Design,synthesis, and characterization of main‐chain,aromatic polyesters based on 3,4‐ethylenedioxythiophene

A series of new thermoplastic polyesters based on 3,4‐ethylenedioxythiophene (EDOT) with flexible aliphatic spacers have been synthesized and characterized for the first time. The thermal properties of these polyesters based on EDOT are comparable to those of conventional polyesters based on the 1,4‐phenyl unit, indicating that EDOT is a viable replacement for the phenyl units. The glass‐transition and melting‐transition temperatures decrease monotonically with an increase in the spacer length. Theoretical calculations have revealed that the core angle for EDOT is comparable to that of unsubstituted thiophene and hence should be compatible with the formation of the mesophase. This has been confirmed experimentally by the synthesis of a main‐chain, thermotropic, liquid‐crystalline polyester based on EDOT that exhibits fluid birefringence. In fact, this is the first report in which a main‐chain, liquid‐crystalline polymer based on 3,4‐disubstituted thiophene has been successfully designed and synthesized. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3479–3486, 2006     

Light-directed radial combinatorial chemistry: orthogonal safety-catch protecting groups for the synthesis of small molecule microarrays

We describe the development of photolabile protecting groups based on the 3,4,5-trimethoxyphenacyl group (TMP). Orthogonal safety-catches were created by introducing an acid-activatible dimethyl ketal (AA-TMP) and an oxidatively activatible 1,3-dithiane (OA-TMP) into the photolabile TMP group. We demonstrate the application of these protecting groups in light-directed synthesis of small molecule microarrays with diversity elements radially attached to a hydroxyproline scaffold.     

Polymerization of vinylcarbazole on copper(II)-13X molecular sieves: Role of copper(II) salts

Copper(II)-exchanged-13X molecular sieves, prepared from four copper(II) salts, namely, sulfate, nitrate, chloride, and acetate, and their activities in the polymerization of N-vinylcarbazole at a fixed copper ion exchange level were studied. From the kinetic characteristics of polymerization it was established that the variation in activities of the Cu-exchanged sieves is due primarily to the difference in the pH of the original salt solution, which is responsible for the varying degree of proton exchange with the zeolite during copper ion exchange. A first-order dependence of the rate of polymerization was observed with respect to H⁺ ion concentration of the original copper-salt solution. It was further established that the rate of polymerization of exchanged copper ion, at a hypothetical zero proton concentration, is low. A mechanism of polymerization of NVC on copper-exchanged-13X zeolite was suggested on the basis of the results obtained.     

LC–MS Determination of Gabapentin from Human Plasma

Gabapentin is an anticonvulsant drug used for the treatment of epilepsy. It is not bound to plasma protein and is not metabolized. A high performance liquid chromatography–mass spectrometric micro method is described in this report for its determination from human plasma. Chromatography was performed on a 50 × 4.6 mm, 4 μm nitrile column and the parent ion detected in the positive ionization mode on single quadrupole analyzer (Q1MI) with atmospheric pressure ionization source. Extraction was carried out on C₁₈, 100 mg/3cc cartridge using 10 μL sample volume. The mean extraction recovery was 97% and within batch and between batch coefficients of variation were <9%. Lack of interference from endogenous substances helped in achieving a highly sensitive method without the need for monitoring fragment ions. The lowest concentration injected on column for calibration curve was 195 pg (range 0.5–64 ng). The method was applied for analysis of samples from a cross-over bio-equivalence study comparing two formulations.     

Mechanism of N-vinylcarbazole polymerization on Co(II)–13X molecular sieve

Kinetics of polymerization of N-vinylcarbazole over Co(II)-13X molecular sieves in toluene have been studied. The rate of polymerization (R_p) has been found to be second order with respect to percent exchange level of Co(II) and also to the NVC concentration at all the reaction temperatures of 40, 50 and 60°C. The rate increases with decreasing pH of the original exchanging salt solution up to a pH of about 3.5, beyond which it falls. The overall activation energy of polymerization has been found to decrease with increase in monomer concentration, exchange level of Co(II), and the hydrogen ion concentration of the original exchange solution. Average degree of polymerization also follows a similar trend. A mechanism of polymerization involving simultaneous propagation on both metal ion Co(II) and proton on a zeolite surface has been suggested. The two propagation routes are characterized by an average activation energy of 10.36 kcal/mol and 5.40 kcal/mol on the metal ion and proton centers, respectively.     

