SEARCH · PubMed Health

Results for “Solvent substitution”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources

PubMed1997

Effectiveness of eucalyptol and d-limonene as gutta-percha solvents.

Eucalyptol and d-limonene were evaluated for their ability to serve as a substitute solvent for chloroform. The amount of time required to soften and remove the gutta-percha in 72 instrumented and filled simulated root canals in epoxy blocks was measured. After preparation to the apices of the block canals with a #60 file, four different filling techniques were used. These obturations were softened with each solvent and then removed, first using a #15 Hedstrom file inserted to full working length, and then removing the remaining filling mass with a #60 reamer. The two instrument placements were timed: one for the Hedstrom file insertion to the apex and the other for the reamer to remove the filling material. Neither the different solvents nor the filling techniques had a significant effect on the times required for the H-files to reach the apex. However, the times for the reamer to remove the filling materials were effected both by the filling techniques and the solvents used.

Chloroform↗

PubMed1982

Freeze-substitution and isothermal freeze-fixation studies to elucidate the pattern of ice formation in smooth muscle at 252 K (-21 degrees C).

Taenia coli muscle was cooled to 252 K in the presence of the cryoprotectant dimethylsulphoxide, at cooling rates known to reduce viability by significantly different amounts. The reduction in viability was known to be related to ice formation. Freeze-substitution and isothermal freeze-fixation studies were carried out to determine the distribution of ice within the muscle at this temperature. Freeze-substitution using ethylene glycol was unsuccessful but a new method, using high concentrations of the cryoprotectant as the substituting solvent, was able to maintain ice configuration at this relatively high substitution temperature. The results of freeze-substitution in dimethylsulphoxide were confirmed by isothermal freeze-fixation when both techniques were conducted under identical cooling conditions. The results indicated that the functional differences produced by cooling muscle at either 0.3 K min-1 or 2 K min-1 were related to the distribution of the ice phase within the tissue.

Animals↗

PubMed2005

Effect of deuterium oxide on the thermodynamic quantities associated with phase transitions of phosphatidylcholine bilayer membranes.

The bilayer phase transitions of three kinds of phospholipids, dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC) and dihexadecylphosphatidylcholine (DHPC), in deuterium oxide (D(2)O) and hydrogen oxide (H(2)O) were observed by differential scanning calorimetry (DSC) under ambient pressure and light-transmittance measurements under high pressure. The DSC measurements showed that the substitution of H(2)O by D(2)O affected the pretransition temperatures and the main-transition enthalpies of all PC bilayers. The temperature-pressure phase diagrams for these PC bilayer membranes in both solvents were constructed by use of the data of light-transmittance measurements. Regarding the main transition of all PC bilayer membranes, there was no appreciable difference between the transition temperatures in D(2)O and H(2)O under high pressure. On the other hand, the phase transitions among the gel phases including the pretransition were significantly affected by the solvent substitution. The thermodynamic quantities of phase transitions for the PC bilayer membranes were evaluated and the differences in thermodynamic properties by the water substitution were considered from the difference of interfacial-free energy per molecule in the bilayer in both solvents. It was proved that the substitution of H(2)O by D(2)O causes shrinkage of the molecular area of phospholipid at bilayer interface due to the difference in bond strength between deuterium and hydrogen bonds and produces the great influence on the bilayer phase with the smaller area. Further, the induction of bilayer interdigitation in D(2)O turned out to need higher pressures than in H(2)O.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1990

Sodium channel activation mechanisms. Insights from deuterium oxide substitution.

