Energy Loss, Electron Screening, and the Astrophysical 3He(d,p)4He cross section

We reanalyze the low-energy 3He(d,p)4He cross section measurements of Engstler et al. using recently measured energy loss data for proton and deuteron beams in a helium gas. Although the new 3He(d,p)4He S-factors are significantly lower than those reported by Engstler et al. they clearly show the presence of electron screening effects. From the new S-factors we find an electron screening energy in agreement with the adiabatic limit.Comment: 8 Page RevTeX document, two postscript figures, now in a self-extracting uufile type archiv

Energetic, structural, and antimicrobial analyses of β-lactam side chain recognition by β-lactamases

AbstractBackground: Penicillins and cephalosporins are among the most widely used and successful antibiotics. The emergence of resistance to these β-lactams, most often through bacterial expression of β-lactamases, threatens public health. To understand how β-lactamases recognize their substrates, it would be helpful to know their binding energies. Unfortunately, these have been difficult to measure because β-lactams form covalent adducts with β-lactamases. This has complicated functional analyses and inhibitor design.Results: To investigate the contribution to interaction energy of the key amide (R1) side chain of β-lactam antibiotics, eight acylglycineboronic acids that bear the side chains of characteristic penicillins and cephalosporins, as well as four other analogs, were synthesized. These transition-state analogs form reversible adducts with serine β-lactamases. Therefore, binding energies can be calculated directly from Ki values. The Ki values measured span four orders of magnitude against the Group I β-lactamase AmpC and three orders of magnitude against the Group II β-lactamase TEM-1. The acylglycineboronic acids have Ki values as low as 20 nM against AmpC and as low as 390 nM against TEM-1. The inhibitors showed little activity against serine proteases, such as chymotrypsin. R1 side chains characteristic of β-lactam inhibitors did not have better affinity for AmpC than did side chains characteristic of β-lactam substrates. Two of the inhibitors reversed the resistance of pathogenic bacteria to β-lactams in cell culture. Structures of two inhibitors in their complexes with AmpC were determined by X-ray crystallography to 1.90 Å and 1.75 Å resolution; these structures suggest interactions that are important to the affinity of the inhibitors.Conclusions: Acylglycineboronic acids allow us to begin to dissect interaction energies between β-lactam side chains and β-lactamases. Surprisingly, there is little correlation between the affinity contributed by R1 side chains and their occurrence in β-lactam inhibitors or β-lactam substrates of serine β-lactamases. Nevertheless, presented in acylglycineboronic acids, these side chains can lead to inhibitors with high affinities and specificities. The structures of their complexes with AmpC give a molecular context to their affinities and may guide the design of anti-resistance compounds in this series

Ablowitz-Ladik system with discrete potential. I. Extended resolvent

Ablowitz-Ladik linear system with range of potential equal to {0,1} is considered. The extended resolvent operator of this system is constructed and the singularities of this operator are analyzed in detail.Comment: To be published in Theor. Math. Phy

The Cross Section of 3He(3He,2p)4He measured at Solar Energies

We report on the results of the \hethet\ experiment at the underground accelerator facility LUNA (Gran Sasso). For the first time the lowest projectile energies utilized for the cross section measurement correspond to energies below the center of the solar Gamow peak ($E_{\rm 0}$=22 keV). The data provide no evidence for the existence of a hypothetical resonance in the energy range investigated. Although no extrapolation is needed anymore (except for energies at the low-energy tail of the Gamow peak), the data must be corrected for the effects of electron screening, clearly observed the first time for the \hethet\ reaction. The effects are however larger than expected and not understood, leading presently to the largest uncertainty on the quoted $S_{\rm b}(E_{\rm 0})$ value for bare nuclides (=5.40 MeV b).Comment: 18 pages, 10 postscript figures, Calculations concerning hypothetical resonanz added, Submitted to Phys. Rev. C., available at this URL: HTTP://www.lngs.infn.it/lngs/htexts/luna/luna.htm

