Abstract Design and test of clay-based formulations of pesticides for solving environmental and e... more Abstract Design and test of clay-based formulations of pesticides for solving environmental and economical problems are described. Organoclays were mainly designed to promote the adsorption of neutral and hydrophobic pesticides and slow their release. Adsorption of organic cations modifies the nature of the clay mineral surface, transforming it from hydrophilic to hydrophobic. The modified clay mineral surface can have enhanced affinity for adsorbing neutral organic molecules of hydrophobic characteristics. The adsorption of the hydrophobic herbicides alachlor, metolachlor norflurazon, and acetochlor, which include a phenyl ring, was maximal for montmorillonite preadsorbed by a small cation, for example, phenyl trimethylammonium at a loading corresponding to 5/8 of the cation-exchange capacity (CEC). Loading of the organic cations above the CEC of the clay can promote the adsorption of certain anionic herbicides, such as imazaquin. Reduction of volatilization and photodegradation of herbicides was also achieved by certain organoclay formulations. In certain cases, an organic cation adsorbed on the clay mineral can act as an energy acceptor of the photoexcited molecule of the pesticide, which returns to its ground state before its photodecomposition occurs, thus becoming photostabilized. Clay mineral–micelle and –liposome formulations were introduced for obtaining slow release formulations of certain anionic herbicides, which could not be achieved by organoclay ones. The procedure involves incubation of the clay mineral with organic cations, which are mostly in micelles or liposomes. The complex formed between ODTMA (octadecyl trimethylammonium) and montmorillonite in the presence of excess of micelles is very different from the complex formed in the exclusive presence of ODTMA monomers, as shown by electron microscopy, XRD, and adsorption measurements. Unlike the monomer–clay mineral complex, which was not efficient for the adsorption of anionic organic molecules, such as sulfometuron, the micelle–clay mineral complex was highly efficient. Liposome–clay mineral formulations were prepared by employing the positively charged didodecylammonium and the neutral and EPA approved phosphatidylcholine. Efforts to develop slow release formulations also focused on encapsulation of herbicides in clay mineral polymer nanocomposites. The efficacy of the bypiridil herbicides paraquat (PQ) and diquat (DQ), which are divalent organic cations, used for post-emergence weed control was enhanced by addition to the herbicide formulation of monovalent organic cations which could compete for adsorption to the dust with DQ and PQ, thus making them more available for herbicidal activity.
Naproxen, a non-steroidal anti-inflammatory drug, commonly used for fever, inflammation and for d... more Naproxen, a non-steroidal anti-inflammatory drug, commonly used for fever, inflammation and for different health problems, as found for many pharmaceuticals has been recently detected in sewage effluents, surface, and ground water, and sometimes even in drinking water. An advanced wastewater treatment plant (WWTP), utilizing ultra-filtration, activated charcoal (AC), and reverse osmosis (RO) after the primary biological treatment, showed that both nano-and micro-ultrafiltration were not sufficient for removing spiked naproxen to a safe level, whereas RO membrane was quite efficient. No naproxen degradation was detected in pure water whereas it underwent biodegradation within three days in activated sludge giving O-desmethylnaproxen. Adsorption performed on micelle-clay complex and AC under steady state conditions, showed that the former adsorbent is highly effective in removing naproxen with fast kinetics. Laboratory micelle-clay complex filters under continuous naproxenspiked water flowing were found to be efficient in removing this drug, suggesting that the efficiency of existing advanced WWTP could be improved by including filtration columns filled with suitable sand/micelle-clay mixtures.
This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides... more This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides alachlor, metolachlor, and norflurazon, which include a phenyl ring in the structure. The adsorbed amounts of herbicides increased severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammonium (BTEA). Fourier transform infrared studies indicated interactions between alachlor molecules and adsorbed BTMA. The adsorption affinity of the herbicides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the CEC. The enhanced adsorbed amounts of herbicides are mainly due to interactions between the phenyl rings of herbicide molecules and organic cations, which are favored with the smaller cation, BTMA. BTMA preadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phenyl rings are available for interacting with herbicide molecules. This effect is small for preloading by BTEA, so that the amounts adsorbed increase with the degree of preloading. Thus, optimization of claybased herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.
