Headlines 2012

Oct 01, 2012

Si un aimant peut être "permanent", la dynamique des spins à l'origine de l'aimantation peut être ultra-rapide à l'échelle nanométrique, dans le domaine femtoseconde (10-15 s). Les possibilités actuelles de génération d’impulsions ultra-brèves dans le domaine X-UV  ouvrent de nouvelles perspectives pour les études dans ce domaine. Elles permettent en particulier d'observer, à cette échelle de temps et à des échelles spatiales de l'ordre de la longueur d'onde (< 100 nm), la réponse d'une structure nanométrique magnétique soumis à une stimulation externe. Il devient par exemple possible d'étudier la dynamique ultra-rapide de la perte d'aimantation sous l'effet d'une forte irradiation lumineuse, sur des échantillons magnétiquement nanostructurés (multicouches de cobalt– palladium). Ces études, réalisées par une collaboration de physiciens franciliens associés à des physiciens de l'Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), permettent de mieux comprendre la dynamique de spin, ouvrant ainsi la voie vers la réalisation de dispositifs magnétiques ultra-rapides.

 

Aug 06, 2012

L'état électronique d'une molécule réagit très rapidement - à l'échelle de la femtoseconde (10-15 s), voire de l'attoseconde (10-18 s) - à toute perturbation telle qu'une excitation laser, une vibration qui modifie la position relative des noyaux atomiques qui la constitue, ou encore au cours d'une réaction chimique. Suivre en temps réel l'évolution des orbitales électroniques demande ainsi des techniques d'observation permettant d'atteindre cette résolution temporelle attoseconde.

Une telle sonde ultra-rapide est fournie par la réponse non linéaire de la molécule à un champ laser intense. Dans cette réponse, l’électron de valence peut être extrait de la molécule (avec une certaine probabilité d'ionisation), accéléré et "renvoyé" vers la molécule ionisée (recollision), où il peut se désexciter (recombinaison vers l’orbitale de valence), en émettant une impulsion intense de lumière UV attoseconde. De l’analyse complète de cette émission lumineuse (mesure de l'amplitude, phase et polarisation), il est possible de reconstruire le paquet d’ondes électronique dans les orbitales de valence avec une résolution spatiale subnanométrique et de suivre leur dynamique avec une résolution temporelle attoseconde.

 

Jul 13, 2012

The stability and conformational dynamics of biomolecules are strongly influenced by the dynamics of hydrogen bonds and by hydrogen transfer processes which occur through these bonds. Not surprisingly, a large activity develops since many years to understand which molecular deformations play a role when a hydrogen atom is transferred through a hydrogen bond. The acetylacetone molecule (noted AcAc in the figure) is often used to approach this question from a fundamental point of view. Under the form drawn in the figure (the most stable one in the gas phase), AcAc is indeed one of the simplest molecule which carries an intramolecular hydrogen bond with the H-atom in proper position to transfer from an atom to another (here, the two oxygen atoms drawn in red in the figure). This issue relates to a more general one in reaction dynamics, a field in physical chemistry: to bring enough an accurate experimental material for modeling and foreseeing how several degrees of freedom couple together to drive to a chemical reaction.

The MOMA group of the Institute of the Molecular Sciences of Orsay (ISMO), in collaboration with the Reaction Dynamics group of the laboratory Francis Perrin (LFP) developed a very attractive method to show (maybe to control in the future) how large amplitude motions are coupled to frustrated rotations of the methyl groups in AcAc (schemed as bend arrows in the left panel of the figure) to stimulate the transfer of the H-atom between the two oxygen atoms (also shown by an arrow in the figure). This collaboration was initiated by the ANR project GOUTTELIUM and is pursued through the NOSTADYNE project funded by Triangle de la Physique. The AcAc molecule was isolated in a matrix of para-hydrogen, a quantum solid that does not perturb the deformation of hosted molecules. At the temperature of the matrix (4K), the relaxation of the nuclear spin of the methyl groups is very slow. This has been taken as an advantage to reveal that the pseudo-rotation of the methyl groups is intricate with the large amplitude motion associated with the H-atom transfer. The experimental method that lead to this result (see the reference) appeared as an elegant way, although indirect, to answer a long controversy on the role of the methyl rotations in the H-atom transfer in AcAc. The issue was important actually, since AcAc is considered as a prototype to unravel the dynamics of molecules possessing an intramolecular hydrogen bond.

