Fractionational reprecipitation

A polysaccharide can be conveniently degraded for purposes of structural determinations in a rather simple way. The polysaccharide to be examined is dissolved in an excess of an oxygen-free solution of a base, usually saturated lime-water, and allowed to stand at 25-37° for several months. Cations are then removed from the solution with a suitable cation-exchange resin. Residual polysaccharide may be precipitated with three volumes of ethanol, and the degradation products separated by cellulose-column chromatography, or by fractional reprecipitation of their calcium salts. ... 

Grayson et al. (10) investigated untreated, vacuum-baked at 120°C, and fractionally reprecipitated PS. They analyzed the various samples for indigenous volatile content by vaporization-gas chromatography/mass spectrometry (33,36,37) and found that fractional reprecipitation was the only effective method to remove the volatile fraction from PS. Further, stress MS experiments with PS samples prepared from the fractionally reprecipitated polymer evolved only a trace of styrene monomer. 

A fractional reprecipitation procedure involving a solution of the polymer in acetone, followed by reprecipitation with light petroleum can be used to separate additives from acrylics. The acetone - light petroleum extract can be used for the determination of plasticisers and lubricants (Figure 2.2). 

The mixture is taken up with water and the base is extracted from the toluene with dilute hydrochloric acid. The hydrochloric solution is rendered alkaline with caustic soda, the base is separated with ether, dried, and after distillation of the ether fractionated in vacuo, BP at 0.05 mm Hg, 150° to 153°C. The basic ether is then dissolved in dry ether, and ether saturated with dry hydrogen chloride is added dropwise with stirring. An excess of hydrogen chloride must be avoided as it may produce decomposition to the corresponding diphenyl ethylene. The ether-moist hydrochloride is preferably dried at once in vacuo and subsequently reprecipitated from acetone-ether and then again dried in vacuo over phosphorus pentoxide. Hydrochloride, MP 12B°C. 

Reprecipitation from acetone/pentane is repeatedly effected until the condensation product suits in flaky form. Further purification is effected in that the crude product is chromatographed on silica gel. The fractions which are uniform in accordance with thin layer chromatography are combined and yield crystals from absolute alcohol. Pure 4 -demethylepipo-dophyllotoxin-/3-D-thenylidene glucoside has a melting point of 242°C to 246°C (last residue up to 255°C). 

When 3(4)-substituted thiophenes (e.g., 3-hexylthiophene) are used as monomers, the polymers are partly or completely soluble in low polar organic solvents like toluene, chloroform, dichloromethane, THF. Therefore, after washing with ethanol, the polymer is dried, then dissolved in chloroform and reprecipitated into methanol or acetone to remove low-molecular-weight fractions. The solid is collected by filtration, washed with methanol, and dried in vacuo. 

Tris-HCl (120 gL), CaCl2 (60 gL), BSA (60 gL), DTT (3 gL), water (87 /iL) and freshly thawed microsomal fraction (30 gL) were mixed well and divided into three reaction tubes one reaction and two control tubes (1 and 2). 40 uL of freshly prepared UDPGA was added to the reaction tube and control 1 and 40 yfL of water to the control 2. The tubes were pre-incubated for 2 min in a water bath shaker at 35 °C. Then, 20 yiL of freshly prepared solutions of substrate in 20 % DMSO was added to the reaction and control 2 tubes, as well as 20 /rL of water to the control 1 mbe. Aliquots of 50 /rL were withdrawn from aU tubes and were quenched in acidified ice-cold methanol (160 gL per each 50 gL aliquot). The quenched aliquots collected during the incubation were left on ice for at least 20 min. After centrifugation for 10 min at 12000 rpm, the supernatants were collected into glass mbes and the remaining proteinaceous pellets were washed twice with 50 /iL of water and reprecipitated with 150 /iL of ice-cold methanol. The combined supernatants were dried under N2 gas and reconstimted into 50 /iL of appropriate mobile phase. 

