Search results for " molecule"

Stereoselective synthesis of carane-based chiral β- and γ-amino acid derivatives via conjugate addition

2015

Abstract Michael addition of dibenzylamine to (−)-tert-butyl isochaminate, prepared in three steps from (−)-perillaldehyde, furnished a carane-based β-amino acid derivative in a highly stereospecific reaction. The resulting amino ester was transformed to the bicyclic amino acid, a promising building block for the synthesis of 1,3-heterocycles and peptidomimetics. The conjugate addition of nitromethane to α,β-unsaturated methyl ester likewise resulted in nitro esters in stereospecific reactions. Catalytic reduction of the nitro group yielded a γ-amino ester. Under acidic conditions, the hydrolysis of the methyl ester resulted in an unexpected aminolactone-type product through rearrangement o…

Comparative analysis of the coordinated motion of Hsp70s from different organelles observed by single-molecule three-color FRET.

2021

Cellular function depends on the correct folding of proteins inside the cell. Heat-shock proteins 70 (Hsp70s), being among the first molecular chaperones binding to nascently translated proteins, aid in protein folding and transport. They undergo large, coordinated intra- and interdomain structural rearrangements mediated by allosteric interactions. Here, we applied a three-color single-molecule Forster resonance energy transfer (FRET) combined with three-color photon distribution analysis to compare the conformational cycle of the Hsp70 chaperones DnaK, Ssc1, and BiP. By capturing three distances simultaneously, we can identify coordinated structural changes during the functional cycle. Be…

Vibronic structure in triatomic molecules : The hydrocarbon flame bands of the formyl radical (HCO). A theoretical study

1998

A theoretical study of the vibrational structure of the math 2A′ ground and math 2A′ excited states of the formyl radical, HCO, and its deuterated form, DCO, has been performed. The potential energy surfaces have been computed by means of a multiconfigurational perturbative method, CASPT2. The computed geometries and the harmonic and anharmonic frequencies are successfully compared to the available experimental information. The vibrational intensities of the transition math 2A′↔math 2A′ have been computed both for absorption and emission. The results lead to accurate determinations of several structural parameters and some reassignments of the vibrational transitions of the so-called hydroc…

Intensities and electronic transition strengths of seven Te2 visible and i.r. band systems

1992

Abstract We present an experimental study of relative intensity distributions in the laser-flourescence spectra of the AO+u-XO+g, AO+u-X1g, AO+u-b1∑+g, A1-u-X1-g, BO+u-XO+g, BO+u-X1g, and BO+u-b1∑+g systems of the 130Te2 molecule. Effective internuclear potentials have been determined by applying the RKR-procedure and accounting for rovibronic interaction. Franck-Condon factors and r-centroids have been calculated and tabulated. By combining expiremental results and calculations, we obtain the dependence of the electronic transition strength on internuclear distance in the r-centroid approximation. Normalization over the lifetimes of the rovibronic levels of the excited electronic states en…

1H,13C and17O NMR spectroscopic study of four bicyclo [3.1.1]heptenes (derivatives of α-pinene) and four bicyclo[3.1.1]heptanes (derivatives of β-pin…

1992

Four derivatives of 2, 6, 6-trimethylbicyclo[3.1.1]hept-2-ene (α-pinene) and four derivatives of 2-methylene-6, 6-dimethylbicyclo[3.1.1]heptane (β-pinene) were synthesized and their 1H, 13C and 17O NMR spectra were measured, analysed and assigned. The 1H NMR spectral parameters were obtained by computer-aided analyses of the very complex multi-spin, second-order spectra. Some of the 13C NMR chemical shift assignments of these compounds reported in the literature should be changed, based on these accurate 1H NMR spectral analyses and carbon-proton chemical shift correlated spectra (COSY). The new assignments are supported by the 1J(C, H) values, which show a strong dependence on the bond ang…

1996

The major new developments in electron crystallography and high resolution imaging of organic molecules are discussed. The aim is to show that the recent rapid advances in computer technology have made it possible to analyze molecules at a level or resolution which was impossible less than a decade ago, thus bringing the concept of molecular design to produce specific physical properties much nearer to realization

Single molecule probing of dynamics in supercooled polymers

2011

6 pages; International audience; Fluorescence experiments with single BODIPY molecules embedded in a poly(methyl acrylate) matrix have been performed at various temperatures in the supercooled regime. By using pulsed excitation, fluorescence lifetime and linear dichroism time trajectories were accessible at the same time. Both observables have been analyzed without data binning. While the linear dichroism solely reflects single particle dynamics, the fluorescence lifetime observable depends on the molecular environment, so that the dynamics from the polymer host surrounding a chromophore contributes to this quantity. We observe that the lifetime correlation decays slightly faster than polar…

Zero-field NMR of urea : spin-topology engineering by chemical exchange

2021

Well-resolved and information-rich J-spectra are the foundation for chemical detection in zero-field NMR. However, even for relatively small molecules, spectra exhibit complexity, hindering the analysis. To address this problem, we investigate an example biomolecule with a complex J-coupling network─urea, a key metabolite in protein catabolism─and demonstrate ways of simplifying its zero-field spectra by modifying spin topology. This goal is achieved by controlling pH-dependent chemical exchange rates of 1H nuclei and varying the composition of the D2O/H2O mixture used as a solvent. Specifically, we demonstrate that by increasing the proton exchange rate in the [13C,15N2]-urea solution, the…

A study on the aminomercuration-nucleophilic demercuration of --1,5-cyclooctadiene; stereoselective synthesis of 2,6-disubstituted-9-aza bicyclo[3.3.…

1992

Abstract The aminomercuration of cis - cis -1,5-Cyclooctadiene with a series of mercury(II) salts followed by nucleophilic displacement of mercury by aromatic amines, water and nitrate ion has been studied. As a result, bicyclic triamines, aminoalcohols and nitrate esters have been obtained respectively in clean processes which occur under total stereoelectronic control by involvement of a tricyclic aziridium ion to afford a single stereoisomer in each case. The influence of the counter ion and the basicity of the amine on the tandem aminomercuration-demercuration is discussed.

Oxyfunctionalization of Aliphatic Esters by Methyl(trifluoromethyl)dioxirane

1996

The oxidation of lineal, cyclic, and bicyclic aliphatic p-chlorobenzoic and p-chorobenzenesulfonic acid esters 2 with methyl(trifluoromethyl)dioxirane (TFDO) (1) occurs at positions in the hydrocarbon chain distant from the directing group with a significant degree of selectivity to give the corresponding keto or hydroxy esters. Compounds 2 are relatively deactivated with respect to this oxidation due to the electron-withdrawing nature of the ester moiety. Methylene Cα−H and Cβ−H bonds remain unchanged in all cases, but tertiary Cβ−H bonds undergo oxidation with TFDO (1). Stereoelectronic factors are used to explain the faster reaction rate in competition experiments for the oxidation of en…