Ein synthetisches Analogon für [2Fe-2S]-Cluster des Rieske-Typs

Mechanistic Aspects of Redox-Induced Assembly and Disassembly of S-Bridged [2M-2S] Structures

Sulfur-bridged binuclear structures [2M-2S] play a pivotal role in a variety of chemical processes such as bond breaking and formation and electron transfer. In general, structural persistence is deemed essential to the respective function but owing to the lack of a suitable molecular model system, the current understanding of the factors that control the thermodynamic and kinetic stability of [2M-2S] cores clearly is limited. This work reports a series of binuclear complexes of nickel derived from a 1,4-terphenyldithiophenol ligand platform that is ideally suited for mechanistic work to overcome this limitation. Redox-induced assembly and disassembly of S-bridged [2M-2S] fragments have been investigated at the molecular level. As part of an extended square scheme, metastable binuclear structures that are significant mechanistically have been identified, characterized, and their reactivity studied quantitatively. Electronic properties that are inherent to [2M-2S] structures and determine thermodynamic and kinetic stability are differentiated from steric effects imposed by co-ligands.

A combined computational and experimental investigation of the [2Fe-2S] cluster in biotin synthase

Biotin synthase was the first example of what is now regarded as a distinctive enzyme class within the radical S-adenosylmethionine superfamily, the members of which use Fe/S clusters as the sulphur source in radical sulphur insertion reactions. The crystal structure showed that this enzyme contains a [2Fe-2S] cluster with a highly unusual arginine ligand, besides three normal cysteine ligands. However, the crystal structure is at such a low resolution that neither the exact coordination mode nor the role of this exceptional ligand has been elucidated yet, although it has been shown that it is not essential for enzyme activity. We have used quantum refinement of the crystal structure and combined quantum mechanical and molecular mechanical calculations to explore possible coordination modes and their influences on cluster properties. The investigations show that the protonation state of the arginine ligand has little influence on cluster geometry, so even a positively charged guanidinium moiety would be in close proximity to the iron atom. Nevertheless, the crystallised enzyme most probably contains a deprotonated (neutral) arginine coordinating via the NH group. Furthermore, the Fe...Fe distance seems to be independent of the coordination mode and is in perfect agreement with distances in other structurally characterised [2Fe-2S] clusters. The exceptionally large Fe...Fe distance found in the crystal structure could not be reproduced.