64 research outputs found
Correlation between sequence hydrophobicity and surface-exposure pattern of database proteins
Hydrophobicity is thought to be one of the primary forces driving the folding
of proteins. On average, hydrophobic residues occur preferentially in the core,
whereas polar residues tends to occur at the surface of a folded protein. By
analyzing the known protein structures, we quantify the degree to which the
hydrophobicity sequence of a protein correlates with its pattern of surface
exposure. We have assessed the statistical significance of this correlation for
several hydrophobicity scales in the literature, and find that the computed
correlations are significant but far from optimal. We show that this less than
optimal correlation arises primarily from the large degree of mutations that
naturally occurring proteins can tolerate. Lesser effects are due in part to
forces other than hydrophobicity and we quantify this by analyzing the surface
exposure distributions of all amino acids. Lastly we show that our database
findings are consistent with those found from an off-lattice hydrophobic-polar
model of protein folding.Comment: 16 pages, 2 tables, 8 figure
Kosmotropes and chaotropes: modelling preferential exclusion, binding and aggregate stability
Kosmotropic cosolvents added to an aqueous solution promote the aggregation
of hydrophobic solute particles, while chaotropic cosolvents act to destabilise
such aggregates. We discuss the mechanism for these phenomena within an adapted
version of the two-state Muller-Lee-Graziano model for water, which provides a
complete description of the ternary water/cosolvent/solute system for small
solute particles. This model contains the dominant effect of a kosmotropic
substance, which is to enhance the formation of water structure. The consequent
preferential exclusion both of cosolvent molecules from the solvation shell of
hydrophobic particles and of these particles from the solution leads to a
stabilisation of aggregates. By contrast, chaotropic substances disrupt the
formation of water structure, are themselves preferentially excluded from the
solution, and thereby contribute to solvation of hydrophobic particles. We use
Monte Carlo simulations to demonstrate at the molecular level the preferential
exclusion or binding of cosolvent molecules in the solvation shell of
hydrophobic particles, and the consequent enhancement or suppression of
aggregate formation. We illustrate the influence of structure-changing
cosolvents on effective hydrophobic interactions by modelling qualitatively the
kosmotropic effect of sodium chloride and the chaotropic effect of urea.Comment: 13 pages, 12 figures; inclusion of review material, parameter
analysis and comparison of kosmotropic and chaotropic effect
The Minority Game with interactions
We partially modify the rules of the Minority Game (MG) by introducing some
degree of local information in the game, which is only available for some
agents, called the interacting agents. Our work shows that, for small values of
the new parameter of the model (the fraction of interacting agents), there is
an improvement of the use of the resources with respect to the MG, while as
this number grows the response of the system changes, and ends up behaving
worst than the usual MG.Comment: 9 pages, 4 figures; typoos corrected; references upadted; Physica A
-like LaTe
The Local Minority Game
Ecologists and economists try to explain collective behavior in terms of
competitive systems of selfish individuals with the ability to learn from the
past. Statistical physicists have been investigating models which might
contribute to the understanding of the underlying mechanisms of these systems.
