Biomedical Engineering Reference
In-Depth Information
microcontact method and low melting point metal, this imprinting strategy is different
significantly from the traditional approaches where organic or inorganic materials are
generally used. It may open new avenues in optical and electronic sensor design resulting
from the combination of the conductive property and the molecular selectivity.
4. Epitope Approach: A Small Structural Element for
the Whole Molecule Recognition
In the development of the protein imprinting materials, many challenges, such as the
accessibility of binding sites, non-specific binding, and flexible conformation of the
macromolecules, remain in producing materials that selectively bind the target molecules. In
addition, proteins are inherently capable of assuming a large number of conformations
depending on a number of factors, including temperature, pH, and ionic strength. Steric
factors also make molecular recognition of proteins difficult, as it is extremely difficult for
their large structures to move freely through highly cross-linked polymer networks. It is
known that well-defined recognition sites are more easily formed by the imprinting of
relatively small molecules with rigid structure. So, to reduce the complications associated
with the imprinting of proteins and peptides, Rachkov and coworkers [73] developed a new
strategy for the synthesis of biomacromolecule recognition polymers, which were termed the
“epitope approach”. Generally, an epitope refers to the small active site located within the
larger protein structure on an antigen, which combines with the antigen-binding site on an
antibody or lymphocyte receptor. In this technique, instead of the whole proteins, a short
peptide sequence, often exposed at the protein surface, was used as a template for molecular
imprinting polymers preparation. Once the matrix has been polymerized the resultant
imprinted material should be able to recognize and bind the whole protein. That is to say,
when the original whole protein is exposed to the imprinted material, the region containing
the peptide used as template can recognize its spatial and chemical mimic in the polymer.
Rachkov et al. [73] prepared an imprinting polymer specific for the nona-peptide
oxytocin, a neurohypophyseal hormone. The polymer was made of methacrylic acid and
ethylene glycol dimethacrylate. A small oxytocin sequence of three amino acids proved to be
enough for recognition of the whole sequence by the molecular imprinting polymer. In this
study, synthesis of the molecular imprinting polymer was performed in an organic
environment, but subsequent rebinding experiments were performed using chromatographic
methods in both aqueous-rich and aqueous-poor mobile phases. In the aqueous-poor mobile
phase, hydrogen bonds and ionic interactions are the dominating factor in creating selective
recognition sites. In the aqueous-rich phase, ionic and hydrophobic interactions provide the
dominant binding interaction. Few years later, they exploited the epitope approach for the
imprinting of the octapeptide hormone angiotensin II in aqueous media [29]. HPLC
evaluation of these polymers with a water-based mobile phase showed their selectivity for the
peptide, [Sar1,Ala8]angiotensin II, that had been used as the template, but not for its parent
peptide angiotensin II. The binding capacity and selectivity of the molecular imprinted
polymers depended on the ratio of template to functional monomer in the polymerization
mixture, as well as on the ionic strength and pH of the chromatographic mobile phase.
