A clickable, highly soluble oligopeptide that easily forms organogels

Article abstract of DOI:10.1080/10610278.2013.766735

An artificial peptide, N₃-GVGV-OMe (G, glycine; V, valine), which mimics the repeating GAGA (A, alanine) sequence in Bombix Mori silk, was synthesised via solution-phase synthesis. Compared with N₃-GAGA-OMe sequence, N₃-GVGV-OMe showed high solubility in common organic solvents (such as CHCl₃, THF and CH₂Cl₂), and easily formed organogels simply by adding poor solvents (such as toluene or ether) to the peptide solution at room temperature. The hierarchical nanostructure of N₃-GVGV-OMe organogel was dependent on the nature of the poor solvents, although in all cases, β-sheets were formed exclusively. Gels formed in ether showed higher level hierarchical assembly, as evidenced by AFM and CD studies. Solution-state FT-IR analysis showed that the pre-organisation of the peptides in solution was not significant, and well-defined antiparallel β-sheets were formed after the addition of the poor solvent. The high solubility and strong tendency for self-assembly of N₃-GVGV-OMe, together with its terminal azide group, might facilitate the modification of functional organic molecules even macromolecules for better nanostructure control.

Full text of DOI:10.1080/10610278.2013.766735

Supramolecular Chemistry, 2013
Vol. 25, No. 5, 269–275, http://dx.doi.org/10.1080/10610278.2013.766735
RESEARCH ARTICLE
A clickable, highly soluble oligopeptide that easily forms organogels
Ruiying Gonga, Yubao Songb, Zongxia Guoa, Ming Lib, Yi Jianga and Xiaobo Wana*
aQingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, 189 Songling Road, Qingdao,
Shandong Province 266101, P.R. China;
bDepartment of Chemistry, Qingdao University of Science and Technology, 53 Zhengzhou Road, Qingdao,
Shandong Province 266042, P.R. China
(Received 19 November 2012; final version received 11 January 2013)
An artificial peptide, N3-GVGV-OMe (G, glycine; V, valine), which mimics the repeating GAGA (A, alanine) sequence in
Bombix Mori silk, was synthesised via solution-phase synthesis. Compared with N3-GAGA-OMe sequence, N3-GVGV-
OMe showed high solubility in common organic solvents (such as CHCl3, THF and CH2Cl2), and easily formed organogels
simply by adding poor solvents (such as toluene or ether) to the peptide solution at room temperature. The hierarchical
nanostructure of N3-GVGV-OMe organogel was dependent on the nature of the poor solvents, although in all cases, b-sheets
were formed exclusively. Gels formed in ether showed higher level hierarchical assembly, as evidenced by AFM and CD
studies. Solution-state FT-IR analysis showed that the pre-organisation of the peptides in solution was not significant, and
well-defined antiparallel b-sheets were formed after the addition of the poor solvent. The high solubility and strong tendency
for self-assembly of N3-GVGV-OMe, together with its terminal azide group, might facilitate the modification of functional
organic molecules even macromolecules for better nanostructure control.
Keywords: organogel; nanostructure; self-assembly
Introduction
We herein report the synthesis and self-assembly
behaviour of an artificial tetrapeptide sequence (N3-
GVGV-OMe), which could be considered as a surrogate of
the GAGA sequence. It showed excellent solubility in
common organic solvents such as CHCl3, CH2Cl2 and
tetrahydrofuran (THF), and easily formed organogels at
room temperature (rt) just by adding poor solvents to the
peptide solution. Atomic Force Microscopy (AFM) and
Circular Dichroism (CD) studies showed that the
hierarchical nanostructure of the gels was dependent on
the nature of the poor solvent. The N-terminal of this
peptide was modified with an azide moiety, which we
believe will facilitate the modification of functionalised
organic molecules even macromolecules via ‘click
chemistry’.
