Biomedical Engineering Reference
In-Depth Information
Covalent linkage of proteins to conductive, semiconductive, or nonconduc-
tive supports often utilizes the availability of functional groups on the surface of
the solid support. Metal oxides such as TiO
2
and SnO
2
contain surface hydroxyl
groups that are useful in the coupling of organic materials. Noble metals (Au, Pt)
are chemically or electrochemically pretreated to generate surface functionalities.
Some of the functional groups potentially available in proteins for immobilization
and the functionalities required on the surfaces are shown in Figure 9.15.
Chemical binding via side chains of amino acids is often random, since it is
based upon residues typically present on the exterior of the protein. As a result,
heterogeneity in the modes of protein linkage and orientation with respect to the
surface is obtained. The structural alignment of the redox protein relative to the
conductive support is essential for many bioelectronic applications, and so the de-
velopment of methods for specific binding and alignment of proteins on surfaces
is important. In some cases, oriented immobilization is obtained if the protein
possesses a single reactive amino acid in the structure. For example, cysteine that
contains a unique thiol (SH) functional group is an uncommon amino acid resi-
due in proteins. A single cysteine residue on the protein periphery or a genetically
engineered cysteine component in the protein allows the linkage of the protein
to the surface at a single point. Chemical attachment is also guided in an orderly
manner to attain oriented immobilization. Site-specific immobilization requires
functionalization of the molecule or altering the surface or both. Thiolated DNA is
also utilized for self-assembly onto gold transducers. Covalent linkage to the gold
surface via functional alkanethiol-based monolayers is an approach used in devel-
oping highly organized DNA monolayers. Proteins are also attached to surfaces in
an ordered structure, allowing reproducibility and conformational stability.
9.5.2.3 Affi nity
Affinity interactions between an enzyme and its substrate, a receptor protein and
its recognition pair, or antigen/antibody pairs are often characterized by high as-
sociation constants of the resulting complexes. This has enabled the use of specific
recognition interactions to construct protein layers on solid supports. Biochemi-
cal affinity reactions offer an oriented immobilization of proteins, providing an
important advantage over other immobilization techniques. Moreover, not only is
Hydroxyl
Amino
Aldehyde
Carboxyl
Figure 9.15
Surfaces with functionalities that can couple with biomolecules.
