr contains a weakly antiferromagnetically coupled hydroxo-bis(carboxylato)-bridged Fe2+-Fe2+ center. Thus, in spite of their structural similarity, there are meaningful differences in electronic structure among the various forms of hemerythrin. To gain insight about the specific electronic structures and magnetic properties of each diiron-oxo protein we carry out electronic structure calculations on their diiron cores as well as on related synthetic compounds that closely resemble their biological counterparts.

Superparamagnetism and Macroscopic Quantum Tunneling in Bacterioferritin

Bacterioferritin from Escherichia coli: The 24 subunit protein shell encloses a central cavity for iron storage. The core contains some n + m = 1500 Fe(III)(S=5/2) ions where n and m correspond to net electronic spin up and down, respectively. The iron core exhibits superparamagnetic behavior characteristic of spin-uncompensated (n > m) antiferromagnetic nanoparticles.

Antiferromagnetic bacterioferritin exhibits superparamagnetic behavior whereby two (non-compensated) sublattice magnetizations (Neel vector) oscillate spontaneously between two preferred crystalline orientations. At high temperatures such oscillations are explained as classical, thermally induced, superparamagnetism with relaxation rates given by an Arrenious law. By contrast, below a certain crossover temperature (Tc), the dynamics of the Neel vector has been attributed to temperature-independent macroscopic quantum coherence (MQC) or magnetic-field-tuned thermally-assisted MQT.