Effect of pH on the self-exchange reactivity of the plant plastocyanin from parsley

The self-exchange rate constant (25 degrees C) for parsley plastocyanin is 5.0 x 10(4) M-1 s-1 at pH* 7.5 (I = 0.10 M). This value is quite large for a higher plant plastocyanin and can be attributed to a diminished upper acidic patch in this protein. The self-exchange rate constant is almost independent of pH* in the range 7.5-5.6, with a value (25 degrees C) of 5.6 x 10(4) M-1 s-1 at pH* 5.6 (I = 0.10 M). At this pH*, the ligand His87 is protonated in approximately 50% of the reduced protein molecules (pKa* 5.6), and this would be expected to hinder electron transfer between the two oxidation states. However, this effect is counterbalanced by the enhanced association of two parsley plastocyanins at lower pH* due to the partial protonation of the acidic patch.     

The Mercury (II) Catalyzed,One-Pot Oxidation of Terminal Alkynes by Sodium Perborate in Acetic Acid

A series of terminal alkynes has been reacted with sodium perborate and mercuric acetate catalyst in acetic acid to produce 1-acetoxyalkan-2-ones in good yield. The reaction constitutes a mild and convenient method for the oxidation of terminal triple bonds.     

The Mechanism of Biochemical NO-Sensing: Insights from Computational Chemistry

The binding of small gas molecules such as NO and CO plays a major role in the signaling routes of the human body. The sole NO-receptor in humans is soluble guanylyl cyclase (sGC) – a histidine-ligated heme protein, which, upon NO binding, activates a downstream signaling cascade. Impairment of NO-signaling is linked, among others, to cardiovascular and inflammatory diseases. In the present work, we use a combination of theoretical tools such as MD simulations, high-level quantum chemical calculations and hybrid QM/MM methods to address various aspects of NO binding and to elucidate the most likely reaction paths and the potential intermediates of the reaction. As a model system, the H-NOX protein from Shewanella oneidensis (So H-NOX) homologous to the NO-binding domain of sGC is used. The signaling route is predicted to involve NO binding to form a six-coordinate intermediate heme-NO complex, followed by relatively facile His decoordination yielding a five-coordinate adduct with NO on the distal side with possible isomerization to the proximal side through binding of a second NO and release of the first one. MD simulations show that the His sidechain can quite easily rotate outward into solvent, with this motion being accompanied in our simulations by shifts in helix positions that are consistent with this decoordination leading to significant conformational change in the protein.     

Comparison of the precision of estimation of copolymerization reactivity ratios by current methods

Six recently proposed methods for analyzing copolymerization composition data have been compared to a nonlinear least-squares analysis to ascertain the precision of the six methods in determining reactivity ratios. Data used were simulated for five hypothetical monomer pairs with three different types of experiment design and contained error structures similar to those observed experimentally. The results of the comparisons suggest that retrospective analyses of existing copolymerization data should only be done with a nonlinear least-squares analysis. For new data, the design of experiments is of great importance, and when done properly allows the use of some of the linear least-squares methods of analysis.     

Excited-state absorption in Er: BaY2F8 and Cs3Er2Br9 and comparison with Er: LiYF4

The influence of Excited-State Absorption (ESA) on the green laser transition and the overlap of Ground-State Absorption (GSA) and ESA for 970 nm upconversion pumping in erbium is investigated in Er³⁺ : BaY₂F₈ and Cs₃Er₂Br₉. Results are compared to Er³⁺ : LiYF₄. In Er³⁺: BaY₂F₈, a good overlap between GSA and ESA is found at 969 nm in one polarization direction. The emission cross section at 550 nm is a factor of two smaller than in LiYF₄. In Cs₃Er₂Br₉, the smaller Stark splitting of the levels shifts the wavelengths of the green emission and ESA from⁴ I _{1 3/2} off resonance. It enhances, however, ground-state reabsorption. The emission cross section at 550 nm is comparable to LiYF₄. Upconversion leads to significant green fluorescence from² H _9/2. A significant population of the⁴ I _11/2 level and ESA at 970 nm are not present under 800 nm pumping.     

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)_10−xSi₆O_26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La_9.33Si₆O₂₆ with Co, Fe, Mn on the Si site, according to the formula La_9.33+x/3Si_6−xM_xO₂₆ (M=Co, Fe, Mn). Substitution limits observed were x≤1.5 (Co), x≤1.25 (Fe), x≤0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La₈Mn₂Si₆O₂₆. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O₂) (0.2–10⁻⁵ atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O₂) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O₂₆ appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La_9.33Si₆O₂₆ with potential SOFC cathode materials (La_1−xSr_xMO₃; M=Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

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