537 research outputs found
More is different: 50 years of nuclear BCS
Many of the concepts which are at the basis of the development associated
with a quantitative treatment of the variety of phenomena associated with the
spontaneous breaking of gauge symmetry in nuclei have been instrumental in
connection with novel studies of soft matter, namely of protein evolution and
protein folding. Although the route to these subjects and associated
development does not necessarily imply the nuclear physics connection, such a
connection has proven qualitatively and quantitatively inspiring. In particular
to model protein evolution in terms of the alignment of quasispins displaying
twenty different projections, one for each of the twenty amino acids occurring
in nature, and the associated symmetry breaking in information (sequence)
space. Emergent properties of the corresponding phase transition are domain
walls which stabilize local elementary structures (LES), few groups of 10-20
aminoacids which become structured already in the denatured state provide the
molecular recognition directing protein folding. In fact, their docking is
closely related to the transition state of the process. While the two-step, yes
or no, folding process, does not provide direct information concerning LES, one
can force LES from virtual to become real, observable final state entities.
Getting again inspiration from the nuclear case (virtual processes contributing
to pair correlations can be forced to become real with the help of a probe
which itself changes particle number by two), one would expect that to make
real virtual LES, that is segments of the protein which already at an early
stage of the folding process flicker in and out of their native conformation,
one needs a probe which itself displays a similar behaviour. Peptides
displaying a sequence identical to LES are such probes.Comment: Contribution to the Volume 50 years of Nuclear BCS edited by World
Scientifi
Direct observation of the glue pairing the halo of the nucleus 11Li
With the help of a unified description of the nuclear structure and of the
direct reaction mechanism we show that a recent 1H(11Li,9Li)3H experiment
provides, for the first time in nuclear physics, direct evidence of phonon
mediated pairing.Comment: 9 pages, 4 figures. Major change
Reading the three-dimensional structure of a protein from its amino acid sequence
While all the information required for the folding of a protein is contained
in its amino acid sequence, one has not yet learnt how to extract this
information so as to predict the detailed, biological active, three-dimensional
structure of a protein whose sequence is known. This situation is not
particularly satisfactory, in keeping with the fact that while linear
sequencing of the amino acids specifying a protein is relatively simple to
carry out, the determination of the folded-native-conformation can only be done
by an elaborate X-ray diffraction analysis performed on crystals of the protein
or, if the protein is very small, by nuclear magnetic resonance techniques.
Using insight obtained from lattice model simulations of the folding of small
proteins (fewer than 100 residues), in particular of the fact that this
phenomenon is essentially controlled by conserved contacts among strongly
interacting amino acids, which also stabilize local elementary structures
formed early in the folding process and leading to the (post-critical) folding
core when they assemble together, we have worked out a successful strategy for
reading the three-dimensional structure of a notional protein from its amino
acid sequence.Comment: misprints eliminated and small mistakes correcte
Difference between stable and exotic nuclei: medium polarization effects
The bare NN-potential, parametrized so as to reproduce the nuclear phase
shifts leads to a sizable Cooper pair binding energy in nuclei along the
stability valley. It is a much debated matter whether this value accounts for
the "empirical" value of the pairing gap or whether a similarly important
contribution arises from the exchange of collective vibrations between Cooper
pair partners. In keeping with the fact that two-particle transfer reactions
are the specific probe of pairing in nuclei, and that exotic halo nuclei like
11Li are extremely polarizable, we find that the recent studied reaction,
namely 11Li+p -> 9Li+t, provides direct evidence of phonon mediated pairing in
nuclei
Pair-transfer probability in open- and closed-shell Sn isotopes
Approximations made to estimate two-nucleon transfer probabilities in
ground-state to ground-state transitions and physical interpretation of these
probabilities are discussed. Probabilities are often calculated by
approximating both ground states, of the initial nucleus A and of the final
nucleus A\pm 2 by the same quasiparticle vacuum. We analyze two improvements of
this approach. First, the effect of using two different ground states with
average numbers of particles A and A\pm2 is quantified. Second, by using
projection techniques, the role of particle number restoration is analyzed. Our
analysis shows that the improved treatment plays a role close to magicity,
leading to an enhancement of the pair-transfer probability. In mid-shell
regions, part of the error made by approximating the initial and final ground
states by a single vacuum is compensated by projecting onto good particle
number. Surface effects are analyzed by using pairing interactions with a
different volume-to-surface mixing. Finally, a simple expression of the
pair-transfer probability is given in terms of occupation probabilities in the
canonical basis. We show that, in the canonical basis formulation, surface
effects which are visible in the transfer probability are related to the
fragmentation of single-particle occupancies close to the Fermi energy. This
provides a complementary interpretation with respect to the standard
quasiparticle representation where surface effects are generated by the
integrated radial profiles of the contributing wave functions.Comment: 12 pages, 7 figure
Hiking in the energy landscape in sequence space: a bumpy road to good folders
With the help of a simple 20 letters, lattice model of heteropolymers, we
investigate the energy landscape in the space of designed good-folder
sequences. Low-energy sequences form clusters, interconnected via neutral
networks, in the space of sequences. Residues which play a key role in the
foldability of the chain and in the stability of the native state are highly
conserved, even among the chains belonging to different clusters. If, according
to the interaction matrix, some strong attractive interactions are almost
degenerate (i.e. they can be realized by more than one type of aminoacid
contacts) sequence clusters group into a few super-clusters. Sequences
belonging to different super-clusters are dissimilar, displaying very small
() similarity, and residues in key-sites are, as a rule, not
conserved. Similar behavior is observed in the analysis of real protein
sequences.Comment: 17 pages 5 figures Corrected typos added auxiliary informatio
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