Schauf and Bullock (1979. Biophys. J. 27:193-208; 1982. Biophys. J. 37:441-452), using Myxicola giant axons, demonstrated that solvent substitution with deuterium oxide (D2O) significantly affects both sodium channel activation and inactivation kinetics without corresponding changes in gating current or tail current rates. They concluded that (a) no significant component of gating current derives from the final channel opening step, and (b) channels must deactivate (during tail currents) by a different pathway from that used in channel opening. By contrast, Oxford (1981. J. Gen. Physiol. 77:1-22) found in squid axons that when a depolarizing pulse is interrupted by a brief (approximately 100 microseconds) return to holding potential, subsequent reactivation (secondary activation) is very rapid and shows almost monoexponential kinetics. Increasing the interpulse interval resulted in secondary activation rate returning towards control, sigmoid (primary activation) kinetics. He concluded that channels open and close (deactivate) via the same pathway. We have repeated both sets of observations in crayfish axons, confirming the results obtained in both previous studies, despite the apparently contradictory conclusions reached by these authors. On the other hand, we find that secondary activation after a brief interpulse interval (50 microseconds) is insensitive to D2O, although reactivation after longer interpulse intervals (approximately 400 microseconds) returns towards a D2O sensitivity similar to that of primary activation. We conclude that D2O-sensitive primary activation and D2O-insensitive tail current deactivation involve separate pathways. However, D2O-insensitive secondary activation involves reversal of the D2O-insensitive deactivation step. These conclusions are consistent with "parallel gate" models, provided that one gating particle has a substantially reduced effective valence.

Animals↗

PubMed2002

Consequences of correlated solvation on the structures and reactivities of RLi-diamine complexes: 1,2-addition and alpha-lithiation reactions of imines by TMEDA-solvated n-butyllithium and phenyllithium.

6Li and (13)C NMR spectroscopic studies were carried out on [(6)Li]n-BuLi and [(6)Li]PhLi (RLi) in toluene-d(8) containing the following diamines: N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetraethylethylenediamine, 1,2-dipyrrolidinoethane, 1,2-dipiperidinoethane, N,N,N',N'-tetramethylpropanediamine, trans-(R,R)-N,N,N',N'-tetramethylcyclohexanediamine, and (-)-sparteine. Dimers of general structure (RLi)(2)S(2) (S = chelating diamine) are formed in each case. Treatment of RLi with two different diamines (S and S') affords homosolvates (RLi)(2)S(2) and (RLi)(2)S'(2) along with a heterosolvate (RLi)(2)SS'. Relative binding constants and associated free energies for the sequential solvent substitutions are obtained by competing pairs of diamines. The high relative stabilities of certain heterosolvates indicate that solvent binding to the RLi dimer can be highly correlated. Rate studies of both the 1,2-addition of RLi/TMEDA to the N-isopropylimine of cyclohexane carboxaldehyde and the RLi/TMEDA-mediated alpha-lithiation of the N-isopropylimine of cyclohexanone reveal monomer-based transition structures, [(RLi)(TMEDA)(imine)], in all cases. The complex relationships of solvent binding constants and relative reactivities toward 1,2-additions and alpha-lithiations are discussed.

Diamines↗

PubMed2000

A comparative analysis of freon substitutes in the purification of reovirus and calicivirus.

Freon 113 (Freon) is an essential component used in some viral purification methods to separate virus from infected cell debris. With its environmental and toxic hazards, Freon's availability is limited and more tightly regulated. Several organic solvent substitutes were selected to identify a suitable Freon replacement for the purification of both cultivable reovirus and fastidious calicivirus. Reovirus was extracted from tissue cultured cells with each solvent tested and purified in cesium chloride gradients by standard techniques. Purified virions were analyzed for conservation of physical and biological properties by morphological examination and infectivity studies. The purification of calicivirus nucleic acid from stool samples using selected solvents was also examined. Solvent-extracted calicivirus RNA was reverse transcribed and quantified by polymerase chain reaction amplification of a standard diagnostic 117 bp amplicon. These studies indicated that Vertrel XF (a newly developed environmentally friendly Freon substitute) and a 7:3 mixture of isopentane/1-chlorobutane are suitable replacements. Considerations of flammability and ease of use suggest that Vertrel XF is the preferred choice as a Freon substitute for the purification of these non-enveloped viruses.

Caliciviridae↗

PubMed1983

Insensitivity of activation delays in potassium and sodium channels to heavy water in Myxicola giant axons.

Substitution of heavy water (D2O) for H2O slows the Na+ and K+ currents in Myxicola giant axons by 40-50% at 5 degrees C. In contrast, the delays in Na+ K+ activation which can be produced by hyperpolarizing pre-pulses are not affected by solvent substitution. We suggest that in the cascade of transitions leading to the opening of Na+ and K+ channels, D2O affects only the final step that establishes the aqueous pore across the membrane.