The overall objective of this study was to develop, optimize and evaluate novel formulations, whi... more The overall objective of this study was to develop, optimize and evaluate novel formulations, which reduce herbicide leaching and enhance agronomic efficacy. Numerous studies have demonstrated that CsT promotes environmental quality and enhances sustainable crop production, yet continued use of CsT-practices appears threatened unless cost effective alternative weed control practices can be found. The problem is pressing in the southern portion of the Atlantic Coastal Plain region of the eastern USA where cotton and peanut are produced extensively. This research addressed needs of the region’s farmers for more effective weed control practices for CsT systems. HUJI: CRFs for sulfentrazone and metolachlor were developed and tested based on their solubilizion in cationic micelles and adsorption of the mixed micelles on montmorillonite. A better understanding of solubilizing anionic and nonionic organic molecules in cationic micelles was reached. Both CRFs demonstrated controlled release...
Photostable formulations of the herbicide norflurazon [4-chloro-5-(methylamino)-2-(R,R,R-trifluor... more Photostable formulations of the herbicide norflurazon [4-chloro-5-(methylamino)-2-(R,R,R-trifluorom-tolyl)pyridazin-3-(2H)-one] were achieved by adsorbing it on pillared clay or on montmorillonite preadsorbed with the organic cation thioflavin T (TFT). Diffuse reflectance Fourier transform infrared spectra showed the existence of strong interactions between the aromatic moieties of preadsorbed TFT and the herbicide, particularly after irradiation. The photostabilization of norflurazon obtained with TFT-clay was mainly due to energy transfer from the herbicide to the organic cation via π-π* interactions. An additional mechanism is the lower production of radicals from the clay when the clay mineral surface is covered with the organic cation. These radicals are responsible for the enhanced photodegradation observed when norflurazon was irradiated in the presence of untreated montmorillonite.
Organically modified clay minerals have been widely developed, tested and employed as sorbents fo... more Organically modified clay minerals have been widely developed, tested and employed as sorbents for organic pollutants. However, the process of pollutant-composite pairing is not commonly addressed, which would be valuable for efficient pollutant filtration by such sorbents. This study presents an approach for achieving efficient pollutant removal by large-scale composite filters, based on pairing chemically compatible pollutants and composites and by employing a predictive filtration model. The removal of three organic pollutants, simazine, sulfentrazone and diclofenac by lab-scale filtration columns containing one of three sorbents, a polymer-, micelle-or liposome-clay composite, was measured. Understanding the factors governing pollutant-organic modifier interactions enabled to pair an efficient sorbent to each pollutant. The high removal (80%) of simazine by the polymer composite, was attributed to hydrogen bonds and π-π interactions, compared to less than 20% removal by the surfactant composites. The removal of the anionic diclofenac (pKa = 4.1) was mainly governed by electrostatic attraction, explaining its high removal by the most positively charge sorbent, the liposome composite. Sulfentrazone (pKa = 6.5) removal was mostly affected by micellar solubilization and upon its removal, the zeta potential of the micelle-composite was not reduced as obtained for diclofenac removal. The filtration of the successful pairs was modelled to determine sorbent capacity and adsorption and desorption rate constants. The pilot filtration experiments were well described by the model and demonstrated efficient removal of paired pollutants and sorbents. Model simulations predicted promising treatment at environmental pollutant concentrations in the μg L −1 range. This pairing approach along with model calculations can be a strong and valid tool for efficient pollutant-sorbent filtration.
