Gels Mimicking Antibodies in Their Selective Recognition of Proteins and Its Potential Use for Protein Crystallization - Molecules: Nucleation, Aggregation and Crystallization - page 24

Classification of chromatographic techniques in terms
of the number of bonds between the solute and the
stationary phase and the strength of these bonds
Proteins and other macromolecules with many charged and/or non-polar
groups bind strongly to a bed which has a high density of ligands of the
same types of groups by virtue of electrostatic and hydrophobic interac-
tions, respectively. However, the adsorption is not selective because such
a bed can, via different combinations of the ligands, strongly bind many
proteins of varying structure. This alternative ( several strong bonds) is
used in conventional gradient elution of macromolecules (proteins) in ion
exchange and hydrophobic-interaction chromatography. For isocratic
elution of proteins these separation methods require few strong bonds
(Hjertén et al. , 1986; Yao, Hjertén, 1987). As for low molecular weight
compounds, the strength of the bonds can (or perhaps, must) be high since
the number of bonds is necessarily small. An example of this alternative
for small molecules is the molecular imprinting method, which has been
employed with great success for the separation of enantiomers and other
low molecular weight compounds, often using strong electrostatic bonds
created by functional monomers (Wulff et al ., 1985; Wulff, Mindrik, 1988;
1990; Wulff, 1995; Shea, 1994; Schweitz et al ., 1997). In ideal molecular
imprinting, all functional monomer molecules in the bed have positions
close to the complimentary groups in the solute molecules. However, a gel
in which only some fraction of the functional monomer molecules fulfills
this requirement also exhibits (a certain degree of ) selectivity because the
binding caused by the rest of these monomer molecules will be weaker,
more or less resembling that occurring in conventional ion exchangers
(when charged functional monomers are used) or beds for hydrophobic-
interaction chromatography (when non-polar functional monomers are
used). The difficulty of synthesizing a gel for ideal molecular imprinting
(or the latter semi-ideal approach) increases with the number of binding
sites, often equivalent to an increase in molecular weight. Therefore, it is
not surprising that no molecular imprinting experiments with high selec-
tivity for proteins have been reported. These considerations indicate that