Silvia Sottini

Copper Sulfide Nanocrystals with Tunable Composition by Reduction of Covellite Nanocrystals with Cu+ Ions

Platelet-shaped copper sulfide nanocrystals (NCs) with tunable Cu stoichiometry were prepared from Cu-rich covellite (Cu1.1S) nanoplates through their reaction with a Cu(I) complex ([Cu(CH3CN)4]PF6) at room temperature. Starting from a common sample, by this approach it is possible to access a range of compositions in these NCs, varying from Cu1.1S up to Cu2S, each characterized by a different optical response: from the metallic covellite, with a high density of free carriers and strong localized surface plasmon resonance (LSPR), up to Cu2S NCs with no LSPR. In all these NCs the valency of Cu in the lattice stays always close to +1, while the average -1 valency of S in covellite gradually evolves to -2 with increasing Cu content; i.e., sulfur is progressively reduced. The addition of copper to the starting covellite NCs is similar to the intercalation of metal species in layered transition metal dichalcogenides (TMDCs); i.e., the chalcogen-chalcogen bonds holding the layers are progressively broken to make room for the intercalated metals, while their overall anion sublattice does not change much. However, differently from the TMDCs, the intercalation in covellite NCs is sustained by a change in the redox state of the anion framework. Furthermore, the amount of Cu incorporated in the NCs upon reaction is associated with the formation of an equimolar amount of Cu(II) species in solution. Therefore, the reaction scheme can be written as: Cu1.1S + 2γCu(I) → Cu1.1+γS + γCu(II).

A Multifrequency High-Field Electron Paramagnetic Resonance Study of CoIIS4 Coordination

Advanced electron paramagnetic resonance (EPR) methods have been employed in the study of two high-spin cobalt(II) complexes, Co[(SPPh(2))(2)N](2) (Co(Ph,Ph)L(2)) and Co[(SPPh(2))(SP(i)Pr(2))N](2) (Co(iPr,Ph)L(2)), in which the bidentate disulfidoimidodiphosphinato ligands make up for a pseudotetrahedral sulfur coordination of the transition metal. The CoS(4) core in the two complexes has slightly different structure, owing to the different peripheral groups (phenyl or isopropyl) bound to the phosphorus atoms. To determine the zero-field splitting, notoriously difficult for high-spin cobalt(II), the two complexes required different approaches. For Co(Ph,Ph)L(2), the study of the X-band EPR spectrum of a single crystal as a function of temperature revealed a nearly axial character of the zero-field splitting (ZFS; E/D approximately -0.05). For Co(iPr,Ph)L(2), the combination of the EPR spectra at 9, 95, and 275 GHz revealed a rhombic character of the ZFS (E/D approximately -0.33). The energy difference between the Kramers doublets in Co(Ph,Ph)L(2) and Co(iPr,Ph)L(2) amounts to 24 cm(-1) and 30 cm(-1), respectively. From the X-band EPR spectra of diamagnetically diluted single crystals at fields up to 2.5 T for Co(Ph,Ph)L(2) and 0.5 T for Co(iPr,Ph)L(2), the effective g tensors and cobalt hyperfine tensors have been determined, including the direction of the principal axes in the cobalt sites. The values of the EPR observables are discussed in relation to the structural characteristics of the first (CoS(4)) and second coordination sphere in the complexes.

Involvement of Tyr108 in the enzyme mechanism of the small laccase from Streptomyces coelicolor.

The enzyme mechanism of the multicopper oxidase (MCO) SLAC from Streptomyces coelicolor was investigated by structural (XRD), spectroscopic (optical, EPR), and kinetics (stopped-flow) experiments on variants in which residue Tyr108 had been replaced by Phe or Ala through site-directed mutagenesis. Contrary to the more common three-domain MCOs, a tyrosine in the two-domain SLAC is found to participate in the enzyme mechanism by providing an electron during oxygen reduction, giving rise to the temporary appearance of a tyrosyl radical. The relatively low k(cat)/K(M) of SLAC and the involvement of Y108 in the enzyme mechanism may reflect an adaptation to a milieu in which there is an imbalance between the available reducing and oxidizing co-substrates. The purported evolutionary relationship between the two-domain MCOs and human ceruloplasmin appears to extend not only to the 3D structure and the mode of binding of the Cus in the trinuclear center, as noted before, but also to the enzyme mechanism.

