X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character.     

Inner-sphere electron-transfer reorganization energies of zinc porphyrins

Inner-sphere electron-transfer reorganization energies of Zn(protoporphyrin IX) and Zn(octaethylporphyrin) are determined from band-shape analyses of the first ionization obtained by gas-phase valence photoelectron spectroscopy. The experimentally determined total inner-sphere reorganization energies for self-exchange (120-140 meV) indicate that structural changes upon oxidation are largely confined to the porphyrin ring, and substituents on the ring or solvent and other environmental factors make smaller contributions. Computational estimates by different models vary over a wide range and are sensitive to numerical precision factors for these low reorganization energies. Of current computational models that are widely available and practical for molecules of this size, functionals that contain a mixture of Hartree-Fock exchange and DFT exchange-correlation appear to be the most applicable.     

Ab initio study of AmCl(+): f-f spectroscopy and chemical binding

A valence full configuration interaction study with a polarized double-zeta quality basis set has been carried out for the lowest 49 electronic states of AmCl(+). The calculations use a pseudopotential treatment for the core electrons and incorporate a one-electron spin-orbit interaction operator. Electrons in the valence s, p, d, and f subshells were included in the active space. The resulting electronic potential energy curves are largely repulsive. The chemical bonding is ionic in character with negligible participation of 5f electrons. The molecular f-f spectroscopy of AmCl(+) arises essentially from an in situ Am(2+) core with states slightly redshifted by the presence of chloride ion. Am(+)+Cl asymptotes which give rise to the few attractive potential energy curves can be predicted by analysis of the f-f spectroscopy of isolated Am(+) and Am(2+). The attractive curves have substantial binding energies, on the order of 75-80 kcal/mol, and are noticeably lower than recent indirect measurements on the isovalent EuCl(+). An independent empirical correlation supports the predicted reduction in AmCl(+) binding energy. The energies of the repulsive curves are strongly dependent on the selection of the underlying atomic orbitals while the energies of the attractive curves do not display this sensitivity. The calculations were carried out using our recently developed parallel spin-orbit configuration interaction software.     

The role of covariance in the analysis of gamma-ray spectra

When a gamma-ray spectrum contains a number of photopeaks originating from the same nuclide, the calculation of nuclide activity depends not only on the values of, and uncertainties in, the photopeak areas, photopeak efficiencies, gamma-ray emission probabilities and peak area correction factors, but also on the correlations that exist between these quantities. When photopeaks corresponding to a group of nuclides overlap or are unresolved, simultaneous estimation of the activities of all the nuclides present in the group is necessary if the information contained in the spectrum is to be used efficiently — a procedure which can prove important when the only prominent photopeak associated with a nuclide of interest overlaps with the photopeak of another nuclide. Two procedures for activity estimation are described: both procedures take correlations into account; one procedure deals with the special case of activity estimation for a single nuclide, where unresolved photopeaks are absent or neglected; the other procedure deals with simultaneous activity estimation using all the photopeaks associated with an interrelated group of nuclides.     

Quantum mechanics versus realism

Quantum theory accepts the point-like indivisible (classical) character of a particle as a mere product of a measuring process, or what has become known as a collapse. Following the notion of empty waves, which accepts the particle as a real existent entity without regard to the measurement process, we propose an experiment that may shed some light on the reality of the particle and the consequences of that reality.