How can a single second sphere amino acid substitution cause reduction midpoint potential changes of hundreds of millivolts?

The active site metal ion of superoxide dismutase (SOD) is reduced and reoxidized as it disproportionates superoxide to dioxygen and hydrogen peroxide. Thus, the reduction midpoint potential (Em) is a critical determinant of catalytic activity. In E. coli Fe-containing SOD (FeSOD), reduction of Fe3+ is accompanied by protonation of a coordinated OH-, to produce Fe2+ coordinated by H2O. The coordinated solvent's only contact with the protein beyond the active site is a conserved Gln residue. Mutation of this Gln to His or Glu resulted in elevation of the Em by 220 mV and more than 660 mV, respectively [Yikilmaz et al., Biochemistry 2006, 45, 1151-1161], despite the fact that overall protein structure was preserved, His is a chemically conservative replacement for Gln, and neutral Glu is isostructural and isoelectronic with Gln. Therefore, we have investigated several possible bases for the elevated Em's, including altered Fe electronic structure, altered active site electrostatics, altered H-bonding and altered redox-coupled proton transfer. Using EPR, MCD, and NMR spectroscopies, we find that the active site electronic structures of the two mutants resemble that of the WT enzyme, for both oxidation states, and Q69E-FeSOD's apparent deviation from WT-like Fe3+ coordination in the oxidized state can be explained by increased affinity for a small anion. Spontaneous coordination of an exogenous anion can only stabilize oxidized Q69E-Fe3+SOD and, therefore, cannot account for the increased Em of Q69E FeSOD. WT-like anion binding affinities and active site pK's indicate that His69 of Q69H-FeSOD is neutral in both oxidation states, like Gln69 of WT-FeSOD, whereas Glu69 appears to be neutral in the oxidized state but ionized in the reduced state of Q69E-FeSOD. A 1.1 A resolution crystal structure of Q69E-Fe2+SOD indicates that Glu69 accepts a strong H-bond from coordinated solvent in the reduced state, in contrast to the case in WT-FeSOD where Gln69 donates an H-bond. These data and DFT calculations lead to the proposal that the elevated Em of Q69E-FeSOD can be substantially explained by (1) relief from enforced H-bond donation in the reduced state, (2) Glu69's capacity to provide a proton for proton-coupled Fe3+ reduction, and (3) strong hydrogen bond acceptance in the reduced state, which stabilizes coordinated H2O. Our results thus support the hypothesis that the protein matrix can apply significant redox tuning via its influence over redox-coupled proton transfer and the energy associated with it.     

Mass‐Spectrometric Studies of Providencia SR‐Form Lipopolysaccharides and Elucidation of the Biological Repeating Unit Structure of Providencia rustigianii O14‐Polysaccharide

Enterobacteria Providencia are opportunistic human pathogens causing multiple types of infections. Earlier we have studied the S‐ and R‐form lipopolysaccharides (LPSs) of Providencia strains of various O‐serogroups and established the structures of the O‐polysaccharides (O‐antigens) and core‐region oligosaccharides, respectively. Now we report on mass spectrometric studies of oligosaccharides consisting of the core moiety with one O‐polysaccharide repeating unit attached, which were derived from the SR‐form LPSs of Providencia strains. The site of attachment of the O‐polysaccharide to the core and the structure of the O‐polysaccharide biological repeating unit were elucidated in Providencia rustigianii O14 using NMR spectroscopy.     

Computational studies of boron‐ and nitrogen‐doped single‐walled carbon nanotubes as potential sensor materials of hydrogen halide molecules HX (X = F,Cl, Br)

Potential applicability of undoped, B‐, and N‐doped carbon nanotubes (CNTs) for elaboration of the working materials of gas sensors of hydrogen halide molecules HX (X = F, Cl, Br) is analyzed in computational studies of molecular adsorption on the CNTs surfaces. Density Functional Theory (DFT)‐based geometry‐optimized calculations of the electronic structure of undoped, B‐, and N‐doped CNTs of (3,3) and (5,5) chiralities with adsorbed HX (X = F, Cl, Br) molecules are performed within molecular cluster approach. Relaxed geometries, binding energies between the adsorbates and the nanotubes, charge states of the adsorbates and the electronic wave function contours are calculated and analyzed in the context of gas sensing applications. Obtained results are supplemented by calculations of adsorption of hydrogen halides on B(N)‐doped graphene sheets which are considered as model approximation for large‐diameter CNTs. It is found that the B‐doped CNTs are perspective for elaboration of sensing materials for detection of HCl and HBr molecules. The undoped and the N‐doped CNTs are predicted to be less suitable materials for detection of hydrogen halide gases HX (X = F, Cl, Br). © 2015 Wiley Periodicals, Inc.     

On genera of polyhedra

We consider the stable homotopy category S of polyhedra (finite cell complexes). We say that two polyhedra X,Y are in the same genus and write X ∼ Y if X _p ≅ Y _p for all prime p, where X _p denotes the image of Xin the localized category S _p . We prove that it is equivalent to the stable isomorphism X∨B ₀ ≅Y∨B ₀, where B ₀ is the wedge of all spheres S ⁿ such that π _n ^S (X) is infinite. We also prove that a stable isomorphism X ∨ X ≅ Y ∨ X implies a stable isomorphism X ≅ Y.     

Matrix problems and stable homotopy types of polyhedra

This is a survey of the results on stable homotopy types of polyhedra of small dimensions, mainly obtained by H.-J. Baues and the author [3, 5, 6]. The proofs are based on the technique of matrix problems (bimodule categories). Dedicated to C. M. Ringel.     

A peculiarity of silver-based corona wire heating on ozone generation

The effect of corona wire temperature on the ozone generation in the positive dc corona electrostatic precipitator is studied experimentally. The external heating of the corona wire can suppress the ozone generation. In this study, nichrome and two kinds of silver-based wires 0.1 mm diameter were tested as discharges electrodes. The nichrome corona wire heating shows a well-known monotonic decreasing the rate of ozone production. In the case of the tested silver-based wires the rate of ozone production decreases nonlinearly and passes through a local minimum in the range from 35 to 55 °C with increasing the wire temperature. At the wire temperature about 46 °C ozone generation by positive dc corona discharge is decreased by 53% with Ag:Mn = 0.85:0.15 wire and by 25% with Ag:Ni = 0.7:0.3 wire as compared to the same wire at 26 °C. Under these conditions the corona wire heating increases slightly the corona current and speed of airflow.     

Phosphorus halide free synthesis of 1,2,3,4-tetrahydroisophosphinoline 2-oxides

1. Download high-res image (88KB)
2. Download full-size image