Design and synthesis of push-pull chromophores for second-order nonlinear optics derived from rigidified thiophene-based pi-conjugating spacers.

Two series of push-pull chromophores built around thiophene-based pi-conjugating spacers rigidified either by covalent bonds or by noncovalent intramolecular interactions have been synthesized and characterized by UV-vis spectroscopy, electric field induced second harmonic generation (EFISH) and differential scanning calorimetry. Comparison of the linear and second-order nonlinear optical properties of chromophores based on a covalently bridged dithienylethylene (DTE) spacer with those of their analogues based on open chain DTE shows that the rigidification of the spacer produces a considerable bathochromic shift of the absorption maximum together with a dramatic enhancement of the molecular quadratic hyperpolarizability (mu beta) which reaches values among the highest reported so far. A second series of NLO-phores has been derived from a 2,2'-bi(3,4-ethylenedioxythiophene) (BEDOT) pi-conjugating spacer. As indicated by X-ray and UV-vis data, rigidification of the spacer originates in that case, from noncovalent intramolecular interactions between sulfur and oxygen atoms. Again, comparison with the parent compounds based on an unsubstituted bithiophene spacer reveals a marked red shift of the absorption maximum and a large enhancement of mu beta. In an attempt to distinguish the contribution of the electronic and geometrical effects of the ethylenedioxy group, a third series of NLO-phores based on 3,4-ethylenedioxythiophene (EDOT) and 3,4-dihexyloxythiophene spacers has been synthesized. Comparison with compounds based on unsubstituted thiophene shows that, despite a red shift of lambda(max), introduction of alkoxy groups leads to a decrease of mu beta. Theoretical calculations indicate that this effect results from a decrease of the dipole moment (mu) caused by the auxiliary electron-donor alkoxy groups on the thiophene ring. In contrast, replacement of BT by BEDOT produces an increase of mu, which associated with the noncovalent rigidification of the BT system accounts for the observed enhancement of mu beta.     

Following the aggregation of amyloid-forming peptides by computer simulations

There is experimental evidence suggesting that the toxicity of neurodegenerative diseases such as Alzheimer's disease may result from the soluble intermediate oligomers. It is therefore important to characterize extensively the early steps of oligomer formation at atomic level. As these structures are metastable and short lived, experimental data are difficult to obtain and they must be complemented with numerical simulations. In this work, we use the activation-relaxation technique coupled with a coarse-grained energy model to study in detail the mechanisms of aggregation of four lys-phe-phe-glu (KFFE) peptides. This is the shortest peptide known to form amyloid fibrils in vitro. Our simulations indicate that four KFFE peptides adopt a variety of oligomeric states (tetramers, trimers, and dimers) with various orientations of the chains in rapid equilibrium. This conformational distribution is consistent with all-atom molecular-dynamics simulations in explicit solvent and is sequence dependent; as seen experimentally, the lys-pro-gly-glu (KPGE) peptides adopt disordered structures in solution. Our unbiased simulations also indicate that the assembly process is much more complex than previously thought and point to intermediate structures which likely are kinetic traps for longer chains.     

Chiral induction in quinoline-derived oligoamide foldamers: assignment of helical handedness and role of steric effects

Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiral helical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83% could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact that diastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sense favored by the terminal chiral group could be determined unambiguously using X-ray crystallography. Assignment of chiral induction was performed in an original way using the strong tendency of racemates to cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that the stereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiral helical induction was rationalized on the basis of sterics. Upon assigning an Rs or Ss chirality to the stereogenic center using a nomenclature where the four substituents are ranked according to decreasing sizes, it is observed that Rs chirality always favors left-handed helicity and Ss chirality favors right-handed helicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction. The preferred conformation at the stereocenter is apparently one where the bulkiest group should preferentially point away from the helix, the second largest group should be aligned with the helix backbone, and the smallest should point to the helix.     

Comparative dielectric and thermotropic properties of liquid crystalline polyesters as a function of mesogen group and flexible thioether group length

Two polyesters containing rigid biphenyl and hydroquinone bisbenzoate groups and presenting flexible thioether moieties with different lengths (14 elements in the flexible group) have been carried out using a reaction of the Michaël type. The properties of these polyesters have been compared to those of polyesters of the same type presenting shorter flexible groups (11 elements). All these polymers present thermotropic properties: the biphenyl ones are smectic and the bisbenzoate ones are nematic. The biphenyl polyesters present two types of dielectric relaxations: α and β. The bisbenzoate ones show three relaxations, α and two β (β and β^′). The lengthening of the flexible group increases significantly the flexibility of the molecular chains.     