Identification of a Radical Intermediate in the Enzymatic Reduction of Oxygen by a Small Laccase

The enzyme mechanism of the Cu-containing small laccase (SLAC) from Streptomyces coelicolor has been investigated by optical and electron paramagnetic resonance spectroscopy. A new intermediate was identified after the reaction of molecular oxygen with the reduced trinuclear site of the type-1-depleted (T1D) form of the enzyme. It has the fingerprint of a biradical with a triplet ground state. One of the spins resides on a Cu in the trinuclear site, tentatively identified as the type-2 site, while the other spin derives from a protein-based radical. The latter is tentatively identified as a tyrosyl radical on the basis of the similarity of the optical characteristics with those observed for a Cu tyrosyl radical pair. The spin-spin distance was found to be 5.0 +/- 0.2 A.

High-frequency EPR study of the high-spin FeII complex Fe[(SPPh2)2N]2.

We report continuous-wave electron-paramagnetic-resonance (EPR) spectra of the high-spin Fe(II) complex Fe[(SPPh(2))(2)N](2) at 275.7 GHz, 94.1 GHz and 9.5 GHz. Combined analysis of these EPR spectra shows that the complex occurs in multiple conformations. For two main conformations the spin-Hamiltonian parameters, which reflect the electronic structure of the complex, are accurately determined: (1) D=9.17 cm(-1) (275 GHz), E/D=0.021 and (2) D=8.87 cm(-1) (266 GHz), E/D=0.052. The EPR spectra obtained at 275.7 GHz on single crystals of the complex are essential for the analysis and in addition they reveal that the two main conformations occur at two magnetically distinguishable sites.

The type 1 copper site of pseudoazurin: Axial and rhombic

We report on a high-frequency electron-paramagnetic-resonance study of the type 1 copper site of pseudoazurin. The spectra fully resolve the contribution of a nearly axial spectrum besides the rhombic spectrum, which unequivocally proves the existence of two conformations of the copper site. Pseudoazurins have been considered from Achromobacter cycloclastes including eight mutants and from Alcaligenes faecalis. The two conformations are virtually the same for all pseudoazurins, but the rhombic/axial population varies largely, between 91/9 and 33/67. These observations are discussed in relation to optical absorption spectra and X-ray diffraction structures. A similar observation for fern plastocyanin from Dryopteris crassirhizoma suggests that dual conformations of type 1 copper sites are more common.

A comment on the pseudo-nuclear Zeeman effect.

For high-spin systems whose magnetic sublevels are arranged in doublets at zero field, the electron-paramagnetic-resonance (EPR) spectra are commonly described by an effective spin Hamiltonian. We show that also in this approach, if the mixing of the electron spin states by the hyperfine interaction is negligible, a proper description of electron-nuclear double resonance (ENDOR) spectra can be obtained using a nuclear spin Hamiltonian in which the electron spin angular momentum operator is replaced by its expectation value. Appropriate values of this expectation value can be obtained from a wave function correct to first-order in the electron Zeeman interaction. In terms of perturbation theory, such a description is more logical than the conventional practice based on the inclusion of a second-order cross term, the so-called pseudo-nuclear Zeeman effect, which involves both the electron Zeeman interaction and the hyperfine interaction. We illustrate our analysis with calculations of the expectation value of the electron spin angular momentum and of the energies of the hyperfine levels for a high-spin cobalt complex, which we studied by EPR and ENDOR recently.

A modification of the commercial ESR900 cryostat to enable three-dimensional electron-paramagnetic-resonance studies of crystals

Complete orientation studies of X-band electron-paramagnetic-resonance spectra of crystals largely benefit from the possibility to measure the spectrum for any orientation of the magnetic field with respect to the crystal without the need to remount the crystal. We report on a modification of a commercial cryostat to allow such experiments down to liquid helium temperatures and demonstrate its performance.