Apr 03, 2012
Pour observer des phénomènes ultrarapides tels que le mouvement des électrons au sein de la matière, les chercheurs ont besoin de sources capables de produire des rayonnements lumineux extrêmement brefs et énergétiques. Si des dispositifs capables d’émettre des impulsions dans le domaine de l’attoseconde (10-18 seconde) existent déjà, de nombreuses équipes s’efforcent de repousser les limites de leur intensité et de leur durée.

 

May 31, 2012

The description of the interactions controlling the shape of a protein is crucial in understanding the cellular mechanisms, but is still difficult to achieve on biological systems because of their complexity.

In this context, the use of model molecules makes accessible to experiments many biological problems lying at the heart of current societal issues. Gas-phase IR/UV spectroscopy of small peptides is an outstanding example. The study of peptides containing the methionine residue has recently shown that NHamide---Smethionine hydrogen bonds are particularly strong. They are, for example, as strong as the "classical" NHamide---OCamide hydrogen bonds which define the secondary structure of proteins.

Analysis of protein structures identified to date reveals that the type of NHamide---Smethionine bond observed experimentally occurs on 12% of methionines. Comparison of the parameters defining the NHamide---Smethionine bond shows that the hydrogen bonds formed in the gas phase faithfully reproduce those observed in proteins. This strongly suggests that the properties of the NHamide---Smethionine bonds highlighted in the gas phase, especially their strength, are identical to those naturally occurring in proteins.

The study of the constraints imposed to the backbone by these NHamide---Smethionine bonds shows that they reduce the range of possible values ​​for the Ramachandran angles. This phenomenon common to gas-phase peptides and proteins having these NHamide---Smethionine bonds thus illustrates the effect that these bonds can have on the rigidity of the peptide backbone. These results already open interesting paths in understanding the way antitumor drugs act.  Journal of Physical Chemistry Letters 2012, 3, 755−759

Dec 12, 2012

Ionized molecules are involved in many chemical reactions, and participate for an important part in the chemistry of the upper atmosphere and interstellar clouds. Data on the vibrational spectroscopy of these ions are thus needed to better understand the dynamics and energetics of so diluted matter.

Photoelectron spectroscopies are method of choice to characterize these molecules and their vibrational states, but are often ineffective when the structure of the neutral molecule is very different from that of the ion. The Laboratoire Francis Perrin (URA 2453, CEA - CNRS) in collaboration with the team of Chimie Théorique du Laboratoire Modélisation et Simulation Multi Echelle (MSME UMR 8208 CNRS, Univ Paris-Est Marne-La-Vallée) participated in the development of a new spectroscopic method to achieve the desired data, not accessible by conventional methods.

 

May 31, 2012

The description of the interactions controlling the shape of a protein is crucial in understanding the cellular mechanisms, but is still difficult to achieve on biological systems because of their complexity.

In this context, the use of model molecules makes accessible to experiments many biological problems lying at the heart of current societal issues. Gas-phase IR/UV spectroscopy of small peptides is an outstanding example. The study of peptides containing the methionine residue has recently shown that NHamide---Smethionine hydrogen bonds are particularly strong. They are, for example, as strong as the "classical" NHamide---OCamide hydrogen bonds which define the secondary structure of proteins.

Analysis of protein structures identified to date reveals that the type of NHamide---Smethionine bond observed experimentally occurs on 12% of methionines. Comparison of the parameters defining the NHamide---Smethionine bond shows that the hydrogen bonds formed in the gas phase faithfully reproduce those observed in proteins. This strongly suggests that the properties of the NHamide---Smethionine bonds highlighted in the gas phase, especially their strength, are identical to those naturally occurring in proteins.

