Biomimetic self-assembly of tetrapeptides into fibrillar networks and organogels

Article abstract of DOI:10.1002/ejoc.200800547

The self-assembly features of a family of tetrapeptides with a silk-inspired structure are presented. An exhaustive study of the influence of the terminal alkyl chain length in this process is undertaken. Scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), FTIR spectroscopy, and circular dichroism were used for structural analysis. These compounds, as in the natural model, self-assemble into antiparallel β-sheet structures that further organize to form fibrillar aggregates. Furthermore, some of them are capable of forming a crowded network that entraps the solvent leading to physical gels with different microscopic morphologies. A model for the assembly process is proposed. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800547

FULL PAPER
DOI: 10.1002/ejoc.200800547
Biomimetic Self-Assembly of Tetrapeptides into Fibrillar Networks and
Organogels
Sajid Iqbal,[a] Juan F. Miravet,*[a] and Beatriu Escuder*[a]
Keywords: Self-assembly / Gels / Peptides / Silk / Fibers / Protein folding
The self-assembly features of a family of tetrapeptides with
a silk-inspired structure are presented. An exhaustive study
of the influence of the terminal alkyl chain length in this pro-
cess is undertaken. Scanning electron microscopy (SEM),
wide-angle X-ray diffraction (WAXD), FTIR spectroscopy,
and circular dichroism were used for structural analysis.
These compounds, as in the natural model, self-assemble
into antiparallel β-sheet structures that further organize to
form fibrillar aggregates. Furthermore, some of them are
capable of forming a crowded network that entraps the
solvent leading to physical gels with different micro-
scopic morphologies. A model for the assembly process is
proposed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
from low molecular weight molecules has been studied ex-
tensively in the recent past. Many efforts in this field are
directed towards an understanding of the role that the dif-
ferent structural elements present in the gelators play in the
noncovalent association that yields the final fibrillar struc-
tures.[9–11]
In the last years we have been involved in the study of
low-molecular weight gelators based on amino acids. These
small peptidomimetic compounds self-assemble into sheet-
like extended structures that form fibrous aggregates and
gels in different solvents.[12] Recently, we have reported the
design of a novel tetrapeptidic organogelator (4) based on
a minimal sequence found in silks, Gly–-Ala–Gly–-Ala,
and incorporating C12 alkyl chains at both ends of the tetra-
peptide.[13] We studied its self-assembly in organic solvents
and it was revealed that this sequence showed a high prefer-
ence for the antiparallel assembly into β-sheets (Scheme 1).
Introduction
Nature’s high complexity and efficiency has always at-
tracted the attention of scientists, firstly to unravel its fasci-
nating structures and processes and subsequently intending
to imitate the beauty and harmony of them. For instance,
mimicking the natural control of the conformation of biom-
olecules at the different levels of complexity and function
has been an appealing goal. In this sense, the study of pro-
tein structure and function has created a wide interest.[1,2]
It is well known that the different organization levels play
an important role in different structural and functional pro-
teins. For example, structural proteins (i.e. collagen, elastin,
and silks) base their physical properties in different aspects
of their secondary structure. On the other hand, precisely
folded switch points determine the active sites in catalytic
proteins and enzymes.[3] Furthermore, some devastating dis-
eases such as amyloidogenic pathologies are related to
anomalous protein folding that induces precipitation of
toxic plaques. Recently, intense research has been devoted
to the understanding of the amyloid aggregation mecha-
nisms as well as to the development of strategies to prevent
this unwanted process.[4]
In this context, the study of model systems, including
peptides and small peptidomimetics, that self-assemble into
fibrillar aggregates may help to determine the molecular ba-
sis for such processes.[5–8] In connection with these studies,
the formation of self-assembled fibrillar networks and gels
[a] Departament de Química Inorgànica i Orgànica, Universitat
Jaume I,
12071 Castelló, Spain
Fax: +34-964729155
E-mail: escuder@qio.uji.es
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 1.
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Self-Assembly of Tetrapeptides into Fibrillar Networks and Organogels
In natural silk proteins, specific amino acids induce a hydrogen bonding in the other case) is expected to allow
turn in the polypeptidic chain causing its preorganization fine tuning of the aggregation behavior with the use, for
for an antiparallel β-sheet. In contrast, it is remarkable example, of a variety of solvents. We are interested in as-
that in the reported gelator the “folding effect” was driven sessing how properties such size and aspect of the micro-
only by noncovalent interactions. We believe that this natu- scopic fibers, gelation of different solvents, antiparallel pep-
rally selected one-dimensional assembly motif has a great tide arrangement, or microcrystalline packing are changed
potential for the design of new functional fibrous nano- through a family of structurally related molecules.
materials.
Here we report on the preparation and study of a library
of compounds based on the variation of the terminal
groups attached to the silk-mimetic tetrapeptide motif Gly–
Results and Discussion
-Ala–Gly–-Ala (Scheme 2). Our aims include to clarify
how the interplay of the noncovalent interactions provided
by the apolar end moieties and the peptide core affect the
self-assembly of these molecules. The different nature of the
intermolecular interactions provided by these moieties
(van der Waals and solvophobic/solvophylic in one case and
A library of molecules containing different aliphatic (C12,
C6, or C3 alkyl radicals) and aromatic (phenyl and 4-ni-
trophenyl) end chains were prepared. Most of the synthe-
sized compounds bear different end groups on the peptide
termini and a few of them are “symmetrically” substituted,
bearing analogous side chains on both ends of the peptide
(compounds 4, 5, and 6). The synthesis of these molecules
was carried out by using conventional peptide chemistry
methods (Scheme 2). A convergent approach was used and
tetrapeptide derivatives 4–18 were obtained by coupling the
corresponding dipeptide derivatives with the use of EDC or
DCC as coupling reagents. The preparation of N-acylated
dipeptides 3 was carried out by a Schotten–Baumann pro-
cedure. Compounds 2a–c were prepared by amide forma-
tion on the C-terminus of the Z-protected dipeptides and
subsequent removal of the protecting group. All these com-
pounds were fully characterized by NMR spectroscopy and
mass spectrometry (see Experimental Section).