Biomacromolecules, such as DNA and proteins, are
perfect models in nature that achieve elegant functions by
self-assembly. A long-term goal in material science is to
learn from nature how to precisely control the properties of
synthesised small organic molecules or macromolecules
by self-assembly. Low-molecular weight oliogopeptides,
inspired from the repeating units in proteins, are still of
interest as organogelators (1–3). For instance, GAGA
sequence, presented in silk protein as the repeating
unit, forms antiparallel b-sheet structures by self-assembly
(4–6). It was used to adjust the nanostructure of
conventional synthetic polymers (7–9) and p-conjugated
oligomers (10–14). These reports strongly supported the use
of oligopeptides as versatile building blocks for facilitating
self-assembled supramolecular structures with definite
properties. However, the tendency of self-assembly of b-
sheet forming oligopeptides is also a double-edged sword: in
many cases, it dramatically decreases the solubility of those
oligopeptides in common organic solvents. For example, a
modified GAGA tetrapeptide which mimicked Bombix
Mori silk showed poor solubility in most of organic solvents
(15, 16). Long alphatic chains (17) were needed to increase
its solubility. This problem presents obstacles in utilising
such oligopeptides to modify small molecules or macro-
molecules in large scale.
Experimental
Tetrapeptide synthesis
Three tetrapeptides, namely N3-GVGV-OMe, N3-GAGA-
OMe and butyryl-GVGV-OMe (as shown in Scheme 1),
were synthesised by solution-phase peptide synthesis
methodology. All the reactants used for peptide synthesis
were purchased from commercial suppliers and used as
received. Solvents such as CH2Cl2 and THF was pre-dried
before use. All glasswares were thoroughly oven-dried.
*Corresponding author. Email: wanxb@qibebt.ac.cn
q 2013 Taylor & Francis

270
R. Gong et al.
at rt overnight. The mixture was washed with brine, and
1. N3-GVGV-OMe
the organic layer was separated and concentrated under
vacuum. The crude product was eluted by flash
chromatography (ethyl acetate/petroleum ether) to afford
compound 6 (5.42 g) in 94% yield.
O
O
O
O
H
N
H
N
N3
N
N
O
O
H
H
O
O
2. N3-GAGA-OMe
O
O
H
N
H
N
Boc-GV-COOH (compound 7)
N3
N
N
H
H
To a 25 mL of methanol solution of Boc-GV-OMe
(compound 6, 2.88 g, 0.01 mol) was added lithium
hydroxide (3.78 g, 9.0 equiv.) all at once. The solution
was stirred at rt overnight. The pH value of reaction
mixture was adjusted to 1 with diluted HCl solution and
the resulting mixture was extracted with CH2Cl2 three
times. The combined CH2Cl2 layers were dried over
anhydrous MgSO4 and concentrated under reduced
pressure to afford compound 7 (2.60 g) in 95% yield.
O
O
3. Butyryl-GVGV-OMe
O
O
O
H
H
N
N
N
H
N
O
H
O
O
Scheme 1. Molecular structures of tetrapeptides synthesised.
The detail synthetic route towards tetrapeptide N3-GVGV-
OMe is shown in Scheme 2. N3-GAGA-OMe and butyryl-
GVGV-OMe were synthesised in a similar manner and
their characterisation was provided in the Supplementary
Information.
NH2-GV-OMe (compound 8)
Boc-GV-OMe (compound 6, 0.14 g, 0.5 mmol) was
dissolved in 10 mL of CH2Cl2 and then 1 mL of
trifluoroacetic acid (TFA) was added. The solution was
stirred at rt overnight. The solvent was removed to afford a
syrup in almost quantitative yield. The crude product was
used in the next step without further purification.
Boc-GV-OMe (compound 6)
Boc-glycine (compound 4, 3.50 g, 0.02 mol) was dissolved
in 40 mL of dry CH2Cl2 at 08C. HOBt (1-hydroxybenzo-
trizole) (2.97 g, 1.1 equiv.) and EDC z HCl (1-ethyl-3-(3-
dimethyllaminopropyl) carbodiimide hydrochloride)
(4.22 g, 1.1 equiv.) were added to the above solution and
stirred for 15 min, followed by the addition of the valine
methyl ester 5 (2.62 g, 1.0 equiv.). The solution was stirred
Boc-GVGV-OMe (compound 9)
Boc-GV-COOH (compound 7, 5.49 g, 0.02 mol) was
dissolved in 40 mL of dry CH2Cl2 at 08C. HOBt
O
O
O
EDC / HOBt
CH2Cl2 , rt.
