Titration Curves of p-Aminobenzoic Acid-Formaldehyde-p-Bromoaniline Copolymers in Nonaqueous Media

Copolymers obtained by condensation of p-aminobenzoic acid, p-bromoaniline, and formaldehyde in presence of acid catalyst have been separated into five fractions. Quantitative estimation of Br in each of the fractions was carried out, and the total amount of acidic and basic functional groups in given weight of the copolymer was calculated. Each of the fractions were titrated potentiometrically and conductometrically in nonaqueous solvents, and an excellent correlation was obtained for calculated and observed functional groups. The titration curves also indicated a series of breaks which bear an integral relation to each other. These features have been interpreted in terms of intramolecular hydrogen bonding, homoconjugation, and composition of the copolymers.     

Progress report on the boundedness character of third-order rational equations

An interpretation of Cull's enveloping method used to determine global asymptotic stability of one dimensional population models is given. This is done by relating the enveloping property with the existence of a global Lyapunov function. Following this spirit we revisit a result of Liz.     

Effect of Hydrogen Bonding on the Dissociation of Some Mixtures of Phenolic Oligomers in Nonaqueous Media

Some structurally and molecularly uniform phenolic oligomers with additional functional groups have been prepared by known methods of stepwise synthesis, and their dissociation behavior in nonaqueous solvent has been studied by electrometric titration techniques. Hyperacidity and stepwise neutralization of some of the functional groups in the oligomer molecules have been interpreted in terms of intramolecular hydrogen bonding, homoconjugation, and ion association in a medium of low dielectric constant.     

Molecular Mechanisms of Amylin Turnover,Misfolding and Toxicity in the Pancreas

Amyloidosis is a common pathological event in which proteins self-assemble into misfolded soluble and insoluble molecular forms, oligomers and fibrils that are often toxic to cells. Notably, aggregation-prone human islet amyloid polypeptide (hIAPP), or amylin, is a pancreatic hormone linked to islet β-cells demise in diabetics. The unifying mechanism by which amyloid proteins, including hIAPP, aggregate and kill cells is still matter of debate. The pathology of type-2 diabetes mellitus (T2DM) is characterized by extracellular and intracellular accumulation of toxic hIAPP species, soluble oligomers and insoluble fibrils in pancreatic human islets, eventually leading to loss of β-cell mass. This review focuses on molecular, biochemical and cell-biology studies exploring molecular mechanisms of hIAPP synthesis, trafficking and degradation in the pancreas. In addition to hIAPP turnover, the dynamics and the mechanisms of IAPP–membrane interactions; hIAPP aggregation and toxicity in vitro and in situ; and the regulatory role of diabetic factors, such as lipids and cholesterol, in these processes are also discussed.     

Mixed Chalcopyrites as Useful Solar Cell Materials

I1n view of the present global energy crisis the advent of the ternary chalcopyrites with some important useful properties stimulate us to study these compounds together with their mixed versions with a view to find their suitability as efficient solar cell materials. We finally selected out a few promising materials, as some of them have desirable characteristics and satisfy the stringent requirements for photovoltaic energy conversion.     

Structural features of the La-Sr-Fe-Co-O system

A structural study has been performed on the La_0.8Sr_0.2Fe_xCo_1-xO₃ (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. ⁵⁷Fe M?ssbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La_0.8Sr_0.2Fe_xCo_1-xO₃ compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe³⁺. Received 22 January 2001     

Self-assembled four-membered networks of trimesic acid forming organic channel structures

By the co-crystallization of trimesic acid, TMA, with molecules such as dimethylamine, N,N,N′,N′-tetramethylethylenediamine and methanol, it has been possible to generate hydrogen-bonded four-membered networks of TMA. The three-dimensional arrangement of the four-membered networks gives rise to channels occupied by the guest molecules. It has also been possible to generate a four-membered network by co-ordination of TMA with Co(II).