Release of organic cations from cationic micelle-clay complexes in slow release herbicide formula... more Release of organic cations from cationic micelle-clay complexes in slow release herbicide formulations or in column-filters for water purification may pose a problem. In this study we determined the release of octadecyltrimethylammonium (ODTMA) or benzyldimethylhexadecylammonium (BDMHDA) from micelle-clay complexes, and devised a method to retain the released cations by using a column filled with sand/clay mixtures. The adsorption of 5 mM ODTMA or BDMHDA on montmorillonite added at 5 g/l clay was complete. The release was examined at different times and complex concentrations. The percents of release of ODTMA after 24 h were 13.2, 3.8 and 0 for 0.1, 1 and 5 g/L, respectively, whereas for BDMHDA the corresponding values were 31.2, 19.4 and 7.2. Measurements of dissolved organic carbon (DOC) yielded the amount of the released organic cation from micelle-clay complexes in column-filters for water purification after adding a layer (13 cm) of quartz mixed with clay (150:1 w/w ratio) at the bottom of the column. The amount of the released organic cation from columns including the micelle-clay complexes was less (or equal within the experimental error) than the amount released from those including quartz alone or a quartz-clay mixture. Hence, the carbon analysis showed that in the ppm range all the released cations (ODTMA or BDTMA) were retained by the column-filter. The water passing through a 25 cm column filled with quartz mixed with micelleclay was also passed sequentially through three columns filled with quartz-clay (150:1) and was concentrated 1000-fold. Measurements by GC-MS showed no traces of the organic cations. The conclusion is that the released organic cations from micelle-clay complexes can be completely retained by adding quartz-clay layers.
The binding of Ca"+ and Jig'+ to phosphatidylserine (PS) vesicles is studied esperimentalIg; and ... more The binding of Ca"+ and Jig'+ to phosphatidylserine (PS) vesicles is studied esperimentalIg; and analyzed. The surface potential and charge density are found for any given concentration of both monovalent and divalent cations in solution from a modified GOW-CHAPMAN equation. An anaI_ytical expression is given for the amount of cations concentrated in the double-layer region, which is treated distinctly from the amount of tightly bound cations. The explanation for the binding data of Ca2+ and Big"+ to PS required taking into account the binding of Na+ to PS. The computed binding of Na+ to PS is in accord with recent results of NMR and vesicle aggregation studies. The calculated surface potentials of PS in the presence of Ca'+ and Na+ are in a reasonable agreement with the previouslv measured zeta-potentials. Our results indicate that Ca"+ has a tenfold greater intrinsic binding constant than Jig"+ for PS vesicles_ * Discussed at the IV International Symposium on Bioelectrochemistry held at Xk-oods HoIe, Mass. (U.S._\_), .2-S October 1977.
A number of amphiphiles which raise the bilayer to hexagonal phase transition temperature (TH) of... more A number of amphiphiles which raise the bilayer to hexagonal phase transition temperature (TH) of phosphatidylethanolamine (PE) have been shown to inhibit viral fusion. In this study we have further evaluated the mechanism of this inhibition. Several anionic amphiphiles, including cholesterol sulfate, a component of mammalian plasma membranes, lower the final extent of Sendai virus fusion with both human erythrocyte ghosts and liposomes composed of PE and 5% of the ganglioside, GD1a. A cationic amphiphile slightly increased the final extent of fusion. The fusion rate constant is not greatly affected by the presence of as much as 20% cholesterol sulfate or other charged amphiphiles. The zwitterionic amphiphile, cholesterol phosphorylcholine has no effect on the final extent of fusion but it lowers the fusion rate constant. This amphiphile is potent in raising TH. The amphiphile cholesterol hemisuccinate (CHEMS) stabilizes the bilayer relative to the hexagonal phase at neutral pH, while at acidic pH the formation of the hexagonal phase is promoted. When CHEMS is added to vesicles of egg PE containing 5% GD1a, the rate of Sendai virus fusion is little affected at neutral pH but the rate is significantly enhanced at pH 5.0. These results demonstrate that viral fusion can be modulated, in part, by the tendency of the membrane to convert to the hexagonal phase.