H
O
OH
+
N
H2N
O
N
H
O
O
N
O
O
H
O
O
5
4
6
Boc-GV-OMe
O
O
LiOH
CH3OH, rt.
H
N
O
N
OH
H
O
O
O
EDC / HOBt
CH2Cl2, rt.
H
H
7 Boc-GV-COOH
N
N
O
N
N
H
O
H
O
O
O
H
CF3COOH
CH2Cl2, rt.
N
H2N
O
9 Boc-GVGV-OMe
O
8 NH2-GV-OMe
OH
N3
O
H
O
O
O
O
O
H
H
H
O
CF3COOH
CH2Cl2 / rt.
N
N
N3
N
N
H2N
N
H
N
N
H
O
EDC / HOBt,CH2Cl2/acetone, rt.
H
O
O
O
O
10 NH2-GVGV-OMe
1 N3-GVGV-OMe
Scheme 2. The synthetic routes of stepwise synthesis of tetrapeptide N3-GVGV-OMe.

Supramolecular Chemistry
271
(1.1 equiv.) and EDC z HCl (1.1 equiv.) were added to the
above solution and stirred for 15 min, followed by the
addition of NH2-GV-OMe (compound 8, 3.76 g, 1.0 equiv.).
The solution was stirred at rt overnight. The organic layer
was then washed with brine and concentrated under
reduced pressure. The crude product was purified by flash
chromatography (ethyl acetate/petroleum ether) to give
compound 9 (7.64 g) in 86% yield.
pressure for 15 h, and then used for characterisations of
AFM and SEM. CD spectra were measured with JASCO-
815 spectrophotometer. The prepared gel was casted onto
a quartz slide and dried in a vacuum oven for 15 h. UV–vis
spectra of N3-GVGV-OMe in CHCl3 and N3-(CH2)4-
COOH were recorded on Cary 50 UV-Vis spectropho-
tometer. HRMS (EI) were carried out on Bruker maXis Q-
TOF mass spectrometer.
NH2-GVGV-OMe (compound 10)
Results and discussions
Boc-GVGV-OMe (compound 9, 0.22 g, 0.5 mmol) was
dissolved in 10 mL of CH2Cl2 and then 1 mL of TFA was
added. The solution was stirred at rt overnight. The solvent
was removed to afford a syrup in almost quantitative yield.
The residue was used for further synthesis without further
purification.
The goal of this research was to find a short peptide
sequence that fulfilled three criteria: (a) it was easily self-
assembled into b-sheets, (b) it was highly soluble in
organic solvents and (c) it had an anchor that can be
attached to other molecules in high yield. For efficient self-
assembly in organic solvents, a tetrapeptide was estimated
to be sufficient (17). Although the GAGA sequence
was widely used with different materials to regulate
their assembly behaviours (8, 15, 16, 18–20), compared
with L-valine, L-alanine was a weaker b-sheets former
(21). It was reasonable to postulate that tetrapeptide
GVGV, with the alanine residue replaced with a valine
residue, would not deteriorate the b-sheets formation
ability. Moreover, since valine residues bear larger
hydrophobic side chains (isopropyl) than alanine (methyl),
the replacement might increase the solubility of the
peptide in organic solvents.
N3-GVGV-OMe (compound 1)
4-Azidobutyric acid (1.29 g, 0.01 mol) was dissolved in
35 mL of dry CH2Cl2 at 08C. HOBt (1.49 g, 1.1 equiv.) and
EDC z HCl (2.11 g, 1.1 equiv.) were added to the above
solution and stirred for 15 min, followed by the addition of
NH2-GVGV-OMe (compound 10, 3.44 g, 1.0 equiv.). The
solution was stirred at rt overnight. The reaction mixture
was pipetted into 200 mL of Et2O and the resulting white
gel was filtered and dried under reduced pressure to afford
compound 1 (3.19 g) in 70% yield.
