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Roger Rousseau

Publications and source records attributed to Roger Rousseau.

16 recordsLinked to original sources
PubMed2006

Inverse temperature transition of a biomimetic elastin model: reactive flux analysis of folding/unfolding and its coupling to solvent dielectric relaxation.

The inverse temperature transition (ITT) of a biomimetic model for elastin, capped GVG(VPGVG) in liquid water, is investigated by a comprehensive classical molecular dynamics study. The temperature dependence of the solvation structure and dynamics of the octapeptide are compared using three common force fields, CHARMM, GROMOS, and OPLS. While these force fields differ in quantitative detail, they all predict this octapeptide to undergo a "folding transition" to closed conformations upon heating and a subsequent "unfolding transition" to open conformations at still higher temperatures, thus reproducing the ITT scenario. The peptide kinetics is analyzed within the reactive flux formalism applied to the largest-amplitude mode extracted from principal component analysis, and the solvent's dielectric fluctuations are obtained from the total water dipole autocorrelations. Most importantly, preliminary evidence for an intimate coupling of peptide folding/unfolding dynamics, and thus the ITT, and dielectric relaxation of bulk water is given, possibly being consistent with a "slave mode" picture.

Biomimetic Materials↗
PubMed2005

Electronic origin of disorder and diffusion at a molecule-metal interface: self-assembled monolayers of CH3S on Cu111.

The relationship between structure, interfacial electrostatics, bonding, and dynamics of organic molecules on metals is studied using a self-assembled monolayer of methylthiolate, CH3S, on Cu(111). The flat adsorption energy landscape of CH(3)S/Cu(111) results from metal-to-molecule charge redistribution which allows for a high mobility of CH3S. This contributes a nonuniform diffuse background to Bragg scattering, which needs to be considered in diffraction analyses. Ramifications on the interpretation of experimental data and the potential impact on the design of metal-organic interfaces are discussed.

Journal Article↗
PubMed2004

Detaching thiolates from copper and gold clusters: which bonds to break?

The interaction of alkanethiolates with small coinage metal clusters of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu(n)-thiolate (n = 1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n = 1 to 1.4 eV for n = 9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu(n) cluster. For the gold analogues, this effect is not observed and fragmentation at the S-C bond experiences only a slight 10% destabilization as n increases from 3 to 9. The relativistic origin of this difference between Cu and Au is discussed, and an analysis of bonding considerations is presented.

Journal Article↗
PubMed2004

Quantum and thermal fluctuation effects on the photoabsorption spectra of clusters.

A first principles approach for the calculation of photoabsorption cross sections of clusters and molecules is presented which includes both quantum and thermal fluctuations. The method relies on an ab initio path integral representation of the nuclear quantum motion in conjunction with time-dependent density-functional theory for electronic excitations. It is shown that quantum fluctuations of cold lithium clusters, such as Li8 below 50 K, affect significantly their photoabsorption spectra.

Journal Article↗
PubMed2004

Folding and unfolding of an elastinlike oligopeptide: "inverse temperature transition," reentrance, and hydrogen-bond dynamics.

The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy.

Elastin↗
PubMed2004

Temperature-dependent conformational transitions and hydrogen-bond dynamics of the elastin-like octapeptide GVG(VPGVG): a molecular-dynamics study.

A joint experimental/theoretical investigation of the elastin-like octapeptide GVG(VPGVG) was carried out. In this article a comprehensive molecular-dynamics study of the temperature-dependent folding and unfolding of the octapeptide is presented. The current study, as well as its experimental counterpart (see companion article in this issue) find that this peptide undergoes an inverse temperature transition (ITT), leading to a folding at approximately 40-60 degrees C. In addition, an unfolding transition is identified at unusually high temperatures approaching the normal boiling point of water. Due to the small size of the system, two broad temperature regimes are found: the ITT regime at approximately 10-60 degrees C and the unfolding regime at approximately T > 60 degrees C, where the peptide has a maximum probability of being folded at T approximately 60 degrees C. A detailed molecular picture involving a thermodynamic order parameter, or reaction coordinate, for this process is presented along with a time-correlation function analysis of the hydrogen-bond dynamics within the peptide as well as between the peptide and solvating water molecules. Correlation with experimental evidence and ramifications on the properties of elastin are discussed.

Biomimetic Materials↗
PubMed2002

Pulling monatomic gold wires with single molecules: an Ab initio simulation.

Car-Parrinello molecular dynamics simulations demonstrate that pulling a single thiolate molecule anchored on a stepped gold surface does not preferentially break the sulfur-gold chemical bond. Instead, it is found that this process leads to the formation of a monoatomic gold nanowire, followed by breaking a gold-gold bond with a rupture force of about 1.2 nN. The simulations also indicate that previous single-molecule thiolate-gold and gold-gold rupture experiments both probe the same phenomenon, namely, the breaking of a gold-gold bond within a gold nanowire.

