Design of a "new motif" with beta-amino acids and alpha-aminoxy acids: synthesis of hybrid peptides with 12/10-helix

Hybrid peptides are prepared from a C-linked carbo-beta-amino acid ester (R-beta-Caa) and an alpha-aminoxy acid (R-Ama) derived from S-lactic acid. Extensive NMR (in CDCl 3 solution), CD, and MD studies on the tetra- and hexapeptides led to identification of robust 12/10-mixed helices. The dipeptide repeat having an R-beta-Caa and an R-Ama thus provides a "new motif" to realize a 12/10-mixed helix, for the first time, in oligomers containing R-Ama. To understand the impact of side chains in the mixed helix formation, R-beta-Caa/Ama (with no substitution in Ama) and S-beta-hAla/R-Ama oligomers were investigated. NMR studies revealed the existence of 12/10-helices in these hybrid peptides, and the side chains of monomers were found to have a profound influence on their stabilities. These observations imply that the propensity of beta-amino acid to prefer a mixed 12/10-helix governs the structural behavior in these peptides. The structural consequences of the lone-pair repulsion between nitrogen and oxygen atoms result in a new and interesting structural motif which behaves like "pseudo" beta (3),beta(2)-peptides in generating 12/10-mixed helices.     

Indium-mediated alkynylation of Baylis-Hillman acetates: a novel route to 1,4-enynes

Baylis-Hillman acetates undergo smooth alkynylation with aryl-susbstituted iodoalkynes in the presence of indium metal in refluxing dichloromethane to furnish 1,4-enynes in high yields with (E)-stereoselectivity. In the absence of Lewis acid, the reaction follows both S_N2 and S_N2′ pathways affording 1:1 mixtures of 1,4-enynes. Upon addition of 10 mol % of InBr₃, the reaction proceeds preferably in the S_N2′ manner. In the case of adducts derived from acrylonitrile, the corresponding products are obtained in fairly good yields and with (Z)-stereoselectivity.     

Color response of tri-armed azo host colorimetric sensors and test kit for fluoride

Five new chromogenic tripodal receptors (2a-e) containing electron withdrawing and donating groups appended to the azophenol moiety were synthesized, characterized, and their chromogenic behaviors toward various anions were investigated. These tripodal receptors showed a distinct color change only when treated with fluoride ions in CH₃CN solution. Yet, other anions such as Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻, AcO⁻, HSO₄⁻, and H₂PO₄⁻ could not cause any color change. Thus, the receptors 2a-e can be used as a colorimetric chemosensor for the determination of fluoride ion. In addition, ¹H NMR experiments were carried out to explore the nature of interaction between tripodal receptors and fluoride. Finally, analytical application and the use of test strip of the receptor 2b to detect fluoride was also reported.     

Fluorescent carbazolyldithiane as a highly selective chemodosimeter via protection/deprotection functional groups: a ratiometric fluorescent probe for Cd(II)

A carbazole-based bis-dithiane (1) was rationally constructed in a straightforward manner for the selective and ratiometric fluorescent detection of Cd²⁺ with the concept of aldehyde group protection/deprotection. The probe showed a ratiometric fluorescent response to Cd²⁺ with a large emission wavelength shift (>50 nm) and displayed high selectivity for Cd²⁺ over other metal ions due to distinct deprotection conditions. In addition, a Cd²⁺-promoted dethioacetalization mechanism was proposed.     

Structural and spectroscopic studies on some chloropyrimidine derivatives by experimental and quantum chemical methods

Ab initio quantum chemical and experimental spectroscopic studies in the infrared (4000-60 cm(-1)) and UV spectral regions are being reported on 4-chloro-2,6-dimethylsulfanyl pyrimidine-5-carbonitrile and 4-chloro-2-methylsulfanyl-6-(2-thienyl) pyrimidine-5-carbonitrile. Optimized geometries, electronic charge distribution, dipole moments and three-dimensional molecular electrostatic potential surfaces have been obtained. These have been used to understand the structure and spectral characteristics of the two compounds. A complete assignment of vibrational spectra on the basis of DFT/6-311G** and electronic spectra on the basis of TD-DFT/6-31+G* 5D calculations have been attempted for the two molecules.     

