461 research outputs found
Calibration of White Dwarf cooling sequences: theoretical uncertainty
White Dwarf luminosities are powerful age indicators, whose calibration
should be based on reliable models. We discuss the uncertainty of some chemical
and physical parameters and their influence on the age estimated by means of
white dwarf cooling sequences. Models at the beginning of the white dwarf
sequence have been obtained on the base of progenitor evolutionary tracks
computed starting from the zero age horizontal branch and for a typical halo
chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear
reaction rates, convection, mass loss and initial chemical composition are
discussed. Then, various cooling sequences for a typical white dwarf mass
(M=0.6 Mo) have been calculated under different assumptions on some input
physics, namely: conductive opacity, contribution of the ion-electron
interaction to the free energy and microscopic diffusion. Finally we present
the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much
effort has been spent to extend the equation of state down to the low
temperature and high density regime. An analysis of the latest improvement in
the physics of white dwarf interiors is presented. We conclude that at the
faint end of the cooling sequence (log L/Lo=-5.5) the present overall
uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr.
We suggest that this uncertainty could be substantially reduced by improving
our knowledge of the conductive opacity (especially in the partially degenerate
regime) and by fixing the internal stratification of C and O.Comment: 14 figures, accepted by Ap
DESIGN AND SYNTHESIS OF NOVEL BIOACTIVE PEPTIDES AND PEPTIDOMIMETICS
Nowadays there\u2019s a growing interest in biologically active peptides for the development of new therapeutics; however in some cases, they could not directly use as drugs, due to their inherent limitations, such as rapid metabolism and low oral activity. As a result, peptides are modified into peptidomimetics with specific characteristics, in a rational design.
The present PhD project is focused on the synthesis of several peptides and peptidomimetics, structurally different and presenting individual features, properties, targets and pharmaceutical applications. In particular, two are the research studies we\u2019ve developed during the three years, these are the design of novel Carnosine-like derivatives and of new Farnesyl Transferase Inhibitors (FTIs).
Concerning the first topic, we investigated how Carnosine (\u3b2-alanyl-L-histidine) structural changes influence its role as scavenger of HNE (4-hydroxy-trans-2,3-nonenal) and other toxic aldehydes.
For this reason we modified the carnosine structure firstly replacing the Hystidinil- portion with different aromatic system, secondly substituting the \u3b2-alanyl portion with ten different amino acids, chosen in order to cover exhaustively the available chemical space. Finally we rigidified the whole structure, inserting a 2-oxazolidinone; the entire compound underwent biological evaluation, testing their ability to quench HNE.
As a result, some of the twenty dipeptides showed impressing scavenging activities and great selectivity towards toxic aldehydes, suggesting us that they can represent truly promising candidates for the design of improved carnosine derivatives.
Regarding the second subject, we designed, synthesized and tested several peptidomimetics of the CAAX box, where CAAX is the sequence Cysteine-Valine-Isoleucine-Methionine, able to block the farnesylation of RAS proteins and therefore cell proliferation.
The design started from a nanomolar range FTI, previously synthesized by our group, where the central dipeptide (AA) is replaced with a 4-amino-2-o-tolylbenzoyl spacer and the Cysteine (C) with the residue 2-amino-4-thiazolylacetyl. The synthesis of the novel FTIs followed two separate approaches; at first we kept the aromatic spacer and modified the N-terminal residue with other heterocycles; the unimproved antiproliferative activity suggested us to apply other kind of modification. Therefore we replaced the o-tolyl with six heteroaromatic residues, in addition the synthesized compounds presented, as N- terminal residue, the 2-amino-4-thiazolylacetyl itself or the 1,4-benzodioxan-2-ylmethyl or the 1,4-benzodioxan-2-ylformyl. In all the three series of compounds, the 2-thienyl, 1-naphtyl and the 3-furanyl derivatives showed the highest FTase inhibition, at low micromolar level.
Taken together, our biological activities provide interesting results, confirming that peptides and peptidomimetics should be employed as therapeutics
The fate of Li and Be in stars and in the laboratory
We connect the observed under-abundances of Li and Be in dwarfs, with recent
results on nuclear cross sections at low energies: for collisions of protons
with atomic or molecular targets, the measured cross sections seem too high
with respect to extrapolations for bare nuclei. Phenomenologically, these
anomalous nuclear interactions can be described in terms of an effective
screening potential in the range of few hundred eV: in the presence
of the electron cloud, nuclei become more transparent to each other as if the
effective collision energy is aumented by . This implies that fusion
cross sections are enlarged and at the same time elastic cross sections are
lowered. If something similar occurs in stellar plasma, the nuclear burning
temperatures are lowered, whereas diffusion processes are enhanced. We find
that the observed Li and Be abundances in the Hyades and in the Sun can be
reproduced for effective screening potentials of the plasma in the range of
600-700 eV, close to that found by experiments in the lab.Comment: 21 pages plus 11 figures, uuencoded postscript file
Targeting bacterial cell division: A binding site-centered approach to the most promising inhibitors of the essential protein FtsZ
Binary fission is the most common mode of bacterial cell division and is mediated by a multiprotein complex denominated the divisome. The constriction of the Z-ring splits the mother bacterial cell into two daughter cells of the same size. The Z-ring is formed by the polymerization of FtsZ, a bacterial protein homologue of eukaryotic tubulin, and it represents the first step of bacterial cytokinesis. The high grade of conservation of FtsZ in most prokaryotic organisms and its relevance in orchestrating the whole division system make this protein a fascinating target in antibiotic research. Indeed, FtsZ inhibition results in the complete blockage of the division system and, consequently, in a bacteriostatic or a bactericidal effect. Since many papers and reviews already discussed the physiology of FtsZ and its auxiliary proteins, as well as the molecular mechanisms in which they are involved, here, we focus on the discussion of the most compelling FtsZ inhibitors, classified by their main protein binding sites and following a medicinal chemistry approach
Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models
We investigate the lifting effect of rotation on the thermal evolution of CO
WDs accreting CO-rich matter. We find that rotation induces the cooling of the
accreting star so that the delivered gravitational energy causes a greater
expansion with respect to the standard non-rotating case. The increase in the
surface radius produces a decrease in the surface value of the critical angular
velocity and, therefore, the accreting WD becomes gravitationally unbound
(Roche instability). This occurrence is due to an increase in the total angular
momentum of the accreting WD and depends critically on the amount of specific
angular momentum deposited by the accreted matter. If the specific angular
momentum of the accreted matter is equal to that of the outer layers of the
accreting structure, the Roche instability occurs well before the accreting WD
can attain the physical conditions for C-burning. If the values of both initial
angular velocity and accretion rate are small, we find that the accreting WD
undergoes a secular instability when its total mass approaches 1.4 Msun. At
this stage, the ratio between the rotational and the gravitational binding
energy of the WD becomes of the order of 0.1, so that the star must deform by
adopting an elliptical shape. In this case, since the angular velocity of the
WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss
of rotational energy and angular momentum via GWR. We find that, independent of
the braking efficiency, the WD contracts and achieves the physical conditions
suitable for explosive C-burning at the center so that a type Ia supernova
event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical
Journa
C/O white dwarfs of very low mass: 0.33-0.5 Mo
The standard lower limit for the mass of white dwarfs (WDs) with a C/O core
is roughly 0.5 Mo. In the present work we investigated the possibility to form
C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution
of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf
Workshop" (to be published JPCS). 7 pages including 13 figure
First Replication of the Involvement of OTUD6B in Intellectual Disability Syndrome With Seizures and Dysmorphic Features
The First Fluorine Abundance Determinations in Extragalactic AGB Carbon Stars
Fluorine (19F) abundances (or upper limits) are derived in six extragalactic
AGB carbon stars from the HF(1-0) R9 line at 2.3358 mu in high resolution
spectra. The stars belong to the Local Group galaxies LMC, SMC and Carina dwarf
spheroidal, spanning more than a factor 50 in metallicity. This is the first
study to probe the behaviour of F with metallicity in intrinsic extragalactic
C-rich AGB stars. Fluorine could be measured only in four of the target stars,
showing a wide range in F-enhancements. Our F abundance measurements together
with those recently derived in Galactic AGB carbon stars show a correlation
with the observed carbon and s-element enhancements. The observed correlations
however, display a different dependence on the stellar metallicity with respect
to theoretical predictions in low mass, low metallicity AGB models. We briefly
discuss the possible reasons for this discrepancy. If our findings are
confirmed in a larger number of metal-poor AGBs, the issue of F production in
AGB stars will need to be revisited.Comment: 15 pages, 4 figures, accepted in ApJ Letter
Evolution of low-mass metal-free stars including effects of diffusion and external pollution
We investigate the evolution of low-mass metal-free Population III stars.
Emphasis is laid upon the question of internal and external sources for
CNO-elements, which - if present in sufficient amounts in the hydrogen-burning
regions - lead to a strong modification of the stars' evolutionary behavior.
For the production of carbon due to nuclear processes inside the stars, we use
an extended nuclear network, demonstrating that hot pp-chains do not suffice to
produce enough carbon or are less effective than the triple3-alpha-process. As
an external source of CNO-elements we test the efficiency of pollution by a
nearby massive star combined with particle diffusion. For all cases
investigated, the additional metals fail to reach nuclear burning regions
before deep convection on the Red Giant Branch obliterates the previous
evolution. The surface abundance history of the polluted Pop III stars is
presented. The possibilities to discriminate between a Pop II and a polluted
Pop III field star are also discussed.Comment: Accepted for publication in Ap
Evolution and Nucleosynthesis of Zero Metal Intermediate Mass Stars
New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are
presented. The models have been evolved from the pre Main Sequence up to the
Asymptotic Giant Branch (AGB). At variance with previous claims, we find that
these updated stellar models do experience thermal pulses in the AGB phase. In
particular we show that: a) in models with mass larger than 6 Mo, the second
dredge up is able to raise the CNO abundance in the envelope enough to allow a
"normal" AGB evolution, in the sense that the thermal pulses and the third
dredge up settle on; b) in models of lower mass, the efficiency of the CNO
cycle in the H-burning shell is controlled by the carbon produced locally via
the 3alpha reactions. Nevertheless the He-burning shell becomes thermally
unstable after the early AGB. The expansion of the overlying layers induced by
these weak He-shell flashes is not sufficient by itself to allow a deep
penetration of the convective envelope. However, immediately after that, the
maximum luminosity of the He flash is attained and a convective shell
systematically forms at the base of the H-rich envelope. The innermost part of
this convective shell probably overlaps the underlying C-rich region left by
the inter-shell convection during the thermal pulse, so that fresh carbon is
dredged up in a "hot" H-rich environment and a H flash occurs. This flash
favours the expansion of the outermost layers already started by the weak
thermal pulse and a deeper penetration of the convective envelope takes place.
Then, the carbon abundance in the envelope rises to a level high enough that
the further evolution of these models closely resembles that of more metal rich
AGB stars. These stars provide an important source of primary carbon and
nitrogen.Comment: 28 pages, 5 tables and 17 figures. Accepted for publication in Ap
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