TIME RESOLVED REFLECTION MEASUREMENTS OF CuCl UNDER HIGH EXCITATION

La réflection des échantillons de CuCl excités par un faisceau pompe intense au voisinage de la résonance excitonique Z3, est étudiée en utilisant deux sources laser pulsé. L'évolution temporelle des spectres montre que les polaritons excitoniques créés en grande densité ainsi que les processus de collisions avec les phonons acoustiques thermalisés et non thermalisés produisent d'importantes modifications des spectres de réflection.We study the reflection of CuCl samples, excited by an intense pump beam near the Z3 excitonic resonance, using a pump and probe technique. Its time evolution leads us to conclude that the density of excitonic polaritons as well as scattering processes with thermalized and non-thermalized acoustical phonons lead to important modifications of the spectra

Uneven Twins: Comparison of two enantiocomplementary hydroxynitrile lyases with a/ß--hydrolase fold

Hydroxynitrile lyases (HNLs) are applied in technical processes for the synthesis of chiral cyanohydrins. Here we describe the thorough characterization of the recently discovered R-hydroxynitrile lyase from Arabidopsis thaliana and its S-selective counterpart from Manihot esculenta (MeHNL) concerning their properties relevant for technical applications. The results are compared to available data of the structurally related S-HNL from Hevea brasiliensis (HbHNL), which is frequently applied in technical processes. Whereas substrate ranges are highly similar for all three enzymes, the stability of MeHNL with respect to higher temperature and low pH-values is superior to the other HNLs with alpha/beta-hydrolase fold. This enhanced stability is supposed to be due to the ability of MeHNL to form tetramers in solution, while HbHNL and AtHNL are dimers. The different inactivation pathways, deduced by means of circular dichroism, tryptophan fluorescence and static light scattering further support these results. Our data suggest different possibilities to stabilize MeHNL and AtHNL for technical applications: whereas the application of crude cell extracts is appropriate for MeHNL, AtHNL is stabilized by addition of polyols. In addition, the molecular reason for the inhibition of MeHNL and HbHNL by acetate could be elucidated, whereas no such inhibition was observed with AtHNL

Acoustical Spectroscopy of Carbohydrate Aqueous Solutions: Saccharides; Alkyl Glycosides; Cyclodextrins. Part II. Association and Complexation

Results from broadband acoustical spectroscopy for aqueous solutions of mono- and disaccharides with salts added, of various alkyl glycosides, and of α-cyclodextrin with n-octyl-β-D-glucopyranoside added are briefly summarized in view of their relevance in the study of molecular association and complexation processes. Mono- and tridentate complexes of alkali earth ions with saccharides are discussed as well as the monomer exchange between micellar structures and the suspending phase in the alkyl glycoside surfactant solutions. Particular attention is given to the behaviour at solute concentrations close to the critical micelle concentration or aggregate concentration, respectively. Also described is the competition between inclusion complex formation and self-aggregation in solutions containing cyclodextrin and alkyl glycoside surfactant

Exchange of polymer molecules between block copolymer micelles studied by emission spectroscopy. A method for the quantification of unimer exchange rates

Amphiphilic block copolymers, composed of poly(sodium methacrylate) (PMANa) as the water soluble sequence and poly((dimethylamino)ethyl methacrylate) (PDMAEMA) as the hydrophobic sequence, were anionically synthesized. Due to their amphiphilic character, these polymers aggregate into micelles in aqueous solutions. Of particular interest is the possibility of exchange of free chains, called unimers, between micelles and the bulk and the quantification of the exchange rate. The exchange of unimers between the micellar aggregates was determined spectroscopically. A donor molecule, naphthalene, was covalently bound to the hydrophobic blocks of the copolymers, while the acceptor, pyrene, was dissolved in aggregates built up from an identical, but unlabeled, copolymer. The energy transfer from an excited donor to a ground-state acceptor, with subsequent emission from the acceptor, was used to follow the migration of donor-labeled chains into micelles containing the dissolved acceptor. These data were analyzed according to a kinetic model, and the respective kinetic parameters could be determined. The influence of the relative composition of the block copolymer on the exchange rate was investigated to illustrate the usability of the kinetic model. While keeping the relative amount of comonomers as well as the molecular weights constant, the molecular architecture was also studied: hence the diblock (PDMAEMA-b-PMANa) and the corresponding triblocks (PDMAEMA-b-PMANa-b-PDMAEMA) and (PMANa-b-PDMAEMA-b-PMANa) were investigated