41 research outputs found
Schematic model for QCD at finite temperature
The simplest version of a class of toy models for QCD is presented. It is a Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector, gluon pairs with spin zero are treated as elementary bosons. The model restricts to mesons with spin zero and to few baryonic states. The corresponding energy spectrum is discussed. We show that ground state correlations are essential to describe physical properties of the spectrum at low energies. Quantum phase transitions are described in an effective manner, by using coherent states. The appearance of a Goldstone boson for large values of the interaction strength is discussed, as related to a collective state. The formalism is extended to consider finite temperatures. The partition function is calculated, in an approximate way, showing the convenience of the use of coherent states. The energy density, heat capacity, and transitions from the hadronic phase to the quark-gluon plasma are calculated.Departamento de Físic
Modeling pentaquark and heptaquark states
A schematic model for hadronic states, based on constituent quarks and antiquarks and gluon pairs, is discussed. The phenomenological interaction between quarks and gluons is QCD motivated. The obtained hadronic spectrum leads to the identification of nucleon and Δ resonances and to pentaquark and heptaquark states. The predicted lowest pentaquark state (Jπ=1/2⁻) lies at the energy of 1.5 GeV and it is associated to the observed Θ⁺(1540) state. For heptaquarks (Jπ=1/2⁺,3/2⁺) the model predicts the lowest state at 2.5 GeVFacultad de Ciencias Exacta
Schematic model for QCD. III. Hadronic states
The hadronic spectrum obtained in the framework of a QCD-inspired schematic model is presented. The model is the extension of a previous version, whose basic degrees of freedom are constituent quarks, antiquarks, and gluons. The interaction between quarks and gluons is a phenomenological interaction and its parameters are fixed from data. The classification of the states, in terms of quark and antiquark and gluon configurations is based on symmetry considerations, and it is independent of the chosen interaction. Following this procedure, nucleon and Δ
resonances are identified, as well as various penta- and hepta-quarks states. The lowest pentaquarks state is predicted at 1.5 GeV and it has negative parity, while the lowest hepta-quarks state has positive parity and its energy is of the order of 2.5 GeV.Facultad de Ciencias Exacta
Predicting Specificities Under the Non-self Gametophytic Self-Incompatibility Recognition Model
Non-self gametophytic self-incompatibility (GSI) recognition system is characterized by
the presence of multiple F-box genes tandemly located in the S-locus, that regulate
pollen specificity. This reproductive barrier is present in Solanaceae, Plantaginacea and
Maleae (Rosaceae), but only in Petunia functional assays have been performed to get
insight on how this recognition mechanism works. In this system, each of the encoded
S-pollen proteins (called SLFs in Solanaceae and Plantaginaceae /SFBBs in Maleae)
recognizes and interacts with a sub-set of non-self S-pistil proteins, called S-RNases,
mediating their ubiquitination and degradation. In Petunia there are 17 SLF genes
per S-haplotype, making impossible to determine experimentally each SLF specificity.
Moreover, domain –swapping experiments are unlikely to be performed in large scale to
determine S-pollen and S-pistil specificities. Phylogenetic analyses of the Petunia SLFs
and those from two Solanum genomes, suggest that diversification of SLFs predate the
two genera separation. Here we first identify putative SLF genes from nine Solanum
and 10 Nicotiana genomes to determine how many gene lineages are present in the
three genera, and the rate of origin of new SLF gene lineages. The use of multiple
genomes per genera precludes the effect of incompleteness of the genome at the
S-locus. The similar number of gene lineages in the three genera implies a comparable
effective population size for these species, and number of specificities. The rate of origin
of new specificities is one per 10 million years. Moreover, here we determine the amino
acids positions under positive selection, those involved in SLF specificity recognition,
using 10 Petunia S-haplotypes with more than 11 SLF genes. These 16 amino acid
positions account for the differences of self-incompatible (SI) behavior described in the
literature. When SLF and S-RNase proteins are divided according to the SI behavior, and the positively selected amino acids classified according to hydrophobicity, charge, polarity and size, we identified fixed differences between SI groups. According to the in silico 3D structure of the two proteins these amino acid positions interact. Therefore, this methodology can be used to infer SLF/S-RNase specificity recognition.This work was financed by the project Norte-01-0145-FEDER-000008-Porto Neurosciences and Neurologic Disease Research Initiative at I3S, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (FEDER). SR is supported by a post-doctoral fellowship under this project. HL-F is supported by a post-doctoral fellowship from Xunta de Galicia (ED481B 2016/068-0). SING group acknowledges Consellería de Educación, Universidades e Formación Profesional (Xunta de Galicia) for the ED431C2018/55-GRC grant and CITI (Centro de Investigación, Transferencia e Innovación) from University of Vigo for hosting its IT infrastructure
ATXN1 N-terminal region explains the binding differences of wild-type and expanded forms
BACKGROUND:
Wild-type (wt) polyglutamine (polyQ) regions are implicated in stabilization of protein-protein interactions (PPI). Pathological polyQ expansion, such as that in human Ataxin-1 (ATXN1), that causes spinocerebellar ataxia type 1 (SCA1), results in abnormal PPI. For ATXN1 a larger number of interactors has been reported for the expanded (82Q) than the wt (29Q) protein.