Actually, a pentapeptide, GVGVP (P, proline), was
found as a part of the repeating sequences in elastin (22).
However, it did not show good self-assembly properties
due to the presence of proline at the end of the sequence
(23). We envisioned that the removal of proline from this
sequence would reinstall its ability to form b-sheets. 4-
Azidobutyric acid was chosen as the anchor to introduce
the azide group to the N-terminal of the peptide.
Gel formation
Typical method to prepare N3-GVGV-OMe gel in toluene:
3.3 mg of N3-GVGV-OMe was dissolved in 0.1 mL of
CHCl3 to afford a transparent solution. To this solution,
0.9 mL toluene was added dropwise. The gel formation
was confirmed by vial inversion method.
Thus, N3-GVGV-OMe (compound 1) was synthesised
via conventional solution-phase peptide synthesis, as shown
in Scheme 2. N3-GAGA-OMe (compound 2), which
mimicked the key repeating unit in Bombix Mori silk, and
butyryl-GVGV-OMe (compound 3), which was used to
investigate the role of the azide group in self-assembly, were
also synthesised in a similar manner (details available in
Supplementary Information) for comparison.
Typical example of formation of N3-GVGV-OMe gel
in Et2O: 6.9 mg of N3-GVGV-OMe was dissolved in
0.4 mL of CHCl3 to afford a transparent solution. To this
solution, 0.4 mL of Et2O was added dropwise. The gel
formation was confirmed by vial inversion method.
Characterisation
FT-IR analysis was carried out on Nicolet 6700 FT-IR
Spectrometer. Freshly prepared N3-GVGV-OMe solution
(in CHCl3) was used for solution-state FT-IR analysis.
Xerogel samples for FT-IR were prepared simply by
drying the wet gels under reduced pressure for 15 h. AFM
was carried out on Agilent 5400 using tapping mode. SEM
was carried out on Hitachi S-4800 with an accelerating
voltage of 2.5 kV. The fresh prepared organogel was
casted on freshly cleaved mica, dried under reduced
Solubility and gel formation ability
Since good solubility of the oligopeptides in organic
solvents is one of the key factors to make them applicable
in modification of functionalised materials, we explored in
detail the solubility of all three peptides (results, see
Table S1 of the Supplementary Information, available
online). We were surprised to find that N3-GVGV-OMe
and butyryl-GVGV-OMe showed excellent solubility in

272
R. Gong et al.
common organic solvents such as CH2Cl2, CHCl3, THF
and dimethylformamide (DMF) at rt. In sharp contrast, N3-
GAGA-OMe was only soluble in polar organic solvents
such as DMF and dimethyl sulfoxide (DMSO). It is not
surprising that GAGA type tetrapeptides showed poor
solubility as evidenced in the literature (15, 16).The
improvement of the peptide solubility in organic solvents
by switching alanine residue to valine residue might be
attributed to the larger hydrophobic isopropyl group in
valine. The solubility of N3-GVGV-OMe was even up to
650 mg/mL in CHCl3, which is an obvious advantage to use
this peptide to modify other functional molecules for
inducing the self-assembly thereof. We noticed that some
oligopeptides for self-assembly studies were also reported to
be soluble in common organic solvents, but all with bulky
Boc protecting group on N-terminals to increase its
solubility (24, 25).
organogels easily by adding poor solvents. The minimum
gelation concentration (MGC) of N3-GVGV-OMe was in
mg/mL scale (e.g. MGC in CHCl3/toluene was
3.3 mg/mL).
Interestingly, although butyryl-GVGV-OMe also
showed good solubility in CHCl3, no obvious gelation
formation was observed by adding ether or toluene to its
CHCl3 solution. This suggested that the azide group also
contributed to the self-assembly of tetrapeptide, maybe via
p–p stacking from azide groups (28, 29). The formation
of organogel for N3-GVGV-OMe but not for N3-GAGA-
OMe or butyryl-GVGV-OMe indicated that a delicate
balance between H-bonding, hydrophobic–hydrophobic
interactions and p–p stacking was needed to achieve
better self-assembly.
