395 research outputs found
A universal chemical potential for sulfur vapours
The unusual chemistry of sulfur is illustrated by the tendency for
catenation. Sulfur forms a range of open and closed S species in the gas
phase, which has led to speculation on the composition of sulfur vapours as a
function of temperature and pressure for over a century. Unlike elemental gases
such as O and N, there is no widely accepted thermodynamic potential
for sulfur. Here we combine a first-principles global structure search for the
low energy clusters from S to S with a thermodynamic model for the
mixed-allotrope system, including the Gibbs free energy for all gas-phase
sulfur on an atomic basis. A strongly pressure-dependent transition from a
mixture dominant in S to S is identified. A universal chemical
potential function, , is proposed with wide utility in
modelling sulfurisation processes including the formation of metal chalcogenide
semiconductors.Comment: 12 pages, 9 figures. Supporting code and data is available at
https://github.com/WMD-Bath/sulfur-model [snapshot DOI:
10.5281/zenodo.28536]. Further data will be available from
DOI:10.6084/m9.figshare.1513736 and DOI:10.6084/m9.figshare.1513833 following
peer-revie
Designability of lattice model heteropolymers
Protein folds are highly designable, in the sense that many sequences fold to
the same conformation. In the present work we derive an expression for the
designability in a 20 letter lattice model of proteins which, relying only on
the Central Limit Theorem, has a generality which goes beyond the simple model
used in its derivation. This expression displays an exponential dependence on
the energy of the optimal sequence folding on the given conformation measured
with respect to the lowest energy of the conformational dissimilar structures,
energy difference which constitutes the only parameter controlling
designability. Accordingly, the designability of a native conformation is
intimately connected to the stability of the sequences folding to them.Comment: in press on Phys. Rev.
Mapping of mutation-sensitive sites in protein-like chains
In this work we have studied, with the help of a simple on-lattice model, the
distribution pattern of sites sensitive to point mutations ('hot' sites) in
protein-like chains. It has been found that this pattern depends on the
regularity of the matrix that rules the interaction between different kinds of
residues. If the interaction matrix is dominated by the hydrophobic effect
(Miyazawa Jernigan like matrix), this distribution is very simple - all the
'hot' sites can be found at the positions with maximum number of closest
nearest neighbors (bulk).
If random or nonlinear corrections are added to such an interaction matrix
the distribution pattern changes. The rising of collective effects allows the
'hot' sites to be found in places with smaller number of nearest neighbors
(surface) while the general trend of the 'hot' sites to fall into a bulk part
of a conformation still holds.Comment: 15 pages, 6 figure
Proofs of some Propositions of the semi-Intuitionistic Logic with Strong Negation
We offer the proofs that complete our article introducing the propositional
calculus called semi-intuitionistic logic with strong negation.Comment: Contains proofs omitted, because of their extention, from an article
published in Studia Logic
The Emergence of Scaling in Sequence-based Physical Models of Protein Evolution
It has recently been discovered that many biological systems, when
represented as graphs, exhibit a scale-free topology. One such system is the
set of structural relationships among protein domains. The scale-free nature of
this and other systems has previously been explained using network growth
models that, while motivated by biological processes, do not explicitly
consider the underlying physics or biology. In the present work we explore a
sequence-based model for the evolution protein structures and demonstrate that
this model is able to recapitulate the scale-free nature observed in graphs of
real protein structures. We find that this model also reproduces other
statistical feature of the protein domain graph. This represents, to our
knowledge, the first such microscopic, physics-based evolutionary model for a
scale-free network of biological importance and as such has strong implications
for our understanding of the evolution of protein structures and of other
biological networks.Comment: 20 pages (including figures), 4 figures, to be submitted to PNA
What thermodynamic features characterize good and bad folders? Results from a simplified off-lattice protein model
The thermodynamics of the small SH3 protein domain is studied by means of a
simplified model where each bead-like amino acid interacts with the others
through a contact potential controlled by a 20x20 random matrix. Good folding
sequences, characterized by a low native energy, display three main
thermodynamical phases, namely a coil-like phase, an unfolded globule and a
folded phase (plus other two phases, namely frozen and random coil, populated
only at extremes temperatures). Interestingly, the unfolded globule has some
regions already structured. Poorly designed sequences, on the other hand,
display a wide transition from the random coil to a frozen state. The
comparison with the analytic theory of heteropolymers is discussed
Role of bulk and of interface contacts in the behaviour of model dimeric proteins
Some dimeric proteins first fold and then dimerize (three--state dimers)
while others first dimerize and then fold (two--state dimers). Within the
framework of a minimal lattice model, we can distinguish between sequences
obeying to one or to the other mechanism on the basis of the partition of the
ground state energy between bulk than for interface contacts. The topology of
contacts is very different for the bulk than for the interface: while the bulk
displays a rich network of interactions, the dimer interface is built up a set
of essentially independent contacts. Consequently, the two sets of interactions
play very different roles both in the the folding and in the evolutionary
history of the protein. Three--state dimers, where a large fraction of the
energy is concentrated in few contacts buried in the bulk, and where the
relative contact energy of interface contacts is considerably smaller than that
associated with bulk contacts, fold according to a hierarchycal pathway
controlled by local elementary structures, as also happens in the folding of
single--domain monomeric proteins. On the other hand, two--state dimers display
a relative contact energy of interface contacts which is larger than the
corresponding quantity associated with the bulk. In this case, the assembly of
the interface stabilizes the system and lead the two chains to fold. The
specific properties of three--state dimers acquired through evolution are
expected to be more robust than those of two--state dimers, a fact which has
consequences on proteins connected with viral diseases
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