Redox-adaptable copper hosts. Pyridazine-linked cryptands accommodate copper in a range of redox States

A series of structurally characterized copper complexes of two pyridazine-spaced cryptands in redox states + (I,I), (II,I), (II), (II,II) are reported. The hexaimine cryptand L(I) [formed by the 2 + 3 condensation of 3,6-diformylpyridazine with tris(2-aminoethyl)amine (tren)] is able to accommodate two non-stereochemically demanding copper(I) ions, resulting in [Cu(I)(2)L(I)](BF(4))(2) 1, or one stereochemically demanding copper(II) ion, resulting in [Cu(II)L(I)()](BF(4))(2) 3. Complex 3 crystallizes in two forms, 3a and 3b, with differing copper(II) ion coordination geometries. Addition of copper(I) to the monometallic complex 3 results in the mixed-valence complex [Cu(I)Cu(II)L(I)](X)(3) (X = PF(6)(-), 2a; X = BF(4)(-), 2b) which is well stabilized within this cryptand as indicated by electrochemical studies (K(com) = 2.1 x 10(11)). The structurally characterized, octaamine cryptand L(A), prepared by sodium borohydride reduction of L(I), is more flexible than L(I) and can accommodate two stereochemically demanding copper(II) ions, generating the dicopper(II) cryptate [Cu(II)(2)L(A)](BF(4))(4) 4. Electrochemical studies indicate that L(A) stabilizes the copper(II) oxidation state more effectively than L(I); no copper redox state lower than II,II has been isolated in the solid state using this ligand.     

Vitality fertilization of Scots pine stands growing along a gradient of heavy metal pollution: short-term effects on microbial biomass and respiration rate of the humus layer

In 1992 forest vitality fertilization experiments were established on a heavy metal deposition gradient with four treatments in three replications at distances of 0.5, 4 and 8 km from a Cu-Ni smelter in order to estimate their impact on the disturbed forest ecosystem. The increase in Cu concentration in the humus (F/H) layer of the Calluna site type Scots pine (Pinus sylvestris) stands from ca. 300 to 8000 mg kg(-1) d.m. (dry matter) along the 8 km long transect towards the smelter resulted in declining soil microbial biomass and soil respiration activity. Three independent measurements of microbial biomass: C(mic)-FE (fumigation extraction), C(mic)-SIR (substrate induced respiration), and ATP have been used together with an indicator of fungal biomass (ergosterol) and microbial activity (soil respiration). Within this Cu pollution range, all the measured microbial biomass levels declined to 10%-28% of the control plot values and activity assessed by respiration was lowered to 16%. Liming has increased the C(mic)-SIR and respiration rate. Treatments with test fertilizer, made from grounded apatite, did not result in different microbial biomass and respiration rate values compared to the respective controls along the whole gradient. Nitrogen + lime treatments resulted in similar changes to lime alone. No changes, as compared to the respective control, could be detected with nitrogen fertilization at the less polluted end of the gradient.     

Sublimation of vertically oriented paradichlorobenzene cylinders in a natural convection environment

Paradichlorobenzene cylinders were cast, then suspended vertically and allowed to sublimate in air. Data on mass versus time were measured, and a sublimation rate was calculated. Three cylinders of different diameters were used: 1 inch (2.54?cm), 1.5 inch (3.81?cm), and 2 inch (5.08?cm). The length of all three cylinders was 10 in. (25.4?cm). Calculations indicate that the Schmidt number was constant. The Sherwood number ranged from 23 to 26, and Rayleigh numbers varied from 11?×?10³ to 88?×?10³. The objective of this study was to develop a correlation for determining the mass transfer coefficient of vertically suspended paradichlorobenzene cylinders in a natural convection environment. An equation relating Sherwood and Rayleigh numbers was derived.     

Investigation of the noncovalent interactions between anti-amyloid agents and amyloid β peptides by ESI-MS

This paper describes an efficient and reproducible screening method for identifying low molecular weight compounds that bind to amyloid β peptides (Aβ) peptides using electrospray ionization mass spectrometry (ESI-MS). Low molecular weight compounds capable of interacting with soluble Aβ may be able to modulate/inhibit the Aβ aggregation process and serve as potential disease-modifying agents for AD. The present approach was used to rank the binding affinity of a library of compounds to Aβ1-40 peptide. The results obtained show that low molecular weight compounds bind similarly to Aβ1-42, Aβ1-40, as well as Aβ1-28 peptides and they underline the critical role of Aβ peptide charge motif in binding at physiological pH. Finally, some elements of structure-activity relationship (SAR) involved in the binding affinity of homotaurine to soluble Aβ peptides are discussed.     

