X-ray Absorption Spectroscopy Combined with Time-Dependent Density

A combination of two elements’ (Au, Zn) X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TD-DFT) allowed the elucidation of differential substitution pathways of Au(I) and Au(III) compounds reacting with biologically relevant zinc fingers (ZnFs). Gold L₃-edge XAS probed the interaction of gold and the C-terminal Cys₂HisCys finger of the HIV-1 nucleocapsid protein NCp7, and the Cys₂His₂ human transcription factor Sp1. The use of model compounds helped assign oxidation states and the identity of the gold-bound ligands. The computational studies accurately reproduced the experimental XAS spectra and allowed the proposition of structural models for the interaction products at early time points. The direct electrophilic attack on the ZnF by the highly thiophilic Au(I) resulted in a linear P–Au–Cys coordination sphere after zinc ejection whereas for the Sp1, loss of PEt₃ results in linear Cys–Au–Cys or Cys–Au–His arrangements. Reactions with Au(III) compounds, on the other hand, showed multiple binding modes. Prompt reaction between [AuCl(dien)]²⁺ and [Au(dien)(DMAP)]³⁺ with Sp1 showed a partially reduced Au center and a final linear His–Au–His coordination. Differently, in the presence of NCp7, [AuCl(dien)]²⁺ readily reduces to Au(I) and changes from square-planar to linear geometry with Cys–Au–His coordination, while [Au(dien)(DMAP)]³⁺ initially maintains its Au(III) oxidation state and square-planar geometry and the same first coordination sphere. The latter is the first observation of a “noncovalent” interaction of a Au(III) complex with a zinc finger and confirms early hypotheses that stabilization of Au(III) occurs with N-donor ligands. Modification of the zinc coordination sphere, suggesting full or partial zinc ejection, is observed in all cases, and for [Au(dien)(DMAP)]³⁺ this represents a novel mechanism for nucleocapsid inactivation. The combination of XAS and TD-DFT presents the first direct experimental observation that not only compound reactivity, but also ZnF core specificity, can be modulated on the basis of the coordination sphere of Au(III) compounds.

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