Heparin coating durability on artificial heart valves studied by XPS and antithrombin binding capacity

The durability and functionality of a heparin coating on artificial heart valve leaflets were evaluated with X-ray photoelectron spectroscopy (XPS) and by the coatings' capacity to bind antithrombin. Current methods for accelerated life-time testing are based on exposing leaflets to water solutions. In this paper a method is explored, in which heart valve leaflets were exposed to a continuous high shear rate (4 L/min) of human citrated plasma. It was found that the heparin coating was stable and wear resistant enough to still be present after 3 weeks and to have about the same antithrombin uptake as coatings not exposed to circulating plasma. It was, however, partly destroyed by the test as found using XPS. We suggest that heparin chains from the upper layer of heparin have been torn off from the carrier chain, in combination with loss of heparin conjugate and plasma deposition in patches. This study showed that XPS provides additional information to biological measurements such as antithrombin uptake. XPS is therefore a valuable technique not only to characterize biomaterials but also to evaluate the effect of a performance test.     

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Electronic structure of a vapor-deposited metal-free phthalocyanine thin film

The electronic structure of a vapor-sublimated thin film of metal-free phthalocyanine (H2Pc) is studied experimentally and theoretically. An atom-specific picture of the occupied and unoccupied electronic states is obtained using x-ray-absorption spectroscopy (XAS), core- and valence-level x-ray photoelectron spectroscopy (XPS), and density-functional theory (DFT) calculations. The DFT calculations allow for an identification of the contributions from individual nitrogen atoms to the experimental N1s XAS and valence XPS spectra. This comprehensive study of metal-free phthalocyanine is relevant for the application of such molecules in molecular electronics and provides a solid foundation for identifying modifications in the electronic structure induced by various substituent groups.     

Mechanism of dioxygen activation in 2-oxoglutarate-dependent enzymes: a hybrid DFT study

The reaction mechanism for dioxygen activation in 2-oxoglutarate-dependent enzymes has been studied by means of hybrid density functional theory. The results reported here support a mechanism in which all chemical transformations take place on a quintet potential-energy surface. More specifically, the activated dioxygen species attacks the carbonyl group of the co-substrate producing the Fe(II)-persuccinate-CO(2) complex, which readily releases the carbon dioxide molecule. The step in which the Fe(II)-peracid-CO(2) complex is formed is found to be rate-limiting and irreversible. Subsequent heterolysis of the Obond;O bond in the Fe(II)-persuccinate complex proceeds in two one-electron steps and produces the high-valent iron-oxo species Fe(IV)dbond;O, which is most likely to be responsible for oxidative reactions catalyzed by 2-oxoglutarate-dependent enzymes. The concerted pathway for simultaneous Obond;O and Cbond;C bond cleavage on the septet potential-energy surface is found to be less favorable. The relative stability of different forms of the active iron-oxo species is assessed, and the quintet five-coordinate complex is found to be most stable.     

Orientational preference of two ethylene ligands bound to a nickel atom

CASSCF and CCI calculations have been performed to analyze the bonding in Ni(C₂H₄)₂. Three different relative orientations of the two olefins have been studied. It is found that a structure with D_2d symmetry, where the C-C bonds in the two olefins make a 90 degree angle to each other, gives the lowest energy. A D_2h form, with the two C-C bonds and Ni in the same plane, is 10.3 kcal/ mol higher in energy. The reason for the preference of the D_2d form is analyzed in terms of valence bond theory, and is found to be due to a d ⁸ structure with two simultaneous d bonds. A C _2v form, for which the two nickel olefin bonds make a 103 degree angle to each other and the C-C bonds are parallel to each other, is 32 kcal/mol higher in energy than the D_2d form. The low binding energy of the C _2v form is due to a poor interaction with inefficient sd hybridization.     

Theoretical determination of the X 2Σ+ and A 2Π potentials of CsO using relativistic effective core potentials

Theoretical potential energy curves are computed for the X ²Σ⁺ and A ²Π states of CsO using a relativistic effective core potential and a large valence Gaussian basis set. Seventeen electrons are correlated by a CI (SD ) calculation from each HF reference. We find the X ²Σ⁺ state lower by 497 and 726 cm^?1 at the HF and CI(SD) levels. Our calculated ω_e of 312 cm^?1 for the X ²Σ⁺ state agrees well with experimental values deduced from studies in matrices.     

Metal-bridging mechanism for O-O bond cleavage in cytochrome C oxidase

Density functional theory (B3LYP) has been applied to large models of the Fe(II)-Cu(I) binuclear center in cytochrome oxidase, investigating the mechanism of O-O bond cleavage in the mixed valence form of the enzyme. To comply with experimental information, the O(2) molecule is assumed to be bridging between iron and copper during the O-O bond cleavage, leading to the formation of a ferryl-oxo group and a cupric hydroxide. In accord with previous suggestions, the calculations show that it is energetically feasible to take the fourth electron needed in this reaction from the tyrosine residue that is cross-linked to one of the copper ligands, resulting in the formation of a neutral tyrosyl radical. However, the calculations indicate that simultaneous transfer of an electron and a proton from the tyrosine to dioxygen during bond cleavage leads to a barrier more than 10 kcal/mol higher than that experimentally determined. This may be overcome in two ways. If an extra proton in the binuclear center assists in the mechanism, the calculated reaction barrier agrees with experiment. Alternatively, the fourth electron might initially be supplied by a residue in the vicinity other than the tyrosine.     

Reaction mechanism of apocarotenoid oxygenase (ACO): a DFT study

The mechanism of the oxidative cleavage catalyzed by apocarotenoid oxygenase (ACO) was studied by using a quantum chemical (DFT: B3 LYP) method. Based on the available crystal structure, relatively large models of the unusual active-site region, in which a ferrous ion is coordinated by four histidines and no negatively charged ligand, were selected and used in the computational investigation of the reaction mechanism. The results suggest that binding of dioxygen to the ferrous ion in the active site promotes one-electron oxidation of carotenoid leading to a substrate radical cation and a Fe-bound superoxide radical. Recombination of the two radicals, which can be realized in at least two different ways, yields a reactive peroxo species that subsequently evolves into either a dioxetane or an epoxide intermediate. The former easily decays into the final aldehyde products, whereas the oxidation of the epoxide to the proper products of the reaction requires involvement of a water molecule. The calculated activation barriers favor the dioxetane mechanism, yet the mechanism involving the epoxide intermediate cannot be ruled out.     

A new method for the precise determination of h/e

Summary A new β-spectrometric method of measuringh/e by using conversion electron lines is proposed. The energy difference between atomic shells is obtained directly in electron volts and this quantity is compared to the corresponding frequency. The accuracy of the method is quantitatively discussed.     

An electron spectroscopic determination of theL I energy in the elements Sodium (Z=11) to Copper (Z=29)

Energies of atomic electrons in theK andL shells of eighteen elements of low atomic numbers have been measured by electron spectroscopy. Tabulated values for theL _I energies are shown to be in need of considerable revision. AluminumKα radiation is shown to have high potentialities for ESCA studies.