Animals↗

PubMed2003

Substrate specificity of mutants of the hydroxynitrile lyase from Manihot esculenta.

Several tryptophan128-substituted mutants of the hydroxynitrile lyase from Manihot esculenta (MeHNL) are constructed and applied in the MeHNL-catalyzed addition of HCN to various aromatic and aliphatic aldehydes as well as to methyl and ethyl ketones to yield the corresponding cyanohydrins. The mutants (especially MeHNL-W128A) are in most cases superior to the wild-type (wt) enzyme when diisopropyl ether is used as the solvent. Substitution of tryptophan128 by an alanine residue enlarges the entrance channel to the active site of MeHNL and thus facilitates access of sterically demanding substrates to the active site, as clearly demonstrated for aromatic aldehydes, especially 3-phenoxybenzaldehyde. These experimental results are in accordance with the X-ray crystal structure of MeHNL-W128A. Aliphatic aldehydes, surprisingly, do not demonstrate this reactivity dependence of mutants on substrate bulkiness. Comparative reactions of 3-phenoxybenzaldehyde with wtMeHNL and MeHNL-W128A in both aqueous citrate buffer and a two-phase system of water/methyl tert-butyl ether again reveal the superiority of the mutant enzyme: 3-phenoxybenzaldehyde was converted quantitatively into a cyanohydrin nearly independently of the amount of enzyme present, with a space-time yield of 57 g L(-1) h(-1).

Alanine↗

PubMed1984

Vapor pressure and solvent vapor hazards.

This paper discusses applications of vapor pressure to industrial hygiene. Vapor pressure is the quantitative term characterizing solvent volatility; it is also equivalent to the concentration of solvent vapor at the liquid surface. The ratio between this concentration at the source and the acceptable concentration at the breathing zone is an intrinsic property of each chemical but varies over 6 orders-of-magnitude among common organic chemicals. The name Vapor Hazard Ratio (VHR) is introduced quantifying this ratio; and its logarithm is termed the Vapor Hazard Index (VHI). Thus, while vapor pressure is directly relatable to the economic effects of solvent evaporation and to airborne concentrations, the VHI has the added component of occupational toxicity. To be used safely and/or acceptably within the workplace, vapors must be diluted from concentrations at the source by a ratio equal to or greater than the Vapor Hazard Ratio, either by general or local exhaust ventilation. It is argued that the VHI can be used currently to predict the relative hazards of vapor concentrations of proposed substitute solvents to be used in similar settings and in hazard communication programs, and in the future with vapor dilution profiles to anticipate achieving quantitative compliance levels with new processes, work practices or ventilation controls. A review of the pertinent vapor pressure literature is also provided.

Air Pollutants, Occupational↗

PubMed2004

Mechanism of the dihydroorotase reaction.

Dihydroorotase (DHO) is a zinc metalloenzyme that functions in the pathway for the biosynthesis of pyrimidine nucleotides by catalyzing the reversible interconversion of carbamoyl aspartate and dihydroorotate. A chemical mechanism was proposed on the basis of an analysis of the effects of pH, metal substitution, solvent isotope effects, mutant proteins, and alternative substrates on the enzyme-catalyzed reaction. The pH-rate profiles for the hydrolysis of dihydroorotate or thiodihydroorotate demonstrated that a single group from the enzyme must be unprotonated for maximal catalytic activity. Conversely, the pH-rate profiles for the condensation of carbamoyl aspartate to dihydroorotate showed that a single group from the enzyme must be protonated for maximal catalytic activity. The native zinc ions within the active site of DHO were substituted with cobalt or cadmium by reconstitution of the apoenzyme with divalent cations in the presence of bicarbonate. The ionizations observed in the pH-rate profiles were dependent on the specific metal ion bound to the active site. Mutation of the residue (Asp-250) that hydrogen bonds to the bridging hydroxide (or water) resulted in the loss of catalytic activity. These results are consistent with the formation of a hydroxide bridge between the two divalent cations that functions as the nucleophile during the hydrolysis of dihydroorotate. In addition, Asp-250 is postulated to shuttle the proton from the bridging hydroxide to the leaving group amide during hydrolysis of dihydroorotate. The X-ray crystal structure of DHO showed that the exocyclic alpha-carboxylate of dihydroorotate is bound to the protein via electrostatic interactions with Arg-20, Asn-44, and His-254. Mutation of these residues resulted in the loss of catalytic activity, indicating that these residues are critical for substrate recognition. The thio analogue of dihydroorotate was found to be a good substrate of the enzyme. A comprehensive chemical mechanism for DHO was proposed on the basis of the experimental findings in this study and the X-ray crystal structure.

