Gelation of aqueous pectin solutions: a dynamic light scattering study

We report the dynamic light scattering study of the gelation of aqueous solutions of the biopolymer, pectin, induced by the addition of calcium chloride. The time correlation function data are analyzed under the framework of the coupling model. As the solution enters the semidilute regime where gelation sets in, the relaxation process shows a stretched exponential behavior. The stretching exponent decreases and the characteristic time of the stretched exponential diverges as the system evolves to a gel. Aqueous pectin solutions in the presence of 0.1 M NaCl show similar behavior. Thus, the molecular relaxation modes of pectin solutions can be well described by the coupling model.     

Reduction of copper in porous matrixes. Stepwise and autocatalytic reduction routes

The reduction of Cu(II) oxide species in siliceous matrixes of different porosity (MFI, FAU, MCM-48) and in alumosilicate MFI was studied by temperature-programmed reduction in hydrogen (TPR), by X-ray absorption fine structure (after stationary hydrogen treatments), and by transmission electron microscopy. It was found that the reduction may proceed in one or in two reduction steps. The two-step scheme known for zeolites was observed also for Cu(II) in siliceous microporous matrixes, with similar temperature of Cu(II) reduction onset as for the alumosilicate MFI. Therefore, the two-step scheme cannot be explained by the stabilization of Cu ions by intra-zeolite electrical fields. CuOx clusters in MCM-48 were reduced in a one-step scheme (similar to bulk CuO) at high Cu content (6 wt %) but in a two-step scheme at low Cu content (1 wt %). The two reduction steps observed with most samples cannot be identified with the transitions of all Cu(II) to Cu(I) and of Cu(I) to Cu(0). Instead, Cu(0) nuclei were observed already at low reduction temperatures and were found to coexist with Cu ions over temperature ranges of different extension. This coexistence range was narrow in materials that favor aggregation of the Cu nuclei into particles: Cu-MCM-48 of low Cu content and Cu-ZSM-5. In the latter, metal segregation from the pore system was found to be accompanied by an autocatalytic initiation of the second reduction step. In the siliceous microporous matrixes, the Cu(0) nuclei were observed to coexist with Cu ions over wide temperature ranges (100 K for MFI) at temperatures far above that of Cu reduction in the bulk oxide. These observations suggest that oligomeric Cu metal nuclei which may have been formed, e.g., at the intersections of the MFI channel system, may be unable to activate hydrogen, which would be required for rapid reduction of the coexisting Cu ions.     

Boronic acid based dynamic click chemistry: recent advances and emergent applications

Recently, reversible click reactions have found numerous applications in chemical biology, supramolecular chemistry, and biomedical applications. Boronic acid (BA)-mediated cis-diol conjugation is one of the best-studied reactions among them. An excellent understanding of the chemical properties and biocompatibility of BA-based compounds has inspired the exploration of novel chemistries using boron to fuel emergent sciences. This topical review focuses on the recent progress of iminoboronate and salicylhydroxamic–boronate constituted reversible click chemistries in the past decade. We highlight the mechanism of reversible kinetics and its applications in chemical biology, medicinal chemistry, biomedical devices, and material chemistry. This article also emphasizes the fundamental reactivity of these two conjugate chemistries with assorted nucleophiles at variable pHs, which is of utmost importance to any stimuli-responsive biological and material chemistry explorations.

Fundamental progress, current developments, and rapidly growing applications of iminoboronate and salicylhydroxamic–boronate conjugate esters are deliberated.     

STUDIES ON HUMAN LENSES: II. DISTRIBUTION AND SOLUBILITY OF FLUORESCENT PIGMENTS IN CATARACTOUS AND NON-CATARACTOUS LENSES OF INDIAN ORIGIN

Abstract— Fluorometric studies of cataractous and non-cataractous human lenses were carried out to study the emission characteristics and the distribution and solubility of lenticular pigments. Most of the detected fluorophores were well distributed over the cortical and nuclear portion of the lens. The decrease in solubility of proteins with aging and cataract formation is concomitant with increasing photolysis of tryptophan. However, this is likely a phenomenon independent of the photochemical transformations of the lens proteins. The number of emitting species in the diseased lenses are higher than in the normal mature lenses. A species emitting around 375 or 388 nm is of particular interest (λ_cx, 330 nm) in that the emission characteristics of this fluorophore resemble kynurenic acid which has a high photosensitizing efficiency. The concentration of fluorescent pigments in the lenses of Indian origin is significantly high. The intense pigmentation could be attributed largely to the formation of photoproducts in the absence of normal endogenous antioxidant accumulation that is dependent on nutrition standard. If, indeed, any of these fluorescent pigments, because of their photosensitizing ability, are responsible for lenticular opacity, it is not the abundance of sunlight alone but also malnutrition that could account for the high incidence of cataract in India.     

SL(2,C)-gauge theory,N=1 supergravity and torsion

It is shown that the SL(2,C)-gauge theory of gravitation may be considered to correspond toN = 1 supergravity and the conserved current gives rise to the Einstein-Cartan action. The torsion term here appears due to the spinorial variable, which is associated with the internal helicity giving rise to the isospin algebra from the conformal reflection group. In this sense, the internal symmetry of hadrons is found to take a dominant role in gravitational phenomena in the microlocal space-time region where the Einstein-Cartan action becomes significant.