Biomedical subjects

M Pretti

Publications and source records attributed to M Pretti.

8 recordsLinked to original sources

PubMed2005

Thermodynamic anomalies in a lattice model of water: solvation properties.

We investigate a lattice-fluid model of water, defined on a three-dimensional body-centered-cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified by removing distinction between "donors" and "acceptors." We focus on the solvation properties, mainly as far as an ideally inert (hydrophobic) solute is concerned. As in our previous analysis, devoted to neat water [J. Chem. Phys. 121, 11856 (2004)], we make use of a generalized first-order approximation on a tetrahedral cluster. We show that the model exhibits quite a coherent picture of water thermodynamics, reproducing qualitatively several anomalous properties observed both in pure water and in solutions of hydrophobic solutes. As far as supercooled liquid water is concerned, the model is consistent with the second critical-point scenario.

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PubMed2005

Hydration of an apolar solute in a two-dimensional waterlike lattice fluid.

In a previous work, we investigated a two-dimensional lattice-fluid model, displaying some waterlike thermodynamic anomalies. The model, defined on a triangular lattice, is now extended to aqueous solutions with apolar species. Water molecules are of the "Mercedes Benz" type, i.e., they possess a D3 (equilateral triangle) symmetry, with three equivalent bonding arms. Bond formation depends both on orientation and local density. The insertion of inert molecules displays typical signatures of hydrophobic hydration: large positive transfer free energy, large negative transfer entropy (at low temperature), strong temperature dependence of the transfer enthalpy and entropy, i.e., large (positive) transfer heat capacity. Model properties are derived by a generalized first order approximation on a triangle cluster.

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PubMed2004

Thermodynamic anomalies in a lattice model of water.

We investigate a lattice-fluid model of water, defined on a three-dimensional body centered cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms, aiming to mimic the formation of hydrogen bonds. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified in that no distinction between bond "donors" and "acceptors" is imposed. Bond formation depends both on orientation and local density. In the ground state, we show that two different ordered (ice) phases are allowed. At finite temperature, we analyze homogeneous phases only, working out phase diagram, response functions, the temperature of maximum density locus, and the Kauzmann line. We make use of a generalized first-order approximation on a tetrahedral cluster. In the liquid phase, the model exhibits several anomalous properties observed in real water. In the low temperature region (supercooled liquid), there are evidences of a second critical point and, for some range of parameter values, this scenario is compatible with the existence of a reentrant spinodal.

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PubMed2004

Two-dimensional lattice-fluid model with waterlike anomalies.

We investigate a lattice-fluid model defined on a two-dimensional triangular lattice, with the aim of reproducing qualitatively some anomalous properties of water. Model molecules are of the "Mercedes Benz" type, i.e., they possess a D3 (equilateral triangle) symmetry, with three bonding arms. Bond formation depends both on orientation and local density. We work out phase diagrams, response functions, and stability limits for the liquid phase, making use of a generalized first order approximation on a triangle cluster, whose accuracy is verified, in some cases, by Monte Carlo simulations. The phase diagram displays one ordered (solid) phase which is less dense than the liquid one. At fixed pressure the liquid phase response functions show the typical anomalous behavior observed in liquid water, while, in the supercooled region, a reentrant spinodal is observed.

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PubMed2002

Semiflexible polymer in the cactus approximation.

We investigate a cactus approximation for the analysis of a lattice polymer model (self-avoiding walk) in two and three dimensions. We focus on the semiflexible model, which incorporates both an attractive short range interaction between monomers that are nonconsecutive along the chain, and a bending energy (stiffness). In agreement with Monte Carlo simulations, we find two different qualitative behaviors. In the low stiffness regime the polymer undergoes two different transitions upon decreasing temperature: an ordinary Theta collapse from a swollen ("coil") state to a disordered compact ("globule") state, and then a first-order transition to an orientationally ordered ("anisotropic") state. In the high stiffness regime the system displays a single first-order collapse from the coil state at high temperature to the anisotropic state at low temperature. We show that the cactus approximation is able to recover even fine qualitative features of the phase diagram, such as the stiffness dependence of the Theta temperature, with a relatively small computational effort.

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PubMed2002

Bethe approximation for a hydrophobic-polar random copolymer.

A previous analysis of the configurational statistics of a lattice self-avoiding walk, based on the pair approximation of the cluster variation method (Bethe approximation), is generalized to describe a random copolymer in dilute solution, in which monomers interact with one another and with the solvent. The phase diagram is worked out numerically and the coil-globule transition is obtained. Entropy and internal energy in the coil phase, as well as the Theta point are evaluated analytically, by means of a Landau expansion of the variational free energy. A detailed analysis is carried out for the case in which the only interaction present is an attraction between monomers of one species. Such a model has been proposed in the literature to mimic a random copolymer made up of hydrophobic and polar monomers in water solution, a physical system which is believed to be relevant to understanding the behavior of proteins.

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PubMed2001

Bethe approximation for a model of polymer solvation.

The phase diagram of a recently proposed model for the solvation of monomers and polymers in water is studied in the homopolymer case, and several thermodynamic quantities are computed by means of pair approximation of the cluster variation method, i.e., the Bethe approximation. The model takes into account the water degrees of freedom in a simplified way, so that they can be integrated out analytically. The resulting effective Hamiltonian contains only a temperature-dependent water-monomer interaction and its phase diagram can be easily studied thanks to the simplicity of the Bethe approximation and exhibits, for a hydrophobic polymer, both cold and warm unfolding transitions in a wide region of the parameter space. This suggests that the present one might be a toy-model description of the phase behavior observed experimentally in water solutions of hydrophobic polymers, such as poly-N-isopropylacrylamide (PNIPAM), as well as a step to understand the mechanism of cold unfolding in proteins.

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PubMed2000

Herringbone ordering and lattice distortions in a planar-molecule model for langmuir monolayers

A model of planar molecules, made up of "atoms" interacting by Lennard-Jones potentials and arranged to mimic the cross section of alkyl chains, is used to study the problem of backbone plane ordering in Langmuir monolayers. It is shown that two minima of the interaction energy are reached if molecules lie on the sites of a centered rectangular lattice in a herringbone configuration with two different dihedral angles. These orientationally ordered phases can be related to the so-called herringbone and pseudoherringbone structures, whose lattice distortions qualitatively agree with those determined by means of grazing incidence x-ray diffraction experiments on Langmuir monolayers. A third energy minimum is obtained for a configuration of parallel molecules on an oblique lattice, which has also been observed in some experiments. The competition between the three phases is investigated, upon varying geometric parameters of the model molecules and surface pressure. The effect of temperature is analyzed in a mean field approximation, by taking into account the orientational entropy contribution on a lattice system with variable unit cell parameters. In this framework the transition to an orientationally disordered phase is also pointed out.

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