Asymptotics of Feynman Diagrams and The Mellin-Barnes Representation

It is shown that the integral representation of Feynman diagrams in terms of the traditional Feynman parameters, when combined with properties of the Mellin--Barnes representation and the so called {\it converse mapping theorem}, provide a very simple and efficient way to obtain the analytic asymptotic behaviours in both the large and small ratios of mass scales.Comment: References added. This is the version published in Physics Letters

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Contains research objectives and reports on two research objectives.National Science Foundation (Grant GP-2495)National Institutes of Health (Grant MH-04737-04)National Aeronautics and Space Administration (Grant NsG-496)Joint Services Electronics Program by the U. S. Army Research Office, Durha

Single-photon absorption of isolated collagen mimetic peptides and triple-helix models in the VUV-X energy range

Cartilage and tendons owe their special mechanical properties to the fibrous collagen structure. These strong fibrils are aggregates of a sub-unit consisting of three collagen proteins wound around each other in a triple helix. Even though collagen is the most abundant protein in the human body, the response of this protein complex to ionizing radiation has never been studied. In this work, we probe the direct effects of VUV and soft X-ray photons on isolated models of the collagen triple helix, by coupling a tandem mass spectrometer to a synchrotron beamline. Single-photon absorption is found to induce electronic excitation, ionization and conversion into internal energy leading to inter- and intra-molecular fragmentation, mainly due to Gly-Pro peptide bond cleavages. Our results indicate that increasing the photon energy from 14 to 22 eV reduces fragmentation. We explain this surprising behavior by a smooth transition from excitation to ionization occurring with increasing photon energy. Moreover, our data support the assumption of a stabilization of the triple helix models by proline hydroxylation via intra-complex stereoelectronic effects, instead of the influence of solvent

Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source

Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond ultraviolet laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theoretically predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold molecular beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths 3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility

Mass Spectral Signatures of Complex Post-Translational Modifications in Proteins: A Proof-of-Principle Based on X-ray Irradiated Vancomycin

Characterizing post-translational modifications (PTM) of proteins is of key relevance for the understanding of many biological processes, as these covalent modifications strongly influence or even determine protein function. Among the different analytical techniques available, mass spectrometry is attracting growing attention because recent instrumental and computational improvements have led to a massive rise of the number of PTM sites that can be identified and quantified. However, multiple PTM occurring at adjacent amino acid residues can lead to complex and dense chemical patterns that are a challenge to characterize. By means of X-ray synchrotron radiation coupled to mass spectrometry, and through the test-case of the glycopeptide antibiotic vancomycin, we show that such a pattern has a unique and robust signature in terms of photon energy and molecular environment. This highlights the potential of this technique in proteomics and its value as a tool to understand the biological roles of PTM

Finite Factorization equations and Sum Rules for BPS correlators in N=4 SYM theory

A class of exact non-renormalized extremal correlators of half-BPS operators in N=4 SYM, with U(N) gauge group, is shown to satisfy finite factorization equations reminiscent of topological gauge theories. The finite factorization equations can be generalized, beyond the extremal case, to a class of correlators involving observables with a simple pattern of SO(6) charges. The simple group theoretic form of the correlators allows equalities between ratios of correlators in N=4 SYM and Wilson loops in Chern-Simons theories at k=\infty, correlators of appropriate observables in topological G/G models and Wilson loops in two-dimensional Yang-Mills theories. The correlators also obey sum rules which can be generalized to off-extremal correlators. The simplest sum rules can be viewed as large k limits of the Verlinde formula using the Chern-Simons correspondence. For special classes of correlators, the saturation of the factorization equations by a small subset of the operators in the large N theory is related to the emergence of semiclassical objects like KK modes and giant gravitons in the dual ADS \times S background. We comment on an intriguing symmetry between KK modes and giant gravitons.Comment: 1+69 pages, harvmac, 38 figures; v2: references added, comment added on next-to-extremal correlator

Divergent antiviral effects of bioflavonoids on the hepatitis C virus life cycle

AbstractWe have previously demonstrated that quercetin, a bioflavonoid, blocks hepatitis C virus (HCV) proliferation by inhibiting NS5A-driven internal ribosomal entry site (IRES)-mediated translation of the viral genome. Here, we investigate the mechanisms of antiviral activity of quercetin and six additional bioflavonoids. We demonstrate that catechin, naringenin, and quercetin possess significant antiviral activity, with no associated cytotoxicity. Infectious virion secretion was not significantly altered by these bioflavonoids. Catechin and naringenin demonstrated stronger inhibition of infectious virion assembly compared to quercetin. Quercetin markedly blocked viral translation whereas catechin and naringenin demonstrated mild activity. Similarly quercetin completely blocked NS5A-augmented IRES-mediated translation in an IRES reporter assay, whereas catechin and naringenin had only a mild effect. Moreover, quercetin differentially inhibited HSP70 induction compared to catechin and naringenin. Thus, the antiviral activity of these bioflavonoids is mediated through different mechanisms. Therefore combination of these bioflavonoids may act synergistically against HCV

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Contains research objectives.National Institutes of Health (Grant 5 POI GM14940-06)National Institutes of Health (Grant 1 P01 GM19428-01)National Science Foundation (Grant GK-33736X)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-0300Grant from the Associated PressPeter Bent Brigham Hospital Purchase Orders F64088Peter Bent Brigham Hospital Purchase Orders F6580

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Contains research objectives, summary of research and reports on two research projects.National Institutes of Health (Grant 5 PO1 GM-14940-02)National Institutes of Health (Grant 5 P01 GM-15006-02)Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Institutes of Health (Grant 5 TO1 GM-01555-02