Chemistry Reference
In-Depth Information
3†4, the formation of 3†6 or 3†7 was entropically more costly, reflecting the
fact that the presence of mismatched sites in these mutant duplexes led to
more flexible backbones. The ITC results demonstrated that the decreased stab-
ilities of 3†6 and 3†7 were largely attributable to the loss of one H bonding
site in each pair, whereas increased entropic barriers also played a minor role.
Note that the observed drop in stability was not due to partial alignment of
the two strands, as indicated by NOESY studies. These results firmly established
the high sequence fidelity of the fully H bonded 3†4.
The high fidelity of the H bonded duplexes was again demonstrated by duplexes
8†8 (see Section 8.5), 9†9, and 3
0
†4
0
, with eight, six, and four intermolecular H
bonds, by gel permeation chromatography (GPC). When mixed in solution, each
of these three duplexes clearly existed independently (Fig. 9.5). When tethered
to various structural units, these duplexes can be used in mixtures to sort out and
assemble the corresponding units in a highly specific fashion.
9.6. UNEXPECTED DISCOVERY: DUPLEXES CONTAINING
FOLDED STRANDS
Having demonstrated the adjustable strength and programmable sequence specificity
of our H bonded duplexes, we decided to probe the application of these information-
carrying associating units for constructing various supramolecular structures. Among
various self-assembling structures, supramolecular polymers have attracted intense
interest in recent years (Moore 1999; Brunsveld et al. 2001; Zimmerman and
Lawless 2001). With the ready tunability of their sequences, the duplexes we devel-
oped should be able to direct not only the association of molecular components but
also the regiospecificity of such an association.
As shown in Figure 9.6, oligoamide strand 10, consisting of two 4-H bonding
DDAD units linked in a head-to-head fashion, should associate with single strand
11, formed by linking two AADA units in a head-to-head fashion (Yang et al.
2003). If 10 and 11 adopted extended conformations, they would only partially
overlap each other because of their unsymmetrical 4-H bonding units, leading to
Figure 9.6 Oligoamide strands 10, 10
0
, 11, and 12 containing two quadruply H bonding
units linked in a head-to-head fashion. Control strand 12
0
is essentially half of 12.