Investigation of nu(N-H) and nu(C-N) stretching modes of propylamine (C3H7NH2) in a binary system C3H7NH2 + CH3OH via concentration dependent Raman study and ab initio calculations

Raman spectra of propylamine (C3H7NH2) and its binary mixtures, C3H7NH2 + CH3OH with varying mole fractions of the reference system, C3H7NH2, C were recorded in two widely apart wavenumber regions, 3100-3600 cm(-1) and 1225-1325 cm(-1). In the former region, the two Raman bands at approximately 3305 and approximately 3326 cm(-1), obtained after the line shape analysis, which were assigned to symmetric nu(N-H) and anti-symmetric nu(N-H) stretching modes, respectively, show a downshift upon dilution. However, whereas the nu(N-H) anti-symmetric mode shows a shift of 18.6 cm(-1), the nu(N-H) symmetric mode shows a much smaller shift (5.7 cm(-1)) between neat liquid and high dilution, C = 0.1. This aspect has been explained using the optimized geometries calculated employing ab initio theory (MP2 level) for the neat C3H7NH2 and its different hydrogen-bonded complexes. The linewidth versus concentration plot for the nu(N-H) anti-symmetric stretching mode, however exhibits a distinct maxima at C = 0.4, which has been explained as a slight departure from the concentration fluctuation model. In the latter region, a symmetric peak is observed, which corresponds to nu(C-N) stretching mode, which shows an upshift upon dilution and an almost linear concentration dependence. This has also been explained in terms of the parameters obtained from the optimized geometries of the different hydrogen-bonded complexes.     

A Synthetic Strategy for Novel Organothiophosphates Incorporating a Pyridine Ring

Eight novel amidothiophosphates have been successfully achieved using cycloiminium salts of pyridine as the starting material. Phosphorylation was done using phosphorus trichloride, and the resulting functionalized iminohalophosphines in situ were further oxidized and substituted through sulfur and a suitable secondary amine (alkyl, alicyclic, and aryl). The reactions were carried out in a dry nitrogen atmosphere. The synthesized products were characterized using different spectroscopic techniques (¹H and ³¹P NMR). The pyridine nucleus incorporating a phosphoryl group is highly bioactive. Hence, the synthesized organothiophosphates should be potential pesticidal molecules.     

Stereopositional Outcome in the Packing of Dissimilar Aromatics in Designed β‐Hairpins

A popular strategy in the de novo design of stable β‐sheet structures for various biomedical applications is the incorporation of aromatic pairs at the non‐hydrogen‐bonding (NHB) position. However, it is important to explicitly understand how aryl pair packing at the NHB region is coordinated with backbone structural rearrangements, and to delineate the benefits and drawbacks associated with stereopositional choice of dissimilar aromatic pairs. Here, we probe the consequences of flipped Trp/Tyr pairs by using engineered permutants at the NHB position of dodecapeptide β‐hairpins, proximal and distal to the turn. Extensive conformational analysis of these peptides using NMR and CD spectroscopy reveal that a classic Edge‐to‐Face and Face‐to‐Edge geometry at the proximal and distal aromatic pairs, respectively, in YW‐WY, is the most stabilizing. Such a preferred packing geometry in YW‐WY results in a highly twisted β‐sheet backbone, with Trp always providing a ‘Face’ orientation to its dissimilar aromatic partner Tyr. Flipping the proximal and/or distal aromatic pair distorts the ideal T‐shaped geometry, and results in alternate aryl arrangements that can adversely affect strand twist and β‐sheet stability. Our study reveals the existence of a strong stereopositional influence on the packing of dissimilar aromatic pairs. Our findings highlight the importance of modeling physical interaction forces while designing protein and peptide structures for functional applications.     

UV spectral characterization of a smectic-C liquid crystal: Theoretical support to the experiment

In the present article, UV spectral characterization of a smectic-C liquid crystal 4,4′-bis(n-alkoxy)azoxybenzene (n = 14) (C₄₀H₆₆N₂O₃) has been carried out. Structure of the molecule has been optimized using the Density functional B3LYP with 6-31+G (d) basis set using crystallographic geometry as input. The absorption spectra have been estimated in the UV region by employing the DFT method, semiempirical CNDO/S and INDO/S parameterizations. The oscillator strength (f) and vertical transition energy (E_V) have been reported corresponding to absorption wavelength (λ_max). These values have been compared with the experimental value reported in the literature to offer theoretical support to the experimental value. Further, some electrochemical properties have been reported for the molecule.     

Synthesis and Characterization of Some New Benzimidazole Derivatives

Synthesis of nine new benzimidazole derivatives was reported. The products were identified by ¹H NMR, mass spectroscopy, and infrared spectroscopy.