Solar fusion cross sections II: the pp chain and CNO cycles

We summarize and critically evaluate the available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for 8B solar neutrinos. We also discuss opportunities for further increasing the precision of key rates, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to Reviews of Modern Physics 70 (1998) 1265.Comment: 54 pages, 20 figures, version to be published in Reviews of Modern Physics; various typos corrected and several updates mad

Structures of FOX-4 cephamycinase in complex with transition-state analog inhibitors

Boronic acid transition-state analog inhibitors (BATSIs) are partners with β-lactam antibiotics for the treatment of complex bacterial infections. Herein, microbiological, biochemical, and structural findings on four BATSIs with the FOX-4 cephamycinase, a class C β-lactamase that rapidly hydrolyzes cefoxitin, are revealed. FOX-4 is an extended-spectrum class C cephalosporinase that demonstrates conformational flexibility when complexed with certain ligands. Like other β-lactamases of this class, studies on FOX-4 reveal important insights into structure–activity relationships. We show that SM23, a BATSI, shows both remarkable flexibility and affinity, binding similarly to other β-lactamases, yet retaining an IC50 value < 0.1 μM. Our analyses open up new opportunities for the design of novel transition-state analogs of class C enzymes

Solar fusion cross sections. II. The pp chain and CNO cycles

We summarize and critically evaluate the available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for 8B solar neutrinos. We also discuss opportunities for further increasing the precision of key rates, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to Reviews of Modern Physics 70 (1998) 1265.Comment: 54 pages, 20 figures, version to be published in Reviews of Modern Physics; various typos corrected and several updates mad

Ligand-Induced Proton Transfer and Low-Barrier Hydrogen Bond Revealed by X-ray Crystallography

Ligand binding can change the pKa of protein residues and influence enzyme catalysis. Herein, we report three sub-Angstrom resolution X-ray crystal structures of CTX-M \u3b2-lactamase, representing three stages of the enzymatic pathway, apo protein (0.79 \uc5), pre-covalent complex (0.89 \uc5), and acylation transition state analog (0.84 \uc5). The binding of a non-covalent ligand induces a proton transfer from the catalytic Ser70 to the general base Glu166, and the formation of a low-barrier hydrogen bond (LBHB) between Ser70 and Lys73. QM/MM reaction path calculations determined the proton transfer barrier between Ser70 and Lys73 to be 1.53 kcal/mol, further confirming the presence of a LBHB. This LBHB is absent in the other two structures. Our data represents the first evidence of a direct and transient LBHB stabilizing a nucleophilic serine, as hypothesized by Cleland and Kreevoy. These results have important implications for the study of enzyme mechanisms as well as protein-inhibitor interactions

Tailoring gold nanoparticle characteristics and the impact on aqueous-phase oxidation of glycerol

Poly(vinyl alcohol) (PVA)-stabilized Au nanoparticles (NPs) were synthesized by colloidal methods in which temperature variations (−75 to 75 °C) and mixed H2O/EtOH solvent ratios (0, 50, and 100 vol/vol) were used. The resulting Au NPs were immobilized on TiO2 (P25), and their catalytic performance was investigated for the liquid phase oxidation of glycerol. For each unique solvent system, there was a systematic increase in the average Au particle diameter as the temperature of the colloidal preparation increased. Generation of the Au NPs in H2O at 1 °C resulted in a high observed activity compared with current Au/TiO2 catalysts (turnover frequency = 915 h−1). Interestingly, Au catalysts with similar average particle sizes but prepared under different conditions had contrasting catalytic performance. For the most active catalyst, aberration-corrected high angle annular dark field scanning transmission electron microscopy analysis identified the presence of isolated Au clusters (from 1 to 5 atoms) for the first time using a modified colloidal method, which was supported by experimental and computational CO adsorption studies. It is proposed that the variations in the populations of these species, in combination with other solvent/PVA effects, is responsible for the contrasting catalytic properties