A naphthalene-containing amino acid enables hydrogelation of a conjugate of nucleobase-saccharide-amino acids

Article abstract of DOI:10.1039/c3cc48946a

Here we report the first example of a hydrogelator made of a conjugate of nucleobase-saccharide-amino acids by incorporating l-3-(2-naphthyl)-alanine to the conjugate, which illustrates a facile and effective method for generating bioactive and functional hydrogelators from the basic biological building blocks.

Full text of DOI:10.1039/c3cc48946a

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A naphthalene-containing amino acid
enables hydrogelation of a conjugate of
nucleobase–saccharide–amino acids†
Cite this: Chem. Commun., 2014,
50, 1992
Received 22nd November 2013,
Accepted 18th December 2013
Dongdong Wu, Jie Zhou, Junfeng Shi, Xuewen Du and Bing Xu*
DOI: 10.1039/c3cc48946a
www.rsc.org/chemcomm
Here we report the first example of a hydrogelator made of a Thus, we decided to introduce a naphthalene containing unnatural
conjugate of nucleobase–saccharide–amino acids by incorporating amino acid, 3-(2-naphthyl)-alanine (Nal), into the amino acid
L-3-(2-naphthyl)-alanine to the conjugate, which illustrates a facile segment via solid phase peptide synthesis (SPPS) because Nal
and effective method for generating bioactive and functional hydro- provides enhanced aromatic–aromatic interactions for molecular
gelators from the basic biological building blocks.
self-assembly in water.15 In addition, Nal already finds applications
in the preparation of bioactive molecules. For example, Nal has
Supramolecular hydrogels of small molecules, as an emerging, served as a residue for making antagonists (e.g., SB-75) of luteinizing
new type of biomaterials, have potential for a wide range of hormone-releasing hormone (LH-RH),16 inhibitors of protease,17
applications in biomedicine. For example, hydrogels formed by polyvalent inhibitors of influenza-mediated hemagglutination,18
the self-assembly of oligopeptides1 and derivatives of small and enantioselective catalysts.19 In fact, Nal is also a residue which
peptides2 have shown promise in tissue engineering,3 drug enables lanreotide to form autogels.20 Despite these applications,
delivery,4 cancer therapy,5 protein separation,6 binding RNA7 it is yet to be used in the conjugates of nucleobase, saccharide,
or proteins,8 and wound healing.9 These advances and other and amino acids.
developments in gelators made of small molecules10 have encour-
We found that the introduction of L-Nal to a conjugate of
aged us to explore supramolecular hydrogelators constructed from NSA type enables the conjugate (1) to act as a supramolecular
the basic biological building blocks (i.e., nucleobase, saccharide, hydrogelator (2) (Scheme 1). The nucleobase on the hydrogelator
and amino acid). We have recently demonstrated that the covalent preserves its ability to interact with single strand nucleic acids
conjugates of nucleobase and amino acids11,12 or the conjugates (e.g., ssDNA). The incorporation of D-Nal to a D-amino acid
of nucleobase, amino acids, and saccharide13 are not only able to conjugate (3) yields a new conjugate, 4, which is unable to act
self-assemble in water to form hydrogels, but are also cell- as a hydrogelator. According to in vitro cell viability tests, these
compatible.11,13,14 Moreover, a conjugate of nucleobase, amino NSA conjugates are cell compatible. These results, as the
acids, and saccharide is able to assist single strand nucleic acids first demonstration of converting a conjugate of nucleobase,
into cells.13 These successful results indicate that it is feasible to saccharide, and amino acids into a hydrogelator to form supra-
generate bioactive and functional hydrogelators from the pool of molecular nanofibers, illustrate a facile and effective method
the basic building blocks used by nature.
Encouraged by the hydrogelation of a conjugate of nucleo-
base, amino acids, and saccharide (denoted as NAS type in this
communication), we mutated the sequence of the building
blocks to make a conjugate of nucleobase, saccharide, and
amino acids (denoted as NSA type) to expand the diversity of
the hydrogelators. However, the covalent linkage of a nucleo-
side derivative to a dipeptide fails to produce a hydrogelator.
Department of Chemistry, Brandeis University, 415 South St., Waltham, MA 02454,
USA. E-mail: bxu@brandeis.edu
† Electronic supplementary information (ESI) available: Synthesis and character-
ization, supplementary figures and schemes, 1H NMR of all the conjugates. See
Scheme 1 The representation of a conjugate of nucleobase, saccharide,
and amino acids, and the structures of the molecules explored in this work.
DOI: 10.1039/c3cc48946a
1992 | Chem. Commun., 2014, 50, 1992--1994
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for generating diverse functional biomaterials from the basic
biological building blocks.
Scheme 1 shows four conjugates of NSA type investigated in
this work. Both 1 and 3 consist of nucleobase (adenine),
saccharide (ribofuranose) and amino acids (L-Phe-L-Phe for 1
and D-Phe-D-Phe for 2). The addition of an L-Nal at the C-terminal of
1 gives 2, and a D-Nal onto 3 affords 4. Since a nucleoside consists
of both nucleobase and ribofuranose, the covalent linkage of a
nucleoside to a small peptide provides a facile synthetic route for
making these NSA type conjugates. We choose adenosine as an
example to build the conjugates because of an excellent procedure
to convert adenosine the o-diol-protected adenosine acid derivative
