A novel mixed-component molecular hydrogel system with excellent stabilities

Article abstract of DOI:10.1039/c2cc32348f

We report a novel mixed-component molecular hydrogel system with excellent stabilities against dilution and enzyme digestion.

Full text of DOI:10.1039/c2cc32348f

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COMMUNICATION
A novel mixed-component molecular hydrogel system with
excellent stabilitiesw
Dongxia Li,a Jinjian Liu,b Liping Chu,b Jianfeng Liu*b and Zhimou Yang*a
Received 1st April 2012, Accepted 28th April 2012
DOI: 10.1039/c2cc32348f
We report a novel mixed-component molecular hydrogel system
with excellent stabilities against dilution and enzyme digestion.
co-assembly of a C-terminal PEG-functionalized gelator and
the gelator resulted in gels with higher rigidities and better
solvolytic stabilities.9 Though several groups have demon-
strated the better properties of mixed-component gels than
single-component ones, the simple mixing strategy cannot
guarantee the full incorporation of functional molecules into
self-assembled nanofibers because the functional molecules are
not the requisite building blocks for the formation of self-
assembled structures and some of the functional molecules
might be in soluble form. In this study, we report a novel
strategy to prepare a mixed-component hydrogel system with
most of the functional molecules being incorporated into
nanofibers, thus leading to a better stability of the functional
molecules against proteinase K digestion in the mixed-
component gel.
Molecular hydrogels have attracted extensive research interests
recently because of their great potential for tissue engineering,1
mineration of organic and inorganic materials,2 drug delivery,3
etc. Molecular hydrogels are formed by the self-assembly of
small molecules (molecular hydrogelators) and those composed
of one kind and two kinds of molecular hydrogelators are
called single-component and two-component molecular hydrogels,
respectively. Compared with single-component molecular
hydrogels, two-component ones possess more functionality,
tunability, and control.4 However, the examples of two-
component molecular hydrogels are much fewer than single-
component ones and it is difficult to rationally design a
two-component hydrogel. Indeed, there is a relatively easy
approach to produce not very strictly two-component molecular
hydrogels, mixed-component ones.
The development of the mixed-component hydrogels was
based on our recent observations.10,11 We had reported that
Nap-GFFY-ss-EEE could be converted to 1 (Scheme 1) by
dithiothreitol (DTT) and we observed the formation of
molecular hydrogels during the conversions.11 However, the gels
were not stable because more and more Nap-GFFY-ss-EEE
was converted to the hydrophobic 1—the gels could only last
for less than one hour and would change into precipitates after
one hour.11 We also found that bovine serum albumin (BSA)
could stabilize the gels of 1 for more than 2 months because
BSA had a hydrophobic pocket that could interact with 1 and
help to stabilize hydrophobic self-assembled nanofibers of 1.11
The phenomenon of gel formation at low conversion percentages
of Nap-GFFY-ss-EEE suggested that Nap-GFFY-ss-EEE
could also co-assemble with 1 and stabilize the hydrophobic
self-assembled nanofibers of 1, which stimulated us to use
Nap-GFFYEG-D-Ala-D-Ala (4 in Scheme 1) to stabilize gels
of 1. The reason for us choosing 4 was due to the fact that
Mixed-component hydrogels are formed by mixing a gelator
with a functional molecule with similar chemical structure to
the gelator. The introduction of functional molecules into the
nanofibers formed by gelators will give novel properties and
functionalities of gels. Several groups have conducted pioneering
works in this field: Stupp’s group reported a mixed-component
nanofiber system with good control of the density of functional
groups,5 Gasiorowski and Collier have developed a mixed-
component hydrogel as an immune adjuvant,6 and both
groups have demonstrated that functional molecules present
at the surface of self-assembled nanofibers exhibit much higher
activities than those in the solution phase;7 Ulijn’s group also
introduced a bioactive RGD ligand into the self-assembled
fibers formed by a short peptide, thus promoting cells spreading
and proliferation in gels;8 Nilsson’s group demonstrated that
a State Key Laboratory of Medicinal Chemical Biology and
College of Life Sciences, Nankai University, Tianjin 300071,
P. R. China. E-mail: yangzm@nankai.edu.cn
b Tianjin Key Laboratory of Molecular Nuclear Medicine, Institute of
Radiation Medicine, Chinese Academy of Medical Science &
Peking Union Medical College, Tianjin 300192, P. R. China.
E-mail: lewis78@163.com
w Electronic supplementary information (ESI) available: Synthesis
and characterization of Nap-GFFYEG-D-Ala-D-Ala, dynamic
strain/time sweeps, and procedures to prepare the gels and determine
the stability of compounds against proteinase K digestion. See DOI:
10.1039/c2cc32348f
Scheme 1 Chemical structures of peptides used for molecular
hydrogelations.
c
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dipeptide D-Ala-D-Ala was the terminal peptide on the bacterial
cell wall and the self-assembled fibers bearing it might mimick
the bacterial cell wall for the study of antibiotics–bacterial cell
wall interactions. We envisioned that the mixed-component gels
formed by compounds 1 and 4 would possess excellent stability
against dilution because 4 is a requisite building block for the