We also noticed that the polymorphism of the gel was
strongly influenced by the poor solvents. Regardless the
type of good solvents used to dissolve the peptide, gels
formed in ether were opaque while gels formed in toluene
were transparent, which inferred the existence of different
hierarchical nanostructures. An example (images of
organogels obtained in CHCl3/Et2O and CHCl3/toluene,
insertion) is shown in Figure 1. The difference in gel
polymorphism was studied by AFM. Figure 1(a) clearly
showed that the organogel formed in CHCl3/Et2O
consisted of both thin and thick fibrils. An enlarged
image (Figure 1(b)) showed that a thick left-handed-
helices fibre (ca. 45 nm in width and 25 nm in pitch) was a
higher level hierarchical structure, composed of at least
two or more twisted filaments. The AFM images of
organogel obtained in CHCl3/toluene (Figure 1(c)), on the
other hand, showed more uniformed but thinner nano-
fibres. SEM images (see Figure S1 of the Supplementary
Information, available online) also gave similar results.
The difference in the nanostructures of the same
oligopeptide in different solvents showed that the self-
assembly process was strongly dependent on solvents (30,
31). The coexistence of both thin and thick nanofibres in
organogel formed in CHCl3/Et2O might be accounted for
its opaque appearance, which was supported by a recent
Moreover, unlike most of peptides, which had to be
dissolved in hot solvents and then cooled down to rt to
form organogel, N3-GVGV-OMe formed organogel
simply by adding poor solvents (ether or toluene) to the
peptide solution (in CHCl3, CH2Cl2 DMF or THF) at rt.
On the other hand, the solution of N3-GAGA-OMe failed
to form organogels when poor solvents were added, and
only a precipitate was observed instead (see Table S2 of
the Supplementary Information, available online). This
result indicated that when the alanine moiety in the GAGA
sequence was replaced by valine, the tetrapeptide showed
better ability to gelate organic solvents. To the best of our
knowledge, the oligopeptide gel formation by adding a
poor solvent to a peptide solution at rt was scarcely
reported, which might be due to the poor solubility of the
peptides. A phenylalanine dipeptide formed organogels
when it was firstly dissolved in 1,1,1,3,3,3-hexafluoropro-
panol and then diluted with poor solvent (26). Fmoc-
protected dipeptides formed hydrogels when first dis-
solved in DMSO and then diluted with water (27). In both
cases, a highly polar solvent had to be used to dissolve the
peptide. In our case, a wide variety of organic solvents
could be used to dissolve N3-GVGV-OMe and all formed
Figure 1. AFM images of N3-GVGV-OMe xerogels obtained in CHCl3/Et2O (gel concentration: 8.6 mg/mL) (a and b) and in
CHCl3/toluene (gel concentration: 3.3 mg/mL). (c) The images of the organogels were inserted at the up left corner in (a) and (c).

Supramolecular Chemistry
report on the polymorph changes of gels formed at
different temperatures (32).
273
FT-IR study on the gel formation
Despite their appearance differences, the nanofibres
obtained in both gel systems consisted mostly of b-sheets,
as indicated by FT-IR analysis. It was generally accepted
that the absorptions in the amide A, amide I and amide II
regions were indicative for peptide’s secondary structures,
such as a-helix and b-sheet (33). The non-assembled
peptides normally had absorptions at 3412–3426 cm21
(amide A), 1659–1678 cm21 (amide I) and 1503–
1516 cm21 (amide II), while peptides forming b-sheet
showed absorptions at ,3280, ,1630 and ,1540 cm21
.
Figure 3. FT-IR spectra of N3-GVGV-OMe in CHCl3 solution
at different concentrations: (a) 1.1 mg/mL, (b) 3.3 mg/mL, (c)
9.0 mg/mL and (d) 33.0 mg/mL.
