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F J Bermejo

Publications and source records attributed to F J Bermejo.

13 recordsLinked to original sources
PubMed2006

Evidence of the presence of opticlike collective modes in a liquid from neutron scattering experiments.

Inelastic neutron scattering data from liquid DF close to the melting point show, in addition to spectra comprising quasielastic and heavily damped acoustic motions, an intense, nondispersive band centered at about 27 meV along with a broader higher energy feature. Observation of the former band provides the first direct verification of the existence within the liquid state of collective opticlike excitations as predicted by molecular dynamics simulations. The latter corresponds to mainly reorientational motions assigned from mode eigenvector analysis carried out by computer simulations.

Journal Article↗
PubMed2005

Direct experimental assessment of the strength of orientational correlations in polar liquids.

The strength of molecular orientational correlations in polar liquids is assessed by means of comparison of the diffuse scattering patterns of a liquid composed by molecules devoid of permanent electric dipole but having a weak quadrupole moment and those for a liquid composed by permanent molecular dipoles. The extent of orientational correlations within the liquid phases is in both cases assessed by comparison of the liquid radial distributions to those present in the rotator-phase (plastic) crystal phases of both compounds. For such disordered-crystal phases, information concerning orientational correlations is directly derived from the experimental scattering patterns by means of analysis of the diffuse scattering background present beneath the Bragg peaks. The results show that rather than long-ranged, orientational correlations in polar or polarizable liquids are confined within distances comprising the second coordination sphere.

Journal Article↗
PubMed2004

Unconventional density dependence of the stochastic dynamics in an organic liquid.

The density dependence of the diffusive rotational and center-of-mass dynamics of 2-methyl-pyridine is investigated by means of the concurrent use of quasielastic neutron scattering and molecular dynamics simulations. The dependence of both translation and rotational diffusion coefficients shows a distinctive change of slope with increasing density taking place about rho=0.975 g/cm3. Such a change in the dynamics can be related to observations made in other liquids composed of oblate-spheroidal particles.

Journal Article↗
PubMed2003

Microscopic dynamics of liquid aluminum oxide.

Collective excitations have been observed in liquid aluminum oxide at high temperatures by combining a containerless sample environment with inelastic x-ray scattering. The excitation spectra show a well-defined triplet peak structure at lower wave vectors Q (1 to 6 nanometers-1) and a single quasi-elastic peak at higher Q. The high-Q spectra are well described by kinetic theory. The low-Q spectra require a frequency-dependent viscosity and provide previously unknown experimental constraints on the behavior of liquids at the interface between atomistic and continuum theory.

Journal Article↗
PubMed2002

How well do we know atomic motions of simple liquids?

Microscopic motions in molten potassium spanning three frequency decades are studied by neutron-scattering techniques. These comprise well-defined density oscillations and stochastic particle rearrangements and both are modeled on microscopic grounds. While vibratory motions are shown to share characteristics with those of their parent crystals, dynamic correlations between a diffusing particle and its neighbors can be accounted for only semiquantitatively.

Journal Article↗
PubMed2002

Chemical isomerism as a key to explore free-energy landscapes in disordered matter.

The effects of a minor chemical modification on the microscopic structure of a material in its glass and crystal phases are investigated by the concurrent use of neutron diffraction and computer simulation. Significant changes in short-, intermediate-, and long-range order are found, resulting from the change in molecular structure. These differences are explainable by a shift in the balance between directional and excluded-volume interactions.

Journal Article↗
PubMed2001

Reentrant miscibility in fluids with spherical interactions.

We have obtained the closed-loop fluid-fluid immiscibility in the phase diagram of a binary mixture with interactions with spherical symmetry. That topology appears when a short-range attractive interaction is considered between unlike pair molecules. We present "exact" results obtained from Monte Carlo simulation on different ensembles and results from the application of a first-order perturbation theory.

Journal Article↗
PubMed1987

1H-n.m.r. study of the folding of ribonuclease 12-(beta-(3-pyridyl)-L-Ala) S-peptide (1-14).

The 1H-n.m.r. spectra (360 MHz) of 12-(beta-(3-pyridyl)-L-Ala) ribonuclease S-peptide (1-14), a tetradecapeptide incorporating (beta-3-pyridyl-L-Ala) instead of His at position 12, have been assigned. The shift vs. temperature dependence has been analyzed at three different pD's in terms of a two-state helix (3-13) in equilibrium coil equilibrium, and the corresponding values for the thermodynamic quantities delta H degrees and delta S degrees determined. Helix populations at 0 degrees C have been measured as a function of pD, showing their dependence on two apparent pKa's at approximately 3.3 and 5.5, with a maximum at pD approximately 4.2. All the obtained results show that the new peptide has very similar folding properties to those shown by S-peptide and particularly to those of C-peptide. The 3-13 helix formed is stabilized by two interactions: a salt-bridge Glu 2-...Arg 10+ and a partial stacking between the aromatic rings of residues Phe 8 and His 12. Calculations involving ring current shifts and potential energies validate the possible existence of this latter interaction, which must present a local geometry defined by chi 81 180 degrees, chi 82 100 degrees, chi 121-60 and chi 122 80.

Kinetics↗
PubMed1986

Assignment and conformation of neurotensin in aqueous solution by 1H NMR.

A complete assignment of exchangeable and unexchangeable proton resonances of neurotensin 1-13 in aqueous solution has been carried out with the help of its 1-8 and 8-13 fragments. To detect formation of a secondary structure, the effects of peptide fragmentation, temperature decrease, pH changes and addition of denaturing agents on the neurotensin 1H NMR spectrum were investigated. The small changes observed in all cases support the conclusion that neurotensin exists mainly as a flexible random coiled polypeptidic chain in aqueous solution in agreement with previous CD studies.

Hydrogen-Ion Concentration↗
PubMed1984

On the fundamental role of the Glu 2- ... Arg 10+ salt bridge in the folding of isolated ribonuclease A S-peptide.

The fundamental role of the Glu 2- ... Arg 10+ salt bridge in the folding of isolated S-peptide (1-19 N-terminal fragment of Ribonuclease A) is demonstrated from the comparison of the helix contents, at 0 degrees C, of S-peptide and related peptides. Helix contents have been determined from the analysis of proton chemical shift vs. temperature curves. The observed data can be accounted for by assuming that two side-chain interactions contribute to stabilize the 3-13 helix of S-peptide, the salt bridges Glu 2- ... Arg 10+ and Glu 9-... His 12+, the former being more effective. The salt bridge Glu 9- ... Arg 10+ turns to a weaker interaction, a hydrogen bond Glu 2 (C delta = 0) ... Arg 10+, on protonation or esterification of the Glu 2 carboxylate.

Amino Acid Sequence↗
PubMed1983

Low-temperature 1H-NMR evidence of the folding of isolated ribonuclease S-peptide.

The temperature (-7 degrees C to 45 degrees C, pH 5.4) and pH (0 degrees C) dependence of 1H chemical shifts of ribonuclease S-peptide (5 mM, 1 M NaCl) has been measured at 360 MHz. The observed variations evidence the formation of a partial helical structure, involving the fragment Thr-3-Met-13. Two salt-bridges stabilize the helix: those formed by Glu-9- ...His-12+ and Glu-2- ...Arg-10+. The structural features deduced from the 1H-NMR at low temperature for the isolated S-peptide are compatible with the structure shown by the same molecule in the ribonuclease S crystal.

Amino Acid Sequence↗