Surface tension increment

Table I. Relative Effectiveness of Various Ions in Stabiliiinq the Native" Form of Collaqen and Ribonuclease <61 and the Predicted Relative Order Using the Molal Surface Tension Increment (7)...

The combination of the hydrophobic salting out and the electrostatic salting in terms explains very nicely the Class I type behavior. The data for carboxyhemoglobin and fibrinogen are well fitted by the theory (Figure 10). Also the order of decreasing molal surface tension increment generally follows the lyotropic order (7). 

The retention caused by different salts in equal concentration is in accordance with the Hofmeister series. The higher the molar surface tension increment of a salt, the greater is its effect on protein retention 55). Higher column temperatures produce prolonged retention in HIC, in contrast to RPC where retention decreases with increasing temperature56). 

B, C, and D are constants, the electrostatic term is expressed by A = Dfi/RT in which fl is the dipole moment of the protein, and the hydrophobic term is expressed by Q, = [N + 4.8N1 3V2/3(Ke -1)]/RT, where N is Avogadro s number, is the non-polar surface area of the protein, V the molar volume of the solvent, and <7 is the surface tension increment, i.e., the difference between the surface tensions with and without salt x6 is a correction factor for the surface tension to take account of the curvature of the protein surface at molecular dimensions. 

TABLE 6 Characteristic Molal Surface Tension Increments of Different Salts Used in HP-HIC... 

Salt Molal surface tension increment a X IO]dyne g cm" 1 mole - 1... 

Based on the position of an ion in the Hoftneister series, it is possible to foretell the relative effectiveness of anions or cations in an enormous number of systems. The rank of an ion was related to its kosmotropicity, surface tension increments, and salting in and salting out of salt solutions (see below) [25]. A quantitative physical chemistry description of this phenomenon is not far off. Molecular dynamics simulations that considered ionic polarizability were found to be valuable tools for elucidating salt effects [26,27]. 

Figure 4.17. Surface tension increments of aqueous solutions of alkylammonium chlorides influence of the chain length. Temperature 25°C. (Redrawn from Tamakl )...

It is shown 30) that the analysis of a relationship between the salting-out constants of a given protein and the molal surface tension increments makes it possible to estimate the relative surface hydrophobicity which is calculated as the ratio of the nonpolar surface area to the molecular weight of the protein. According to the concept developed by Melander et al 30), the hydrophobic character of a protein is believed to be constant at both high and physiological concentrations of different inorganic salts, which seems to be untrue in most cases (see below). 

The increase in retention at a given concentration depends on both the anion and the cation of the salt. Speciflcally, the retention increment depends on the molal surface tension increment of the salt. Table 13.1 contains this value for various salts. The higher the value, the greater is the retention at a pven concentration. It should be noted that the order in the table parallels the Hofmeister salting-out series (12). 

Theoretical discussions of the surface tension increments try to explain the trends noted for the various ions. Aveyard and Saleem (1976) related the 577 of an electrolyte solution to the product of its molality and its osmotic coefficient (p ... 

Table 2 Representative Examples of Salts Employed in HIC and Their Modal Surface Tension Increments, a...

Thus, sodium citrate, on the basis of its molal surface tension increment and high solubility, would appear to be a good choice as an HIC kosmotropic salt. However, citrate salts are well known for their ability to bind selectively to many proteins at low concentrations, a feature that can be exploited [189-191] in the stabilization of multimeric NAD-dependent enzymes. 

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