Figure 2 showed the IR spectra of the xerogels of N3-
GVGV-OMe obtained in different gelation systems. All
the xerogels showed very similar absorption peaks,
regardless of the solvents. The absorptions at 3281, 1627
and 1544 cm21 clearly indicated the formation of b-sheet,
and the weak absorption at 1688 cm21 inferred that the b-
sheet was in an antiparallel manner (33). No characteristic
peaks of either a-helix or random coil were seen in these
spectra, indicating the strong tendency towards antiparallel
b-sheet formation of this oligopeptide.
It would be interesting to investigate how the antiparallel
b-sheet was formed: whether there was a pre-organisation of
the oligopeptide in the solution and the nanofibre was
formed simply because of the solubility decrease due to the
addition of the poor solvent, or the peptide was reorganised
and formed b-sheet after the addition of poor solvent? So,
the solution-state FT-IR spectra of N3-GVGV-OMe at
different concentrations ranging from 1.1 to 33.0mg/mL
in CHCl3 solution were recorded and analyzed
(as shown in Figure 3). Since CHCl3 showed a broad
absorption around 3000 cm21 (see Figure S2 of the
Supplementary Information, available online), which
overlapped with the absorption of N3-GVGV-OMe in this
region, only the absorption evolution in amide I and amide II
regions was discussed. At lower concentrations, the broad
absorption around 1642cm21 was predominant in amide I
region, which was not a typical antiparallel b-sheet absorp-
tion, indicating that the pre-organisation of the peptide into
antiparallel b-sheet was not obvious (Figure 3(a),(b)). At
higher concentrations, certain degree of assembly appeared,
as evidenced by the splitting of the absorptions in amide I
region, but the non-bonded peptide absorptions, for example
absorption at 1518cm21, still existed (Figure 3(c)). When
the concentration was increased to 33.0 mg/mL, the
formation of b-sheet still could not be clearly observed
(Figure 3(d)). Hence, the addition of the poor solvent was the
major reason for the formation of well-organised b-sheet
structure in gels. This observation was different from the
reported pre-organisation of oligopeptides to form b-sheet in
solution before the gel was formed (33–35).
Circular dichroism study on the details of the self-
assembly
Xerogels were placed perpendicular to the light path and
rotated with a constant speed within the film plane to avoid
polarisation-dependent reflections and eliminate the
possible angle dependence of CD signal (36). Normally,
peptides which formed b-sheets had a weak positive CD
signal around 190–195 nm and a stronger negative peak
around 220 nm. However, for the gels obtained from
CHCl3/toluene, the characteristic peak around 220 nm for
b-sheets was barely seen (Figure 4(a)), maybe due to
the overlap with the strong azide absorption at 228 nm
Figure 2. FT-IR spectra of N3-GVGV-OMe xerogels obtained
in: (a) THF/toluene (gel concentration: 3.3 mg/mL), (b)
CHCl3/toluene (gel concentration: 3.3 mg/mL) and (c)
CHCl3/Et2O (gel concentration: 8.6 mg/mL).

274
R. Gong et al.
Possible explanation for gel formation
As described in previous sections, slight structural changes
dramatically changed the ability of the peptide to form
organogels. H-bonding, p–p stacking and hydrophobic–
hydrophobic interaction have to be dedicately balanced to
achieve good self-assembly. N3-GAGA-OMe was unable
to form organogels, which was ascribed to weaker
hydrophobic–hydrophobic interactions compared with
N3-GVGV-OMe; butyryl-GVGV-OMe also failed to
form hydrogels, which might be due to the absence of
azide–azide interaction. Azid–azide interactions were
observed in both organic molecules and organic–
inorganic complexes, hence also played an important
role here in the tetrapeptide assembly (28, 29, 37). A
plausible stacking model is proposed in Scheme 3 to
explain the gel formation process. These nanofibrils of N3-
GVGV-OMe in organogels state were composed of stacks
of molecules formed from coefficient interaction of H-
bonding and p–p stacking direction. The different
gelation states of N3-GVGV-OMe in Et2O and toluene
might also be explained by p–p stacking: stronger azide
stacking in ether would be expected since Et2O would not
interfer with azide group; on the other hand, toluene,
which is able to interact with azide group via p–p
stacking (37), would weaken the p–p stacking between
azide groups. As shown in Scheme 3, p–p stacking might
play an important role for b-sheets to form large bundles.