Microscopic and macroscopic approaches to calculations of long-range Van der Waals interactions b... more Microscopic and macroscopic approaches to calculations of long-range Van der Waals interactions between bodies are reviewed. Expressions are presented for various geometries, including planar-layered structures, sheathed spheres and rods. Retardation effects are shown to reduce dispersion interactions in a similar fashion in both approaches. Pair summation procedure gives 10-25% greater values of dispersion interactions than the macroscopic approach. Orientation effects, previously neglected in microscopic approaches are strongly dependent on many-body effects. When orientation effects are included in a pair summation procedure, its calculated values are close to those calculated with the macroscopic approach. Experimentally determined force values are in agreement with calculated ones for distances of separation above 15 A in vacuum. In general, the theory is insufficient for yielding forces at distances of separation below 20 A in water. Determination of Van der Waals parameters from refractive indices of pure liquids and solutions is described. Within 5%, dispersion coefficients are independent of concentration of solution, and isotropic electronic polarizabilities agree with those obtained by the addition of bond polarizabilities. Van der Waals parameters of several major components of cellular surfaces and intercellular media are arranged according to an ascending sequence: water < alkanes < phospholipids < proteins and cholesterol < sugars. Variations in compositions, distances of separation, and layer thicknesses are considered in the calculation of the interactions between cellular surfaces, both in planar and spherical systems, including phospholipid vesicles. In planar-cellular systems, Hamaker coefficients vary between 4 × 10 15 and 6 × 10 14 erg; at 50 A distance of separation the free energies and forces are 210 to 1600 kT/ixm 2, and 4 × 10 ~ to 3 × 10-'dynes/tzm 2 respectively. The total potential curve, including electrostatic interactions, is calculated and the questions of cellular adhesion and fusion of phospholipid vesicles are discussed. Van der Waals interactions between surfaces of biological interest uncertainty in Van der Waals energies, which may be expected with a given set of data. B. Biological applications One of the main purposes of this review is to provide a means for estimating Van der Waals energies and forces of interaction between macroscopic bodies of biological interest, e.g., cell-cell; cell-substratum; cell-virus; cell-vesicle, etc. For this purpose, we present energy and force expressions for many geometries of interest which can approximate the real system. We also outline the way to treat the interaction between bodies of irregular shapes. The article contains an extensive table of Van der Waals parameters of several substances which compose the cell surfaces and substrata. These include water, several sugars, phospholipids, proteins, glass and others. Other tables give a spectrum of calculated values of free energies and forces, which illustrate their dependence on geometrical factors and composition of bodies and the medium in between them. The reader can compare these values with typical energies, say, kT, or with mechanical forces, i.e., gravitational or centrifugal forces. We will also consider the total potential curve obtained by a sum of the Van der Waals attractive potential, and the electrostatic repulsive potential, which is due to the fact that cells typically carry a net negative charge on their surfaces. The range of distances of separation between bodies considered here is, in general, between 20 and 500 ~, and this restriction should be taken into account in applications, although a certain extension of the range in both directions might be permissible. The upper bound corresponds to the case when energies and forces of interaction are of the same order of magnitude of kT, and mg, respectively, in the case of apposed planar surfaces of an area of 1/~m 2. The lower bound was arbitrarily chosen to indicate that the treatment is applicable when the distances of separation between bodies are large compared with the dimensions of atoms or chemical bonds, so that macroscopic field equations can be applied. Even if the lower range of applicability can be extended down, say, to 10 A, it is still beyond the range where chemical bonds or hydrogen bonds can be effective. The restriction on the lower bound implies that, within the formalism discussed here, it is not possible to treat the question of adhesion or separation between biological bodies at the molecular level. Hence, a quantitative discussion of specific interactions on the molecular level is generally beyond the scope of this review. Recently, there has been significant progress regarding the question of cohesion <16' 1s' 33) and the study of conformations ~3.~°> of molecules or of molecular associations in solutions, c41' 42> In order to extend the study in this direction, detailed information, which is presently lacking, would be required on the arrangement and conformations of such specific molecules on the cell surface. It should be
A kinetic model for loading of proto-tRNA is presented. The kinetic parameters were first estimat... more A kinetic model for loading of proto-tRNA is presented. The kinetic parameters were first estimated from the results of Francklyn &amp; Schimmel (1989;Nature337, 478--481), who studied the aminoacylation of both tRNA and its minihelix. Then these parameters were reduced several-fold, as is more appropriate for the prebiotic world. The simulations revealed a very slow time course of the loading reaction. We also consider a possibility for the proto-tRNA loading without a catalyst and discuss the feasibility of such processes. Analytical approximations are presented for the kinetics of proto-tRNA loading with and without enzyme. An estimate is given for the time required for the development of template- and sequence-directed systems.
