Ruthenium-manganese complexes for artificial photosynthesis: factors controlling intramolecular electron transfer and excited-state quenching reactions

Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.     

Viabilty of atomistic potentials for thermodynamic properties of carbon dioxide at low temperatures

Investigation into volumetric and energetic properties of several atomistic models mimicking carbon dioxide geometry and quadrupole momentum covered the liquid-vapor coexistence curve. Thermodynamic integration over a polynomial and an exponential-polynomial path was used to calculate free energy. Computational results showed that model using GROMOS Lennard-Jones parameters was unsuitable for bulk CO(2) simulations. On the other hand, model with potential fitted to reproduce only correct density-pressure relationship in the supercritical region proved to yield correct enthalpy of vaporization and free energy of liquid CO(2) in the low-temperature region. Except for molar volume at the upper part of the vapor-liquid equilibrium line, the bulk properties of exp-6-1 parametrization of ab initio CO(2) potential were in a close agreement with the experimental results. Copyright 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1772-1781, 2001     

Construction of time-inhomogeneous Markov processes via evolution equations using pseudo-differential operators

For a pseudo-differential operator with symbol which is time-and space-dependent, elliptic and continuous negative definite,the corresponding evolution equation is solved. Further, itis shown that the solution defines a Markov process. In general,this will be a time- and spaceinhomogeneous jump process. Tosolve the evolution equation, we combine a fixed-point methodwith the symbolic calculus for negative definite symbols developedby Hoh. The properties of the fundamental solution which ensurethe existence of a corresponding Markov process are proved alongthe lines of Eidelman, Ivasyshen and Kochubei. However, insteadof hyper-singular integral representations, we use the pseudo-differentialoperator representation together with the positive maximum principleto obtain the required properties.     

(1E)‐2‐(Di­acetyl­amino)‐1‐methyl­prop‐­1‐enyl acetate

The analysis of the title compound, C₁₀H₁₅NO₄, firmly establishes the configuration of the double bond as E, a stereochemistry that had been assigned tentatively by other methods. The di­acetyl­amine and acetate substituents are approximately coplanar to one another, but approximately perpendicular to the planar ethene core. H atoms of the ethene methyl substituents are found within the ethene plane, indicating that hyperconjugation does not play an important role in stabilizing the double bond.     

Numerical methods for oscillatory solutions to hyperbolic problems

Difference approximations of hyperbolic partial differential equations with highly oscillatory coefficients and initial values are studied. Analysis of strong and weak convergence is carried out in the practically interesting case when the discretization step sizes are essentially independent of the oscillatory wave lengths. © 1993 John Wiley & Sons, Inc.     

Algebraic families of nonzero elements of Shafarevich-Tate groups

Principal homogeneous spaces under an abelian variety defined over a number field may have rational points in all completions of the number field without having rational points over . Such principal homogeneous spaces represent the nonzero elements of the Shafarevich-Tate group of the abelian variety.

We produce nontrivial, one-parameter families of such principal homogeneous spaces. The total space of these families are counterexamples to the Hasse principle. We show these may be accounted for by the Brauer-Manin obstruction.

     

The derivatisation of avermectins and milbemycins in milk: new insights and improvement of the procedure

Derivatisation of the avermectines ivermectin (IVM), doramectin (DOR), abamectin (ABA) and eprinomectin (EPR), and the milbemycin moxidectin (MOX) to fluorescent derivatives is commonly used for quantitative analysis at relevant levels using high performance liquid chromatography (HPLC) with fluorescence detection. Problems associated with the differences in reactivity towards derivatisation (EPM) and limited stability of the derived products (IVM, DOR, ABA) may seriously hamper the applicability of the method and the reliability of the obtained results. A study was performed to obtain more insight in this derivatisation process from an organic chemistry point of view. This study demonstrated the occurrence of two main fluorescent derivatives: the trifluoroacetyl esters (flu-TFA) and the derivatives with a free hydroxy group at the glycosidic ring (flu-OH). Optimisation of the derivatisation conditions resulted in a fast and reproducible formation of the fluorescent derivatives for all analytes including EPM. The improved procedure involves the addition of 1-methylimidazole (MI), trifluoroacetic anhydride (TFAA), triethylamine (TEA) and trifluoroacetic acid (TFA) with a subsequent incubation for 30 min at 70 °C. With this procedure for IVM, DOR and ABA flu-TFA derivatives are obtained instead of flu-OH derivatives as generally described in literature. The derivatisation is reproducible in different milk samples and the derivatives proved to be stable for at least 80 h at room temperature. Using the optimised procedure a limit of detection (LoD) of 0.1 μg kg⁻¹ in milk was readily obtained.     

Catalytic asymmetric synthesis of phthioceranic acid, a heptamethyl-branched acid from Mycobacterium tuberculosis

The first total synthesis of phthioceranic acid (1) has been achieved by an iterative catalytic asymmetric 1,4-addition protocol. This method provides a robust and high-yielding route for the preparation of 1,3-oligomethyl (deoxypropionate) arrays. After the desired number of methyl groups has been introduced, these arrays can be further functionalized at both ends to polymethyl-substituted lipids such as phthioceranic acid, a heptamethyl-branched fatty acid from the virulence factor Sulfolipid-I (2), found in Mycobacterium tuberculosis.