Seven primary cellulose triacetate fractions were Isolated by this method. The first primary fraction rich in hemlcellulose was redissolved and reprecipitated into three subfractions in the same way as described above. The refractionation of the first fraction was necessary to isolate the hemlcellulose material for subsequent analysis and characterisation. 

The mucin, after filtration through a bacterial filter, was treated with a 1 % solution of sodium chloride and acetic acid at 96° and the active substance was obtained by fractional precipitation with alcohol. It was then purified by reprecipitation with alcohol in the presence of sodium acetate or with acetic acid. DeproteLnization by... 

Grohn and Vasiliu-Oprea (18,31,32,41) studied the grafting of several vinyl monomers on polyamide, mainly polycaprolactam. [Their work was summarized in the Simionescu and Vasiliu-Oprea book (73).] In each case, the polymer was first purified by extraction, dissolution and reprecipitation. The grinding was performed at 0.42 fraction packing and by using 16 mm diameter steel balls, at an amplitude of 1.75 mm and at 1420 rpm. With vinyl chloride the air was eliminated from the reaction jar and the monomer was introduced at up to two... 

The main difficulty with the classical fractionation process is that, as the fractionation progresses, the number of fractions increases, and their size becomes smaller. With fractional precipitation processes, the number of operations practicable is much smaller than the fractional crystallization, because of the trouble in redisolving the precipitates and following another reprecipitation. In fractional crystallization schemes sometimes the liquor and crystal fractions can be combined with the help of modem analytical techniques by determining their compositions, thus achieving multiplication of stages. 

Solubility behavior of the copolymers in methylene chloride was examined by preparing a 103 solution in warm methylene chloride and allowing the solution to stand for two days at room temperature. The precipitate, if any, was filtered off under nitrogen pressure using a 0.45/t Millipore filter, washed with cold methylene chloride, and dried under vacuum. The soluble fraction was recovered by precipitation with methanol. A similar procedure was followed with m-xylene a 10% solution of the copolymer was heated overnight on a steam bath, the precipitate filtered off, dissolved in chloroform, and reprecipitated with methanol. 

Under the pressure change option, the user can specify a lower and upper pressure range and an incremental pressure interval (AP) at which equilibrium is calculated at a fixed temperature (e.g., 1 to 1001 bars with AP = 100 bars would result in equilibrium calculations at 1, 101,. ..and 1001 bars). Under equilibrium crystallization , solid phases that have precipitated are allowed to dissolve and repreciptate as different solids when environmental drivers such as temperature, pressure, or evaporation change. Under fractional crystallization , a precipitated solid phase is not allowed to subsequently dissolve and reprecipitate when environmental drivers change this option allows for hypothetical removal (layering) of precipitates in basins when environmental drivers change. 

Since pH 5 Supernatant was active in the transfer of sRNA-bound amino acids, it suggested that both of the essential fractions described above were also present in this crude soluble preparation. Experimental verification of this suggestion is presented in Table V (8). Fractionation of the pH 5 Supernatant with ammonium sulfate yielded two fractions, 0 to 35% A.S. residue and 35 to 60% A.S. residue, which by themselves catalyzed aminoacyl transfer in the presence of glutathione. Reprecipitation of the 0 to 35% A.S. fraction, as described in Figure 1,... 

Table I reports the results of typical polymerization runs of pentadiene by cobalt catalysts prepared from Al(C2H5)Cl2 complexed with thiophene or pyridine. The crude polymerization products obtained by these systems have a cis-1,4 content of about 75-80%. Fractions having a higher cis-1,4 content (about 85%) could be isolated by dissolving the crude polymers in benzene and reprecipitating with methylethyl ketone (MEK). This solvent dissolves only the low molecular weight polymers, which, in this case, have also a low cis-1,4 unit content.

The crude polymerization products obtained from the trans isomer have a cis-1,4 unit content of about 75%. Fractionation of the crude products by dissolving them in benzene and reprecipitating with MEK has permitted isolation of fractions (insoluble in MEK) having a cis-1,4 unit content of about 85%. 

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