Gelation Studies
Firstly, the gelation capability of the compounds with
alkyl end groups (4–13) in organic solvents was studied, and
the results are collected in Table 1. In general, all com-
pounds were poorly soluble and had to be heated at the
boiling temperature of the solvent to obtain a clear solu-
tion. After cooling to room temperature many of them
formed gels and some gave place to phase separation by
formation of aggregates that could not be defined as proper
gels but as weak gels or colloidal precipitates. In general,
gels were obtained both in polar and apolar solvents, and
most of them were transparent at their minimum gel con-
centration (mgc).
It is interesting to compare the gelation properties of the
compounds that present analogous alkyl chains on both
ends of the peptide (compounds 4, 5, and 6, which present
respectively C12, C6, and C3 chains). It can be observed that
in the more polar solvents the compound with C6 chains
(5) is in general a better gelator than 4 (C12 chains), which
does not form proper gels in alcohols. It could be argued
that in compound 4 the long C12 end chains preclude gel
formation in polar solvents. Although solvophobic interac-
tions are expected to be important for aliphatic chains in
polar solvents, it seems that the long and flexible C12 chains
may not be contributing significantly to the 1D aggregation
motif shown in Scheme 1.
Scheme 2.
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Table 1. Gelation behavior of compounds 4–12 (5 mg/mL).[a]
Solvent
Compound
4
5
6
7
8
9
10
11
12
Methanol
Ethanol
2-Propanol
Cyclohexanol
Acetone
THF
DME
CH3CN
CH2Cl2
A
WG
A
WG
G (11)
G (7)
G (11)
G (11)
G (11)
G (7)
WG
G (13)
WG
G (13)
G (13)
A
P
G
G (9)
A/P
WG
G (9)
WG
WG
G (9)
G (9)
G (9)
A/P
G (10)
G (10)
G (3)
G (10)
I/A
I/A
WG
G (6)
WG
WG
WG
A
G (9)
G (9)
G (7)
G (9)
A
G (9)
A
G (7)
G (9)
G (9)
WG
G (9)
G (6)
WG
G (12)
G (5)
G (12)
I
I
P
G (7)
G (12)
G (12)
G (12)
P
A
G (10)
A
G (10)
A
Pc
A
A
Pc
WG
Pc
Pc
G (12)
G (12)
G (12)
G (12)
P
Pc
Pc
G (12)
Pc
Pc
Pc
Pc
S
WG
G (8)
A
G (8)
G (8)
G (8)
WG
WG
G (8)
I
G (4)
WG
I
I
A
A
G (11)
G (11)
G (11)
A/Pc
CHCl3
Dioxane
Cyclohexane
Toluene
G (9)
Pc
WG
G (8)
G (10)
WG
Pc
[a] G = gel, WG = weak gel, P = precipitate, Pc = colloidal, I = insoluble, A = aggregates, S = soluble. Minimum gel concentration (m)
in parentheses.
On the other hand, if the gelation capabilities of com- compounds with similar end chains the interactions among
pounds 5 (C6 end chains) and 6 (C3 end chains) are com- the apolar moieties are strengthened relative to the case of
pared it can be noticed that significant differences are ob- unsymmetrical compounds favoring the formation of gels.
served in solvents that present low or moderate polarity.
In order to assess in more detail the role played by the
For example, in toluene compound 5 forms gels at a mgc aliphatic end groups in gelation, the replacement of one of
value of 9 m, whereas compound 6 forms a precipitate. the alkyl tails by an aromatic fragment was studied. In ge-
This behavior is related to that observed, for instance, in neral, the substitution of alkyl moieties by aromatic moie-
dichloromethane where compound 5 forms a gel, whereas ties had a significant effect on the gelation behavior. For
compound 6 is insoluble. These results can be easily ration- example, compound 5 (C6, C6) forms gels in toluene, but
alized by realizing that hydrogen-bonding interactions can replacement of one C6 moiety by a phenyl or 4-nitrophenyl
be dominant in solvents of low-to-moderate polarity and group (compounds 15 and 16, respectively) gives place to
that in these solvents the alkyl chains may play a role in the insoluble materials in this solvent. A similar tendency can
solubilization of the molecules by favorable interactions be observed for compound 4 (C12, C12), which is a good
with the solvent. In the case of molecule 6, the reported gelator in toluene, but it is transformed into a weak gelator
behaviors could be ascribed to the fact that the C3 chains in the case of compound 17 and into insoluble species in
are too short. They would preclude the formation the long the case of 18. Clearly, these results indicate that in apolar
aggregates required for fiber formation as a result of the solvents the aliphatic moieties play a relevant role in the
insoluble nature of the associated species. On the contrary, formation of fibrillar structures that cannot be paralleled
C6 (and C12) chains would impart the required solubility, with the use of aromatic moieties. Most likely, the aliphatic
for example, in toluene. This line of reasoning would ex- moieties provide this family of molecules with more intense
plain the fact that in the most apolar solvent, cyclohexane, intermolecular interactions and a better packing capability
gels are formed only with the compound with C12 chains, relative to those of analogous molecules with aromatic end
4.
