Chemistry Reference
In-Depth Information
supramolecular interaction (hybridization energy) influences the adhesion; weaker
base pairing leads to a drop in the amount of force transferred from one surface to
the other. In fact, a relatively modest change in average free energy (
1 kcal mol
21
)
cuts the adhesion roughly in half for the weaker SP brushes relative to the stronger.
The magnitude of the change in adhesion indicates that the association thermo-
dynamics influence the adhesive properties both through the mechanical response of
the individual associations (presumably related to their dissociation kinetics) and by
influencing the number of adhesive contacts that are formed. Other details of
oligonucleotide-based monomer (OM) structure also influence the mechanics of the
interface. When a flexible spacer is introduced between the molecular recognition
end groups, for example, the adhesion between flexible SP brush surfaces is greater
than that between rigid SP brush surfaces of comparable height, even though the indi-
vidual associations in the flexible brushes are weaker than those in the rigid brush.
The theoretical investigations of van der Gucht and colleagues (2003) predict
that the adhesive interaction should have a long-range component, although the
contributions should drop off with surface separation. This expectation is met in
the experimental systems. When a similarly functionalized AFM tip is held away
from (not in contact with) an SP brush surface, bridges spontaneously form across
5-10 nm gaps (Kersey et al. 2004). Individual bridging events are observed
upon retraction of the AFM tip, and the length of the bridges can be inferred from
the force versus distance retraction curves. The length distribution of the SP
bridges approximates a Flory distribution (Flory 1969) as expected for linear SPs.
The actual distribution of bridge lengths, however, is skewed toward lengths that
are much shorter than the equilibrium distributions in solution. Reversible bridging
is therefore responsive to the spatial constraints of the intersurface gap, and shorter
bridges are preferred to longer ones. The presence of an attractive force provides
an interesting mechanism for self-repair, because surfaces that are mechanically
disjoined might be slowly pulled into closer contact, where adhesion is greatest.
The bridges form across gaps and in surface-surface contact provide snapshots of
different points in that process.
An SP interface of greater complexity is the reversibly cross-linked polymer brush,
in which the addition of SP cross-links might change the brush conformation and
intersurface penetration as well as create entanglements that might contribute, differ-
ently, to lateral and normal forces. In this regard, the pincer coordination motif
described previously has great utility: the ability to change dynamic response at
the level of molecular associations creates the equivalent of a “macromolecular
kinetic isotope effect” that can be applied to the investigation. Although actual iso-
topic substitutions are obviously not involved, the phenomenological similarity
with kinetic isotope effects in reaction mechanisms is quite real. In particular, contri-
butions to rate-determining material processes are revealed by kinetic differences in
two isostructural systems. Here, structure is preserved at two levels: the molecular
structure of the individual SP constituents and the equilibrium association constant
(K
eq
) between molecular recognition partners. The former maintains chemical
composition in the material, and the latter maintains structural composition in the
extended polymer assembly.