The study of the constraints imposed to the backbone by these NHamide---Smethionine bonds shows that they reduce the range of possible values ​​for the Ramachandran angles. This phenomenon common to gas-phase peptides and proteins having these NHamide---Smethionine bonds thus illustrates the effect that these bonds can have on the rigidity of the peptide backbone. These results already open interesting paths in understanding the way antitumor drugs act.  Journal of Physical Chemistry Letters 2012, 3, 755−759

Nov 26, 2012

La dynamique des électrons au sein des atomes et des molécules est extrêmement rapide, typiquement de l'ordre de la centaine d'attosecondes (1 as=10-18 s). Les expériences de type pompe-sonde, où une première impulsion vient exciter le système et une seconde le sonder après un délai variable, peuvent permettre d'explorer cette dynamique, mais nécessitent des impulsions de lumière ultra-brèves, uniques et bien caractérisées à cette échelle de temps (gamme attoseconde). Depuis une dizaine d’années, d’importants efforts de recherche ont permis de générer et mesurer de telles impulsions. Une collaboration entre les chercheurs de l’IRAMIS et du Laboratoire d'Optique Appliqué (LOA) annonce la découverte d'un nouveau procédé d’une grande simplicité pour la génération d'une impulsion attoseconde unique [1], basé sur la génération d'harmoniques en présence d'une rotation ultrarapide du front d'onde de l'impulsion laser incidente. Il est ainsi possible aujourd'hui de disposer d'une source de lumière particulièrement bien adaptée aux expériences pompe-sonde permettant l'exploration de la dynamique électronique.

 

Dec 12, 2012

Ionized molecules are involved in many chemical reactions, and participate for an important part in the chemistry of the upper atmosphere and interstellar clouds. Data on the vibrational spectroscopy of these ions are thus needed to better understand the dynamics and energetics of so diluted matter.

Photoelectron spectroscopies are method of choice to characterize these molecules and their vibrational states, but are often ineffective when the structure of the neutral molecule is very different from that of the ion. The Laboratoire Francis Perrin (URA 2453, CEA - CNRS) in collaboration with the team of Chimie Théorique du Laboratoire Modélisation et Simulation Multi Echelle (MSME UMR 8208 CNRS, Univ Paris-Est Marne-La-Vallée) participated in the development of a new spectroscopic method to achieve the desired data, not accessible by conventional methods.

 

Jul 13, 2012

The stability and conformational dynamics of biomolecules are strongly influenced by the dynamics of hydrogen bonds and by hydrogen transfer processes which occur through these bonds. Not surprisingly, a large activity develops since many years to understand which molecular deformations play a role when a hydrogen atom is transferred through a hydrogen bond. The acetylacetone molecule (noted AcAc in the figure) is often used to approach this question from a fundamental point of view. Under the form drawn in the figure (the most stable one in the gas phase), AcAc is indeed one of the simplest molecule which carries an intramolecular hydrogen bond with the H-atom in proper position to transfer from an atom to another (here, the two oxygen atoms drawn in red in the figure). This issue relates to a more general one in reaction dynamics, a field in physical chemistry: to bring enough an accurate experimental material for modeling and foreseeing how several degrees of freedom couple together to drive to a chemical reaction.

The MOMA group of the Institute of the Molecular Sciences of Orsay (ISMO), in collaboration with the Reaction Dynamics group of the laboratory Francis Perrin (LFP) developed a very attractive method to show (maybe to control in the future) how large amplitude motions are coupled to frustrated rotations of the methyl groups in AcAc (schemed as bend arrows in the left panel of the figure) to stimulate the transfer of the H-atom between the two oxygen atoms (also shown by an arrow in the figure). This collaboration was initiated by the ANR project GOUTTELIUM and is pursued through the NOSTADYNE project funded by Triangle de la Physique. The AcAc molecule was isolated in a matrix of para-hydrogen, a quantum solid that does not perturb the deformation of hosted molecules. At the temperature of the matrix (4K), the relaxation of the nuclear spin of the methyl groups is very slow. This has been taken as an advantage to reveal that the pseudo-rotation of the methyl groups is intricate with the large amplitude motion associated with the H-atom transfer. The experimental method that lead to this result (see the reference) appeared as an elegant way, although indirect, to answer a long controversy on the role of the methyl rotations in the H-atom transfer in AcAc. The issue was important actually, since AcAc is considered as a prototype to unravel the dynamics of molecules possessing an intramolecular hydrogen bond.



Retour en haut