groups. Indeed, compounds bearing two aromatic end
If the gelation capabilities of the compounds that present groups (i.e. compound 19) failed to form gels.
different alkyl chains on both ends are considered (com-
It is interesting to note that in a solvent of intermediate
pounds 6–12) it can be observed again that in apolar sol- polarity, such as THF, compound 6 (C3, C3) is not a gelator
vents the presence of short C3 chains is not favorable for but its aromatic analogue with one phenyl end group (13)
gel formation. For example, compounds 6, 11, and 12 are is capable of forming gels. This fact could be the result of
not capable of gel formation in toluene. The only com- improved solvation of the phenyl moieties as compared to
pound in the series that contains C3 end chains and forms the C3 units, which would avoid precipitation of the aggre-
gels in solvents of low polarity is 8, which presents a C12 gates. Finally, the more polar 4-nitrophenyl analogue (com-
chain that would compensate for the presence of the C3 pound 14) does not form gels in the range of solvents
chain on the other end. The data in Table 1 in general sug- studied.
gest that the compounds with similar alkyl chains on both
ends are better gelators. For example, in acetone proper gels
are only formed by compounds 5 (C6 chains on both ends)
and 6 (C3 chains) and weak gels are formed by compounds
Spectroscopic Studies
4 (C12 chains) and 7 (C12, C6 chains). All the other com-
FTIR spectroscopy is a useful technique to investigate
pounds, which present dissimilar alkyl chains on both ends, the secondary structure of peptides and proteins.[14] For all
do not form gels in acetone. These results suggest that in the studied dialkyl compounds (4–12), the amide I band
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Self-Assembly of Tetrapeptides into Fibrillar Networks and Organogels
appeared at 1625 cm–1, which is typical for the strong H-
Circular dichroism was also used to support the forma-
bonding of the β-sheet secondary structure. Moreover, a tion of β-sheet type-aggregates.[15] Solid-state spectra of
weak band appeared at 1695 cm–1 indicating a high content xerogels were recorded and, in all the cases, a pattern typi-
of antiparallel β-sheet organization, in agreement with our cal for this arrangement was observed: a negative band with
previous results on compound 4 (see for instance Figure 1 a maximum at ca. 220 nm, and crossing zero at ca. 210 nm,
for compound 5 and Supporting Information). It is note- and in some cases a positive lobe at 195 nm (see for instance
worthy that the FTIR spectra of dried gels of compounds Figure 3 for compound 5 and Supporting Information).
with aromatic end units such as compounds 15 and 17
showed the presence of a broad absorption band in the
amide I region, with several overlapping peaks revealing not
only an antiparallel β-sheet organization (1698 cm–1) but
also other randomly organized assemblies (1682, 1668,
1650 cm–1, see Supporting Information). This indicates that
replacement of one of the alkyl units by a phenyl moiety
generates weaker intermolecular interactions between aro-
matic and alkyl groups in the antiparallel arrangement rela-
tive to those found in the compounds with two alkyl groups.
In this way, the clear preference for the antiparallel arrange-
Figure 3. CD spectrum of a xerogel of compound 5 (from a toluene
gel).
ment found in compounds 4–12 is lost in analogues 13–18
with aromatic units, which would present also other aggre-
gation modes including a parallel β-sheet disposition
among them (Figure 2).
X-ray Powder Diffraction
Wide-angle X-ray diffraction (WAXD) spectra of the
xerogels were in good agreement with the proposed β-sheet
arrangement. In all the cases, a low-angle sharp peak corre-
sponding approximately to a calculated distance for a fully
extended molecule was obtained. For example, for com-
pound 5, the WAXD data of xerogels obtained from dif-
ferent solvents provided in all the cases peaks correspond-
ing to a periodicity of 30 Å, which fits nicely with the calcu-
lated distance for an extended molecule. Additionally, in all
the cases, peaks assigned to a periodic distance of 4.4 Å
were found, which can be ascribed to extended two-dimen-
sional layers of β-sheets that stack parallel on top of each
Figure 1. FTIR of the amide C=O region of xerogel of compound
5.
other (see Figure 4 and Supporting Information for other
compounds).
Figure 4. Proposed packing model (top) and WAXD pattern (bot-
tom) for xerogels of compound 5.
For a given compound, there were not important differ-
ences in the position of the diffraction peaks between dif-
ferent solvents. This fact reveals that although macroscopic
properties such as solubility or gelation efficiency are
strongly solvent dependent, the solvent has a limited effect
in the organization at the molecular level. Only some subtle
Figure 2. Proposed antiparallel and parallel arrangements for com-
pound 15.
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S. Iqbal, J. F. Miravet, B. Escuder
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differences were observed for the related low-angle peaks, form networks whereas others merged into rigid and long
probably, with a different degree of mobility of the tails. ribbons, or even fused into globular objects with mem-
Higher angle peaks, corresponding to shorter distances in brane-like aspect.
the microcrystalline structure, were sometimes absent, espe-
cially in xerogels from apolar solvents, which also showed
broader bands than in polar ones. This is probably due to
a higher extent of tail mobility that could provoke the pres-
ence of some folded segments within the chain.
In natural silk fibers, the antiparallel organization is
achieved through an intramolecular turn of the polypep-
tidic chain, and the repeating distance corresponds to the
H-bonding direction of ca. 9.4 Å.[16] In our case, where this
turn is not present, we found two different situations:
(i) compounds bearing C12 tails, which showed a spacing of
ca. 11 Å and (ii) compounds with shorter length tails, which
presented shorter spacing (8.5 Å). In the first case, the
greater chain mobility of the longest tails could explain the
observed larger spacing, whereas in the second, shorter tails
resulted in more compact packing.