Abstract Design and test of clay-based formulations of pesticides for solving environmental and e... more Abstract Design and test of clay-based formulations of pesticides for solving environmental and economical problems are described. Organoclays were mainly designed to promote the adsorption of neutral and hydrophobic pesticides and slow their release. Adsorption of organic cations modifies the nature of the clay mineral surface, transforming it from hydrophilic to hydrophobic. The modified clay mineral surface can have enhanced affinity for adsorbing neutral organic molecules of hydrophobic characteristics. The adsorption of the hydrophobic herbicides alachlor, metolachlor norflurazon, and acetochlor, which include a phenyl ring, was maximal for montmorillonite preadsorbed by a small cation, for example, phenyl trimethylammonium at a loading corresponding to 5/8 of the cation-exchange capacity (CEC). Loading of the organic cations above the CEC of the clay can promote the adsorption of certain anionic herbicides, such as imazaquin. Reduction of volatilization and photodegradation of herbicides was also achieved by certain organoclay formulations. In certain cases, an organic cation adsorbed on the clay mineral can act as an energy acceptor of the photoexcited molecule of the pesticide, which returns to its ground state before its photodecomposition occurs, thus becoming photostabilized. Clay mineral–micelle and –liposome formulations were introduced for obtaining slow release formulations of certain anionic herbicides, which could not be achieved by organoclay ones. The procedure involves incubation of the clay mineral with organic cations, which are mostly in micelles or liposomes. The complex formed between ODTMA (octadecyl trimethylammonium) and montmorillonite in the presence of excess of micelles is very different from the complex formed in the exclusive presence of ODTMA monomers, as shown by electron microscopy, XRD, and adsorption measurements. Unlike the monomer–clay mineral complex, which was not efficient for the adsorption of anionic organic molecules, such as sulfometuron, the micelle–clay mineral complex was highly efficient. Liposome–clay mineral formulations were prepared by employing the positively charged didodecylammonium and the neutral and EPA approved phosphatidylcholine. Efforts to develop slow release formulations also focused on encapsulation of herbicides in clay mineral polymer nanocomposites. The efficacy of the bypiridil herbicides paraquat (PQ) and diquat (DQ), which are divalent organic cations, used for post-emergence weed control was enhanced by addition to the herbicide formulation of monovalent organic cations which could compete for adsorption to the dust with DQ and PQ, thus making them more available for herbicidal activity.
Naproxen, a non-steroidal anti-inflammatory drug, commonly used for fever, inflammation and for d... more Naproxen, a non-steroidal anti-inflammatory drug, commonly used for fever, inflammation and for different health problems, as found for many pharmaceuticals has been recently detected in sewage effluents, surface, and ground water, and sometimes even in drinking water. An advanced wastewater treatment plant (WWTP), utilizing ultra-filtration, activated charcoal (AC), and reverse osmosis (RO) after the primary biological treatment, showed that both nano-and micro-ultrafiltration were not sufficient for removing spiked naproxen to a safe level, whereas RO membrane was quite efficient. No naproxen degradation was detected in pure water whereas it underwent biodegradation within three days in activated sludge giving O-desmethylnaproxen. Adsorption performed on micelle-clay complex and AC under steady state conditions, showed that the former adsorbent is highly effective in removing naproxen with fast kinetics. Laboratory micelle-clay complex filters under continuous naproxenspiked water flowing were found to be efficient in removing this drug, suggesting that the efficiency of existing advanced WWTP could be improved by including filtration columns filled with suitable sand/micelle-clay mixtures.
This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides... more This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides alachlor, metolachlor, and norflurazon, which include a phenyl ring in the structure. The adsorbed amounts of herbicides increased severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammonium (BTEA). Fourier transform infrared studies indicated interactions between alachlor molecules and adsorbed BTMA. The adsorption affinity of the herbicides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the CEC. The enhanced adsorbed amounts of herbicides are mainly due to interactions between the phenyl rings of herbicide molecules and organic cations, which are favored with the smaller cation, BTMA. BTMA preadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phenyl rings are available for interacting with herbicide molecules. This effect is small for preloading by BTEA, so that the amounts adsorbed increase with the degree of preloading. Thus, optimization of claybased herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.