Tracking the Structure-Reactivity Relationship of Zinc Guanidine-Pyridine Hybrid Complexes Initiating Lactide Polymerisation

Summary : Three zinc guanidine-pyridine hybrid complexes [Zn(TMGqu)₂(CH₃SO₃)][CH₃SO₃] ( 1 ), [Zn(DMEGqu)(CH₃SO₃)₂] ( 2 ) and [Zn(DMEGqu)₂(CH₃SO₃)][CH₃SO₃] ( 3 ) were synthesised, completely characterised and investigated on their activity in the solvent-free ring-opening polymerisation of D,L -lactide. It was proven that the bis-chelate trigonal-bipyramidally coordinated compounds 1 and 3 are able to act as initiators for lactide polymerisation, and polylactides with molecular weights (M_w) of around 28000 g/mol could be obtained with relatively narrow polydispersities. The tetrahedral complex 2 does not initiate lactide polymerisation. In an integrated approach of structural studies and DFT calculations, the active complexes 1 and 3 were analysed towards their structural and electronical pre-requisites in comparison to their more active triflate analogues 1 ^OTf and 3 ^OTf . The influence of coordination strength of the anionic component on the charge distribution within the complex and on the substrate accessibility to the zinc centre is highlighted as crucial factor for the polymerisation initiation. As result, it is shown that the mesylate complexes 1 and 3 have less positive charge on the zinc centre and the mesylate is stronger bound than the corresponding triflate in the triflate complexes 1 ^OTf and 3 ^OTf . Consequently, the reactivity of the complexes is directly correlated to the coordinational behaviour of the anionic component.     

Influence of the Metal Work Function on the Photocatalytic Properties of TiO2 Layers on Metals

The photocatalytic properties of titanium dioxide (TiO₂) layers on different metal plates are investigated. The metal–semiconductor interface can be described as a Schottky contact, and is part of a depletion layer for the majority carriers in the semiconductor. Many researchers have demonstrated an increase in the photocatalytic activity, due to the formation of a metal–semiconductor contact that are obtained by deposition of small metal islands on the semiconductor. Nevertheless, the influence of a Schottky contact remains uncertain, sparking much interest in this field. The immobilization of nanoparticulate TiO₂ layers by dip‐coating on different metal substrates results in the formation of a Schottky contact. The recombination rate of photoinduced electron–hole pairs decreases at this interface provided that the thickness of the thin TiO₂ layer has a similar magnitude to the depletion layer. The degradation of dichloroacetic acid in aqueous solution and of acetaldehyde in a gas mixture is investigated to obtain information concerning the influence of the metal work function of the back contact on the efficiency of the photocatalytic process.     

Flexibility in Proteins: Tuning the Sensitivity to O2 Diffusion by Varying the Lifetime of a Phosphorescent Sensor in Horseradish Peroxidase¶

The heme in horseradish peroxidase (HRP) was replaced by phosphorescent Pt‐mesoporphyrin IX (PtMP), which acted as a phosphorescent marker of oxygen quenching and allowed comparison with another probe, Pd‐mesoporphyrin IX (Khajehpour et al. (2003) Proteins 53, 656–666). Benzohydroxamic acid (BHA), a competitive inhibitor of the enzyme, was also used to monitor its effects on phosphorescence quenching. With the addition of BHA, in the presence of oxygen, the phosphorescence intensity of the protein increased. In contrast, the addition of BHA, in the absence of oxygen, reduced the phosphorescence intensity of the protein. K_d= 18 μM when BHA binds to PtMP‐HRP. The effect of BHA can be explained by two factors: ( 1 ) BHA reduces the accessibility of O₂ to the protein interior and ( 2 ) BHA itself quenches the phosphorescence. Consistent with this, the oxygen quenching of the phosphorescence of PtMP‐HRP gave a quenching constant of k_q= 234 mm Hg^?1 s^?1 in the absence of BHA and k_q= 28.7 mm Hg^?1 s^?1 in the presence of BHA. The quenching rate of BHA is 4000 s^?1. The relative quantum yield of the phosphorescence of the Pt derivative is about six times that of the Pd derivative, whereas the phosphorescence lifetime is approximately eight times shorter. The high quantum yield and suitable lifetime make Pt‐porphyrins appropriate as sensors of O₂ diffusion and flexibility in heme proteins.