37 research outputs found
Light 1-+ exotics: molecular resonances
Highlights in the search for nonconventional (non qqbar) meson states are the
pi_1(1400) and pi_1(1600) exotic candidates. Should they exist, mounting
theoretical arguments suggest that they are tetraquark molecular resonances
excitable by meson rescattering.
We report a new tetraquark calculation within a model field theory
approximation to Quantum Chromodynamics in the Coulomb gauge supporting this
conjecture. We also strengthen this claim by consistently contrasting results
with exotic state predictions for hybrid (q qbar g) mesons within the same
theoretical framework.
Our findings confirm that molecular-like configurations involving two color
singlets (a resonance, not a bound state) are clearly favored over hybrid or
color-exotic tetraquark meson (q qbar q qbar atoms) formation.
Finally, to assist needed further experimental searches we document a useful
off-plane correlator for establishing the structure of these exotic systems
along with similar, but anticipated much narrower, states that should exist in
the charmonium and bottomonium spectra.Comment: 12 pages, 8 figure
Meson and tetra-quark mixing
The mixing between q-qbar meson and qqbar-qqbar tetra-quark states is
examined within an effective QCD Coulomb gauge Hamiltonian model. Mixing matrix
elements of the Hamiltonian are computed and then diagonalized yielding an
improved prediction for the low-lying J^{PC} = 0^{+/- +}, 1^{--} isoscalar
spectra. Mixing effects were found significant for the scalar hadrons but not
for the 1^{--} states, which is consistent with the ideal mixing of vector
mesons. A perturbative assessment of the exact QCD kernel is also reported.Comment: 7 pages, 6 figure
ATPase Subdomain IA Is a Mediator of Interdomain Allostery in Hsp70 Molecular Chaperones
The versatile functions of the heat shock protein 70 (Hsp70) family of molecular chaperones rely on allosteric interactions between their nucleotide-binding and substrate-binding domains, NBD and SBD. Understanding the mechanism of interdomain allostery is essential to rational design of Hsp70 modulators. Yet, despite significant progress in recent years, how the two Hsp70 domains regulate each other's activity remains elusive. Covariance data from experiments and computations emerged in recent years as valuable sources of information towards gaining insights into the molecular events that mediate allostery. In the present study, conservation and covariance properties derived from both sequence and structural dynamics data are integrated with results from Perturbation Response Scanning and in vivo functional assays, so as to establish the dynamical basis of interdomain signal transduction in Hsp70s. Our study highlights the critical roles of SBD residues D481 and T417 in mediating the coupled motions of the two domains, as well as that of G506 in enabling the movements of the α-helical lid with respect to the β-sandwich. It also draws attention to the distinctive role of the NBD subdomains: Subdomain IA acts as a key mediator of signal transduction between the ATP- and substrate-binding sites, this function being achieved by a cascade of interactions predominantly involving conserved residues such as V139, D148, R167 and K155. Subdomain IIA, on the other hand, is distinguished by strong coevolutionary signals (with the SBD) exhibited by a series of residues (D211, E217, L219, T383) implicated in DnaJ recognition. The occurrence of coevolving residues at the DnaJ recognition region parallels the behavior recently observed at the nucleotide-exchange-factor recognition region of subdomain IIB. These findings suggest that Hsp70 tends to adapt to co-chaperone recognition and activity via coevolving residues, whereas interdomain allostery, critical to chaperoning, is robustly enabled by conserved interactions. © 2014 General et al
Structure and dynamics of Penetratin’s association and translocation to a lipid bilayer
Penetratin belongs to the important class of small and positively charged peptides, capable of entering
cells. The
determination of the optimal peptidic structure for translocation is challenging; results
obtained so far are varied and dependent on several factors. In this work, we review the
dynamics of association of Penetratin with a modeled dioleoyl-phosphatidylcholine (DOPC)
lipid
membrane
using molecular dynamics simulations with last generation force fields. Penetratin’s
structural preferences are determined using a Markov state model. It is observed that the
peptide retains
a helical form in the membrane associated state, just as in water, with the exception of both
termini which lose helicity, facilitating the interaction of terminal residues with the phosphate groups
on the membrane’s outer layer. The optimal orientation for insertion is found to be with
the peptide’s axis forming a small angle with the interface, and with R1 stretching toward
the bilayer. The interaction between arginine side-chains and phosphate groups is found
to be greater than the corresponding to lysine, mainly due to a higher number of hydrogen
bonds between them. The free
energy profile of translocation is qualitatively studied using Umbrella
Sampling. It is found that there are different paths of penetration, that greatly differ
in size of free
energy barrier. The lowest path is compatible with residues R10 to K13
leading the way through the membrane and pulling the rest of the peptide. When the other side is
reached, the C-terminus overtakes those residues, and finally breaks out of the
membrane.
The peptide’s secondary
structure during this traversal suffers some changes with respect to the
association structure but, overall, conserves its helicity, with both termini in a more
disordered state