Dimère | EPFL Graph Search

redresse=1.15|vignette|Le saccharose, ou sucre de table, est un dimère composé de glucose et de fructose. En chimie, un dimère est une molécule de la famille des polymères ne comportant que deux sous-unités. Certains types de sucre sont parmi les dimères les plus communs : le saccharose, par exemple, est un dimère réunissant une molécule de glucose et une de fructose. Un dimère physique est une situation où deux molécules de structure similaire sont fortement rapprochées par des interactions intermoléculaires, sans former de liaison chimique. En physique de la matière condensée, des systèmes géométriquement frustrés peuvent être représentés et étudiés à l'aide de pavages de dimères. Par exemple, dans le cas de spins sur un réseau triangulaire, et dont les interactions entre voisins sont antiferromagnétiques (favorisant des spins d'orientation opposée de part et d'autre d'un lien), on peut associer à chacune des nombreuses configurations de basse énergie un tel pavage en plaçant p

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

Dante Philippe Sblendorio

This thesis investigates the magnetic properties of single atoms and dimers adsorbed on graphene and oxide decoupling layers supported by single crystal metal substrates, using scanning tunneling microscopy (STM) and spin-polarized scanning tunneling microscopy (SP-STM). The goal is twofold: to use SP-STM to further advance the understanding of the interactions that determine the magnetic stability of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), and to use insights from these systems to motivate the study of new systems-- mainly, IrCo heterodimers on graphene/Ir(111).For Dy adatoms on graphene/Ir(111), the characteristic lifetime of the spin system is measured as a function of temperature and tunnel bias, probing the available magnetization reversal pathways in the energy level diagram. The necessity of including the intra-atomic exchange to correctly describe spin lifetimes is demonstrated. In addition, naturally abundant Dy isotopes possess two possible nuclear spin values. Models of both nuclear spin cases of Dy are compared, and shown to produce similar behavior in the temperature and bias ranges probed by SP-STM. Furthermore, accounting for both nuclear spin cases allows for modeling of X-ray magnetic circular dichroism (XMCD) magnetization sweeps. For Ho adatoms on MgO/Ag(100), novel measurement protocols are used to determine the zero-field stability, and the correct ground state model. These protocols are also used to induce magnetic state reversal via Landau-Zener tunneling at avoided level crossings due to the hyperfine interaction. Finally, the study of the IrCo heterodimer adsorbed on graphene is motivated with stability considerations derived from the studies of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), in addition to calculations based on density function theory (DFT). Strategies of engineering these heterodimers using statistical growth and atomic manipulation are successfully implemented. The heterodimer is highly mobile on graphene/Ir(111) and displays geometric instability, consistent with its predicted upstanding geometry. Despite this, measurement via STM and SP-STM is demonstrated. Observations are consistent with the robust magnetic stability expected from DFT studies, but additional investigation is needed to disentangle the measurements made thus far.

EPFL2022

The Human RNA Helicase DDX21 Presents a Dimerization Interface Necessary for Helicase Activity

Matteo Dal Peraro, Alice Duperrex, Annamaria Kauzlaric, Suliana Manley, Jenny Sülzle, Didier Trono

Members of the DEAD-box helicase family are involved in all fundamental processes of RNA metabolism, and as such, their malfunction is associated with various diseases. Currently, whether and how oligomerization impacts their biochemical and biological functions is not well understood. In this work, we show that DDX21, a human DEAD-box helicase with RNA G-quadruplex resolving activity, is dimeric and that its oligomerization state influences its helicase activity. Solution small-angle X-ray scattering (SAXS) analysis uncovers a flexible multi-domain protein with a central dimerization domain. While the Arg/Gly rich C termini, rather than dimerization, are key to maintaining high affinity for RNA substrates, in vitro helicase assays indicate that an intact dimer is essential for both DDX21 ATP-dependent double-stranded RNA unwinding and ATP-independent G-quadruplex remodeling activities. Our results suggest that oligomerization plays a key role in regulating RNA DEAD-box helicase activity.

2020

Selective degradation of wood lignin over noble-metal catalysts in a two-step process

Paul Joseph Dyson, Cheng Wang, Ning Yan, Chen Zhao

Direct conversion of lignin into alkanes and methanol was carried out in a two-step process (hydrogenolysis and hydrogenation) involving initial treatment of white birch wood sawdust with H2 in dioxane/water/phosphoric acid using carbon supported Ru, Pd, Rh, or Pt as catalysts. The resulting monomers and dimers obtained by selective C-O hydrogenolysis were then hydrogenated in near-crit. water empolying Pd/C as the catalyst. The study is of interest with respect to prodn. of biofuel from lignin.

2008

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

Dante Philippe Sblendorio

EPFL2022