Poly(amideimide)s containing pendant pentadecyl chains: Synthesis and characterization

A new aromatic diacylhydrazide monomer viz., 4-[4′-(hydrazinocarbonyl)phenoxy]-2- pentadecylbenzohydrazide was synthesized starting from cardanol, which in turn is obtainable from cashew nut shell liquid - a renewable resource material. A series of new poly(amideimide)s containing flexibilizing ether linkages and pendant pentadecyl chains was synthesized from 4-[4′-(hydrazinocarbonyl)phenoxy]-2-pentadecylbenzohydrazide and commercially available aromatic dianhydrides, viz., benzene-1,2,4,5-tetracarboxylic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, benzophenone-3,3′,4,4′-tetracarboxylic dianhydride, 4,4′-oxydiphthalic anhydride and 4,4′-(hexafluoro isopropylidene)diphthalic anhydride by a two-step solution polycondensation in N,N-dimethylacetamide via the poly(hydrazide acid) intermediate. Inherent viscosities of poly(amideimide)s were in the range 0.60-0.64 dL/g in N,N-dimethylacetamide at 30 ± 0.1 °C. Poly(amideimide)s could be solution cast into tough, transparent and flexible films from their N,N-dimethylacetamide solutions. The solubility of poly(amideimide)s was significantly improved by incorporation of pendant pentadecyl chains and were found to be soluble in N,N-dimethylacetamide, 1-methyl-2-pyrrolidinone, pyridine and m-cresol at room temperature or upon heating. Wide angle X-ray diffraction patterns of poly(amideimide)s revealed a broad halo at around 2θ = ∼19° suggesting that polymers were amorphous in nature. In the small-angle region, diffuse to sharp reflections of a typically layered structure resulting from the packing of pentadecyl side chains were observed. The temperature at 10% weight loss (T₁₀), determined by TGA in nitrogen atmosphere, of poly(amideimide)s was in the range of 388-410 °C indicating their good thermal stability. Glass transition temperatures of poly(amideimide)s were in the range 162-198 °C. It was observed that the plasticization effect of attached pentadecyl side chains induced the depression of T_g.     

Quality profile of litchi (Litchi chinensis) cultivars from India and effect of radiation processing

Litchi (Litchi chinensis) is a non-climacteric tropical fruit. The fruit has a short shelf-life making its marketing difficult. Physical, biochemical, microbiological, and organoleptic properties of two major commercially grown Indian cultivars of litchi, ‘Shahi’ and ‘China’ were studied. The effect of gamma radiation processing and low temperature storage on the above parameters was evaluated to standardize the optimal process parameters for shelf-life extension of litchi. Physical and biochemical parameters analyzed included weight, moisture, pH, titratable acidity, texture, color, total and reducing sugar, total soluble solids, vitamin C, and flavonoid content. Weight, moisture content, and pH in the fresh fruit ranged between 21–26 g, 74–77%, and 3.7–4.4, respectively, whereas, total and reducing sugar ranged 10–15, and 10–13 g%, respectively. In ‘Shahi’ vitamin C content was found to be around 17–19 mg%, whereas, in ‘China’ it was 22–28 mg%. Flavonoid content was in the range of 26–34 μg catechin equivalents/g of fresh fruit. Total surface and internal bacterial load was around 4 and 3 log cfu/g, respectively. Surface yeast-mold count (YMC) was ～3 log cfu/g whereas internal YMC was ～2 log cfu/g. Radiation treatment reduced microbial load in a dose dependent manner. Treatment at 0.5 kGy did not significantly affect the quality parameters of the fruit. Treated fruits retained the “good” organoleptic rating during storage. Thus, radiation treatment (0.5 kGy) in combination with low temperature (4 °C) storage achieved a shelf-life of 28 days for litchi fruit.     

