Fractionation by Selective Precipitation

When a cold, dilute, autoclaved starch sol is overlaid with n-butyl alcohol, a cloudiness develops in the aqueous sol at the alcohol interface. Eventually, a microcrystalline precipitate settles out, appearing as minute dumb-bell shaped particles under the microscope. The particle size can be substantially increased by slowly cooling a hot starch sol in contact with butyl alcohol, in which case the precipitate appears as slender needles, or more usually as birefringent six-petalled crystalline rosettes, some 15-50 microns in diameter with corn starch and 50-80 microns with potato starch. This is the A-fraction. 

The formation of crystalline precipitates from starch sols by treatment with alcohol has been repeatedly mentioned in the literature. Alsberg describes birefringent crystal clusters obtained in this manner from autoclaved starch sols. Wiegel has recently reported crystalline formations obtained by treating starch pastes with various aliphatic alcohols, dioxane or ethylene chloride. However, the significance of these crystalline formations as a definite starch component has not been generally recognized, and little attempt has been made to isolate and purify this material. 

As originally reported, selective precipitation of the A-fraction was accomplished by saturating a starch paste with butyl alcohol, then autoclaving for 2-3 hours at 18-20 lb. pressure. On cooling to room temperature, the A-fraction separated in characteristic crystalline form and could be collected by supercentrifuging. The yield was 22-23% from either corn or potato starch. The iodine adsorption of the crude precipitated A-fraction was 16.5% for the non-precipitated B-fraction it was 1.5-1.7%. 

It was recognized that other alcohols might likewise effect a separation of starch components. Development of the iodine adsorption analysis has made it possible to evaluate the effectiveness of other precipitants as fractionating agents. According to recent tests, almost any monohydroxy alcohol will accomplish a separation under suitable conditions, and several of these are preferred to butyl alcohol. It is difficult to assign relative fractionating efficiencies to the various alcohols, since the 

Excellent n-propyl alcohol (30% by volume aqueous solution), n-hexyl alcohol, 2-ethyl-l-butanol, 2-ethyl-l-hexanol, lauryl alcohol, cyclo-hexanol 

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Separations of polysaccharides by fractionation on a preparative scale were also examined. Stemming from earlier work in his laboratory on the isolation of acidic polysaccharides by precipitation as their insoluble Cetavlon salts, Stacey and coworkers showed that it was possible to fractionate neutral polysaccharides by selective precipitation with Cetavlon after the formation of borate complexes. 

The further fractionation of this mixture by selective precipitation with Pb++ and Hg++ ions, undertaken by Bondzynski et al. (B7), proved unsuccessful, since no distinct separation of peptides was attained. 

In an effort to quantify the effect of photolysis, polysilanes 2 and 8 were selected as typical models. For this experiment, samples of 2 were fractionated by repeated precipitation from toluene using isopropanol. No additional... 

Fractional Precipitation. A preliminary enrichment of certain lanthanides can be carried out by selective precipitation of the hydroxides or double salts. The lighter lanthanides (La, Ce, Pr, Nd, Sm) do not easily form soluble double sulfates, whereas those of the heavier lanthanides (Ho, Er, Tm, Yb, Lu) and yttrium are soluble. Generally, the use of this method has been confined to cmde separation of the rare-earth mixture into three groups light, medium, and heavy. 

Schoch, T. J. (1942). Fractionation of starch by selective precipitation with butanol./. Am. Chem. Soc. 64 2957-2961. 

The second fractionation was carried out by selective precipitations. A whole polymer was dissolved in acetone, and methanol was added until a haze developed. The solution was cooled overnight at — 20°C. 

The nomenclature of starch components has become highly confused through the indiscriminate use of such terms as a and /3-amylose, amylose and amylopectin, amyloamylose and erythroamylose, etc. It does not seem advisable to apply any of these older terms to the new and unique fractions obtained by selective precipitation methods. It is therefore proposed that the term A-fraction be applied to that portion of the starch which is preferentially precipitated by various alcohols or by higher fatty acids, and which exhibits a high affinity for iodine. Correspondingly, the residual non-precipitated portion of the starch, which possesses low iodine affinity, is termed the B-fraction. These designations apply to laboratory-prepared samples, and do not carry any a priori connotations of purity, homogeneity, or structure. 

Analysis of plasma lipids. HDL was separated by selective precipitation of veiy-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) by dextran sulphate and manganese chloride (20), and HDL cholesterol was measured by cholesterol determination of the clear supernatant fraction. Cholesterol and triglycerides were analysed by enzymic procedures (21,22). 

The polymer mixtures formed from the reaction of poly(NPMAm) radicals with MMA and MA were fractionated into three parts by selective precipitation. Part A... 

Similar to the experiences shown at the recrystaUization of enantiomeric mixtures, the conglomerate behaviour can also be observed at fractioned predpitation. An apphcation of this method was effectuated at the resolution of racemic tiserdne (IIS) with half an equivalent of (R,R)-tartaric acid. The (S)-TIS enantiomer, the active pharmaceutical ingredient, remained in the filtrate of the diastereoisomeric salt formation process. Consequently, it contaminated with its mirror imge isomer. The enantiomeric enrichment was accomplished by selective precipitation. The (S>R)-TIS mixture was dissolved in water as a hydrochloric acid salt, then less than an equivalent amount of potassium hydroxide was added to the solutionin order to liberate the excess of (S)-TIS. The pure (S)-TIS base precipitated from the solution and an almost racemic hydrochloride salt remained in the solution..33... 

Lepage, S., Bonnefont-Rousselot, D., Assogba, U. et al. (1994) Distribution of P-carotene in serum lipoprotein fractions separated by selective precipitation. Ann. Biol. Clin., 52,139. 

Solvent deasphalting. This is an extraction of the heaviest fractions of a vacuum residue or heavy distillate. The extract is used to produce the bitumen. The separation is based on the precipitation of asphaltenes and the dissolution of the oil in an alkane solvent. The solvents employed are butane or propane or a butane-propane mixture. By selecting the proper feedstock and by controlling the deasphalting parameters, notably temperature and pressure, it is possible to obtain different grades of bitumen by this process. 

The first fractionation of urinary ampholytes in this way was carried out by Boulanger et al. (BIO) in 1952 with the use of ion-exchange resins. They had designed this procedure previously for the fractionation of ampholytes in blood serum (B8). According to this method, deproteinized urine was subjected to a double initial procedure aiming at the separation of low-molecular weight substances from macro-molecular ones. One of the methods consisted of the fractionation of urinary constituents by means of dialysis, the second was based on the selective precipitation of urinary ampholytes with cadmium hydroxide, which, as had previously been demonstrated, permits separation of the bulk of amino acids from polypeptides precipitated under these circumstances. Three fractions, i.e., the undialyzable part of urine, the dialyzed fraction, and the so-called cadmium precipitate were analyzed subsequently. 

Suppose a chemist has a solution that contains several similar ions. The chemist wants to remove some of these ions, while leaving other ions in solution. One way to do this is by fractional precipitation a process in which ions are selectively precipitated from solution, leaving other ions. 

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