University of Fribourg

About: University of Fribourg is a education organization based out in Fribourg, Freiburg, Switzerland. It is known for research contribution in the topics: Population & Context (language use). The organization has 6040 authors who have published 14975 publications receiving 542500 citations. The organization is also known as: UNIFR & Universität Freiburg.

Absolute configuration of chirally deuterated neopentane.

Abstract: A molecule is 'chiral' when it cannot be superimposed on its mirror image; this property is of decisive importance throughout nature, as DNA, proteins, and many small molecules are chiral. Anomalous X-ray scattering has become the definitive method for determining the absolute configuration of a molecule, but this approach cannot be used unless the molecule contains a 'heavy' atom (for example, bromine). Haesler et al. now show that instrumental advances in Raman optical activity, combined with quantum chemical computations, have made it possible to determine the absolute configuration of molecules that do not contain a heavy atom — in this case, a specially synthesized chirally deuterated version of neopentane that represents the archetype of all molecules that are chiral as a result of a dissymmetric mass distribution. The unequivocal spectroscopic determination of the absolute configuration of this molecule presented a major challenge, one that was at the very limit of what is possible. Decisive instrumental advances in Raman optical activity (ROA), combined with quantum chemical computations, make it possible to determine the absolute configuration of (R)-[2H1, 2H2, 2H3]-neopentane. The relationship between macroscopic chirality and chirality on the molecular level was unequivocally established in 1951 through anomalous X-ray scattering1. Although this technique became the definitive method for determining the absolute configuration of a molecule, one important limitation of the approach is that the molecule must contain ‘heavy’ atoms (for example, bromine). The direct determination of absolute configurations for a wider range of molecules has recently become possible by measuring a molecule’s vibrational optical activity2,3. Here we show that instrumental advances in Raman optical activity4,5, combined with quantum chemical computations6,7,8, make it possible to determine the absolute configuration of (R)-[2H1, 2H2, 2H3]-neopentane9. This saturated hydrocarbon represents the archetype of all molecules that are chiral as a result of a dissymmetric mass distribution. It is chemically inert and cannot be derivatized to yield molecules that would reveal the absolute configuration of the parent compound. Diastereomeric interactions with other molecules, optical rotation, and electronic circular dichroism are, in contrast to the well-known case of bromochlorofluoromethane10,11,12, not expected to be measurable. Vibronic effects in the vacuum ultraviolet circular dichroism might reveal that the molecule is chiral, but the presence of nine rotamers would make it extremely difficult to interpret the spectra, because the spatial arrangement of the rotamers’ nuclei resembles that of enantiomers. The unequivocal spectroscopic determination of the absolute configuration of (R)-[2H1, 2H2, 2H3]-neopentane therefore presented a major challenge, one that was at the very limit of what is possible.

First-principles study on the stability of intermediate compounds of LiBH(4)

Abstract: Note: Times Cited: 110 Reference EPFL-ARTICLE-206017doi:10.1103/PhysRevB.74.075110View record in Web of Science URL: ://WOS:000240238800042 Record created on 2015-03-03, modified on 2017-05-12

Insect eggs suppress plant defence against chewing herbivores

Abstract: Plants activate direct and indirect defences in response to insect egg deposition. However, whether eggs can manipulate plant defence is unknown. In Arabidopsis thaliana, oviposition by the butterfly Pieris brassicae triggers cellular and molecular changes that are similar to the changes caused by biotrophic pathogens. In the present study, we found that the plant defence signal salicylic acid (SA) accumulates at the site of oviposition. This is unexpected, as the SA pathway controls defence against fungal and bacterial pathogens and negatively interacts with the jasmonic acid (JA) pathway, which is crucial for the defence against herbivores. Application of P. brassicae or Spodoptera littoralis egg extract onto leaves reduced the induction of insect-responsive genes after challenge with caterpillars, suggesting that egg-derived elicitors suppress plant defence. Consequently, larval growth of the generalist herbivore S. littoralis, but not of the specialist P. brassicae, was significantly higher on plants treated with egg extract than on control plants. In contrast, suppression of gene induction and enhanced S. littoralis performance were not seen in the SA-deficient mutant sid2-1, indicating that it is SA that mediates this phenomenon. These data reveal an intriguing facet of the cross-talk between SA and JA signalling pathways, and suggest that insects have evolved a way to suppress the induction of defence genes by laying eggs that release elicitors. We show here that egg-induced SA accumulation negatively interferes with the JA pathway, and provides an advantage for generalist herbivores.

Modeling market mechanism with minority game

Abstract: Using the minority game model we study a broad spectrum of problems of market mechanism. We study the role of different types of agents: producers, speculators as well as noise traders. The central issue here is the information flow: producers feed in the information whereas speculators make it away. How well each agent fares in the common game depends on the market conditions, as well as their sophistication. Sometimes there is much to gain with little effort, sometimes great effort virtually brings no more incremental gain. Market impact is also shown to play an important role, a strategy should be judged when it is actually used in play for its quality. Though the minority game is an extremely simplified market model, it allows to ask, analyze and answer many questions which arise in real markets.