Half-of-the-sites reactivity of F235C {lambda}-repressor: implications for the structure of the whole repressor

A site-directed mutation, F235C, was created at the penultimateresidue of the -repressor. Measurement of dimer–monomerdissociation constant suggested that dimer–monomer dissociationof the mutant repressor is similar to that of the wild-type.Affinity towards a single operator O_R1 is also similar to thatof the wild-type repressor. The mutant repressor gene in a multi-copyplasmid confers immunity towards infection by a cI^- lambda phage,suggesting preservation of functional integrity. Far-UV circulardichroism spectra show no major change in the secondary structure.Fluorescence quenching experiments, however, suggest increasedexposure of some tryptophan residues. The urea denaturationprofile indicates decreased stability of a part of the C-terminaldomain. Under non-denaturing conditions, cysteine-235 showshalf-of-the-sites reactivity, i.e. on average only one out oftwo cysteine-235 residues in the dimer shows reactivity towardssulfhydryl reagents. Fluorescence energy transfer between randomlylabeled donor and acceptor fluorescent probes indicates thatonly one sulfhydryl per dimer is reactive, suggesting true half-of-the-sitesreactivity. The structural role of the C-terminal tail in thewhole repressor dimer is discussed.     

Studies on semi‐interpenetrating polymer network based on nitrile rubber and poly(methyl methacrylate)

Sequential interpenetrating polymer networks (IPNs) based on nitrile rubber and poly(methyl methacrylate) (PMMA) were synthesized. IPN compositions were varied by varying the swelling time. Two methods were adopted for making IPNs. The first method is “single‐step IPN” (SIPN) and the second method is “multistep IPN” (MIPN). The compositions were fixed around 90, 80, 70, 60 and 50% of NBR. In SIPN mode, swelling in monomer and subsequent curing was done once. In MIPN mode, swelling in monomer and curing was repeatedly done. Tensile strength of IPNs was found to increase with PMMA content, MIPN showing higher strength compared to SIPN. Dynamic modulus showed a similar trend. The tan δ value was found to decrease with PMMA content. At 62/38 nitrile rubber (NBR)/PMMA, MIPN composition isolated tan δ peaks appeared near glass transition temperatures of NBR and PMMA, respectively. Scanning electron micrograph showed phase‐separated morphology at the same MIPN composition. Solvent resistance increased with IPN formation maintaining higher resistance for MIPN compared to SIPN. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 354–360, 2004     

Pore Structure of Silica Gel

The effect of washing on the texture of silica gel has been investigated in terms of parameters such as porosity, pore size distribution, surface area and absorption of moisture. It was observed that if silica gel (before activation) is washed with water containing dissolved ions such as Ca and Mg wider pores are formed at the expense of smaller ones. This phenomenon has been explained in terms of change in the size distribution of gel particles which is brought about by solution of very fine particles followed by growth, through condensation, of larger particles. The experimental observations have been interpreted to suggest guidelines to control the texture of gels by washing with solutions comaining different cations.     

Polymerization behavior of an organophosphorus monomer for use in dental restorative materials

Vibrational overtone activated polymerization of acrylonitrile (AN) has been demonstrated using two initiators, benzoyl peroxide (BP), and 2,2′‐(azobis)isobutyronitrile (AIBN). Excitation of the fifth vibrational overtone of the CH stretch of AIBN at 627 nm and BP at 604 nm initiates the reaction. Monomer conversions were monitored by a gravimetric method. In both cases, the wavelength selectivity was investigated by irradiating the monomer/initiator mixtures at the absorption maximum of the initiator, the absorption maximum of the monomer, and at a wavelength (580 nm for AN/AIBN and 625 nm for AN/BP) where neither the initiator nor the monomer absorbs light. For the AN/AIBN mixture, after 48 h the monomer conversion for the irradiation on the peak absorption of AIBN (627 nm) is about twice as large as the irradiation at 580 nm. For the AN/BP mixture, after 48 h the monomer conversion for the irradiation on the peak absorption of BP (604 nm) is about a factor of 2.2 larger than for the irradiation at 625 nm. The overall quantum yields of both polymerizations were estimated. After 48 h the overall quantum yield for the AN/AIBN mixture irradiated at the initiator absorption (Φ₆₂₇ = 21083) is about 10 times larger than for the sample irradiated at the pure monomer absorption (Φ₅₉₅ = 1942). For the AN/BP mixture, the 604 nm quantum yield (Φ₆₀₄ = 1096) is about 2.4 times larger than the 595 nm quantum yield (Φ₅₉₅ = 448). The influence of the initiator concentration is also presented. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 565–571, 2003     

Physicochemical Characterization of Chrysin‐Derivative‐Loaded Nanostructured Lipid Carriers with Special Reference to Anticancer Activity

Homologues long‐chain chrysin derivatives (LCD, C _n: 8–18) were synthesized and incorporated into nanostructured lipid carriers (NLC) with the aim to treat human neuroblastoma. Mutual miscibility and attractive interactions among the NLC components, namely tripalmitin (TP), cetyl palmitate (CP), oleic acid (OA), and the chrysin (CHR) derivatives (LCD) at the air–water interface were assessed by the Langmuir monolayer approach. Optimum combination for the NLC formulations was found to be 2:2:1 (M/M/M) for TP/CP/OA, respectively. NLC formulations, both in the absence and presence of LCD, were characterized by combined dynamic light scattering, electron microscopy, atomic force microscopy, and differential scanning calorimetry. The size and zeta potential of the NLC formulations were found in the range 200–350 nm and ?12 to ?18 mV, respectively. Encapsulation efficiency and release kinetics of CHR and LCD when loaded into NLC were also evaluated. LCD exhibited maximum incorporation, drug‐loading capacity, and sustained release because of its enhanced hydrophobicity. Superior incorporation efficiency and sustained‐release profile of LCD were able to enhance their anticancer activity against human neuroblastoma cell lines, compared to CHR, making them promising agents in combating cancer.     