Protic ionic liquids with fluorous anions: physicochemical properties and self-assembly nanostructure

A series of 11 new protic ionic liquids with fluorous anions (FPILs) have been identified and their self-assembled nanostructure, thermal phase transitions and physicochemical properties were investigated. To the best of our knowledge this is the first time that fluorocarbon domains have been reported in PILs. The FPILs were prepared from a range of hydrocarbon alkyl and heterocyclic amine cations in combination with the perfluorinated anions heptafluorobutyrate and pentadecafluorooctanoate. The nanostructure of the FPILs was established by using small- and wide-angle X-ray scattering (SAXS and WAXS). In the liquid state many of the FPILs showed an intermediate range order, or self-assembled nanostructure, resulting from segregation of the polar and nonpolar hydrocarbon and fluorocarbon domains of the ionic liquid. In addition, the physicochemical properties of the FPILs were determined including the melting point (T(m)), glass transition (T(g)), devitrification temperature (T(c)), thermal stability and the density ρ, viscosity η, air/liquid surface tension γ(LV), refractive index n(D), and ionic conductivity κ. The FPILs were mostly solids at room temperature, however two examples 2-pyrrolidinonium heptafluorobutyrate (PyrroBF) and pyrrolidinium heptafluorobutyrate (PyrrBF) were liquids at room temperature and all of the FPILs melted below 80 °C. Four of the FPILs exhibited a glass transition. The two liquids at room temperature, PyrroBF and PyrrBF, had a similar density, surface tension and refractive index but their viscosity and ionic conductivity were very different due to dissimilar self-assembled nanostructure.     

Protic ionic liquids: physicochemical properties and behavior as amphiphile self-assembly solvents

The physicochemical properties of 22 protic ionic liquids (PILs) and 6 protic molten salts, and the self-assembly behavior of 3 amphiphiles in the PILs, are reported. Structure-property relationships have been explored for the PILs, including the effect of increasing the substitution of ammonium cations and the presence of methoxy and hydroxyl moieties in the cation. Anion choices included the formate, pivalate, trifluoroacetate, nitrate, and hydrogen sulfate anions. This series of PILs had a diverse range of physicochemical properties, with ionic conductivities up to 51.10 mS/cm, viscosities down to 5.4 mPa.s, surface tensions between 38.3 and 82.1 mN/m, and densities between 0.990 and 1.558 g/cm3. PILs were designed with various levels of solvent cohesiveness, as quantified by the Gordon parameter. Fourteen PILs were found to promote the self-assembly of amphiphiles. High-throughput polarized optical microscopy was used to identify lamellar, hexagonal, and bicontinuous cubic amphiphile self-assembly phases. The presence and extent of amphiphile self-assembly have been discussed in terms of the Gordon parameter.     

A truncated Newton minimizer adapted for CHARMM and biomolecular applications

We report the adaptation of the truncated Newton minimization package TNPACK for CHARMM and biomolecular energy minimization. TNPACK is based on the preconditioned linear conjugate–gradient technique for solving the Newton equations. The structure of the problem—sparsity of the Hessian—is exploited for preconditioning. Experience with the new version of TNPACK is presented on a series of molecular systems of biological and numerical interest: alanine dipeptide (N-methyl-alanyl-acetamide), a dimer of N-methyl-acetamide, deca-alanine, mellitin (26 residues), avian pancreatic polypeptide (36 residues), rubredoxin (52 residues), bovine pancreatic trypsin inhibitor (58 residues), a dimer of insulin (99 residues), and lysozyme (130 residues). Detailed comparisons among the minimization algorithms available in CHARMM, particularly those used for large-scale problems, are presented along with new mathematical developments in TNPACK. The new TNPACK version performs significantly better than ABNR, the most competitive minimizer in CHARMM, for all systems tested in terms of CPU time when curvature information (Hessian/vector product) is calculated by a finite-difference of gradients (the numeric option of TNPACK). The remaining derivative quantities are, however, evaluated analytically in TNPACK. The CPU gain is 50% or more (speedup factors of 1.5 to 2.5) for the largest molecular systems tested and even greater for smaller systems (CPU factors of 1 to 4 for small systems and 1 to 5 for medium systems). TNPACK uses curvature information to escape from undesired configurational regions and to ensure the identification of true local minima. It converges rapidly once a convex region is reached and achieves very low final gradient norms, such as of order 10^?8, with little additional work. Even greater overall CPU gains are expected for large-scale minimization problems by making the architectures of CHARMM and TNPACK more compatible with respect to the second-derivative calculations. © 1994 by John Wiley & Sons, Inc.     