This might explain why larger bundles were formed when
Et2O was used as poor solvent but not in the case of
toluene. Given that if the nanostructure was not
altered during xerogel preparation, this also would explain
the bathochromic shift of the azide absorption in CD
spectra.
Figure 4. CD spectra of of N3-GVGV-OMe xerogel obtained in
(a) CHCl3/toluene (gel concentration: 3.3 mg/mL) and (b)
CHCl3/Et2O (gel concentration: 8.6 mg/mL).
(azide group has a strong absorption around 230 nm, see
Figure S3 of the Supplementary Information, available
online). The strong negative azide absorption peak
indicated that the azide groups were stacked in a chiral
environment. However, a positive peak at 200 nm and a
negative shoulder around 220 nm were seen in the CD
spectrum for xerogel obtained from CHCl3/Et2O system
(Figure 4(b)), which was indicative for the existence of b-
sheets. Interestingly, the azide absorption was shifted from
228 to 240 nm, indicating the existence of subtle nano-
structure differences caused by switching poor solvent
from toluene to ether. Although FI-IR suggested b-sheets
were formed in all the cases, the redshift of the azide peak
indicated better stacked azide groups in the organogel
obtained from CHCl3/Et2O system. This is in accordance
with its higher level hierarchical nanostructure observed in
AFM study.
π−π Stacking
O
O
H
N
H
N
N
O
O
O
O
N
N
H
N
H
N
N
N
H
O
N
H
O
N
O
N
N
N
H
O
N
H
O
O
O
O
H
N
O
H
N
O
O
O
O
H-bonding
O
N
N
H
H
N
H
N
H
O
N
N
N
N
O
N
N
N
H
N
H
O
O
H-bond
direction
O
O
Hydrophobic interaction
O
O
H
N
H
N
N
O
O
H
N
H
N
N
N
H
O
N
H
O
N
O
O
N
N
N
H
O
N
H
O
O
O
O
H
H
O
O
O
N
N
N
N
H
H
N
H
N
H
O
N
N
N
N
O
N
N
N
H
N
H
O
O
O
O
π−π Stacking direction
Scheme 3. Plausible stacking model for antiparallel b-sheet formation.

Supramolecular Chemistry
275
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Conclusion
In summary, N3-GVGV-OMe, an artificial tetrapeptide
which mimics GAGA sequence in Bombix Mori silk, was
rationally designed and synthesised. It showed good
solubility in most of organic solvents, and easily formed
organogels when poor solvent was added to the peptide
solution at rt. However, N3-GAGA-OMe sequence was
only soluble in highly polar organic solvent such as DMSO
and DMF, and failed to form organogel when added to
poor solvent. This indicated that the artificial tetrapeptide
showed better gelation ability than the natural one under
the same preparation conditions. This might be attributed
to the replacement of alanine moiety with valine moiety,
which not only increased the solubility of the peptide
sequence, but also offered stronger hydrophobic–hydro-
phobic interactions which favoured the formation of
organogels. Further investigation showed that the terminal
azide group also contributed to the gel formation.
Solution-state FT-IR suggested that the formation of
well-defined b-sheets was induced by the addition of poor
solvents. Analysis showed that although all the nanofibres
in the gel consisted of antiparallel b-sheets, the fine
nanostructure was dependent on the nature of the poor
solvents. Nanofibres obtained in ether showed higher level
hierarchical assembly, as evidenced by AFM and CD
studies. We are currently exploring the applications of
this tetrapeptide in modifying conjugated polymers
and other functional materials for better nanostructure
control.
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Acknowledgements
This work is fanancially supported by ‘100 Talents Program’
from the Chinese Accademy of Sciences, and Natural Science
Foundation of Shandong Province (ZR2010BZ007,
BS2009CL026 and Y2008B03). We are also very grateful for
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