A mixed quantum/classical investigation of the dynamical magnetostructural properties, that is, "magnetodynamics," of oxidized Anabaena PCC7119 ferredoxin is carried out at room temperature in two distinct conformational states. This protein hosts a [2Fe-2S] cluster in which two iron centers are antiferromagnetically coupled to an overall low-spin electronic ground state that has a genuine multireference character. To study the magnetodynamics of this prosthetic group, an approximate spin projection method is formulated in the framework of density functional theory that allows for multideterminant ab initio molecular dynamics simulations to be carried out efficiently. By using this scheme, the influence of both thermal fluctuations and conformational motion on the structure of the [2Fe-2S] cluster and on the dynamics of the antiferromagnetic coupling constant, J(t), has been investigated. In addition to demonstrating how sensitively the shape of the [2Fe-2S] core itself is affected by hydrogen bonding, the analyses reveal a complex dynamical coupling of J to both local vibrations and large-amplitude motion. It is shown that this interplay can be understood in terms of specific vibrational modes and distinct hydrogen-bonding patterns between the iron-sulfur cluster and the protein backbone, respectively. This implies going beyond the Goodenough-Kanamori rules for angular magnetostructural correlations of oxidized iron-sulfur prosthetic groups.
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