DNA cleavage by copper-ATCUN complexes. Factors influencing cleavage mechanism and linearization of dsDNA

The reactivity of two [peptide-Cu] complexes ([GGH-Cu](-) and [KGHK-Cu](+)) toward DNA cleavage has been quantitatively investigated. Neither complex promoted hydrolytic cleavage, but efficient oxidative cleavage was observed in the presence of a mild reducing agent (ascorbate) and dioxygen. Studies with scavengers of ROS confirmed hydrogen peroxide to be an obligatory diffusible intermediate. While oxidative cleavage of DNA was observed for Cu(2+)(aq) under the conditions used, the kinetics of cleavage and reaction products/pathway were distinct from those displayed by [peptide-Cu] complexes. DNA cleavage chemistry is mediated by the H(2)O-dependent pathway following C-4'H abstraction from the minor groove. Such a cleavage path also provides a ready explanation for the linearization reaction promoted by [KGHK-Cu](+). Kinetic activities and reaction pathways are compared to published results on other chemical nucleases. Both [peptide-Cu] complexes were found to display second-order kinetics, with rate constants k(2) approximately 39 and 93 M(-1) s(-1) for [GGH-Cu](-) and [KGHK-Cu](+), respectively. Neither complex displayed enzyme-like saturation behavior, consistent with the relatively low binding affinity and residence time expected for association with dsDNA, and the absence of a prereaction complex. However, the intrinsic activity of each is superior to other catalyst systems, as determined from relative k(2) or k(cat)/K(m) values. Linearization of DNA was observed for [KGHK-Cu](+) relative to [GGH-Cu](-), consistent with the increased positive charge and longer residency time on dsDNA.     

ESR Study of Trimethylsilane Plasma Polymer Part II; Effect of Consecutive Treatments and Mixed Gases

An ESR study has indicated that a second plasma treatment on plasma deposited films from trimethylsilane (TMS) monomer gas has the ability to modify the characteristics of the primary plasma polymer significantly in a favorable manner for many applications. The effect of the second plasma polymerization on the primary plasma polymer of TMS depends on the nature of the second monomer. Plasma of F-containing monomer, hexafluoroethane (HFE) and perfluoromethane (CF₄), decreases the ESR signal of TMS and no detectable signal due to F-containing monomer was found. The decay rate of the signal decreased significantly. In contrast to this situation, CH₄ plasma treatment yields an ESR signal that is a composite of that observed from TMS and CH₄ films individually. The overall signal increased in this instance, but didn't show appreciable decay in 24 hr period. A second treatment by nonpolymer forming plasmas also decreased the ESR signal of TMS, and decreased the decay rate, indicating that the second gas plasma treatment yields a somewhat similar effect found with the HFE plasma treatment. Plasma polymerization of mixtures of TMS and nonpolymer-forming gases increased the ESR signal but decreased the decay rate, except in the case of oxygen. A mixture of (TMS + O₂) behaved as a completely different monomer. No ESR signal was found in this system. The ESR analysis was supported by XPS data and an insight into the mechanisms occurring in these thin films are discussed.     

Interpenetrating supramolecular lattices in 4,4′-bi­pyridine–2,3,5,6-tetra­hydroxy-1,4-benzo­quinone (3/2)

4,4′-Bi­pyridine (BPY) and 2,3,5,6-tetra­hydroxy-1,4-benzo­quinone (THBQ) crystallize in a 3:2 ratio as a neutral molecular adduct, 3C₁₀H₈N₂·2C₆H₄O₆, in space group P. There are two independent and centrosymmetric THBQ mol­ecules and two different BPY mol­ecules in the asymmetric unit, one of which lies about an inversion centre. The molecules link together through O—H⃛O and O—H⃛N hydrogen bonds to form three interpenetrating networks which create a `superlattice' of three times the volume of the primitive cell.     

Shallow-underground accelerator sites for nuclear astrophysics: Is the background low enough?

