Showing papers by "Edward I. Solomon published in 1999"

MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S ≥ 1/2. Applications to S = 1/2 and S = 5/2

Abstract: The magnetic circular dichroism (MCD) properties of a spin-allowed transition from an orbitally nondegenerate ground state manifold A to an orbitally nondegenerate excited state manifold J in the presence of spin−orbit coupling (SOC) are derived for any S ≥ 1/2. Three physically distinct mechanisms are identified that lead to MCD intensity and depend on SOC between excited states which leads to a sum rule and SOC between the ground state and other excited states that leads to deviations from the sum rule. The model is valid for any symmetry of the magnetic coupling tensors and arbitrary transition polarizations. The S = 1/2 case is analytically solved, and the determination of linear polarizations from MCD saturation magnetization data is discussed. For all mechanisms the MCD intensity is proportional to the spin-expectation values of the ground state sublevels which are conveniently generated from a spin-Hamiltonian (SH). For Kramers systems with large zero-field splittings (ZFSs) this allows the contrib...

Spectroscopic and Electronic Structural Studies of the Cu(III)2 Bis-μ-oxo Core and Its Relation to the Side-On Peroxo-Bridged Dimer

Abstract: Recently a Cu(III)2 bis-μ-oxo dimer ligated by peralkylated amines has been found to interconvert with the side-on peroxo-bridged, μ-η2:η2 isomer. The Cu(III)2(μ-O)2 dimer exhibits two intense charge transfer (CT) features in the near-UV region of the electronic absorption spectrum. Laser excitation into the lower-energy CT absorption band at 25 000 cm-1 results in intense resonance enhancement of the Raman peaks at 609 and 118 cm-1 which profile this band and give overtone and combination progressions. The combined application of a normal coordinate analysis of the Raman features and a time-dependent Heller theory analysis of the electronic absorption spectrum and resonance Raman profiles provide the excited-state geometry. As this transition corresponds to an oxo-to-Cu(III) CT, this excited state is formally an oxyl−Cu(II) species. Density functional calculations correlated to these data (including the excited-state geometry and the relative CT intensities) allow for an unambiguous assignment of the obs...

Spectroscopic Studies and Electronic Structure Description of the High Potential Type 1 Copper Site in Fungal Laccase: Insight into the Effect of the Axial Ligand

Abstract: A variety of spectroscopic techniques combined with density functional calculations are used to describe the electronic structure of the nonaxially ligated, trigonal planar type 1 copper site in three fungal laccases with substantially different type 1 copper reduction potentials. These methods are also applied to a mutant of the high-potential Polyporus pinsitis laccase in which the nonligating axial phenylalanine (Phe) is changed to methionine (Met). Optical absorption, circular dichroism, and magnetic circular dichroism spectroscopies of all three fungal laccases reveal that, relative to the classic blue copper protein plastocyanin, the ligand field strength at the type 1 Cu center and the oscillator strength of the charge-transfer transitions increase. Resonance Raman spectra show that the envelope of Cu−S(Cys) stretching bands is shifted to higher energy in the fungal laccases, implying a stronger Cu−S(Cys) bond. Differences in the EPR spectra of the fungal laccases and plastocyanin are found to resu...

Investigation of the Electronic Structure of 2Fe−2S Model Complexes and the Rieske Protein Using Ligand K-Edge X-ray Absorption Spectroscopy

Abstract: X-ray absorption spectroscopy at the sulfur K-edge (∼2470 eV) has been applied to a series of 2Fe−2S model complexes to obtain insight into their electronic structures. Since these 2Fe−2S complexes contain both terminal thiolates and bridging sulfides, contributions to covalency from both sets of ligands can be evaluated. Importantly, the pre-edge feature of sulfide can be resolved from that of thiolate due to differences in effective nuclear charge. In our previous studies, the covalency of the metal−thiolate bond in [Fe(SR)4]- was determined. In this study, sulfide covalency is quantified for the first time on the basis of an analysis of previous X-ray photoelectron and X-ray absorption spectroscopic studies of [FeCl4]- which are then applied to the bis-μ2-sulfide compound KFeS2. With references for both sulfide and thiolate covalencies thus established for open d-shell systems, comparisons are made between thiolate and sulfide bonding. Sulfide−Fe covalency in the [Fe2S2(SR)4]2- complexes is higher than...

Circular Dichroism and Magnetic Circular Dichroism Spectroscopy of the Catalytically Competent Ferrous Active Site of Phenylalanine Hydroxylase and Its Interaction with Pterin Cofactor

Abstract: The tetrahydrobiopterin-dependent enzyme phenylalanine hydroxylase (PAH) contains one non-heme iron atom per subunit that is required for reactivity. We have applied circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature, variable-field (VTVH) MCD spectroscopies to investigate the geometric and electronic structure of the catalytically relevant ferrous active site and its interaction with the cofactor analogue 5-deaza-6-methyltetrahydropterin, in the absence and presence of substrate. Excited-state ligand field CD and MCD data indicate that the six-coordinate ferrous active site of the resting and N-ethylmaleimide-activated enzyme is not perturbed by the addition of pterin cofactor in the absence of substrate (Δ5Eg = 1900 cm-1, 10Dq = 9850 cm-1). VTVH MCD analysis yields a ground-state splitting also consistent with an unperturbed six-coordinate ferrous site (Δ5T2g = −285 or −1150 cm-1). Addition of pterin in the presence of l-phenylalanine (l-Phe), however, results in large ...