At a higher level of organization, two different packing
models may be proposed for the arrangement of the anti-
parallel β-sheets, namely, head-to-head or head-to-tail (see
Scheme 3). For symmetrically substituted compounds, only
the head-to-tail packing fits with the observed low-angle
diffraction peaks, coincident with the fully extended molec-
ular dimension. A head-to-head packing would produce
larger repeating distances than those obtained experimen-
tally. Similarly, in the case of unsymmetrically substituted
compounds, head-to-tail packing also fits with the experi-
mental data and a head-to-head packing seems less likely
because it would cause kinks and folds in the chains with
the result of a loss of packing efficiency.
Figure 5. SEM images of xerogels and colloidal precipitates from
compounds 5 (A,B), 6 (C), 8 (D), 10 (E), and 12 (F) gels in toluene.
Although WAXD studies have revealed that all the
studied molecules are present with basically the same or-
ganization at the molecular level, the gel network showed a
diversity of morphologies depending mainly on the solvent.
For instance, the network of xerogels obtained from toluene
is formed by tiny fibrils of less than 100 nm width that are
fused forming bags of micrometer size that were probably
filled by solvent in the original gels (Figure 5).[13] Even for
compounds 6 and 12, which did not form strong gels, a
similar morphology was observed with only subtle differ-
ences at the microscopic level. In difference, xerogels from
acetonitrile (Figure 6) revealed a network made by very
Scheme 3.
long and thin fibers, with several hundreds of nanometers
in width and tens of micrometers in length, that in some
cases appeared highly aligned (5 and 10), whereas in other
cases showed a higher tendency for coiling and cross linking
Electron Microscopy Studies
(7, 8, 9, and 12). For xerogels prepared from alcohols (Fig-
The microscopic aspect of the gels and fibrillar aggre- ure 7), very long and thin fibers were also observed in gene-
gates obtained was studied by scanning electron micro- ral. However in the case of compound 11 a different aspect
scopy. For this purpose, aggregates were dried and the xero- was observed in methanol. In this case, fibers were signifi-
gels were investigated. These compounds presented a high cantly shorter, and in consequence, the formation of a net-
tendency to form fibrillar architectures. Fibers and fibrils work was not complete and isolated aggregates were formed
were observed in most of the cases with differences in instead of a strong gel. The xerogel from compound 8 was
length, stiffness, and branching degree, revealing that these also different, but in this case, a higher degree of cross-
compounds have a great tendency to form elongated objects linking could be observed, in agreement with the formation
(Figures 5, 6, and 7). Some of them further entangled to of a strong gel in that solvent.
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Self-Assembly of Tetrapeptides into Fibrillar Networks and Organogels
From this behavior, it may be concluded that, for those
compounds that formed gels, the nature of the side chains
does not seem to affect significantly the microscopic aspect
of the xerogel for a given solvent being the formation of
fibrillar networks a general trend. Only in the cases where
a strong gel was not formed, namely aggregates or precipi-
tates being formed instead, were different morphologies ob-
served.
Taking into account all the structural information dis-
cussed above and the fact that all these compounds, despite
the fact that they show different specific morphologies, as-
semble into fibrillar aggregates, we may propose a model
for their hierarchical organization into microscopic size fi-
bers. In a first level of organization, molecular entities as-
semble to form an antiparallel β-sheet secondary structure,
with a vague influence of the length of the alkyl chains.
These extended sheets stack into parallel layers separated
by 4.4 Å in all the cases. These layers have a preferential
one-dimensional growth that produces fibers directed by
the formation of H-bonds parallel to their axes. This ar-
rangement is known as the cross-β structure and is typical
for amyloid-like fibrils and some silk fibroins (Fig-
ure 8).[8,17]
Figure 6. SEM images of xerogels from compounds 5 (A), 7 (B), 8
(C), 9 (D), 10 (E), and 12 (F) gels in acetonitrile.
Figure 8. Proposed cross-β packing.
The potential influence of side groups in the self-as-
sembly of oligopeptides and its effect on the fiber packing
has been reported. In our case, as in other reported, it
seems reasonable that the strongest interaction, H-bonding,
runs parallel to the fiber axis and that interactions between
tails have a smaller contribution to the resulting elongated
objects.[18] This model would explain why fibrillar aggre-
gates are always found for those compounds that formed
gels, independently of the chain length. Thus, the effective-
ness of the interaction between tails would affect the thick-
ness of the fibers, and to some extent the length of the fiber.
For instance, packing of symmetrical tails would lead to a
more stable situation than for unsymmetrical ones, where
intercalation or folding may produce packing mistakes and
kinks.
Conclusions
We have studied in detail the aggregation behavior of a
series of silk-inspired low molecular weight compounds
containing a β-sheet-like H-bonding core and apolar end
groups. It has been shown that these molecules present a
high tendency to form fibrillar networks, which in many
cases produced gels. The results have provided useful infor-
mation on the role played by the apolar alkyl chains in the
Figure 7. SEM images of xerogels from compounds 8 (A), 10 (B),
11 (C), and 12 (D) gels in methanol and 5 (E) and 8 (F) in 2-
propanol.