The overall objective of this study was to develop, optimize and evaluate novel formulations, whi... more The overall objective of this study was to develop, optimize and evaluate novel formulations, which reduce herbicide leaching and enhance agronomic efficacy. Numerous studies have demonstrated that CsT promotes environmental quality and enhances sustainable crop production, yet continued use of CsT-practices appears threatened unless cost effective alternative weed control practices can be found. The problem is pressing in the southern portion of the Atlantic Coastal Plain region of the eastern USA where cotton and peanut are produced extensively. This research addressed needs of the region’s farmers for more effective weed control practices for CsT systems. HUJI: CRFs for sulfentrazone and metolachlor were developed and tested based on their solubilizion in cationic micelles and adsorption of the mixed micelles on montmorillonite. A better understanding of solubilizing anionic and nonionic organic molecules in cationic micelles was reached. Both CRFs demonstrated controlled release...
Photostable formulations of the herbicide norflurazon [4-chloro-5-(methylamino)-2-(R,R,R-trifluor... more Photostable formulations of the herbicide norflurazon [4-chloro-5-(methylamino)-2-(R,R,R-trifluorom-tolyl)pyridazin-3-(2H)-one] were achieved by adsorbing it on pillared clay or on montmorillonite preadsorbed with the organic cation thioflavin T (TFT). Diffuse reflectance Fourier transform infrared spectra showed the existence of strong interactions between the aromatic moieties of preadsorbed TFT and the herbicide, particularly after irradiation. The photostabilization of norflurazon obtained with TFT-clay was mainly due to energy transfer from the herbicide to the organic cation via π-π* interactions. An additional mechanism is the lower production of radicals from the clay when the clay mineral surface is covered with the organic cation. These radicals are responsible for the enhanced photodegradation observed when norflurazon was irradiated in the presence of untreated montmorillonite.
Organically modified clay minerals have been widely developed, tested and employed as sorbents fo... more Organically modified clay minerals have been widely developed, tested and employed as sorbents for organic pollutants. However, the process of pollutant-composite pairing is not commonly addressed, which would be valuable for efficient pollutant filtration by such sorbents. This study presents an approach for achieving efficient pollutant removal by large-scale composite filters, based on pairing chemically compatible pollutants and composites and by employing a predictive filtration model. The removal of three organic pollutants, simazine, sulfentrazone and diclofenac by lab-scale filtration columns containing one of three sorbents, a polymer-, micelle-or liposome-clay composite, was measured. Understanding the factors governing pollutant-organic modifier interactions enabled to pair an efficient sorbent to each pollutant. The high removal (80%) of simazine by the polymer composite, was attributed to hydrogen bonds and π-π interactions, compared to less than 20% removal by the surfactant composites. The removal of the anionic diclofenac (pKa = 4.1) was mainly governed by electrostatic attraction, explaining its high removal by the most positively charge sorbent, the liposome composite. Sulfentrazone (pKa = 6.5) removal was mostly affected by micellar solubilization and upon its removal, the zeta potential of the micelle-composite was not reduced as obtained for diclofenac removal. The filtration of the successful pairs was modelled to determine sorbent capacity and adsorption and desorption rate constants. The pilot filtration experiments were well described by the model and demonstrated efficient removal of paired pollutants and sorbents. Model simulations predicted promising treatment at environmental pollutant concentrations in the μg L −1 range. This pairing approach along with model calculations can be a strong and valid tool for efficient pollutant-sorbent filtration.