formation of stable nanofibers and 4 would be predominately
located in self-assembled nanofibers but not in the solution
phase.
upper buffer solutions after 108 hours. The release speed of 4
from SH-gel was much lower than those from OMe-gel and
OH-gel. And the release of 4 from SH-gel reached a balance
after 72 hours (less than 5% of 4 got released). These observa-
tions indicated that more 4 was entrapped in the matrix of
SH-gel than the other two gels. The phenomenon partially
demonstrated our design that most of 4 would be incorporated
into self-assembled nanofibers in SH-gel because it is a requisite
building block for stable 3D self-assembled networks.
We firstly tested whether the addition of 4 could stabilize
gels of 1 or not. The results shown in Fig. S4 (ESIw) indicated
that 4 could stabilize the gel and the minimum percentage of 4
to form a stable gel for more than one week was about 25 wt%
to the total amount of compounds 1 and 4. We then prepared
a mixed-component gel containing 70 wt% of 1 and 30 wt% of
4 (SH-gel, Fig. 2) for the test. We also prepared two other
mixed-component gels formed by incorporation of 4 into gels
Nap-GFFY–COOMe (2, OMe-gel) and Nap-GFFY–COOH
(3, OH-gel) as controls.12
We then added proteinase K to the gels to test the stability
of 4 against enzyme digestion. The results in Fig. 1B show that
the cleavage percentage of 4 increased rapidly in the first
2 hours and reached a balance after about 12 hours in three
gels. However, the cleavage percentages of 4 were different in
three gels at 24 hours time point—about 48, 61, and 70% of 4
got cleaved by the enzyme in SH-gel, OMe-gel, and OH-gel,
respectively. More than 80% and nearly 100% of 4 were
cleaved in solutions of it after 1 hour and 4 hours, respectively
(Fig. S6, ESIw). The cleavage percentage of 1 in SH-gel was
also much lower than that of compounds 2 and 3 in their
corresponding gels (Fig. S7, ESIw). These results demonstrated
that the stability of both the bioactive molecule and gelator
against proteinase K digestion could be enhanced in our stable
mixed-component SH-gel, which was crucial for its future
applications in tissue engineering and regenerative medicine.
We then went back to characterize three mixed-component
gels by several techniques such as rheology, TEM, and
fluorescence.
In order to test the stability of 4 in self-assembled nanofibers
in gels, we added equal volumes of phosphate buffer saline
(PBS, pH = 7.4) on the top of gels in small vials and measured
the released percentages of 4 from gels. The results in Fig. 1A
indicate that 4 kept releasing from OMe-gel and OH-gel at
similar speeds and about 25% of it got released from gels to
The rheological measurements (Fig. 2) indicated that both
the storage modulus (G0) and loss modulus (G00) of three
mixed gels showed weak frequency dependency from 0.1 to
100 rad sꢀ1. And the G0 value of three gels was about ten times
larger than their corresponding G00 value. Both results suggested
the formation of 3D elastic networks and true gels. The G0 values
of three gels followed the order of SH-gel > OMe-gel > OH-gel
Fig. 2 Optical images of gels containing 1.0 wt% of gelators (compound 4
was 30 wt% to total amounts of compounds 1 and 4, compounds 2 and 4,
and compounds 3 and 4 for SH-gel, OMe-gel, and OH-gel, respectively)
and dynamic frequency sweep of gels (triangles: SH-gel, circles:
OMe-gel, and squares: OH-gel, filled symbols: G0 and open symbols: G00).
Fig. 1 (A) Released percentage of Nap-GFFYEG-D-Ala-D-Ala from
gels and (B) cleavage percentage of Nap-GFFYEG-D-Ala-D-Ala in gels
(30 wt% of 4 in all gels, total concentration of gelators is 1.0 wt%).
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and proteinase K digestion. Though we only incorporated
Nap-GFFYG-D-Ala-D-Ala into self-assembled nanofibers of
Nap-GFFY–SH, other bioactive molecules with chemical
structures of Nap-GFFYG-hydrophilic bioactive molecules
could also be used to generate stable mixed-component
molecular hydrogels. Since the activity of functional molecules
could be enhanced when present at the surface of self-
assembled nanostructures,7 our novel mixed-component self-
assembled nanofibers will have great potential for tissue
engineering, cancer therapy, and regenerative medicine.
This work is supported by NSFC (31070856 to Yang’s
group and 81171371 to Liu’s group).
Notes and references
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Fig. 3 TEM images of the gels (from left to right: SH-gel, OMe-gel,
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enzyme digestion.
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Fig. 3 showed networks of fibers in gels. The size of nanofibers
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three mixed-component gels. As shown in Fig. 3, there were
slight red shifts for emission spectra of gels, compared with the
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The red shift wavelength number was small and it was about
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same concentration of gelators, the lower intensity of the emission
peak suggested more efficient aromatic–aromatic stacking in self-
assembled fibers. The intensity of peaks followed the order of
SH-gel o OMe-gel o OH-gel, which was consistent with the
result of reverse order of G0 values of three gels.
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In summary, we have introduced a novel mixed-component
molecular hydrogel system with excellent stability against dilution
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6175–6177 6177