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S. Iqbal, J. F. Miravet, B. Escuder
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TOF+): calcd. for C16H23N3O4Na+ [M + Na]+ 344.1581; found
self-assembly. It has been noticed that the length of the apo-
lar moiety can affect directly the gelation capabilities of
these molecules and this behavior can be associated to fac-
tors such as solubility and intermolecular interactions. The
presence of similar alkyl chains on both ends of the mole-
cule has been shown to improve the gelation properties,
most likely due to favorable packing of the β-sheet like
stacks. Structural studies of the aggregation at the molecu-
lar level with FTIR, circular dichroism, and WAXD re-
vealed that all the alkyl-terminated gelators present an anti-
parallel β-sheet-type arrangement. However, replacement of
the alkyl end groups by aromatic end groups results in the
formation of materials where different structural motifs co-
exist (antiparallel and parallel β-sheet for example). Having
in mind that strong H-bonding interactions are possible
both in the parallel and antiparallel arrangements, these re-
sults indicate that alkyl groups play a key role in the ob-
served antiparallel structures. This fact can be ascribed to
factors such as packing efficiency, solvophobic effects, and
favorable van der Waals interactions. Finally, a variety of
microscopic morphologies associated with really small
changes in molecular structure and solvent have been ob-
served in the SEM study of the xerogels.
Overall, the results reveal that this is a family of versatile
gelators, because the fine tuning of their structure permits
the formation of gels in a broad range of solvents of dif-
ferent polarity. Identically, the microscopic aspect of the
material can also be regulated by simple structural changes
and this property is interesting in fields associated to nano-
fabrication. Most interestingly, the fidelity found for the
antiparallel β-sheet motif opens the door for the use of this
tetrapeptidic structure as a scaffold for the construction of
highly ordered functional materials with application, for ex-
ample, in materials science and catalysis.
344.1587 (∆ = 1.7 ppm), 665.21 [2M + Na]+.
N-Hexyl-(NЈ-benzyloxycarbonylglycyl)-L-alanine Amide (1b): White
solid (2.34 g, 78% yield). M.p. 115–117 °C. Rf = 0.63 (CH2Cl2/
MeOH, 96:4). 1H NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.81
(t, J = 6.3 Hz, 3 H), 1.16 (m, 9 H), 1.34 (t, J = 6.3 Hz, 2 H), 2.97
(q, J = 6.3 Hz, 2 H), 3.61 (d, J = 5.7 Hz, 2 H), 4.21 (m, 1 H), 5.01
(s, 2 H), 7.35 (s, 5 H), 7.43 (t, J = 5.7 Hz, 1 H), 7.77 (s, 1 H), 7.93
(d, J = 7.5 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C):
δ = 14.55, 19.20, 22.68, 26.61, 29.60, 31.60, 39.18, 44.21, 48.83,
66.13, 128.31, 128.46, 129.02, 137.72, 157.21, 169.29, 172.47 ppm.
IR: ν = 3314, 3079, 2930, 2857, 1736, 1674, 1653, 1536, 1256 cm–1.
˜
HRMS (ESI-TOF+): calcd. for C19H29N3O4Na+ [M + Na]+
386.2056; found 386.2049 (∆ = 1.8 ppm), 749.32 [2M + Na]+,
364.22 [M + H]+.
N-Dodecyl-(NЈ-benzyloxycarbonylglycyl)-L-alanine Amide (1c):
White solid (2.3 g, 92% yield). M.p. 121–124 °C. Rf = 0.68
1
(CH2Cl2/MeOH, 90:10). H NMR (300 MHz, [D6]DMSO, 30 °C):
δ = 0.81 (t, J = 6.3 Hz, 3 H), 1.15–1.25 (m, 21 H), 1.34 (t, J =
5.7 Hz, 2 H), 2.97 (q, J = 6.3 Hz, 2 H), 3.60 (d, J = 6 Hz, 2 H),
4.19 (m, 1 H), 5.01 (s, 2 H), 7.33 (s, 5 H), 7.44 (t, J = 6.0 Hz, 1 H),
7.76 (s, 1 H), 7.77 (d, J = 7.5 Hz, 1 H) ppm. 13C NMR (75 MHz,
[D6]DMSO, 30 °C): δ = 14.62, 19.27, 22.76, 26.97, 2C (29.37), 5C
(29.66), 31.96, 39.18, 44.23, 48.81, 66.13, 128.31, 128.44, 129.01,
137.72, 157.19, 169.25, 172.42 ppm. IR: ν = 3321, 3070, 2921, 2850,
˜
1726, 1687, 1653, 1546, 1272 cm–1. HRMS (ESI-TOF+): calcd. for
C25H41N3O4Na+ [M + Na]+ 470.2995; found 470.3002 (∆ =
1.4 ppm), 917.4327 [2M + Na]+, 448.3202 [M + Na]+.
General Procedure for Compounds 2a–c: Compound 1 (1.43 mmol)
was dissolved in methanol (30 mL) containing palladium on acti-
vated carbon (5 wt.-%, 50 mg) in a two-necked, round-bottomed
flask. Then a balloon filled with hydrogen gas was connected to
one neck of the flask and the system was saturated with hydrogen
by slightly opening and closing the second neck to remove air. After
complete deprotection, the solution was filtered through Celite, and
the filtrate was evaporated under vacuum. The product was puri-
fied through a silica column (CH2Cl2/MeOH).
N-Propyl-(NЈ-glycyl)-L-alanine Amide (2a): White solid (1.51 g,
98% yield). M.p. 290 °C (decomp.). Rf = 0.37 (MeOH/CH2Cl2,
25:75). H NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.83 (t, J =
Experimental Section
1
7.2 Hz, 3 H), 1.17 (d, J = 7.5 Hz, 3 H), 1.35–1.41 (m, 2 H), 2.99
(m, 2 H), 3.10 (s, 2 H) 4.55 (m, 1 H), 7.90 (s, 1 H), 8.02 (d, 1 H)
ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = 11.96, 19.70,
General Procedure for Compounds 1a–c: Benzyloxycarbonylglycyl–
-alanine (3.57 mmol) was dissolved in freshly dried DME (80 mL)
in a round-bottomed flask with a stirrer and immersed in an ice
bath. Dicyclohexylcarbodiimide (DCC; 4.6 mmol) dissolved in
DME (100 mL) was then added dropwise to the solution over a
period of 1 h, and the mixture was stirred for 2 h. Then, the amine
(3.5 mmol) dissolved in dry DME was added slowly, and the mix-
ture was warmed to room temperature overnight. Afterwards, the
white suspension was filtered, and the solvent was evaporated un-
der vacuum. The product was purified by column chromatography
on (silica gel; CH2Cl2/MeOH).
