Recent Research Developments
|Index of Recent Research News|
|October 26th, 2005|
|New Copper-Sulfur Complexes With Unusual Electronic Structures: Progress Toward Synthetic Models of the Tetracopper-Sulfide Catalytic Site in Nitrous Oxide Reductase|
Copper-sulfur interactions play a critical role in the function of several classes of biological molecules, with the soft, polarizable nature of sulfur being uniquely suited for modulating the electronic structure and reactivity of copper protein active sites. For example, electronic delocalization has been postulated to be a role played by the inorganic sulfur recently identified in the "CuZ" site of nitrous oxide reductase (Figure 1), a key enzyme involved in the environmentally important process of biological denitrification. In order to gain insight into the properties of the unique CuZ catalytic cluster, N-donor ligand supported copper-sulfur chemistry needs to be developed.
Figure 1.Depiction of the CuZ site in nitrous oxide reductase from its X-ray crystal structure (PDB 1FWX).
In efforts toward this aim, postdoctoral associate Dr. Eric Brown and graduate student John York in the laboratory of Professor William Tolman have recently succeeded in preparing two novel complexes 1 and 2 (Figure 2), which have been found to have unusual electronic structures through spectroscopic and theoretical studies. 1,2 Compound 1 features three copper ions capped by sulfides, a motif that models structural attributes of the CuZ site. The [Cu3(μ-S)2]3+ core has a formal oxidation state description of Cu(II)2Cu(III), but in contrast to a previously reported analog with oxides instead of sulfides that has a localized mixed-valent electronic structure, 3 1 was determined to be fully delocalized on the basis of low temperature X-ray crystallography and EPR spectroscopy. Theoretical calculations performed by collaborators E. Ruiz and S. Alvarez at the University of Barcelona rationalize this intriguing result. Complex 2 forms under slightly different reaction conditions, and features a [Cu2(μ-1,2-S2)2]2+ core in which the S-S bonds remain intact. This core may be envisoned as either (a) two Cu(III) ions bridged by disulfides (S22-), or (b) two Cu(II) ions bridged by disulfide(-) ions (S2-). It was concluded that the latter description (b) is correct on the basis of X-ray crystallographic and resonance Raman spectroscopic measurements. Current work is focused on assessing the redox chemistry of 1 and 2 and on further developing new copper-sulfur complex motifs.
Figure 2. The X-ray structures of (left) the cluster 1, [(TMEDA)3Cu3(μ-S)2]3+, and (right) the complex 2, [(TMEDA)2Cu2(S2)2(O3SCF3)2], with all nonhydrogen atoms shown as 50% thermal ellipsoids. Atom colors are: Cu, green; S, yellow; N, blue; O, red; F, purple. TMEDA = N,N,N',N'-tetramethylethylenediamine.
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Next scheduled update: Nov 9th, 2005.