51 research outputs found
Lattice QCD Simulations in External Background Fields
We discuss recent results and future prospects regarding the investigation,
by lattice simulations, of the non-perturbative properties of QCD and of its
phase diagram in presence of magnetic or chromomagnetic background fields.
After a brief introduction to the formulation of lattice QCD in presence of
external fields, we focus on studies regarding the effects of external fields
on chiral symmetry breaking, on its restoration at finite temperature and on
deconfinement. We conclude with a few comments regarding the effects of
electromagnetic background fields on gluodynamics.Comment: 31 pages, 10 figures, minor changes and references added. To appear
in Lect. Notes Phys. "Strongly interacting matter in magnetic fields"
(Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Ye
The Chiral MagnetoHydroDynamics of QCD fluid at RHIC and LHC
The experimental results on heavy ion collisions at RHIC and LHC indicate
that QCD plasma behaves as a nearly perfect fluid described by relativistic
hydrodynamics. Hydrodynamics is an effective low-energy Theory Of Everything
stating that the response of a system to external perturbations is dictated by
conservation laws that are a consequence of the symmetries of the underlying
theory. In the case of QCD fluid produced in heavy ion collisions, this theory
possesses anomalies, so some of the apparent classical symmetries are broken by
quantum effects. Even though the anomalies appear as a result of UV
regularization and so look like a short distance phenomenon, it has been
realized recently that they also affect the large distance, macroscopic
behavior in hydrodynamics. One of the manifestations of anomalies in
relativistic hydrodynamics is the Chiral Magnetic Effect (CME). At this
conference, a number of evidences for CME have been presented, including i) the
disappearance of charge asymmetry fluctuations in the low-energy RHIC data
where the energy density is thought to be below the critical one for
deconfinement; ii) the observation of charge asymmetry fluctuations in Pb-Pb
collisions at the LHC. Here I give a three-page summary of some of the recent
theoretical and experimental developments and of the future tests that may
allow to establish (or to refute) the CME as the origin of the observed charge
asymmetry fluctuations.Comment: 4 pages, talk at Quark Matter 2011 Conference, Annecy, France, 23-28
May 201
A Model of a MAPK•Substrate Complex in an Active Conformation: A Computational and Experimental Approach
The mechanisms by which MAP kinases recognize and phosphorylate substrates are not completely understood. Efforts to understand the mechanisms have been compromised by the lack of MAPK-substrate structures. While MAPK-substrate docking is well established as a viable mechanism for bringing MAPKs and substrates into close proximity the molecular details of how such docking promotes phosphorylation is an unresolved issue. In the present study computer modeling approaches, with restraints derived from experimentally known interactions, were used to predict how the N-terminus of Ets-1 associates with ERK2. Interestingly, the N-terminus does not contain a consensus-docking site ((R/K)2-3-X2-6-ΦA-X-ΦB, where Φ is aliphatic hydrophobic) for ERK2. The modeling predicts that the N-terminus of Ets-1 makes important contributions to the stabilization of the complex, but remains largely disordered. The computer-generated model was used to guide mutagenesis experiments, which support the notion that Leu-11 and possibly Ile-13 and Ile-14 of Ets-1 1-138 (Ets) make contributions through binding to the hydrophobic groove of the ERK2 D-recruiting site (DRS). Based on the modeling, a consensus-docking site was introduced through the introduction of an arginine at residue 7, to give the consensus 7RK-X2-ΦA-X-ΦB13. This results in a 2-fold increase in kcat/Km for the phosphorylation of Ets by ERK2. Similarly, the substitution of the N-terminus for two different consensus docking sites derived from Elk-1 and MKK1 also improves kcat/Km by two-fold compared to Ets. Disruption of the N-terminal docking through deletion of residues 1-23 of Ets results in a 14-fold decrease in kcat/Km, with little apparent change in kcat. A peptide that binds to the DRS of ERK2 affects Km, but not kcat. Our kinetic analysis suggests that the unstructured N-terminus provides 10-fold uniform stabilization of the ground state ERK2•Ets•MgATP complex and intermediates of the enzymatic reaction
Ammoniated electron as a solvent stabilized multimer radical anion
The excess electron in liquid ammonia ("ammoniated electron") is commonly
viewed as a cavity electron in which the s-type wave function fills the
interstitial void between 6-9 ammonia molecules. Here we examine an alternative
model in which the ammoniated electron is regarded as a solvent stabilized
multimer radical anion, as was originally suggested by Symons [Chem. Soc. Rev.
1976, 5, 337]. In this model, most of the excess electron density resides in
the frontier orbitals of N atoms in the ammonia molecules forming the solvation
cavity; a fraction of this spin density is transferred to the molecules in the
second solvation shell. The cavity is formed due to the repulsion between
negatively charged solvent molecules. Using density functional theory
calculations for small ammonia cluster anions in the gas phase, it is
demonstrated that such core anions would semi-quantitatively account for the
observed pattern of Knight shifts for 1-H and 14-N nuclei observed by NMR
spectroscopy and the downshifted stretching and bending modes observed by
infrared spectroscopy. It is speculated that the excess electrons in other
aprotic solvents (but not in water and alcohols) might be, in this respect,
analogous to the ammoniated electron, with substantial transfer of the spin
density into the frontier N and C orbitals of methyl, amino, and amide groups
forming the solvation cavity.Comment: 34 pages, 12 figures; to be submitted to J Phys Chem
Quark Matter in a Strong Magnetic Background
In this chapter, we discuss several aspects of the theory of strong
interactions in presence of a strong magnetic background. In particular, we
summarize our results on the effect of the magnetic background on chiral
symmetry restoration and deconfinement at finite temperature. Moreover, we
compute the magnetic susceptibility of the chiral condensate and the quark
polarization at zero temperature. Our theoretical framework is given by chiral
models: the Nambu-Jona-Lasinio (NJL), the Polyakov improved NJL (or PNJL) and
the Quark-Meson (QM) models. We also compare our results with the ones obtained
by other groups.Comment: 34 pages, survey. To appear in Lect. Notes Phys. "Strongly
interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K.
Landsteiner, A. Schmitt, H.-U. Ye
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