5 (Scheme S1, ESI†),21 which makes it compatible with SPPS.22
We also use D-amino acids for the construction of the conjugates
(e.g., 3 and 4) for increasing their proteolytic resistance.23 The
structural difference between 1 and 3 (or 2 and 4) also provides
insights on the effect of stereochemistry on the self-assembly of the
conjugates. Scheme S1 (ESI†) shows the typical synthetic routes for
making the conjugates. The synthesis of 1 or 2 starts with the
linkage of N-Fmoc-protected amino acids to 2-chlorotrityl resin.
After the construction of the peptide chains (e.g., L-Phe-L-Phe (for 1)
and L-Phe-L-Phe-L-Nal (for 2)) from the C-terminal towards the
N-terminal through a step-by-step peptide chain elongation
procedure, the direct coupling of the o-diol-protected adeno-
Fig. 2 Transmission electron micrographs (TEM) of the hydrogels of (A) 2
(pH 7.0, 1.0 wt%), (B) 2 + T10 (pH 7.0, 1.0 wt%). Scale bar is 100 nm.
Fig. 3 (A) Strain dependence of the storage moduli (G0) and the loss
moduli (G00) of the hydrogels of 2 and 2 + T10; (B) frequency dependence of
the dynamic storage moduli (G0) and the loss moduli (G00) of the hydrogels
of 2 and 2 + T10
.
sine acid derivative 5 to the peptide chains on the beads allows (with mgc of 0.8 wt% at pH 7.0), which turns into a solution
the formation of the protected conjugates. TFA cleaves the at 59 1C or at pH above 9.0. To evaluate the ability of the
conjugates from the resin and deprotects the o-diol-protected nucleobase on 2 to interact with nucleic acids after the self-
group from the ribofuranose. Using D-amino acids and following assembly of 2, we added a single-strand deoxyribonucleic acid
the same SPPS protocols, we obtain 3 and 4.
(T10, Scheme S2, ESI†) to the hydrogel of 2 and examined the
After the synthesis of the NSA conjugates, we tested their morphological and rheological changes of the hydrogel. The
gelation properties. 1 affords a transparent solution (Fig. 1A) at addition of T10 appears to result in a mechanically stronger
pH 7.0 and concentration 1.0 wt%. The change in the pH value hydrogel (Fig. 1C), which is confirmed by the subsequent TEM
of the solution fails to lead to a hydrogel, except the precipitation and rheological measurements. The TEM image of the hydrogel
occurring at pH 4–5. Increasing the concentration of 1 up to of 2 + T10 shows that the width of the nanofibers increases to
3.0 wt% also does not result in hydrogelation. The transmission 17 ꢀ 2 nm (Fig. 2B), confirming that the addition of T10 is
electron microscopy (TEM)24 image of the solution of 1 (1.0 wt%) responsible for the morphology change of the matrices of the
hardly shows any ordered nanostructures except few aggregates hydrogel. Another difference between the TEM images of 2 and
(Fig. S1A, ESI†). Unlike 1, 2 is able to form a transparent hydrogel 2 + T10 is that non-fibrillar aggregates are observed in the TEM
at pH 7.0 and 1.0 wt%, (Fig. 1B). TEM reveals the long and images of 2 + T10, which might be T10 since most of the aggregates
flexible nanofibers (7 ꢀ 2 nm in diameter, Fig. 2A) as the attach or twine on the nanofibers. TEM reveals that T10 alone is
matrices of hydrogel of 2. The dynamic strain sweep of rheome- unable to form any fibers (Fig. S1D, ESI†). The rheological test
try shows that the hydrogel of 2 has a critical strain value of (Fig. 3A and B) shows that the addition of T10 increases the
0.86%, indicating a relative weak network in the hydrogel. The maximum dynamic storage moduli of the hydrogel to about two
hydrogel has a maximum dynamic storage moduli (G0) of 181 Pa, orders of magnitudes (i.e., from 181 Pa of the hydrogel of 2 to
which is almost ten times higher than its dynamic loss moduli 12 317 Pa of the hydrogel 2 + T10). The frequency sweep of the
G00, suggesting that the material behaves as a solid (Fig. 3A). In hydrogel of 2 + T10 (Fig. 3B) also shows frequency-independent
addition, the G0 and G00 values of the hydrogel of 2 are almost G0 values, confirming the solid-like behaviour of the hydrogel of
independent with a frequency from 0.1 to 50 rad sꢁ1 (Fig. 3B). 2 + T10. Interestingly, TEM reveals that the addition of 2 (not the
These results confirm that
2
forms
a
typical hydrogel hydrogel of 2) to the solution of T10 results in nanofibers with a
diameter of 8 ꢀ 2 nm (Fig. S2, ESI†).
Like 1, 3 dissolves in water to form a transparent solution
(Fig. 1D) under the same conditions (pH 7.0 and 1.0 wt%). The
TEM image of the solution of 3 shows some aggregates (Fig. S1B,
ESI†). As a diastereomer of 2, 4 is unable to form a hydrogel. At
1.0 wt% and pH 7, 4 affords a solution (Fig. 1E), though the TEM
image of the solution of 4 reveals non-fibrillar aggregates in the
Fig. 1 Optical images of solution of (A) 1, hydrogels of (B) 2 and (C) 2 + T10
(2 : T10 = 10 : 1), and solutions of (D) 3 and (E) 4. All at pH 7.0 and 1.0 wt%.
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gelator from a conjugate of nucleobase, saccharide, and amino
acids. The use of 3-(2-naphthyl)-alanine and derivatives of
nucleoside for SPPS illustrates a facile route to introduce
nucleobase to peptides for making biofunctional soft materials.
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generating functional hydrogelators and exploring supramolecular
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This work was partially supported by NIH (R01CA142746).
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This journal is ©The Royal Society of Chemistry 2014