METHODS:
To understand how the expanded polyQ affects PPI, protein structures were predicted for wt and expanded ATXN1, as well as, for 71 ATXN1 interactors. Then, the binding surfaces of wt and expanded ATXN1 with the reported interactors were inferred.
RESULTS:
Our data supports that the polyQ expansion alters the ATXN1 conformation and that it enhances the strength of interaction with ATXN1 partners. For both ATXN1 variants, the number of residues at the predicted binding interface are greater after the polyQ, mainly due to the AXH domain. Moreover, the difference in the interaction strength of the ATXN1 variants was due to an increase in the number of interactions at the N-terminal region, before the polyQ, for the expanded form.
CONCLUSIONS:
There are three regions at the AXH domain that are essential for ATXN1 PPI. The N-terminal region is responsible for the strength of the PPI with the ATXN1 variants. How the predicted motifs in this region affect PPI is discussed, in the context of ATXN1 post-transcriptional modifications.This work was financed by the project Norte-01-0145-FEDER-000008 -Porto
Neurosciences and Neurologic Disease Research Initiative at I3S, supported
by Norte Portugal Regional Operational Programme (NORTE 2020), under the
PORTUGAL 2020 Partnership Agreement, through the European Regional Development
Fund (FEDER). Sara Rocha is supported by a post-doctoral fellowship
under this project. Hugo López-Fernández is supported by a postdoctoral
fellowship from Xunta de Galicia (ED481B 2016/068–0). SING group
thanks Consellería de Educación, Universidades e Formación Profesional
(Xunta de Galicia) for the ED431C2018/55-GRC grant and CITI (Centro de
Investigación, Transferencia e Innovación) from University of Vigo for hosting
its IT infrastructure. The funding bodies played no role in the design of the
study and collection, analysis, and interpretation of data and in writing the
manuscript
Neutron-proton pairing in the BCS approach
We investigate the BCS treatment of neutron-proton pairing involving
time-reversed orbits. We conclude that an isospin-symmetric hamiltonian,
treated with the help of the generalized Bogolyubov transformation, fails to
describe the ground state pairing properties correctly. In order for the np
isovector pairs to coexist with the like-particle pairs, one has to break the
isospin symmetry of the hamiltonian by artificially increasing the strength of
np pairing interaction above its isospin symmetric value. We conjecture that
the np isovector pairing represents part (or most) of the congruence energy
(Wigner term) in nuclear masses.Comment: 9 pages, RevTex, submitted to Phys. Rev.
A schematic model for QCD at finite temperature
The simplest version of a class of toy models for QCD is presented. It is a
Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector,
gluon pairs with spin zero are treated as elementary bosons. The model
restricts to mesons with spin zero and to few baryonic states. The
corresponding energy spectrum is discussed. We show that ground state
correlations are essential to describe physical properties of the spectrum at
low energies. Phase transitions are described in an effective manner, by using
coherent states. The appearance of a Goldstone boson for large values of the
interaction strength is discussed, as related to a collective state. The
formalism is extended to consider finite temperatures. The partition function
is calculated, in an approximate way, showing the convenience of the use of
coherent states. The energy density, heat capacity and transitions from the
hadronic phase to the quark-gluon plasma are calculated.Comment: 33 pages, 11 figure
A schematic model for QCD I: Low energy meson states
A simple model for QCD is presented, which is able to reproduce the meson
spectrum at low energy. The model is a Lipkin type model for quarks coupled to
gluons. The basic building blocks are pairs of quark-antiquarks coupled to a
definite flavor and spin. These pairs are coupled to pairs of gluons with spin
zero. The multiplicity problem, which dictates that a given experimental state
can be described in various manners, is removed when a particle-mixing
interaction is turned on. In this first paper of a series we concentrates on
the discussion of meson states at low energy, the so-called zero temperature
limit of the theory. The treatment of baryonic states is indicated, also.Comment: 29 pages, 6 figures. submitted to Phys. Rev.