Deuterium↗

PubMed1990

Effect of chemical fixatives on accurate preservation of Escherichia coli and Bacillus subtilis structure in cells prepared by freeze-substitution.

Five chemical fixatives were evaluated for their ability to accurately preserve bacterial ultrastructure during freeze-substitution of select Escherichia coli and Bacillus subtilis strains. Radioisotopes were specifically incorporated into the peptidoglycan, lipopolysaccharide, and nucleic acids of E. coli SFK11 and W7 and into the peptidoglycan and RNA of B. subtilis 168 and W23. The ease of extraction of radiolabels, as assessed by liquid scintillation counting during all stages of processing for freeze-substitution, was used as an indicator of cell structural integrity and retention of cellular chemical composition. Subsequent visual examination by electron microscopy was used to confirm ultrastructural conformation. The fixatives used were: 2% (wt/vol) osmium tetroxide and 2% (wt/vol) uranyl acetate; 2% (vol/vol) glutaraldehyde and 2% (wt/vol) uranyl acetate; 2% (vol/vol) acrolein and 2% (wt/vol) uranyl acetate; 2% (wt/vol) gallic acid; and 2% (wt/vol) uranyl acetate. All fixatives were prepared in a substitution solvent of anhydrous acetone. Extraction of cellular constituents depended on the chemical fixative used. A combination of 2% osmium tetroxide-2% uranyl acetate or 2% gallic acid alone resulted in optimum fixation as ascertained by least extraction of radiolabels. In both gram-positive and gram-negative organisms, high levels of radiolabel were detected in the processing fluids in which 2% acrolein-2% uranyl acetate, 2% glutaraldehyde-2% uranyl acetate, or 2% uranyl acetate alone were used as fixatives. Ultrastructural variations were observed in cells freeze-substituted in the presence of different chemical fixatives. We recommend the use of osmium tetroxide and uranyl acetate in acetone for routine freeze-substitution of eubacteria, while gallic acid is recommended for use when microanalytical processing necessitates the omission of osmium.

Bacillus subtilis↗

PubMed2007

Stability of globular proteins in H2O and D2O.

In several experimental techniques D2O rather then H2O is often used as a solvent for proteins. Concerning the influence of the solvent on the stability of the proteins, contradicting results have been reported in literature. In this paper the influence of H2O-D2O solvent substitution on the stability of globular protein structure is determined in a systematic way. The differential scanning calorimetry technique is applied to allow for a thermodynamic analysis of two types of globular proteins: hen's egg lysozyme (LSZ) with relatively strong internal cohesion ("hard" globular protein) and bovine serum albumin (BSA), which is known for its conformational adaptability ("soft" globular protein). Both proteins tend to be more stable in D2O compared to H2O. We explain the increase of protein stability in D2O by the observation that D2O is a poorer solvent for nonpolar amino acids than H2O, implying that the hydrophobic effect is larger in D2O. In case of BSA the transitions between different isomeric forms, at low pH values the Nm and F forms, and at higher pH values Nm and B, were observed by the presence of a supplementary peak in the DSC thermogram. It appears that the pH-range for which the Nm form is the preferred one is wider in D2O than in H2O.

Animals↗

PubMed2006

Effect of co-solvents on the characteristics of enkephalin microcapsules.