Capillary electrophoresis - mass spectrometry: survey on developments and applications 2003-2004

The major developments and applications related to CE-MS over the last two years (2003-2004) and most of the reviews and applications found in the ISI Web of Science and publisher data bases are presented in a tabulated way. This article complements our previous review "Capillary electrophoresis - mass spectrometry: 15 years of developments and applications", Electrophoresis, 2003, 24, 3837-3867 for the last two years 2003-2004. All cited articles were analyzed in a way to illustrate (i) in which journals CE-MS-related papers were mostly found over the last decades and (ii) which commercial CE-, MS-instrumentations or CE-MS combinations were mostly used in the European, Asian, and American continent. Additionally, like it was done in our last review, the reader will rapidly find applications classified as forensics, environment, bioanalytics, pharmaceutics, and metabolites.     

New insights into ethene epoxidation on two oxidized Ag[111] surfaces

Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications are that both surfaces are active and that epoxidation can take place over a wide O coverage regime.     

Chemotaxonomic features associated with flavonoids of cannabinoid-free cannabis (Cannabis sativa subsp. sativa L.) in relation to hops (Humulus lupulus L.)

The major flavonoids present in the leaves and flowers of the cannabinoid-free cannabis (Cannabis sativa subsp. sativa L.) cultivars Felina and Futura are orientin (1), vitexin (2), luteolin-7-O-beta-D-glucuronide (3), and apigenin-7-O-beta-D-glucuronide (4), while prenylated flavonoids, to which the potent estrogenicity of hops (Humilus lupulus L.) is associated, are absent. The different composition of flavonoids has chemotaxonomic value.     

Congresses,Conferences, Symposia,Meetings, and Seminars in the Field of Chemical Sciences held in 1995–1996

Information

Congresses, Conferences, Symposia, Meetings, and Seminars in the Field of Chemical Sciences held in 1995–1996     

Oil core-polymer shell microcapsules prepared by internal phase separation from emulsion droplets. I. Characterization and release rates for microcapsules with polystyrene shells

Microcapsules with an oil core surrounded by a polymeric shell have been prepared by the controlled phase separation of polymer dissolved within the oil droplets of an oil-in-water emulsion. The dispersed oil phase consists of the shell polymer (polystyrene), a good solvent for the polymer (dichloromethane), and a poor solvent for the polymer (typically hexadecane). Removal of the good solvent results in phase separation of the polymer within the oil droplets. If the three interfacial tensions between the core oil, the shell-forming polymer, and the continuous phase are of the required relative magnitudes, a polymer shell forms surrounding the poor solvent. A UV-responsive organic molecule was added to the oil phase, prior to emulsification, to investigate the release of a model active ingredient from the microcapsules. This molecule should be soluble in the organic core but also have some water solubility to provide a driving force for release into the continuous aqueous phase. As the release rate of the active ingredient is a function of the thickness of the polymeric shell, for controlled release applications, it is necessary to control this parameter. For the preparative method described here, the thickness of the shell formed is directly related to the mass of polymer dissolved in the oil phase. The rate of volatile solvent removal influences the porosity of the polymer shell. Rapid evaporation leads to cracks in the shell and a relatively fast release rate of the active ingredient. If a more gentle evaporation method is employed, the porosity of the polymer shell is decreased, resulting in a reduction in release rate. Cross-linking the polymer shell after capsule formation was also found to decrease both the release rate and the yield of the active ingredient. The nature of the oil core also affected the release yield.     

Direct probing of zwitterion formation in unsolvated peptides

Molecular beam electric deflection measurements have been used to determine electric susceptibilities for small unsolvated alanine-based peptides. The electric susceptibility provides information about the charge distribution within the peptide and can be used to distinguish between zwitterionic and canonical forms. Measured electric susceptibilities for WAn peptides (n = 1-5) are similar to those for capped Ac-WAn-NH2 peptides (which cannot form zwitterions). Susceptibilities calculated using a simulated tempering-based approach are substantially larger for the zwitterionic form than for the canonical form. The measured susceptibilities are in good agreement with those calculated for the canonical form. For the larger peptides, the lowest potential energy structure found in the simulations is hairpin-like, while the lowest free energy structure found at room temperature is extended. The zwitterionic form is constrained by intramolecular interactions which make it entropically unfavorable.