Release of organic cations from cationic micelle-clay complexes in slow release herbicide formula... more Release of organic cations from cationic micelle-clay complexes in slow release herbicide formulations or in column-filters for water purification may pose a problem. In this study we determined the release of octadecyltrimethylammonium (ODTMA) or benzyldimethylhexadecylammonium (BDMHDA) from micelle-clay complexes, and devised a method to retain the released cations by using a column filled with sand/clay mixtures. The adsorption of 5 mM ODTMA or BDMHDA on montmorillonite added at 5 g/l clay was complete. The release was examined at different times and complex concentrations. The percents of release of ODTMA after 24 h were 13.2, 3.8 and 0 for 0.1, 1 and 5 g/L, respectively, whereas for BDMHDA the corresponding values were 31.2, 19.4 and 7.2. Measurements of dissolved organic carbon (DOC) yielded the amount of the released organic cation from micelle-clay complexes in column-filters for water purification after adding a layer (13 cm) of quartz mixed with clay (150:1 w/w ratio) at the bottom of the column. The amount of the released organic cation from columns including the micelle-clay complexes was less (or equal within the experimental error) than the amount released from those including quartz alone or a quartz-clay mixture. Hence, the carbon analysis showed that in the ppm range all the released cations (ODTMA or BDTMA) were retained by the column-filter. The water passing through a 25 cm column filled with quartz mixed with micelleclay was also passed sequentially through three columns filled with quartz-clay (150:1) and was concentrated 1000-fold. Measurements by GC-MS showed no traces of the organic cations. The conclusion is that the released organic cations from micelle-clay complexes can be completely retained by adding quartz-clay layers.
The binding of Ca"+ and Jig'+ to phosphatidylserine (PS) vesicles is studied esperimentalIg; and ... more The binding of Ca"+ and Jig'+ to phosphatidylserine (PS) vesicles is studied esperimentalIg; and analyzed. The surface potential and charge density are found for any given concentration of both monovalent and divalent cations in solution from a modified GOW-CHAPMAN equation. An anaI_ytical expression is given for the amount of cations concentrated in the double-layer region, which is treated distinctly from the amount of tightly bound cations. The explanation for the binding data of Ca2+ and Big"+ to PS required taking into account the binding of Na+ to PS. The computed binding of Na+ to PS is in accord with recent results of NMR and vesicle aggregation studies. The calculated surface potentials of PS in the presence of Ca'+ and Na+ are in a reasonable agreement with the previouslv measured zeta-potentials. Our results indicate that Ca"+ has a tenfold greater intrinsic binding constant than Jig"+ for PS vesicles_ * Discussed at the IV International Symposium on Bioelectrochemistry held at Xk-oods HoIe, Mass. (U.S._\_), .2-S October 1977.
A number of amphiphiles which raise the bilayer to hexagonal phase transition temperature (TH) of... more A number of amphiphiles which raise the bilayer to hexagonal phase transition temperature (TH) of phosphatidylethanolamine (PE) have been shown to inhibit viral fusion. In this study we have further evaluated the mechanism of this inhibition. Several anionic amphiphiles, including cholesterol sulfate, a component of mammalian plasma membranes, lower the final extent of Sendai virus fusion with both human erythrocyte ghosts and liposomes composed of PE and 5% of the ganglioside, GD1a. A cationic amphiphile slightly increased the final extent of fusion. The fusion rate constant is not greatly affected by the presence of as much as 20% cholesterol sulfate or other charged amphiphiles. The zwitterionic amphiphile, cholesterol phosphorylcholine has no effect on the final extent of fusion but it lowers the fusion rate constant. This amphiphile is potent in raising TH. The amphiphile cholesterol hemisuccinate (CHEMS) stabilizes the bilayer relative to the hexagonal phase at neutral pH, while at acidic pH the formation of the hexagonal phase is promoted. When CHEMS is added to vesicles of egg PE containing 5% GD1a, the rate of Sendai virus fusion is little affected at neutral pH but the rate is significantly enhanced at pH 5.0. These results demonstrate that viral fusion can be modulated, in part, by the tendency of the membrane to convert to the hexagonal phase.
