C O MMU N I C A T I O N S
Figure 2. (A) Hydrogel of 1 ([1] ) 3.6 mM in H2O). The phase transitions
of the hydrogel of 1 induced by the addition of (B) 0.01, (C) 0.1, and (D)
1.0 equiv of Van solid into the hydrogel.
Figure 4. Possible ligand-receptor interactions that induce the gel-sol
phase transition.
dipeptides whose hydrogels respond to a ligand-receptor interaction
and exhibit chiral recognition. The vast pool of ligand-receptor
pairs should offer many useful candidates for the design of
supramolecular hydrogels that respond to specific interactions.
Acknowledgment. This work was partially supported by RGC
(Hong Kong), HIA (HKUST), and a DuPont Young Faculty Grant.
Supporting Information Available: Details of the synthesis, the
response of the gels to pH and thermal stimuli, and additional electron
micrograph (PDF). This material is available free of charge via the
Figure 3. Cryo-SEM images11 of the nanofibers in the hydrogels of (A)
1, (B) 2, and (C) 3, and the SEM images of the mixtures after 1 equiv of
Van solid was added to the hydrogels of 1 (D), 2 (E, inset: arrow indicated
the precipitate of Van), or 3 (F).
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enantiomer of 1, this result also represents the first example that a
ligand destroys the hydrogel of its receptor only based on
stereochemistry.
To elucidate the distribution of the Van in the hydrogels, we
performed the morphological study of the hydrogels before and
after adding Van. As shown in Figure 3, scanning electron
micrographs (SEM) indicate that the nanofibers (∼50 nm wide) of
the dipeptides constitute the matrices of the corresponding hydro-
gels, which agree with the transmission electron micrographs
(TEM)11 of the hydrogels. The morphology of the nanofibers formed
by 3 differs from that of the nanofibers formed by 1 and 2 in terms
of persistent length, which is consistent with their stacking modes
and chirality. After adding 1 equiv of Van solid, SEM images show
that the network in the hydrogel of 2 remains unaffected, while
the matrices in the hydrogels of 1 and 3 are completely destroyed
by the ligand-receptor binding between Van and 1 or 3.14 The
(almost) uniform distribution of the aggregates of Van on the surface
of the nanofibers of 2 indicates that Van dissolves homogeneously
in the water phase of the hydrogel of 2. These results agree with
the fact that D-Ala-D-Ala has strong ligand-receptor interaction
with Van, while L-Ala-L-Ala has no obvious binding.14 On the basis
of CD and emission spectra, we suggest the following plausible
mechanism for the response: Van binds to the D-Ala-D-Ala via
four to five hydrogen bonds (Figure 4), and the biphenyl moiety
on the Van blocks the π-π interactions between fluorenyl groups,
thus preventing the formation of the supramolecular polymers
needed for gelation. In addition, one equivalent of teicoplanin solid,9
an analogue of Van, converts the hydrogels of 1, 3, or 4 to
suspensions but exerts no influence on the hydrogels of 2 or 5,
which further supports the proposed mechanism. In conclusion, we
have shown a new kind of small molecular hydrogelators based on
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(11) Supporting Information.
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