Journal Article↗
PubMed2002

An in-depth correlation of perturbation of the organic-inorganic interface topology, electronic structure, and transport properties within beta''-(BEDT-TTF)(4) x (guest)(n) x [Re(6)Q(6)Cl(8)], (Q=S, Se).

An in-depth analysis of a set of 21 layered structures of metallic pseudopolymorphs of general formulation, beta''-(BEDT-TTF)(4) x (guest)(n) x [Re(6)Q(6)Cl(8)], (BEDT-TTF=bis-ethylenedithiotetrathiafulvalene; Q = S, Se; guest = H(2)O, 1,4-dioxane, THF, CCl(4), C(2)H(5)OH, CHCl(3), CH(2)ClI, CH(2)ClBr, CH(2)Cl(2), CH(2)OH-CH(2)OH, C(5)H(5)N, CH(3)COCH(3), 2-hydroxy-tetrahydrofuran, CH(3)CN, CS(2), C(6)H(6)), with diverse low-temperature behaviors, which differ solely by the nature of the cosolvent molecule selectively included during the electrocrystallization process, reveals a precise set of weak HO-H...Cl-mu-Re, (C-H)(BEDT-TTF)...Cl-mu-Re, C-H...O(guest), (C-H)(guest)...Cl-mu-Re hydrogen bonds at the organic-inorganic interface, none of which dominates any of the others and whose balance is adjusted upon substitution of one guest molecule by another. The electronic structure of the host adjusts to the weak perturbation imposed by exchanging the guest molecules and by balancing the former interfacial interactions; this correlates to a net activation of up to 0.1 eV of the energy of the HOMO level of one of the two donors, while keeping the pattern of HOMO-HOMO intermolecular interactions in the donor layer essentially unaltered. It is suggested that this controls the stability of the metallic state at low temperature or the occurrence of a metal-to-insulator phase transition for particular guests along the series. It is concluded that by allowing for numerous tiny modifications at the organic-inorganic interface within a single, robust host structure, one sees a concerted, inherently weak structural response of the system that is proportional to the magnitude of the underlying, equally weak activation of the HOMO energy of a fraction of the pi-donor molecules within the slabs; this has a sizeable influence on the macroscopic transport properties of the system.

Journal Article↗
PubMed2002

Structure and phase stability of binary zintl-phase compounds: lithium-group 13 intermetallics and metal-doped group 14 clathrate compounds.

The structure/bonding relationship in a series of intermetallic phases of Li with Al, Ga, and In was investigated by density functional theory and complemented by a model based on tight-binding theory and the method of moments. The combination of these two approaches provides a simple scheme which allows for both a comprehensive understanding of structural trends and the ability to predict low-energy structures for a given composition. This analysis gives a straightforward picture of phase stability in terms of local geometric features such as triangular, square, and hexagonal arrangements of atoms. The approach was extended to examine the structural properties of metal-doped clathrate compounds of C, Si, Ge, and Sn. Clathrate-type phases based on the frameworks Si172, Ge172, Si40, and Ge40 are not only likely to be energetically favorable but may also exhibit high thermoelectric efficiency.

Journal Article↗
PubMed1997

Origin of the Metal-to-Insulator Transition in H(0.33)MoO(3).

The electronic structure of the double octahedral layers present in H(0.33)MoO(3) has been studied. It is shown that, depending on structural details, three bands, two of them having a two-dimensional character and one having a one-dimensional character, can be in competition at the bottom of the t(2g)-block band structure. Both qualitative arguments and detailed computations show that the Fermi surface of the double octahedral layers has a two-dimensional character and does not exhibit nesting vectors. Consequently, the metal-to-insulator transition exhibited by H(0.33)MoO(3) cannot be a Fermi surface driven electronic instability, as recently proposed. An order-disorder transition of the protons is suggested as a more likely origin of this resistivity anomaly.

Journal Article↗
PubMed1996

Electronic Structure of Layered Oxides Containing M(2)O(7) (M = V, Nb) Double Octahedral Slabs.

The electronic structure of M(2)O(7) double octahedral slabs with low d electron counts has been studied. It is shown that the nature of the low d-block bands is strongly dependent on the d electron count and the distortions of the layer. All d(1) systems are expected to be similar and to exhibit Fermi surfaces which result from the superposition of both one-dimensional (1D) and two-dimensional (2D) contributions. For lower d electron counts the electronic structure is quite sensitive to the existence of M-O bond alternations perpendicular to the layer and off-plane distortions of the equatorial O atoms. The Fermi surface of these systems can either be purely 2D or have 1D and 2D portions like those of the d(1) systems. It is suggested that the recently reported phase Rb(2)LaNb(2)O(7) could be a 2D metal. It is also proposed that chemical reduction of the A'[A(n)()(-)(1)Nb(n)()O(3)(n)()(+1)] Dion-Jacobson phases with n = 3 could lead to metallic conductivity, in contrast with the results for the n = 2 phases.

Journal Article↗