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Handbook of Size Exclusion Chromatography and Related ...

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<strong>and</strong> these distributions are by no means always symmetrical. Chromatographic<br />

behavior may reflect attraction between charged packing materials <strong>and</strong> local<br />

patches <strong>of</strong> opposite charges on the protein, even when the net charge on the protein<br />

as a whole has the same sign as the charge on the packing material (62). Smallmolecule<br />

examples <strong>of</strong> such local interactions are to be found in the binding <strong>of</strong><br />

polyelectrolytes to proteins even at pH values such that the net charges on the<br />

polyelectrolytes <strong>and</strong> proteins are <strong>of</strong> the same sign (63).<br />

4.1.2 Hydrophobic Interactions<br />

Significant hydrophobic interactions between proteins <strong>and</strong> packing material may<br />

be inferred from increases in elution volume as ionic strength is increased to fairly<br />

high values (generally 0.5 or higher, for ionic strength supported by NaCl). A<br />

strong “salting out” salt, such as ammonium sulfate, is especially useful in<br />

assessing the potential for such interactions in the case <strong>of</strong> a particular protein/<br />

matrix pair (51). Hydrophobic adsorption <strong>of</strong> proteins may be reduced by<br />

decreasing the ionic strength <strong>of</strong> the mobile phase (which may concurrently<br />

increase electrostatic interactions, however) or by adding organic solvents (64).<br />

4.2 Geometrical Changes in the Packing Material<br />

Gels such as Sephadex w <strong>and</strong> the BioGel w -P series (BioRad, Hercules, California,<br />

U.S.A.) depend upon swelling <strong>of</strong> the gel material in solvent for the formation <strong>of</strong><br />

pores; the pores collapse completely upon removal <strong>of</strong> the solvent (65). This critical<br />

dependence <strong>of</strong> the gel structure on solvation <strong>of</strong> the polymeric material raises the<br />

possibility <strong>of</strong> changes in effective pore size when the chemical nature <strong>of</strong> the mobile<br />

phase is changed significantly in an attempt to suppress adsorptive effects, as in the<br />

addition <strong>of</strong> detergents or organic solvents (64) for the chromatography <strong>of</strong><br />

hydrophobic proteins. Such considerations may also apply to hybrid gels (65) in<br />

which a hydration-dependent material has been bonded inside the large pores <strong>of</strong> a<br />

macroreticular supporting framework. A second type <strong>of</strong> pore-size change may<br />

affect rigid, permanent-pore packing materials as well as the solvent-swollen<br />

materials, in that detergents added to the mobile phase in order to solubilize or<br />

denature proteins, or to suppress hydrophobic interactions between proteins <strong>and</strong><br />

packings, may bind to the surfaces inside the pores to such an extent that the<br />

effective pore size is significantly decreased (66). The straightforward, though<br />

laborious, countermeasure to both <strong>of</strong> these effects is the calibration <strong>of</strong> the SEC<br />

column under each <strong>and</strong> every set <strong>of</strong> conditions employed experimentally.<br />

4.3 Changes in the Physico-Chemical State <strong>of</strong> the Proteins<br />

When changing the pH <strong>of</strong> the mobile phase to eliminate electrostatic interactions<br />

between protein <strong>and</strong> packing material, one should keep in mind the tendency <strong>of</strong><br />

© 2004 by Marcel Dekker, Inc.

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