Role of the Tyr-Cys Cross-link to the Active Site Properties of Galactose Oxidase

The catalytically relevant, oxidized state of the active site [Cu­(II)-Y·-C] of galactose oxidase (GO) is composed of antiferromagnetically coupled Cu­(II) and a post-translationally generated Tyr-Cys radical cofactor [Y·-C]. The thioether bond of the Tyr-Cys cross-link has been shown experimentally to affect the stability, the reduction potential, and the catalytic efficiency of the GO active site. However, the origin of these structural and energetic effects on the GO active site has not yet been investigated in detail. Here we present copper and sulfur K-edge X-ray absorption data and a systematic computational approach for evaluating the role of the Tyr-Cys cross-link in GO. The sulfur contribution of the Tyr-Cys cross-link to the redox active orbital is estimated from sulfur K-edge X-ray absorption spectra of oxidized GO to be about 24 ± 3%, compared to the values from computational models of apo-GO (15%) and holo-GO (22%). The results for the apo-GO computational models are in good agreement with the previously reported value for apo-GO (20 ± 3% from EPR). Surprisingly, the Tyr-Cys cross-link has only a minimal effect on the inner sphere, coordination geometry of the Cu site in the holo-protein. Its effect on the electronic structure is more striking as it facilitates the delocalization of the redox active orbital onto the thioether sulfur derived from Cys, thereby reducing the spin coupling between the [Y·-C] radical and the Cu­(II) center (752 cm^–1) relative to the unsubstituted [Y·] radical and the Cu­(II) center (2210 cm^–1). Energetically, the Tyr-Cys cross-link lowers the reduction potential by about 75 mV (calculated) allowing a more facile oxidation of the holo active site versus the site without the cross-link. Overall, the Tyr-Cys cross-link confers unique ground state properties on the GO active site that tunes its function in a remarkably nuanced fashion

Multiedge X‑ray Absorption Spectroscopy Part II: XANES Analysis of Bridging and Terminal Chlorides in Hexachlorodipalladate(II) Complex

X-ray absorption spectroscopy is a unique experimental technique that can provide ground state electronic structure information about transition metal complexes with unoccupied d-manifold. The quantitative treatments of pre-edge and rising-edge features have already been developed for the sulfur- and chlorine-ligand K-edge excitations. The complementarity of using multiple core excitation edges from hard, tender, and soft X-ray energy regions has been defined for the first paper of this series. The given study provides compelling evidence for the transferability of the empirical transition dipole integral from ligand K-edge to metal L-edge and back to ligand K-edge in the tender X-ray energy range. The case study was performed for a series of homoleptic chloropalladium compounds at the chlorine K- and palladium L-edges. We propose the method described here to be generally applicable for other core level excitations, where complementarity of ground state electronic structural information from XANES analysis can provide the complete electronic structure description

Conversion of Carbon Dioxide to Oxalate by α‑Ketocarboxylatocopper(II) Complexes

The α-ketocarboxylatocopper­(II) complex [{Cu­(L1)}­{O₂CC­(O)­CH­(CH₃)₂}] can be spontaneously converted into the binuclear oxalatocopper­(II) complex [{Cu­(L1)}₂(μ-C₂O₄)] upon exposure to O₂/CO₂ gas. ¹³C-labeling experiments revealed that oxalate ions partially incorporated ¹³CO₂ molecules. Furthermore, the bicarbonatocopper­(I) complex (NEt₄)­[Cu­(L1)­{O₂C­(OH)}] in an Ar atmosphere and the α-ketocarboxylatocopper­(I) complex Na­[Cu­(L1)­{O₂CC­(O)­CH­(CH₃)₂}] in an O₂ atmosphere were also transformed spontaneously into the oxalato complex [{Cu­(L1)}₂(μ-C₂O₄)]

