Stephan Binder

Catalytic phenol hydroxylation with dioxygen: extension of the tyrosinase mechanism beyond the protein matrix.

A pinnacle of bio-inorganic chemistry is the ability to leverage insights gleaned from metalloenzymes toward the design of small analogs capable of effecting catalytic reactivity outside the context of the natural system.[1,2] Structural mimicry of active sites is an attempt to insert a synthetic catalyst into an enzymatic mechanism. Such a mechanism evolves by selection pressures for efficiency and traverses an energetic path with barriers and wells neither too high nor too deep in energy – a critical factor of catalytic turnover.[3] An advantage of metalloenzymes over small metal complexes is the site-isolation of the metal center in the protein matrix with its attendant ability to attenuate destructive decay processes – reaction sinks. This protection provides access to thermal regimes that allows barriers and wells to be traversed. Synthetic complexes too must avoid any deleterious reactions, often necessitating deliberate incorporation of protective superstructures.[4,5] Such limitations make reproducing enzymatic catalytic reactivity in a synthetic complex with native substrates a significant challenge, as evidenced by the dearth of good examples, despite decades of effort.

Catching an Entatic State—A Pair of Copper Complexes

The structures of two types of guanidine-quinoline copper complexes have been investigated by single-crystal X-ray crystallography, K-edge X-ray absorption spectroscopy (XAS), resonance Raman and UV/Vis spectroscopy, cyclic voltammetry, and density functional theory (DFT). Independent of the oxidation state, the two structures, which are virtually identical for solids and complexes in solution, resemble each other strongly and are connected by a reversible electron transfer at 0.33 V. By resonant excitation of the two entatic copper complexes, the transition state of the electron transfer is accessible through vibrational modes, which are coupled to metal-ligand charge transfer (MLCT) and ligand-metal charge transfer (LMCT) states.

Stabilisation of a Highly Reactive Bis(μ-oxo)dicopper(III) Species at Room Temperature by Electronic and Steric Constraint of an Unconventional Nitrogen Donor Ligand

Herein we present an innovative combination of EXAFSspectroscopy and resonant Raman scattering for the charac-terisation of the ground state and structural dynamics of athermally stable binuclear bisACHTUNGRE(m-oxo) dicopperACHTUNGRE(III) species.Peralkylated bis(guanidine)-based ligands are used in thesynthesis of this compound.

UV-light-induced conversion and aggregation of prion proteins.

Increasing evidence suggests a central role for oxidative stress in the pathology of prion diseases, a group of fatal neurodegenerative disorders associated with structural conversion of the prion protein (PrP). Because UV-light-induced protein damage is mediated by direct photo-oxidation and radical reactions, we investigated the structural consequences of UVB radiation on recombinant murine and human prion proteins at pH 7.4 and pH 5.0. As revealed by circular dichroism and dynamic light scattering measurements, the observed PrP aggregation follows two independent pathways: (i) complete unfolding of the protein structure associated with rapid precipitation or (ii) specific structural conversion into distinct soluble beta-oligomers. The choice of pathway was directly attributed to the chromophoric properties of the PrP species and the susceptibility to oxidation. Regarding size, the oligomers characterized in this study share a high degree of identity with oligomeric species formed after structural destabilization induced by other triggers, which significantly strengthens the theory that partly unfolded intermediates represent initial precursor molecules directing the pathway of PrP aggregation. Moreover, we identified the first suitable photo-trigger capable of inducing refolding of PrP, which has an important biotechnological impact in terms of analyzing the conversion process on small time scales.

Optical response of the Cu2S2 diamond core in Cu2II(NGuaS)2Cl2

Density functional theory (DFT) and time‐dependent DFT calculations are presented for the dicopper thiolate complex Cu2(NGuaS)2Cl2 [NGuaS=2‐(1,1,3,3‐tetramethylguanidino) benzenethiolate] with a special focus on the bonding mechanism of the Cu2S2Cl2 core and the spectroscopic response. This complex is relevant for the understanding of dicopper redox centers, for example, the CuA center. Its UV/Vis absorption is theoretically studied and found to be similar to other structural CuA models. The spectrum can be roughly divided in the known regions of metal d‐d absorptions and metal to ligand charge transfer regions. Nevertheless the chloride ions play an important role as electron donors, with the thiolate groups as electron acceptors. The bonding mechanism is dissected by means of charge decomposition analysis which reveals the large covalency of the Cu2S2 diamond core mediated between Cu dz2 and S‐S π and π* orbitals forming Cu‐S σ bonds. Measured resonant Raman spectra are shown for 360‐ and 720‐nm excitation wavelength and interpreted using the calculated vibrational eigenmodes and frequencies. The calculations help to rationalize the varying resonant behavior at different optical excitations. Especially the phenylene rings are only resonant for 720 nm.

Characterization of the optically excited state of a bis (μ-oxo)-dicopper(III) species mimicking the hemocyanin and tyrosinase active sites

Optical excited molecules play an increasingly important role in research at light sources. Here we compare two approaches to structurally characterize such states, pumped-XAS and an innovative combination of EXAFS spectroscopy and resonant Raman scattering. The later combination allows to study efficiently charge-transfer complexes in their ground and excited state. The design of the experimental setups for pumped-XAS and resonant Raman scattering at different temperatures as well as results obtained are presented. We receive twofold information on the structural and electronic properties of both states elucidating the alterations upon induced charge transfer in the Cu2O2-core of a system mimicking the active site of tyrosinase and hemocyanin.