A simple 1,10-phenanthroline-based fluorescent receptor in solution and 1,10-phenanthroline in solid state for urea recognition

1,10-phenanthroline-based fluorescent chemosensor has been designed and synthesized for the recognition of urea and 1,3-dimethyl urea in CH₃CN. Interactions of this compound with urea and 1,3-dimethyl urea are studied with UV-visible and fluorescence spectroscopic methods. 1,10-phenanthroline was found to be capable of distinguishing urea from 1,3-dimethyl urea. The formation of a highly stable hydrogen bonded complex between 1,10-phenanthroline and urea was confirmed by single crystal X-ray diffraction analysis.     

A simple polyacrylamide gel electrophoresis procedure for separation of polyamidoamine dendrimers

A simple, inexpensive, and rapid electrophoresis technique was developed for use as a routine tool for evaluating purity of polyamidoamine (PAMAM) dendrimers. A variety of factors influencing migration of generations 0-7 dendrimers on nongradient polyacrylamide gels were evaluated. The low generation dendrimers were found to be very sensitive to diffusion during or after electrophoresis. The proposed method incorporates steps that minimize diffusion, in order to obtain improved resolution and sensitivity, especially for the lower-molecular-weight dendrimers. This was accomplished by inclusion of a dendrimer fixation step with glutaraldehyde and performing the electrophoresis separation, fixation, staining, and destaining at 4 degrees C. PAMAM dendrimer separation was studied under basic and acidic conditions. Electrophoresis under acidic conditions gave increased resolution and sensitivity over separation at alkaline pH. Oligomers and trailing generations could be clearly separated and visualized under these conditions. The smallest PAMAM dendrimer, generation 0, was visible at 1.5 microg under the optimized acidic conditions. With slight modifications, this technique should be applicable to separation of other water-soluble dendrimers.     

Constrained self-consistent-field wave functions with improved long-range behavior

The recent suggestion that the long-range behavior of energy-optimized Gaussian basis sets can be improved by augmenting them with a Gaussian chosen to satisfy a constraint involving a linearly averaged position moment is explored. Calculations indicate that the high-order moments 〈r^k〉, with k > 4, in He, Be, and Li^?, and 〈x^kz^L?k〉, with L > 4 and k ≤ L, in H₂ are improved by the constraint, but that lower-order moments and dipole polarizabilities are not. In H₂, the higher moments with a given L improve by different amounts for different k, and, hence, the multipole moments do not improve. The basis-set superposition error in He? He and Be? Be interaction energy calculations decreases if the internuclear distance is large enough. Thus, the constraint procedure improves the very long range behavior of the self-consistent-field wave functions. © 1992 John Wiley & Sons, Inc.     

Sphere‐to‐Rod Transitions in Cationic Micelles: Quaternary Ammonium Halides+Phenol+Water Systems

Viscosity measurements on aqueous micellar solutions of cationic surfactants containing phenol with and without sodium bromide were made to study the sphere‐to‐rod transitions. Effect of surfactant structure (nonpolar tail, polar head group sizes and counterion), temperature, and phenol and sodium bromide concentration on the viscosity behavior of the surfactant solution is discussed. The sphere‐to‐rod transition is usually indicated by a marked increase in viscosity. While low temperature, high surfactant concentration, presence of salt, and hydrophobic nature of surfactant all favor the formation of rod‐like micelles, the presence of phenol showed peculiar behavior. Initial additions of phenol (up to about 2.5 wt%) showed a marked increase in viscosity, independent of the nature and concentration of surfactant and temperature; lower viscosities were observed at higher phenol concentration. Conductance and sound velocity results support the viscosity results.