Molecular Treatment of Nano-Kaolinite Generations

A procedure is developed for defining a compositionally and structurally realistic, atomic-scale description of exfoliated clay nanoparticles from the kaolinite family of phylloaluminosilicates. By use of coordination chemical principles, chemical environments within a nanoparticle can be separated into inner, outer, and peripheral spheres. The edges of the molecular models of nanoparticles were protonated in a validated manner to achieve charge neutrality. Structural optimizations using semiempirical methods (NDDO Hamiltonians and DFTB formalism) and ab initio density functionals with a saturated basis set revealed previously overlooked molecular origins of morphological changes as a result of exfoliation. While the use of semiempirical methods is desirable for the treatment of nanoparticles composed of tens of thousands of atoms, the structural accuracy is rather modest in comparison to DFT methods. We report a comparative survey of our infrared data for untreated crystalline and various exfoliated states of kaolinite and halloysite. Given the limited availability of experimental techniques for providing direct structural information about nano-kaolinite, the vibrational spectra can be considered as an essential tool for validating structural models. The comparison of experimental and calculated stretching and bending frequencies further justified the use of the preferred level of theory. Overall, an optimal molecular model of the defect-free, ideal nano-kaolinite can be composed with respect to stationary structure and curvature of the potential energy surface using the PW91/SVP level of theory with empirical dispersion correction (PW91+D) and polarizable continuum solvation model (PCM) without the need for a scaled quantum chemical force field. This validated theoretical approach is essential in order to follow the formation of exfoliated clays and their surface reactivity that is experimentally unattainable

Spin-Polarization-Induced Preedge Transitions in the Sulfur K‑Edge XAS Spectra of Open-Shell Transition-Metal Sulfates: Spectroscopic Validation of σ‑Bond Electron Transfer

Sulfur K-edge X-ray absorption spectroscopy (XAS) spectra of the monodentate sulfate complexes [M^II(itao)­(SO₄)­(H₂O)_0,1] (M = Co, Ni, Cu) and [Cu­(Me₆tren)­(SO₄)] exhibit well-defined preedge transitions at 2479.4, 2479.9, 2478.4, and 2477.7 eV, respectively, despite having no direct metal–sulfur bond, while the XAS preedge of [Zn­(itao)­(SO₄)] is featureless. The sulfur K-edge XAS of [Cu­(itao)­(SO₄)] but not of [Cu­(Me₆tren)­(SO₄)] uniquely exhibits a weak transition at 2472.1 eV, an extraordinary 8.7 eV below the first inflection of the rising K-edge. Preedge transitions also appear in the sulfur K-edge XAS of crystalline [M^II(SO₄)­(H₂O)] (M = Fe, Co, Ni, and Cu, but not Zn) and in sulfates of higher-valent early transition metals. Ground-state density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that charge transfer from coordinated sulfate to paramagnetic late transition metals produces spin polarization that differentially mixes the spin-up (α) and spin-down (β) spin orbitals of the sulfate ligand, inducing negative spin density at the sulfate sulfur. Ground-state DFT calculations show that sulfur 3p character then mixes into metal 4s and 4p valence orbitals and various combinations of ligand antibonding orbitals, producing measurable sulfur XAS transitions. TDDFT calculations confirm the presence of XAS preedge features 0.5–2 eV below the rising sulfur K-edge energy. The 2472.1 eV feature arises when orbitals at lower energy than the frontier occupied orbitals with S 3p character mix with the copper­(II) electron hole. Transmission of spin polarization and thus of radical character through several bonds between the sulfur and electron hole provides a new mechanism for the counterintuitive appearance of preedge transitions in the XAS spectra of transition-metal oxoanion ligands in the absence of any direct metal–absorber bond. The 2472.1 eV transition is evidence for further radicalization from copper­(II), which extends across a hydrogen-bond bridge between sulfate and the itao ligand and involves orbitals at energies below the frontier set. This electronic structure feature provides a direct spectroscopic confirmation of the through-bond electron-transfer mechanism of redox-active metalloproteins

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)­FeCl₂] (<b>1</b>) (PNP = <i>N</i>[2-P­(CHMe₂)₂-4-methylphenyl]₂^–), prepared from one-electron oxidation of [(PNP)­FeCl] with ClCPh₃, displays an unexpected <i>S</i> = 3/2 to <i>S</i> = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex <b>1</b> to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of <i>U</i>_eff = 32–36 cm^–1 (47–52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and Δ<i>E</i>_Q values for <b>1</b>, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of <b>1</b> reveals the <i>S</i> = 3/2 to be likely the ground state with the <i>S</i> = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of <b>1</b> to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of <b>1</b> collected between 30 and 300 K also reveals elongation of the Fe–P bond lengths and increment in the Cl–Fe–Cl angle as the <i>S</i> = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d­(<i>z</i>²) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet (⁴B) and the sextet (⁶A) states are close in energy