During the last three years one intuitive model, commonly referred to as the
Minority Game, has attracted broad attention. Powerful yet simple, the minority
game has produced encouraging results which can explain the temporal behaviour
of competitive systems. Here we switch the interest to phenomena due to a
distribution of the individuals in space. For analyzing these effects we modify
the Minority Game and the Local Minority Game is introduced. We study the
system both numerically and analytically, using the customary techniques
already developped for the ordinary Minority Game
Order and disorder in the Local Evolutionary Minority Game
We study a modification of the Evolutionary Minority Game (EMG) in which
agents are placed in the nodes of a regular or a random graph. A neighborhood
for each agent can thus be defined and a modification of the usual relaxation
dynamics can be made in which each agent updates her decision scheme depending
upon the options made in her immediate neighborhood. We name this model the
Local Evolutionary Minority Game (LEMG). We report numerical results for the
topologies of a ring, a torus and a random graph changing the size of the
neighborhood. We focus our discussion in a one dimensional system and perform a
detailed comparison of the results obtained from the random relaxation dynamics
of the LEMG and from a linear chain of interacting spin-like variables at a
finite temperature. We provide a physical interpretation of the surprising
result that in the LEMG a better coordination (a lower frustration) is achieved
if agents base their actions on local information. We show how the LEMG can be
regarded as a model that gradually interpolates between a fully ordered,
antiferromagnetic system and a fully disordered system that can be assimilated
to a spin glass.Comment: 12 pages, 8 figures, RevTex; omission of a relevant reference
correcte
Solvent-induced micelle formation in a hydrophobic interaction model
We investigate the aggregation of amphiphilic molecules by adapting the
two-state Muller-Lee-Graziano model for water, in which a solvent-induced
hydrophobic interaction is included implicitly. We study the formation of
various types of micelle as a function of the distribution of hydrophobic
regions at the molecular surface. Successive substitution of non-polar surfaces
by polar ones demonstrates the influence of hydrophobicity on the upper and
lower critical solution temperatures. Aggregates of lipid molecules, described
by a refinement of the model in which a hydrophobic tail of variable length
interacts with different numbers of water molecules, are stabilized as the
length of the tail increases. We demonstrate that the essential features of
micelle formation are primarily solvent-induced, and are explained within a
model which focuses only on the alteration of water structure in the vicinity
of the hydrophobic surface regions of amphiphiles in solution.Comment: 11 pages, 10 figures; some rearrangement of introduction and
discussion sections, streamlining of formalism and general compression; to
appear in Phys. Rev.
Boolean Game on Scale-free Networks
Inspired by the local minority game, we propose a network Boolean game and
investigate its dynamical properties on scale-free networks. The system can
self-organize to a stable state with better performance than random choice
game, although only the local information is available to the agent. By
introducing the heterogeneity of local interactions, we find the system has the
best performance when each agent's interaction frequency is linear correlated
with its information capacity. Generally, the agents with more information gain
more than those with less information, while in the optimal case, each agent
almost has the same average profit. In addition, we investigate the role of
irrational factor and find an interesting symmetrical behavior.Comment: 12 pages and 6 figure
The Emergence of Leadership in Social Networks
We study a networked version of the minority game in which agents can choose
to follow the choices made by a neighbouring agent in a social network. We show
that for a wide variety of networks a leadership structure always emerges, with
most agents following the choice made by a few agents. We find a suitable
parameterisation which highlights the universal aspects of the behaviour and
which also indicates where results depend on the type of social network.Comment: 22 pages (as in Physica A but with a few extra references to
supplementary material) plus 11 pages of supplementary material not in
Physica A versio
Structure-preserving desynchronization of minority games
Perfect synchronicity in -player games is a useful theoretical dream, but
communication delays are inevitable and may result in asynchronous
interactions. Some systems such as financial markets are asynchronous by
design, and yet most theoretical models assume perfectly synchronized actions.
We propose a general method to transform standard models of adaptive agents
into asynchronous systems while preserving their global structure under some
conditions. Using the Minority Game as an example, we find that the phase and
fluctuations structure of the standard game subsists even in maximally
asynchronous deterministic case, but that it disappears if too much
stochasticity is added to the temporal structure of interaction. Allowing for
heterogeneous communication speeds and activity patterns gives rise to a new
information ecology that we study in details.Comment: 6 pages, 7 figures. New version removed a section and found a new
phase transitio
Statistical Mechanics of Dilute Batch Minority Games with Random External Information
We study the dynamics and statics of a dilute batch minority game with random
external information. We focus on the case in which the number of connections
per agent is infinite in the thermodynamic limit. The dynamical scenario of
ergodicity breaking in this model is different from the phase transition in the
standard minority game and is characterised by the onset of long-term memory at
finite integrated response. We demonstrate that finite memory appears at the
AT-line obtained from the corresponding replica calculation, and compare the
behaviour of the dilute model with the minority game with market impact
correction, which is known to exhibit similar features.Comment: 22 pages, 6 figures, text modified, references updated and added,
figure added, typos correcte
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