Hydrophobic interactions, elimination

Hydrophobic interactions, on the other hand, are strong, indifferent to local details, and are relatively long range. If transient direct or water-separated contacts occur between nonpolar side chains, the net effect could be local organization and an overall compaction of the polypeptide chain. Whereas the strengths of hydrophobic interactions must be considerably reduced in 8 M urea, they clearly are not eliminated, as evidenced by the persistence of lipid bicelles. Thus hydrophobic interactions probably play some role in persistent global structure, the importance of which can be tested by replacing multiple hydrophobic side chains with similarly shaped polar ones. 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

Adsorption. Hydrophobic interactions, which may occur using aqueous mobile phases, usually can be eliminated by the addition of an organic modifier to the aqueous mobile phase (30,33) or by a reduction of ionic strength (3A 25.)- Recently, Haglund and Marsden (36-AO) have undertaken a systematic study on the chromatographic behavior of low molecular weight solutes on Sephadex packings and explained these results in terms of hydrophobic interactions. 

If both the analytes and the EOF move in the same direction, as with CIE, the system is classified as coelectroosmosis, and the analytes reach the detector faster than they would as a result of their own mobilities. If the analytes move in the opposite direction from the analytes, the system is classified as counterelectroosmosis, and the analytes reach the detector later than they would as a result of their own mobilities. If the EOF is suppressed by eliminating the effective charge on the capillary wall, then the analytes reach the detector solely as a result of their own mobility. The last approach is often taken for the analysis of large peptides and proteins, where ionic or hydrophobic interactions between the analyte and the capillary wall result in peak tailing or total adsorption. 

A third option for reducing solute-wall interactions is to operate at extremes of pH. At pH 1.5, the silanol groups at the capillary wall are not ionized. Although proteins are cationic at that pH, electrostatic interactions are eliminated. Operating at pH values that are within 1-2 units of the pi of the protein will also reduce wall effects, but these approaches are limited by hydrophobic interactions and poor selectivity. 

M-0.2M) and/or the addition of 5-10% organic solvent to remove hydrophobic binding. Due to their variations in physical properties, it is sometimes difficult to eliminate all the ionic and hydrophobic interactions of a mixture of peptides using a single mobile phase. 

The binding forces are electrostatic attractions between sites with opposite charges such as -NH3 carried by a lysine residue and a carboxylate, van der Waals forces, and hydrogen bonds. It is predicted that the first two types of bonds, which increase rapidly as the distances decrease, will be all the more efficient as the complementarity is better. The expression hydrophobic interactions is used because an exact fit drives away water molecules present close to the hydrophobic residues such as valine, leucine, isoleucine, etc. This concept, developed further in Chapter 11, implies that a decrease in surface contact with water is in itself a stabilizing factor. All these forces are reversible. When the hapten-antibody complex in solution is introduced into a dialysis bag, only the hapten can cross over the membrane, and its elimination from the interior bag causes complete dissociation of the complex through equilibrium displacement. 

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