The objective of this project was to study the effect of the presence of co-solvents with dichloromethane on the properties of leu-enkephalin microcapsules. Six co-solvents, including ethyl acetate, methanol, ethanol, acetone, isopropanol and acetonitrile, at three concentrations of 5%, 10% and 20% (v/v), respectively, of dichloromethane were selected for this study. The resulting microcapsules were evaluated for morphology and particle size, surface area, thermal characteristics and efficiency of encapsulation. The analysis of particle size distribution showed bi- and tri-modal distribution of the microcapsules. The median particle size of the microcapsules was between 17 microm and 57 microm. All microcapsules were smaller than 90 microm. Approximately 10% of the microcapsules were smaller than 500 nm. In general, the microcapsules prepared with co-solvents showed relatively smaller median size. The microcapsules were spherical in shape. DSC analysis of the microcapsules showed that there were no significant differences in the glass transition temperatures. There were significant changes in the efficiency of encapsulation due to the addition of co-solvents. Substitution with 20% methanol resulted in 40% increase in the efficiency of encapsulation (12% vs. 17%). Furthermore, substitution with 20% ethyl acetate, isopropanol, or acetonitrile, reduced the efficiency of encapsulation to as low as 6%.

Biocompatible Materials↗

PubMed2006

Factors controlling BTEX and chlorinated solvents plume length under natural attenuation conditions.

Natural attenuation is presently used at numerous sites where groundwater is contaminated. In order to simulate this attenuation, reactive transport models are often used but they are quite complex and depend on both physical and chemical conditions in the aquifer. As complex numerical models cannot be used to study all possible cases, we develop here analytical solutions to draw general conclusions. Our strategy, called MIKSS (Mixed Instantaneous and Kinetics Superposition Sequence), allows the calculation of the concentrations of all reacting substances in a plume. It is an extension of the superimposition principle that is able to treat the case of joint kinetics and instantaneous reactions. The basic equations have been extended to treat different reactions that occur in the plume core and at its fringe. At first we consider one organic substance degraded under all oxidising conditions (toluene for instance). For this problem the size of the plume depends on the reduced source width and on the ratio of the organic substance concentration to the sum of the electron acceptors' concentrations. For several BTEX substances having different degradation behaviour the formulation is similar, but leads to quite different plume lengths for each substance. Contrary to the case of one substance, the plumes can be quite long and may not satisfy the target risk level. For chlorinated solvents we developed a specific approach to take under consideration all reactions and particularly the competition for hydrogen. A formula is given to assess the size of the plume core, i.e. the zone with highly reducing conditions. The factors influencing the core length are the same as for BTEX (source width, dispersivity, organic carbon content). The size of the TCE plume is calculated from the plume core length and the kinetic constant of TCE degradation. Using assumptions of degradation constants for DCE and VC it is also possible to calculate the longitudinal concentration profile of these substances. The degradation of moderately substituted solvents under oxic conditions reduces the size of their plumes but under these conditions TCE becomes the major threat. Among the conditions studied in this paper, very few chlorinated solvents sites can lead to a negligible risk at an acceptable distance from the source.

Aerobiosis↗

PubMed1991

Preservation of EDTA-expanded grid-mounted chromosomes and nuclei for electron microscopy using a specially designed freeze-dryer.

We describe methods for freezing and drying EDTA-expanded, fixed metaphase chromosomes and nuclei, attached to grids as whole-mounts, for transmission electron microscopy. These methods use a special apparatus that is simple to construct. While separate freezers and dryers are commercially available, one for freezing blocks of tissue by slamming them against a cold metal surface, and the other for vacuum drying the frozen tissue, our apparatus is designed for gentler, cryogenic liquid plunge freezing and drying, sequentially, in the same apparatus, thus avoiding any compression or damage to the specimen. Use of a cryoprotectant is not essential; however, good results are obtained more often when 20% ethanol is used. Freezing is accomplished by rapid propulsion of the grid, with specimens attached, into slushy N2 (-210 degrees C) within the drying chamber; drying is automatic, by either sublimation under vacuum or by solvent substitution using absolute ethanol followed by acetone, which, in turn, is removed with a critical-point dryer. The apparatus offers a means of drying chromosomes and nuclei in an expanded state, and avoids the shrinkage of these structures that occurs during stepwise passage through increasing concentrations of ethanol or acetone.

Cell Nucleus↗

PubMed1991

The use of freeze-substitution and LR gold in the study of rye grass (Lolium perenne) pollen.