In order to reliably estimate the rate of a charged particle induced nuclear reaction in a non-explosive astrophysical scenario, its cross-section must be measured far below the Coulomb barrier. However, at the corresponding energies the cross-section values are very low, so that the experimental counting rate is dominated by cosmic-ray induced background, even if a suitable anticoincidence shield is applied. This problem can be overcome by performing an accelerator-based experiment in a deep underground site, as has been done with great success at the LUNA 0.4MV accelerator in Gran Sasso, Italy. Several underground accelerators with higher beam energy are in the planning phase worldwide. All of them are shielded by over 1000m of rock, a depth at which cosmic-ray effects are negligible for the purposes of nuclear astrophysics experiments. It is shown here that a combined approach, using a shallow-underground laboratory below 47m of rock and an active shield to veto surviving muons in simple detectors, results in a background level that is not far from that of deep underground sites. Data have been obtained using two ??traveling?? ??-detectors. They have been transported both shallow underground, to the Dresden Felsenkeller in Germany, and deep underground, to the Gran Sasso laboratory in Italy. As shallow-underground facilities are more easily accessible than deep-underground ones, the present finding holds the promise of greatly accelerated progress in the field of cross-section measurements for nuclear astrophysics.     

Recurrence Relationships for the Mean Number of Faces and Vertices for Random Convex Hulls

This paper studies the convex hull of n random points in R^d\mathsf{R}^{d} . A recently proved topological identity of the author is used in combination with identities of Efron and Buchta to find the expected number of vertices of the convex hull—yielding a new recurrence formula for all dimensions d. A recurrence for the expected number of facets and (d−2)-faces is also found, this analysis building on a technique of Rényi and Sulanke. Other relationships for the expected count of i-faces (1≤i<d) are found when d≤5, by applying the Dehn–Sommerville identities. A general recurrence identity (see (3) below) for this expected count is conjectured.     

Control of cytochrome C redox potential: axial ligation and protein environment effects

Axial iron ligation and protein encapsulation of the heme cofactor have been investigated as effectors of the reduction potential (E degrees ') of cytochrome c through direct electrochemistry experiments. Our approach was that of partitioning the E degrees ' changes resulting from binding of imidazole, 2-methyl-imidazole, ammonia, and azide to both cytochrome c and microperoxidase-11 (MP11), into the enthalpic and entropic contributions. N-Acetylmethionine binding to MP11 was also investigated. These ligands replace Met80 and a water molecule axially coordinated to the heme iron in cytochrome c and MP11, respectively. This factorization was achieved through variable temperature E degrees ' measurements. In this way, we have found that (i) the decrease in E degrees ' of cytochrome c due to Met80 substitution by a nitrogen-donor ligand is almost totally enthalpic in origin, as a result of the stronger electron donor properties of the exogenous ligand which selectively stabilize the ferric state; (ii) on the contrary, the binding of the same ligands and N-acetylmethionine to MP11 results in an enthalpic stabilization of the reduced state, whereas the entropic effect invariably decreases E degrees ' (the former effect prevails for the methionine ligand and the latter for the nitrogenous ligands). A comparison of the reduction thermodynamics of cytochrome c and the MP11 adducts offers insight on the effect of changing axial heme ligation and heme insertion into the folded polypeptide chain. Principally, we have found that the overall E degrees ' increase of approximately 400 mV, comparing MP11 and native cytochrome c, consists of two opposite enthalpic and entropic terms of approximately +680 and -280 mV, respectively. The enthalpic term includes contributions from both axial methionine binding (+300 mV) and protein encapsulation of the heme (+380 mV), whereas the entropic term is almost entirely manifest at the stage of axial ligand binding. Both terms are dominated by the effects of water exclusion from the heme environment.     

Ultralow emittance, multi-MeV proton beams from a laser virtual-cathode plasma accelerator

The laminarity of high-current multi-MeV proton beams produced by irradiating thin metallic foils with ultraintense lasers has been measured. For proton energies >10 MeV, the transverse and longitudinal emittance are, respectively, <0.004 mm mrad and <10(-4) eV s, i.e., at least 100-fold and may be as much as 10(4)-fold better than conventional accelerator beams. The fast acceleration being electrostatic from an initially cold surface, only collisions with the accelerating fast electrons appear to limit the beam laminarity. The ion beam source size is measured to be <15 microm (FWHM) for proton energies >10 MeV.