Synthesis and characterization of aromatic polyazomethines bearing pendant pentadecyl chains

Aromatic diamine monomer viz., 4-(4′-aminophenoxy)-2-pentadecylbenzenamine containing pendant pentadecyl chain was synthesized starting from cashew nut shell liquid - a renewable resource material and was characterized by FTIR, ¹H and ¹³C NMR spectroscopy. A series of new (co) polyazomethines containing pendant pentadecyl chains and flexibilizing ether linkages was synthesized by polycondensation of 4-(4′-aminophenoxy)-2-pentadecyl benzenamine with commercially available aromatic dialdehydes viz., terephthaldehyde (TPA), isophthaldehyde (IPA) and varying mixture of TPA and IPA. Inherent viscosities and number average molecular weights of (co) polyazomethines were in the range 0.50-0.70 dL/g and 10,490-40-800 (GPC, polystyrene standard), respectively indicating formation of medium to reasonably high molecular weight polymers. (Co) polyazomethines containing pendant pentadecyl chains were found to be soluble in common organic solvents such as chloroform, dichloromethane, tetrahydrofuran, pyridine, m-cresol and could be cast into transparent and stretchable films from their solution in chloroform. (Co) polyazomethines were essentially amorphous in nature and the formation of loosely-developed layered structure was observed arising from the packing of pendant pentadecyl chains. Polyazomethines exhibited glass transition temperatures (T_g) in the range 21-48 °C. The observed depression of glass transition temperature could be ascribed to the “internal plasticization” effect of pentadecyl chains. The temperature at 10% wt loss (T₁₀), determined from TGA in nitrogen atmosphere of polyazomethines were in the range 434-441 °C indicating their good thermal stability.     

Hyperaromatic stabilization of arenium ions: acid-catalyzed dehydration of 2-substituted 1,2-dihydro-1-naphthols

Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = -0.49 - 8.8σ(I), with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σ(I) and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C-H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that k(cis)/k(trans) rate ratios for substituents (Me, 8.4; Bu(t), 12.7; Ph, 3.8; NH(3)(+), 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N(3)) or suspected (EtSO, EtSO(2)) of stabilizing the cation by a π or σ neighboring group effect. The good Taft correlation is taken to indicate that cis substuents are reacting normally, differentiated only by their inductive effects. The slower reactions of the trans isomers are the judged to be "abnormal". This is confirmed by comparing effects of cis and trans β-OH substituents on the reactivities of dihydro phenols, naphthols, and phenanthrols. Whereas k(H)/k(OH) for cis substituents varies by less than 8-fold and is consistent with the influence of an inductive effect of the OH group (k(H)/k(OH) ≈ 2000), k(H)/k(OH) for the trans substituents varies by 3 orders of magnitude, reflecting the additional influence of the lesser hyperconjugating ability of a C-OH bond compared to a C-H bond. The magnitude and variation of this difference is consistent with C-H hyperconjugation conferring aromatic character on the arenium ions.     

An analysis of the structural and energetic properties of deoxyribose by potential energy methods

We discuss the three fundamental issues of a computational approach in structure prediction by potential energy minimization, and analyze them for the nucleic acid component deoxyribose. Predicting the conformation of deoxyribose is important not only because of the molecule's central conformational role in the nucleotide backbone, but also because energetic and geometric discrepancies from experimental data have exposed some underlying uncertainties in potential energy calculations. The three fundamental issues examined here are: (i) choice of coordinate system to represent the molecular conformation; (ii) construction of the potential energy function; and (iii) choice of the minimization technique. For our study, we use the following combination. First, the molecular conformation is represented in cartesian coordinate space with the full set of degrees of freedom. This provides an opportunity for comparison with the pseudorotation approximation. Second, the potential energy function is constructed so that all the interactions other than the nonbonded terms are represented by polynomials of the coordinate variables. Third, two powerful Newton methods that are globally and quadratically convergent are implemented: Gill and Murray's Modified Newton method and a Truncated Newton method, specifically developed for potential energy minimization. These strategies have produced the two experimentally-observed structures of deoxyribose with geometric data (bond angles and dihedral angles) in very good agreement with experiment. More generally, the application of these modeling and minimization techniques to potential energy investigations is promising. The use of cartesian variables and polynomial representation of bond length, bond angle and torsional potentials promotes efficient second-derivative computation and, hence, application of Newton methods. The truncated Newton, in particular, is ideally suited for potential energy minimization not only because the storage and computational requirements of Newton methods are made manageable, but also because it contains an important algorithmic adaptive feature: the minimization search is diverted from regions where the function is nonconvex and is directed quickly toward physically interesting regions.