Pollen grains of Lolium perenne (rye grass) were prepared for transmission electron microscopy by rapid freezing in liquid propane, substitution in acetone, methanol or diethyl ether, and embedment in the acrylic resin London Resin gold. These were compared to pollen chemically fixed (CF) in aldehyde/osmium tetroxide and embedded in the epoxy resin Quetol 651. Ultrastructural preservation was superior in freeze-substituted (FS) pollen, particularly with the use of acetone or methanol. Optimally preserved FS pollen displayed a homogeneous aspect of the cytoplasm and nucleoplasm, and smooth, uninterrupted contour or organelles. A striking difference was also seen in the preservation of inclusions in the intine. Varied forms and sizes of intine inclusions were evident in FS pollen but these were not discernible in the CF image. The FS scheme studied here presents enormous potential for both ultrastructural and immunolabelling studies in rye grass pollen. Problems discussed include artifacts associated with each of the substitution solvents used, and a gradient of freezing damage observed within the pollen grain.

Acrylic Resins↗

PubMed1995

Interaction stabilizing tertiary structure of bacteriorhodopsin studied by denaturation experiments.

The structural stability of bacteriorhodopsin was studied by denaturation experiments, using aliphatic alcohol as denaturants. The disappearance of a positive peak at 285 nm of the circular dichroism spectra, the change in the intrinsic fluorescence decay time, and the decrease of the regeneration activity bacteriorhodopsin indicated the denaturation of the tertiary structure of this protein at a methanol concentration of about 3 M. The circular dichroism band at 222 nm was unchanged by the denaturation. It was concluded that the alcohol-denatured state in water was similar to the molten globule state of soluble proteins, in which only the tertiary structure was destroyed. Solvent substitution from water to hexane did not cause denaturation of bacteriorhodopsin. However, further addition of alcohol destroyed the secondary as well as the tertiary structures. Comparing the alcohol effects of bacteriorhodopsin in water to that in hexane, the dominant interactions for the structure formation of this protein could be revealed: the hydrophobic interaction that arose from the structure of water is essential for the stability of membrane spanning helices, while the interaction which binds the helices is polar in nature.

Alcohols↗

PubMed2005

Biomonitoring of 2-(2-alkoxyethoxy)ethanols by analysing urinary 2-(2-alkoxyethoxy)acetic acids.

2-Methoxyacetic and 2-ethoxyacetic acids are well known toxic metabolites of 2-alkoxyethanols. The use of 2-alkoxyethanols is now restricted, and the regulations have forced manufacturers to find substitutive solvents, 2-(2-alkoxyethoxy)ethanols. 2-(2-Alkoxyethoxy)ethanols resemble 2-alkoxyethanols, and their most hazardous similarity is their ability to metabolize to the 2-(2-alkoxyethoxy)acetic acids. In the present study, floor lacquerers' (n = 22) inhalation and total exposure to 2-(2-alkoxy)ethoxyethanols was measured. The measurements of inhalation exposure were done with charcoal tubes, and total exposure was biomonitored by urinalysis of 2-(2-alkoxyethoxy)acetic acids. The 8h inhalation exposures of floor lacquerers to 2-(2-methoxyethoxy)ethanol (DEGME), 2-(2-ethoxyethoxy)ethanol (DEGEE) and 2-(2-butoxyethoxy)ethanol (DEGBE) were in average 0.23 +/- 0.07 ppm (average+/-S.D., n = 3), 0.08 +/- 0.07 ppm (n = 16), and 0.05 +/- 0.03 ppm (n = 16), respectively. The excretions of 2-(2-methoxyethoxy)acetic acid (MEAA), 2-(2-ethoxyethoxy)acetic acid (EEAA) and 2-(2-butoxyethoxy)acetic acid (BEAA) were in average 4.9 +/- 4.3 mmol/mol creatinine, 9.3 +/- 8.0 mmol/mol creatinine and 9.2 +/- 7.4 mmol/mol creatinine, respectively. A linear relationship was found between the urinary 2-(2-alkoxyethoxy)acetic acid concentrations and the preceding 8-h occupational exposure to 2-(2-alkoxyethoxy)ethanol.

Acetates↗