Similarities and differences between azomethines and ketimines: synthesis, materials characterization and structure of novel imines compounds

Imines (ketimines and azomethines) derived from p-dibenzoylbenzene (DB) and terephthalic aldehyde (TA) and two aromatic amines: aniline and 2,6-dimethylaniline have been investigated. Compounds were synthesized via condensation of amines with carbonyl monomers in DMA or amine solution. When using DMA as a solvent, azomethines with high yields were obtained. On the other hand, the amines used as a monomers served also as an effective solvent for the synthesis of the ketanils. This different reactivity of the aldehyde and ketone groups in DMA and in amine depends on the dehydration mechanism being dominated by a kinetic process or thermodynamic one. On the basis of FTIR, 13C and 1H NMR, UV-vis spectra, thermal characteristic and theoretical calculations conclusions are drawn regarding the similarities and differences between azomethines and ketimines.     

A stilbene that binds selectively to transthyretin in cells and remains dark until it undergoes a chemoselective reaction to create a bright blue fluorescent conjugate

We describe a non-fluorescent, second generation stilbene that very selectively binds to transthyretin in complex biological environments and remains dark until it chemoselectively reacts with the pK(a)-perturbed Lys-15 ε-amino group of transthyretin to form a bright blue fluorescent conjugate. Stilbene A2 is mechanistically unusual in that it remains non-fluorescent in cell lysates lacking transthyretin, even though there is likely some proteome binding. Thus, it is especially useful for cellular imaging, as background fluorescence is undetectable until A2 reacts with transthyretin. The mechanistic basis for the effective lack of environment-sensitive fluorescence of A2 when bound to, but before reacting with, transthyretin is reported. Stilbene A2 exhibits sufficiently rapid transthyretin conjugation kinetics at 37 °C to enable pulse-chase experiments to be performed, in this case demonstrating that transthyretin is secreted from HeLa cells. As the chase compound, we employed C1, a cell-permeable, highly selective, non-covalent, transthyretin-binding dihydrostilbene that cannot become fluorescent. The progress reported is viewed as a first and necessary step toward our long-term goal of creating a one-chain, one-binding-site transthyretin tag, whose fluorescence can be regulated by adding A2 or an analogous molecule. Fusing proteins of interest to a one-chain, one-binding-site transthyretin tag regulated by A2 should be useful for studying folding, trafficking, and degradation in the cellular secretory pathway, utilizing pulse-chase experiments. Immediate applications of A2 include utilizing its conjugate fluorescence to quantify transthyretin concentration in human plasma, reflecting nutritional status, and determining the binding stoichiometry of kinetic stabilizer drugs to transthyretin in plasma.     

Intermolecular interactions in electron transfer through stretched helical peptides

The helical peptide Cys-Ala-Lys-(Glu-Ala-Ala-Ala-Lys)(2)-Ala-NH-(CH(2))(2)-SH has been organized forming a self-assembled monolayer on gold (0.602 peptides per nm(2)), its conductance behavior under stretching conditions being studied using scanning tunnelling microscopy and current sensing atomic force microscopy. The helical conformation of the peptide has been found to play a fundamental role in the conductance. Moreover, variation of the current upon molecular stretching indicates that peptides can be significantly elongated before the conductance drops to zero, the critical elongation being 1.22 ± 0.47 nm. Molecular dynamics simulations of a single peptide in the free state and of a variable number of peptides tethered to a gold surface (i.e. densities ranging from 0.026 to 1.295 peptides per nm(2)) have indicated that the helical conformation is intrinsically favored in solvated environments while in desolvated environments it is retained because of the fundamental role played by peptide-peptide intermolecular interactions. The structure obtained for the system with 24 tethered peptides, with a density of 0.634 peptides per nm(2) closest to the experimental one, is in excellent agreement with experimental observations. On the other hand, simulations in which a single molecule is submitted to different compression and stretching processes while the rest remain in the equilibrium have been used to mimic the variation of the tip-substrate distance in experimental measures. Results allowed us to identify the existence, and in some cases coexistence, of intermolecular and intramolecular ionic ladders, suggesting that peptide-mediated electron transfer occurs through the hopping mechanism. Finally, quantum mechanical calculations have been used to investigate the variation of the electronic structure upon compression and stretching deformations.     

Aggregation of sulfonated poly(p-phenylene terephthalamide) in dilute solutions

The viscosities of dilute solutions of poly(p-phenylene terephthalamide), PPTS, in dimethylacetamide, water, and their mixtures were determined. The reduced viscosity plot in dimethylacetamide shows a negative slope. When the water content in the mixed solvent in 90% or higher, there is an upswing in the reduced viscosity values at concentrations below 0.1 g/dL. The latter behavior suggests a “polyelectrolyte” effect. However, an association model was found to be able to explain the viscosity behaviors in both solvents. ©1995 John Wiley & Sons, Inc.     

Influence of the structure of the diol on the properties of aromatic polyesterimides

A series of aromatic polyesterimides with ester bonds in the side-chains has been obtained by the partial esterification of a polyamic acid prepared from pyromellitic dianhydride and 4,4′-diaminodiphenyl ether with the following aromatic diols: 4,4′-di(2-hydroxyethoxy)-1,1′-binaphthyl, 2,2′-di(2-hydroxyethoxy)-1,1′-binaphthyl.di(2-hydroxyethoxy-1 -naphthyl)methane,2,2-di[4-(2-hydroxyethoxy)phenyl] propane.Influence of the structure of the diol on the thermal, mechanical and dielectric properties of resultant polymer foils has been studied. Properties of these foils have been compared with those obtained for the polypyromellitimide film of the 4,4′-diaminodiphenyl ether.     

Testing the concept of a modulation filter bank: the audibility of component modulation and detection of phase change in three-component modulators

Two experiments were performed to test the concept that the auditory system contains a "modulation filter bank" (MFB). Experiment 1 examined the ability to "hear out" the modulation frequency of the central component of a three-component modulator applied to a 4-kHz sinusoidal carrier. On each trial, three modulated stimuli were presented. The modulator of the first stimulus contained three components. Within a run the frequencies of the outer two components were fixed and the frequency of the central ("target") component was drawn randomly from one of five values. The modulators of second and third stimuli contained one component. One had a frequency equal to that of the target and the other had a frequency randomly selected from one of the other possible values. Subjects indicated whether the target corresponded to the second or third stimulus. Scores were around 80% correct when the components in the three-component modulator were widely spaced and when the frequencies of the target and comparison differed sufficiently. Experiment 2 examined the ability to hear a change in the relative phase of the components in a three-component modulator with harmonically spaced components, using a 31FC task. The frequency of the central component, f(c), was either 50 or 100 Hz. Scores were 80%-90% correct when the component spacing was < or = 0.5 f(c), but decreased markedly for greater spacings. Performance was only slightly impaired by randomizing the overall modulation depth from one stimulus to the next. The results of both experiments are broadly consistent with what would be expected from a MFB with a Q value of 1 or slightly less.     

Crystal structure of Cu(I) π-complexes from the system CuBr−HBr-diallylamine

L'vov State University. Institute of Applied Physics, Moldavian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 1, pp. 121–125, January–February, 1992.