Circular footings resting on geotextile-reinforced sand bed

The note pertains to an experimental study made on circular footings resting on semi-infinite layer of sand reinforced with geotextiles. Using the concept of homogenization of such soils, both analytical and numerical analyses have also been conducted to predict the load-settlement behavior and compared with experimental observations. The study highlights the effect of the footing size, number of reinforcing layers, reinforcement placement pattern and bond length and the relative density of the soil on the load-settlement characteristics of the footings.     

Network-Wide Optimal Scheduling of Transit Systems Using Genetic Algorithms

The primary objective of any transit system is to provide a better level of service to its passengers. One of the good measures of level of service is the waiting time of passengers during their journey. The waiting time consists of an initial waiting time (the time a passenger waits to board a vehicle at his or her point of origin) and a transfer time (the time a passenger waits at a transfer station while transferring from one vehicle to another). An efficient schedule minimizes the overall transfer time (TT) of passengers transferring between different routes as well as the initial waiting time (IWT) of the passengers waiting to board the vehicle at their point of origin. This paper uses genetic algorithm (GA)—a search and optimization procedure—to find optimal/near-optimal schedules of vehicles in a transit network. The main advantage of using GA is that the transit network scheduling problem can be reformulated in a manner that is computationally more efficient than the original problem. Further, the coding aspect of GA inherently takes care of most of the constraints associated with the scheduling problem. Results from a number of test problems show that GAs are able to find optimal/near-optimal schedules with minimal computational resources.     

The preparation of high conversion polymeric systems containing eugenol residues and their rheological characterization

Copolymeric systems bearing eugenol covalently linked to the macromolecular chains have been prepared and characterised in terms of thermal, rheological and in vitro behaviour. Eugenyl methacrylate (EgMA) and ethoxyeugenyl methacrylate (EEgMA) were polymerized and copolymerized with ethyl methacrylate (EMA) in a wide range of feed composition by bulk at high conversion. Glass transition temperature (T_g) increased with the content of EgMA in the copolymer whereas T_g decreased with the content of EEgMA. The presence of any of the eugenyl monomers in the copolymer increased the thermal stability of the materials. Rheological analysis showed that EMA-co-EgMA copolymers present a higher density of crosslinking or entanglements among the chains compared with EMA-co-EEgMA materials, and copolymers containing more than 40% EgMA exhibited a pseudo-solid like behaviour with G′ relatively independent of the frequency. The average molecular weight between cross-links (M_c) for the latter copolymers was in the order of 10⁴ Da. For EEgMA copolymers, tan δ was lower than unity and G′ showed a gradual and slight dependency with frequency over the whole range of composition. In vitro behaviour showed a higher hydrophilicity of the EEgMA containing copolymers manifested in higher water sorption values, lower water contact angle values and higher surface free energy due to an increase in polarity. All systems presented a good cytocompatibility with human fibroblasts assessed through direct and indirect assays.     

Poly(acrylic acid) modified calcium phosphate cements: the effect of the composition of the cement powder and of the molecular weight and concentration of the polymeric acid

Polymer modified calcium phosphate cements made with cement powders of varying tetracalcium phosphate [TTCP] content were prepared using two different molecular weight fractions of poly(acrylic acid) at four different concentrations. The ratio of the precursors (TTCP:DCPA) in the cement powder was found to influence the initial setting which decreased with increasing concentration of TTCP in the powder phase. It was also observed that cements derived from the higher molecular weight containing PAA yielded significantly (P < 0.05) shorter initial setting time (Ti) than cements containing the lower molecular weight, poly(acrylic acid) [GE7 PAA] The effect of the varying the TTCP content in the three different cement types PCPC-A, PCPC-B and PCPC-C showed that the trends of the compressive strength were specific to the concentration and molecular weight of the poly (acrylic acid). A 20% concentration of Glascol-E7 with a cement powder composed of an equimolar ratio of precursors (PCPC-B) resulted in optimal compressive strength within the range investigated. The TTCP content of the cement powder could also be varied to improve the diametral tensile strengths of the cements; the specific effects however, were again governed by both the concentration and molecular weight of the constituent poly (acrylic acid). The influence of TTCP on both the initial setting time and diametral tensile strength was related to the Ca ²⁺ ion concentration, which determined the rate and amount of cross-linking in the cement.     

Toughening of epoxy resin using acrylate‐based liquid rubbers

Carboxyl‐terminated poly(2‐ethylhexyl acrylate) (CTPEHA) liquid rubbers of different molecular weights and functionalities (LR‐1 to LR‐6) were synthesized by bulk and solution polymerization techniques. The liquid rubbers were characterized by nonaqueous titration, vapor pressure osmometry, and gel permeation chromatography. The CTPEHA oligomers were prereacted with the epoxy resin, and the modified epoxy networks were made by curing with an ambient‐temperature curing agent. The impact properties of the modified epoxy networks were evaluated, and the effects of molecular weight, functionality of the liquid rubber, and ductility of the matrix on the impact strength of the modified networks were investigated. The morphology of the toughening behavior was analyzed using a scanning electron microscope. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 716–723, 2000