In situ formation of the first proteinogenically functionalized [TeW6O24O2(Glu)]7- structure reveals unprecedented chemical and geometrical features of the Anderson-type cluster

The chemistry of polyoxometalates (POMs) in a protein environment is an almost unexplored but highly relevant research field as important biological and pharmacological attributes of certain POMs are based on their interactions with proteins. We report on the A-type Anderson-Evans polyoxotungstate, [TeW6O24](6-) (TEW), mediated crystallization of Coreopsis grandiflora aurone synthase (cgAUS1) using similar to 0.24 mM protein and 1.0 mM TEW. The 1.78 angstrom crystal structure reveals the covalent binding of TEW to the protein under the formation of an unprecedented polyoxotungstate cluster, [TeW6O24O2(Glu)](7-) (GluTEW). The polyoxotungstate-protein complex exhibits the first covalent bond between a protein and the A-type Anderson-Evans cluster, an archetype where up to now no hybrid structures exist. The polyoxotungstate is modified at two of its six addenda tungsten atoms, which covalently bind to the carboxylic oxygen atoms of glutamic acid (Glu157), leading to W-O distances of similar to 2.35 angstrom. This ligand substitution reaction is accompanied by a reduction of the coordination number of two mu(3) polyoxotungstate oxygen atoms. This is so far unique since all known hybridizations of the Anderson-Evans POM with organic units have been obtained via the functionalization of the B-type Anderson-Evans structure through its bridging oxygen atoms. The structure reported here proves the reactivity of this POM archetype's addenda atoms as it has been administered into the protein solution as a pre-assembled cluster. Moreover, the novel cluster [TeW6O24O2(Glu)](7-) displays the great versatility of the Anderson-Evans POM class.