Pulsed Electron−Electron Double Resonance on Multinuclear Metal Clusters: Assignment of Spin Projection Factors Based on the Dipolar Interaction

Abstract
The interaction between two paramagnetic metal centers, a [3Fe−4S]+ cluster and a [NiFe] center, is investigated in the hydrogenase from Desulfovibrio vulgaris Miyazaki F by pulsed ELDOR (electron−electron double resonance). The distance between the metal centers is known from X-ray crystallography. The experimental dipolar spin−spin interaction deviates from the value expected for two point-dipoles located at the centers of the metal clusters. An extended spin-coupling model accounting for the spin coupling in the [3Fe−4S]+ cluster yields the observed interaction under the assumption of a particular magnetic coupling scheme for the three Fe ions. These results demonstrate that pulsed ELDOR can be used to gain insight into the inner structure of a multinuclear metal cluster.

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