22.93, 39.36, 44.93, 48.46, 172.46, 172.63 ppm. IR: ν = 3292, 2967,
˜
2850, 1646, 1545, 1458, 1254 cm–1. HRMS (ESI-TOF+): calcd. for
C8H17N3O2Na+ [M + Na]+ 210.1213; found 210.1176 (∆ =
19.0 ppm), 397.2142 [2M + Na]+.
N-Hexyl-(NЈ-glycyl)-L-alanine Amide (2b): White solid (2.2 g, 97%
yield). M.p. 82–84 °C. Rf = 0.42 (MeOH/CH2Cl2, 25:75). 1H NMR
(300 MHz, [D6]DMSO, 30 °C): δ = 0.82 (t, J = 6.6 Hz, 3 H), 1.16
(m, 11 H), 1.34 (t, J = 6.6 Hz, 2 H), 3.00 (m, 2 H), 3.03 (m, 2 H),
4.24 (m, 1 H), 7.88 (t, J = 5.1 Hz, 1 H), 8.02 (s, 1 H) ppm. 13C
NMR (75 MHz, [D6]DMSO, 30 °C): δ = 14.77, 19.95, 22.92, 26.84,
N-Propyl-(NЈ-benzyloxycarbonylglycyl)-
L-alanine
Amide
(1a):
White solid (1.55 g, 62% yield). M.p. 117–120 °C. Rf = 0.59
(CH2Cl2/MeOH, 96:4). 1H NMR (300 MHz, [D6]DMSO, 30 °C): δ
= 0.78 (t, J = 7.2 Hz, 3 H), 1.16 (d, J = 6.9 Hz, 3 H), 1.30 (m, 2
H), 2.93 (q, J = 6.6 Hz, 2 H), 3.61 (d, J = 5.7 Hz, 2 H), 4.19 (m, 1
H), 5.01 (s, 2 H), 7.35 (s, 5 H), 7.41 (t, J = 5.4 Hz, 1 H), 7.80 (s, 1
29.85, 31.82, 39.35, 45.29, 48.62, 172.75, 172.83 ppm. IR: ν = 3283,
˜
2930, 2853, 1647, 1511, 1458, 1251 cm–1. HRMS (ESI-TOF): calcd.
for C11H23N3O2Na+ [M + Na]+ 252.1682; found 252.1639 (∆ =
17.0 ppm), 481.2881 [2M + Na]+.
H), 7.94 (d, J = 7.2 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6] N-Dodecyl-(NЈ-glycyl)-
L-alanine Amide (2c): White solid (2.2 g,
1
DMSO, 30 °C): δ = 11.97, 19.23, 22.92, 44.21, 48.85, 66.14, 128.32,
98% yield). M.p. 88–89 °C. Rf = 0.51 (MeOH/CH2Cl2, 25:75). H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.81 (t, J = 6.3 Hz, 3 H),
1.17–1.23 (m, 23 H), 1.34 (t, J = 6.6 Hz, 2 H), 2.97 (m, 2 H), 3.11
128.46, 129.03, 137.73, 157.21, 169.30, 172.55 ppm. IR: ν = 3307,
˜
3268, 2961, 2850, 1723, 1691, 1646, 1542, 1267 cm–1. HRMS (ESI-
4586
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4580–4590

Self-Assembly of Tetrapeptides into Fibrillar Networks and Organogels
(s, 2 H), 4.26 (m, 1 H), 7.91 (t, J = 5.7 Hz, 1 H), 8.28 (d, J = (4-Nitrobenzoyl)glycyl-L-alanine (3e): White solid (0.89 g, 89%
6.9 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = yield). M.p. 194–198 °C. Rf = 0.54 (MeOH/CH2Cl2, 10:90). 1H
14.62, 19.48, 22.75, 26.96, 2C (29.37), 5C (29.65), 31.95, 39.15, NMR (300 MHz, [D6]DMSO, 30 °C): δ = 1.25 (d, J = 8.7 Hz, 3
42.62, 48.81, 168.88, 172.28 ppm. IR: ν = 3298, 2930, 2849, 1647,
H), 3.92 (m, J = 6 Hz, 2 H), 4.20 (m, 1 H), 8.07 (d, J = 8.7 Hz, 2
H), 8.25 (m, 3 H), 9.03 (t, J = 6 Hz, 1 H), 12.47 (br. s, 1 H) ppm.
13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = 22.73, 47.75, 52.95,
˜
1561, 1459, 1248 cm–1. HRMS (ESI-TOF+): calcd. for
C17H35N3O2Na+ [M + Na]+ 336.2621; found 336.2663 (∆ =
10.0 ppm).
128.94, 134.28, 145.16, 154.53, 170.27, 173.68, 179.38 ppm. IR: ν
˜
= 3518, 3290, 3075, 2929, 1724, 1687, 1656, 1531, 1348 cm–1.
HRMS (ESI-TOF+): calcd. for C12H13N3O6Na+ [M + Na]+
318,0697; found 318.0710 (∆ = 4.0 ppm).
