Solubility of macromolecules

For m -> oo, the critical value is identical with that in a d- solvent, i.e., A2 = 0 and X = 0-5- Since the solubility of macromolecules decreases with increasing molecular weight, it is possible to separate these materials with respect to their molecular weights by changing the composition of the solvent and/or the temperature. In general, one roughly distinguishes between two methods, namely fractional precipitation and fractional extraction. 

Fractionation depends on the differential solubility of macromolecules with different sizes. It has been displaced in many cases by size exclusion chromatography as a means for measuring molecular weight distributions, but it is till often the only practical way of obtaining narrow fractions in sufficient quantities for the study of physical properties of well-characterized specimens. It is also part of the original procedure for the calibration of solution viscosity measurements for estimation of molecular weights. 

The solubility of macromolecules like proteins in water generally increases in the presence of a suitable concentration of salt, so-called salting in. Furthermore, increasing the concentration of salt further leads to a decrease in the solubility of the proteins and their precipitation (salting out). 

Soluble polymers also can be separated by liquid/liquid separations. These liquid/liquid separations can involve membrane filtrations that use to advantage the relative size differences of macromolecules and low molecular weight substrates. Alternatively, the physical size or the phase-selective solubility of macromolecules can be used to advantage, separating a solution of a macro-molecule-bound ligand or catalyst from a low molecular weight product on the basis of size or phase-selective solubility (Fig. 2). 

Thermal precipitation by cooling is the scheme chemists normally use in recrystallizations and is the normal behavior of small molecules. Macromolecules are different in that they can often be phase separated from solution by heating [ 119,120]. Thermal precipitation by heating is a process that produces a solid polymer without addition of anything other than heat. It is the inverse of the process used with the polyethylene oligomers discussed above. This inverse temperature-dependent solubility of macromolecules is a phenomenon that is most simply ascribed to the unfavorable entropy of solvation of a macro-... 

Zhang Y, Furyk S, Bergbreiter DE, Cremer PS (2005) Specific ion effects on the water solubility of macromolecules PNIPAM and the Hofmeister series. J Am Chem Soc 127 14505-14510 Zhao HJ (2005) Effects of ions and other compatible solutes on enzymatic activity, and its implication for biocatalysis using ionic liquids. Mol Catal B Enzym 37 16-25... 

The discussion of real (i.e., non-ideal) polymer solutions will be deferred till section 6 d. We shall first consider the solubility" of macromolecules from the point of view of ideal solutions. This aspect has played an important part in the physical chemistry of high polymers especially in the theory of fractional precipitation. 

The solubility of macromolecules as a rule improves with the rising temperature. Solvent - polymer mixtures usually exhibit the upper consolute temperature or upper critical solution temperature, UCST, with a maximum on the plot of system concentration versus temperature. Above the critical solution temperature, polymer is fully soluble at any concentration. For practical work, the systems with UCST below ambient temperature are welcome. There are, however numerous polymer - solvent systems, in which the solvent quality decreases with increasing temperature. The plot of system concentration versus temperature exhibits a minimum. The phenomenon is called lower consolute temperature or lower critical solution temperature, LCST Polymer is only partially soluble or even insoluble above lower critical solution temperature. This unexpected behavior can be explained by the dominating effect of entropy in case of the stiff polymer chains or by the strong solvent - solvent interactions. The possible adverse effect of rising temperature on polymer solubility must be kept in mind when woiking with low solubility polymers and with multicomponent mobile phases. It may lead to the unforeseen results especially in the polymer HPLC techniques that combine exclusion and interaction retention mechanisms, in coupled methods of polymer HPLC (see section 11.8, Coupled Methods of Polymer HPLC). 

Liquid chromatography at the critical adsorption point (LC LCS) was investigated. The solubility of macromolecules, the sensitivity of LC LCS to such variables as the injection volume, concentration injected, mobile phase flow rate and mixing between the injection zone, were investigated, using polymethyl methacrylate (PMMA) and polyacrylamide standards. It is proposed that using tetrahydrofuran and n-hexane as eluents for PMMA, at the LC LCS, limited polymer insolubility or even local precipitation is combined with the size exclusion of macromolecules. 12 refs. 

A carboxylic acid can be represented as R — CO2 H. Many different carboxylic acids participate in organic chemistry and biochemishy. Although carboxylic acids react in many different ways, breaking the C—OH bond is the only reaction that is important in polymer formation. A carboxylic acid is highly polar and can give up H to form a carboxylate anion, R — CO2. The carboxyl group (— CO2 H) also forms hydrogen bonds readily. These properties enhance the solubility of carboxylic acids in water, a particularly important property for biochemical macromolecules. 

Laurent, T. C., The interaction between polysaccharides and other macromolecules. 5. The solubility of proteins in the presence of dextran, Biochem.., 89, 253, 1963. 

Inamura I, Isshiki M, and Araki T. 1989. Solubilization of (3-carotene in water by water-soluble linear macromolecules. Bulletin of the Chemical Society of Japan 62(5) 1671-1673. 

Several block and graft copolymers have been shown to form stable aggregates under thermodynamically poor solvent conditions, as a result of differences in the solubility of different parts of a macromolecule. Whereas in a good solvent the experimentally measured value of A2 for a copolymer represents the balance of all the multiple interactions, under thermodynamically poor conditions A2 is mainly determined by the interaction of the groups situated on the polymer-solvent interface. Groups which form the hydrophobic core and are not in a contact with the solvent do not contribute significantly to the solution properties of the copolymer. 

Despite the fact that the water-solubility of the s-fractions during the heating of their aqueous solutions (above the PST for the p-fraction) is an essential property of protein-like macromolecules, other structural features could also be responsible for such behaviour, including the following ... 

---