We have determined the crystal structure of YodA, an Escherichia coli protein of unknown function. YodA had been identified under conditions of cadmium stress, and we confirm that it binds metals such as cadmium and zinc. We have also found nickel bound in one of the crystal forms. YodA is composed of two domains: a main lipocalin/calycin-like domain and a helical domain. The principal metal-binding site lies on one side of the calycin domain, thus making YodA the first metal-binding lipocalin known. Our experiments suggest that YodA expression may be part of a more general stress response. From sequence analogy with the Cterminal domain of a metal-binding receptor of a member of bacterial ATP-binding cassette transporters, we propose a three-dimensional model for this receptor and suggest that YodA may have a receptor-type partner in E. coli.

We report the exceptional reactivity towards dioxygen of a nanostructured organic-inorganic hybrid material due to the confinement of copper cyclam within a silica matrix. The key step is the metalation reaction of the ligand, which can occur before or after xerogel formation through the sol-gel process. The incorporation of a Cu(II) center into the material after xerogel formation leads to a bridged Cu(I)/Cu(II) mixed-valence dinuclear species. This complex exhibits a very high affinity towards dioxygen, attributable to auto-organization of the active species in the solid. The remarkable properties of these copper complexes in the silica matrix demonstrate a high cooperative effect for O(2) adsorption; this is induced by close confinement of the two copper ions leading to end-on mu-eta(1):eta(1)-peroxodicopper(II) complexes. The anisotropic packing of the tetraazamacrocycle in a lamellar structure induces an exceptional reactivity of these copper complexes. We show for the first time that the organic-inorganic environment of copper complexes in a silica matrix fully model the protecting role of protein in metalloenzymes. For the first time an oxygenated dicopper(II) complex can be isolated in a stable form at room temperature, and the reduced Cu(2) (I,I) species can be regenerated after several adsorption-desorption cycles. These data also demonstrate that the coordination scheme and reactivity of the copper cyclams within the solid are quite different from that observed in solution.

The Escherichia coli protein YodA was overexpressed, puri®ed and crystallized in several crystal forms. The function of this protein is not known, although it has been identi®ed under conditions of bacterial stress. Three of the four crystal forms were obtained in the presence of divalent cations (zinc, nickel and cadmium), suggesting that YodA may be a metal-binding protein.

X‐ray absorption spectroscopy was used to elucidate how hybrid xerogels complexed with CuCl2 could reversibly bind molecular dioxygen. Difference EXAFS analyses at the Cu K‐edge suggest that dioxygen could bridge two Cu atoms in a μ‐η1:η1 peroxo‐like conformation with unequal Cu···O distances. Only the short distance (RCu–O1 = 1.86 ± 0.01 Å) was unambiguously determined and looks typical of a CuII site. The Cu···Cu internuclear distances would be rather long: RCu–Cu ≈ 4.0 Å (3.9 Å) for the oxygenated (oxygen‐free) xerogels. Cl K‐edge EXAFS spectra revealed the pre‐existence in the oxygen‐free xerogels of CuI sites with short Cl–Cu bonds (2.11 ± 0.03 Å). Pentacoordinate CuII sites with a longer Cl–Cu bond (2.45 ± 0.03 Å) were also identified. Another signal at a further distance (2.73 ± 0.03 Å) suggests that the Cl ions could bridge mixed‐valence {CuI,CuII} sites. In oxygenated xerogels complexed with CuCl2, ca. 75 % of the Cu atoms would be singly bound to dioxygen while the signature of the pentacoordinate CuII sites would totally vanish. Substituting CuBr2 for CuCl2 yields xerogels with a poor capability to bind dioxygen. This is remarkably correlatedwith a decrease of the signatures at RCu–O1 = 1.86 Å and RCu–Cu 4.0 Å. EXAFS spectra of the oxygen‐free xerogel complexed with CuBr2 again exhibit signatures assigned to short (2.30 Å)/long (2.49 Å) Cu–Br bonds as expected for mixed‐valence {CuI,CuII} systems. Inactive xerogels prepared with metallated cyclams seem to undergo a structural change during hydrolysis and polycondensation. Deconvoluted XANES spectra of the active xerogels exhibit a strong pre‐edge resonance which is characteristic of mixed‐valence CuI sites. This interpretation was supported by ab initio XANES simulations carried out beyond the muffin‐tin approximation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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