Chemistry Reference
In-Depth Information
Figure 3.10 Reversible base pairing defines the rigid structure of a reversible polymer
system.
oligonucleotide sequences that are covalently linked directly or through a synthetic
spacer (Fig. 3.10). Duplex formation creates a linear, polymeric assembly that
resembles larger duplex DNA, but in which the main chain is defined by the revers-
ible base pairing. These SPs are intrinsically modular: 1) the thermodynamics and
kinetics of the association are determined by the variable base sequence; 2) the
linear density of the reversible interactions, and the conformational flexibility along
the polymer backbone, are each dependent on a spacer in which much variation is
possible; and 3) inter- and intrachain interactions may be tuned by salt concentration
in the buffer. Furthermore, enzymatic covalent capture and characterization are
possible (Xu et al. 2004).
SP bridging was examined using atomic force microscopy (AFM; Kim et al.
2005). On patterned gold substrates, SP brushes reversibly assembled in solution
from different surface densities of a thiol SP anchor. The adhesion between the
SP-functionalized tip and four different substrate chemistries was characterized in
quasiparallel by AFM. Using a series of comparative control experiments, the contri-
butions of SP-mediated adhesion were quantified and attributed to the formation of
multiple surface-tip bridges during contact. The DNA-based monomers enable
differentiation between bridges that are formed through molecular recognition of
one brush layer with the other and those that are formed by nonmolecular recognition
mediated adsorption of the brushes onto opposing surfaces. When similar, but
noncomplementary, brushes are displayed on opposing surfaces, for example, the
adhesive interaction is much lower than that between the same tip and a complemen-
tary substrate, supporting direct SP bridging (
15-20 bridging contacts present with
the polymer that are not present without) as a dominant contributor to adhesion in
these systems.
Using the modularity of the DNA-based SPs, several structure-activity relation-
ships have been probed. For example, it was determined that the strength of the
