Dynamic Modulations of Heisenberg Exchange Coupling
Protein molecules containing small magnetic centres of iron atoms play an important role in maintaining the flow of energy in living organisms. Research scientists working in Prof. Dr. Dominik Marx's group (Department of theoretical chemistry at the Ruhr-University, Bochum) have been able to show for the first time that the magnetic properties of these centres are intricately coupled with the dynamics of the surrounding protein. These results, gained from a new, state-of-the-art multiscale simulation technique, have now been published in the renowned periodical ‘Proceedings of the National Academy of Sciences’ (PNAS).
Iron-Sulfur Centres in Proteins
The so-called iron-sulfur proteins ensure that the large quantities of energy formed during respiration or photosynthesis are made available to the organism in small, well-controlled portions. This process relies on tiny atomic clusters composed of "typical inorganic atoms" like iron and sulfur, which play an important role as they are able to take up and release electrons. “These atoms can basically be brought to rust and derust in a controlled way in the protein”, explains principal author Dr. Eduard Schreiner. In addition to this, these clusters possess fascinating magnetic properties as a result of their embedded iron atoms. In contrast to the iron atoms encountered in daily life, which are ferromagnetic, these exhibit a more complex, antiferromagnetic coupling and are even employed by nature as nanomagnets.
Antiferromagnetism, Heisenberg and Multiscales
The antiferromagnetic interaction between the iron atoms of the iron-sulfur-cluster is conveyed through the so-called Heisenberg exchange coupling and described quantitatively by means of a “coupling constant”. Up until now it has only been possible to investigate these effects in purely static calculations, which, however, delivers a very unrealistic picture, as the proteins and the cluster are, in fact, in perpetual motion. The theoretical chemists at the RUB have now developed a new, state-of-the-art multiscale computer simulation technique to calculate the effect of these dynamical movements on the Heisenberg coupling constant.
Protein dynamic and magnetic coupling
And behold, the antiferromagnetic coupling constant was found to be strongly influenced by the protein's dynamics, which lead to an, albeit slight, but continual change in the protein's structure. This quantity, suggested to be a constant per designation and, up until now, tacitly taken to be as such, is thus in “real life” actually not constant at all, but rather fluctuates broadly around an average value. This average value is, on the one hand, dependant on the immediate protein surroundings, a fact which has been demonstrated on two states (conformations) of a ferrodoxin. Additionally, the dynamical modulations of the antiferromagnetic coupling could be spectrally decomposed and the single components extracted from this analysis could be analysed. Interestingly, these investigations led to the discovery that only certain vibrational modes of the protein have an influence on the coupling. The next step will be to measure this effect predicted by theoretical methods experimentally. Even in this respect the theoreticians have a few ideas, which they introduce in their publication, and therefore place the ball in the experimenters' court.
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