Microscopic and macroscopic approaches to calculations of long-range Van der Waals interactions b... more Microscopic and macroscopic approaches to calculations of long-range Van der Waals interactions between bodies are reviewed. Expressions are presented for various geometries, including planar-layered structures, sheathed spheres and rods. Retardation effects are shown to reduce dispersion interactions in a similar fashion in both approaches. Pair summation procedure gives 10-25% greater values of dispersion interactions than the macroscopic approach. Orientation effects, previously neglected in microscopic approaches are strongly dependent on many-body effects. When orientation effects are included in a pair summation procedure, its calculated values are close to those calculated with the macroscopic approach. Experimentally determined force values are in agreement with calculated ones for distances of separation above 15 A in vacuum. In general, the theory is insufficient for yielding forces at distances of separation below 20 A in water. Determination of Van der Waals parameters from refractive indices of pure liquids and solutions is described. Within 5%, dispersion coefficients are independent of concentration of solution, and isotropic electronic polarizabilities agree with those obtained by the addition of bond polarizabilities. Van der Waals parameters of several major components of cellular surfaces and intercellular media are arranged according to an ascending sequence: water < alkanes < phospholipids < proteins and cholesterol < sugars. Variations in compositions, distances of separation, and layer thicknesses are considered in the calculation of the interactions between cellular surfaces, both in planar and spherical systems, including phospholipid vesicles. In planar-cellular systems, Hamaker coefficients vary between 4 × 10 15 and 6 × 10 14 erg; at 50 A distance of separation the free energies and forces are 210 to 1600 kT/ixm 2, and 4 × 10 ~ to 3 × 10-'dynes/tzm 2 respectively. The total potential curve, including electrostatic interactions, is calculated and the questions of cellular adhesion and fusion of phospholipid vesicles are discussed. Van der Waals interactions between surfaces of biological interest uncertainty in Van der Waals energies, which may be expected with a given set of data. B. Biological applications One of the main purposes of this review is to provide a means for estimating Van der Waals energies and forces of interaction between macroscopic bodies of biological interest, e.g., cell-cell; cell-substratum; cell-virus; cell-vesicle, etc. For this purpose, we present energy and force expressions for many geometries of interest which can approximate the real system. We also outline the way to treat the interaction between bodies of irregular shapes. The article contains an extensive table of Van der Waals parameters of several substances which compose the cell surfaces and substrata. These include water, several sugars, phospholipids, proteins, glass and others. Other tables give a spectrum of calculated values of free energies and forces, which illustrate their dependence on geometrical factors and composition of bodies and the medium in between them. The reader can compare these values with typical energies, say, kT, or with mechanical forces, i.e., gravitational or centrifugal forces. We will also consider the total potential curve obtained by a sum of the Van der Waals attractive potential, and the electrostatic repulsive potential, which is due to the fact that cells typically carry a net negative charge on their surfaces. The range of distances of separation between bodies considered here is, in general, between 20 and 500 ~, and this restriction should be taken into account in applications, although a certain extension of the range in both directions might be permissible. The upper bound corresponds to the case when energies and forces of interaction are of the same order of magnitude of kT, and mg, respectively, in the case of apposed planar surfaces of an area of 1/~m 2. The lower bound was arbitrarily chosen to indicate that the treatment is applicable when the distances of separation between bodies are large compared with the dimensions of atoms or chemical bonds, so that macroscopic field equations can be applied. Even if the lower range of applicability can be extended down, say, to 10 A, it is still beyond the range where chemical bonds or hydrogen bonds can be effective. The restriction on the lower bound implies that, within the formalism discussed here, it is not possible to treat the question of adhesion or separation between biological bodies at the molecular level. Hence, a quantitative discussion of specific interactions on the molecular level is generally beyond the scope of this review. Recently, there has been significant progress regarding the question of cohesion <16' 1s' 33) and the study of conformations ~3.~°> of molecules or of molecular associations in solutions, c41' 42> In order to extend the study in this direction, detailed information, which is presently lacking, would be required on the arrangement and conformations of such specific molecules on the cell surface. It should be
A kinetic model for loading of proto-tRNA is presented. The kinetic parameters were first estimat... more A kinetic model for loading of proto-tRNA is presented. The kinetic parameters were first estimated from the results of Francklyn &amp; Schimmel (1989;Nature337, 478--481), who studied the aminoacylation of both tRNA and its minihelix. Then these parameters were reduced several-fold, as is more appropriate for the prebiotic world. The simulations revealed a very slow time course of the loading reaction. We also consider a possibility for the proto-tRNA loading without a catalyst and discuss the feasibility of such processes. Analytical approximations are presented for the kinetics of proto-tRNA loading with and without enzyme. An estimate is given for the time required for the development of template- and sequence-directed systems.
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Papers by S. Nir