General Procedure for Compounds 3a–c: Glycyl--alanine
(6.8 mmol) dissolved in NaOH (1 , 50 mL) was placed in a round-
bottomed flask with an efficient magnetic stirrer. The flask was
completely immersed in an ice bath and during the reaction the
stirrer was set in very rapid motion. To the solution was dropwise
added pure acyl chloride (13.6 mmol) and NaOH (1 , 10 mL) at
the same rate in such a way that the pH of the reaction mixture
did not decrease below 10. When the reaction was complete, the
suspension was acidified carefully with 2  HCl. The residue ob-
tained after filtration was extracted with petroleum ether to remove
the fatty acid. The final dipeptide derivative was chromatographed
on a silica gel column (MeOH/CH2Cl2).
General Procedure for Compounds 4–12
Method 1: Compound 3 (0.567 mmol) and HOBT (0.684 mol) were
placed in a round-bottomed flask with a stirrer and dissolved in
freshly dried DMF (10 mL). The flask was immersed in an ice bath.
To the mixture was dropwise added DCC (0.73 mmol) dissolved in
dry DMF (5 mL). The mixture was stirred for 2 h and then, amine
2 (0.567 mmol) dissolved in dry DMF was added dropwise. The
mixture was further stirred overnight at room temperature. After-
wards, the reaction mixture was quenched by adding an excess
amount of diethyl ether and it was then washed with water. The
white product was purified through 4–5-cm packed column with
silica by (CH2Cl2/MeOH, 90:10) firstly and then with pure meth-
anol or ethanol.
Propanoylglycyl-
104–107 °C. Rf
L
-alanine (3a): White solid (0.8 g, 82% yield). M.p.
=
0.25 (MeOH/CH2Cl2, 10:90). 1H NMR
(300 MHz, [D6]DMSO, 30 °C): δ = 0.94 (t, J = 7.5 Hz, 3 H), 1.22–
1.25 (m, 3 H), 2.07 (q, J = 7.8 Hz, 2 H), 3.66 (m, 2 H), 4.22 (m, 1
H), 7.96 (d, J = 5.4 Hz, 1 H), 8.07 (d, J = 7.2 Hz, 1 H), 12.55 (br.
s, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = 10.29,
17.80, 17.83, 28.80, 42.15, 47.93, 47.99, 169.30, 173.72, 174.49 ppm.
Method 2: Compound 3 (0.567 mmol), HOBT (0.684 mmol), EDC
(0.627 mmol), and amine 2 (0.567 mmol) were stirred overnight in
freshly dried DMF (10 mL). Then, DMF was evaporated under
vacuum, and the solid material was ground, washed with an excess
amount of water, and dried under high vacuum.
IR: ν = 3386, 3273, 3098, 2984, 2531, 1743, 1633, 1577, 1210 cm–1.
˜
HRMS (ESI-TOF+): calcd. for C8H14N2O4Na+ [M + Na]+
225.0846; found 225.0861 (∆ = 6.6 ppm).
N-Dodecyl-(NЈ-dodecanoylglycyl-
L-alanylglycyl)-L-alanine Amide
(4): Described previously.[13]
Hexanoylglycyl-L-alanine (3b): White solid (0.98 g, 89% yield).
M.p. 125–127 °C. 1H NMR (300 MHz, [D6]DMSO, 30 °C): δ =
0.81 (t, J = 6.6 Hz, 3 H), 1.21–1.25 (m, 7 H), 1.45 (q, J = 7.2 Hz,
2 H), 2.07 (t, J = 7.5 Hz, 2 H), 3.66 (m, 2 H), 4.16 (m, 1 H), 7.95
(t, J = 5.4 Hz, 1 H), 8.05 (d, J = 7.5 Hz, 1 H), 12.54 (br. s, 1 H)
ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = 14.52, 18.00,
22.55, 25.52, 31.55, 35.82, 42.32, 48.12, 169.44, 173.20, 174.63 ppm.
N-Hexyl-[NЈ-hexanoylglycyl-L-alanylglycyl]-L-alanine Amide (5):
Method 1: White solid (0.35 g, 89% yield). M.p. 244–248 °C. 1H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.81 (t, J = 6.0 Hz, 3 H),
1.15–1.36 (m, 21 H), 1.46 (m, 2 H), 2.06 (t, J = 7.2 Hz, 2 H), 2.99
(m, 2 H), 3.64 (m, 4 H), 4.17 (m, 2 H), 7.73 (m, 1 H), 7.79 (m, 1
H), 8.00 (m, 1 H), 8.09 (m, 1 H), 8.19 (m, 1 H) ppm. 13C NMR
(75 MHz, [D6]DMSO, 30 °C): δ = 14.42, 14.48, 14.53, 18.37, 18.88,
22.51, 22.22, 25.48, 26.56, 29.54, 31.51, 31.57, 35.78, 42.65, 42.84,
IR: ν = 3292, 3102, 2859, 2570, 1734, 1641, 1570, 1209 cm–1.
˜
HRMS (ESI-TOF+): calcd. for C11H20N2O4Na+ [M + Na]+
267.1315; found 267.1324 (∆ = 3.3 ppm), 511.2747 [2M + Na]+.
48.94, 49.31, 169.03, 169.94, 172.55, 173.49, 173.64 ppm. IR: ν =
˜
3292, 2929, 2859, 1696, 1626, 1545, 1448, 1233 cm–1. HRMS (ESI-
Dodecanoylglycyl-L-alanine (3c): White solid (0.94 g, 94% yield).
+
TOF+): calcd. for C22H41N5NaO5 [M + Na]+ 478.30; found
M.p. 133–135 °C. 1H NMR (300 MHz, [D6]DMSO, 30 °C): δ =
0.81 (t, J = 6.0 Hz, 3 H), 1.20–1.25 (m, 19 H), 1.46 (s, 2 H), 2.06
(t, J = 7.2 Hz, 2 H), 3.62 (m, 2 H), 4.19 (m, 1 H), 7.91 (t, J =
5.4 Hz, 1 H), 8.03 (d, J = 7.5 Hz, 1 H), 12.45 (s, 1 H) ppm. 13C
NMR (75 MHz, [D6]DMSO, 30 °C): δ = 14.62, 18.00, 22.77, 25.86,
29.35, 29.38, 29.50, 29.61, 29.68, 29.71, 31.98, 35.86, 42.30, 48.08,
478.2986 (∆ = 2.9 ppm).
N-Propyl-[NЈ-propanoylglycyl-L-alanylglycyl]-L-alanine Amide (6):
Method 2: White solid (0.14 g, 72% yield). M.p. 274–275 °C. 1H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.79 (t, J = 4.5 Hz, 3 H),
0.95 (t, J = 4.5 Hz, 3 H), 1.15–1.21 (m, 6 H), 1.34–1.42 (m, 2 H),
2.09 (q, J = 4.5 Hz, 2 H), 2.96–3.02 (m, 2 H), 3.62 (m, 4 H), 4.22
(m, 2 H), 7.71 (t, J = 3.3 Hz, 1 H), 7.79 (d, J = 4.5 Hz, 1 H), 7.94
(t, J = 3.3 Hz, 1 H), 8.09 (d, J = 4.5 Hz, 1 H), 8.15 (t, J = 3.3 Hz,
1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = 10.64,
12.18, 18.77, 19.21, 23.15, 29.16, 39.61, 42.87, 43.07, 49.13, 49.43,
169.41, 172.15, 174.64 ppm. IR: ν = 3289, 3102, 2850, 2570, 1726,
˜
1641, 1570, 1208 cm–1. HRMS (ESI-TOF+): calcd. for
C17H32N2O4Na+ [M + Na]+ 351.2254; found 351.2254 (∆ =
0.0 ppm), 679.4482 [2M + Na]+.
Benzoylglycyl-
187–189 °C. Rf
L
-alanine (3d): White solid (0.91 g, 91% yield). M.p.
169.17, 170.06, 172.72, 173.61, 174.29 ppm. IR: ν = 3291, 3088,
˜
=
0.51 (MeOH/CH2Cl2,10:90). 1H NMR
2970, 2876, 1694, 1626, 1543, 1444, 1235 cm–1. HRMS (ESI-
(300 MHz, [D6]DMSO, 30 °C): δ = 1.26 (d, J = 7.2 Hz, 3 H), 3.87
(m, 2 H), 4.20 (m, 1 H), 7.43–7.53 (m, 3 H), 7.85 (dd, J = 6.6,
1.8 Hz, 2 H), 8.19 (d, J = 5.1 Hz, 1 H), 8.65 (t, J = 7.5 Hz, 1 H),
12.14 (br. s, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ =
TOF+): calcd. for C16H29N5NaO5 [M + Na]+ 394.2061; found
+
394.2068 (∆ = 1.7 ppm).
N-Hexyl-[NЈ-dodecanoylglycyl-L-alanylglycyl]-L-alanine Amide (7):
18.02, 42.94, 48.20, 128.00, 128.96, 132.00, 134.74, 167.13, 169.38, Method 1: White solid (0.29 g, 89% yield). M.p. 258–263 °C. 1H
174.68 ppm. IR: ν = 3518, 3382, 3075, 2929, 1715, 1626, 1492,
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.81 (t, J = 6.9 Hz, 3 H),
1.19 (s, 31 H), 1.33 (t, J = 6.6 Hz, 2 H), 1.43 (t, J = 6.6 Hz, 2 H),
2.06 (t, J = 6.9 Hz, 2 H), 3.00 (s, 2 H), 3.64 (m, 4 H), 4.15 (m, 2
H), 7.70 (m, 2 H), 7.95 (s, 1 H), 8.06 (m, 2 H) ppm. 13C NMR
˜
1293 cm–1. HRMS (ESI-TOF+): calcd. for C12H14N2O4Na+ [M +
Na]+ 273.0846; found 273.08567 (∆ = 3.9 ppm), 523.1891 [2M +
Na]+.
Eur. J. Org. Chem. 2008, 4580–4590
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4587

S. Iqbal, J. F. Miravet, B. Escuder
FULL PAPER
(75 MHz, [D6]DMSO, 30 °C): δ = 14.54, 14.60, 18.36, 18.39, 18.47, 2 H), 2.06 (q, J = 7.8 Hz, 2 H), 2.98 (d, J = 7.8 Hz, 2 H), 3.66 (m,
18.97, 22.68, 22.74, 25.81, 26.60, 29.35, 29.47, 29.58, 29.65, 29.68, 4 H), 4.17 (m, 2 H), 7.72 (t, J = 3.3 Hz, 1 H), 7.79 (d, J = 4.0 Hz,
31.60, 31.94, 35.82, 39.19, 42.65, 42.85, 48.90, 49.23, 168.90, 1 H), 7.96 (t, J = 3.3 Hz, 1 H), 8.10 (d, J = 4.0 Hz, 1 H), 8.16 (t,
169.84, 172.42, 173.35, 173.40 ppm. IR: ν = 3296, 2925, 2854, 1696,
J = 3.6 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ
= 10.45, 14.59, 18.57, 19.04, 22.73, 26.64, 28.95, 29.65, 31.64, 39.21,
42.67, 42.86, 48.90, 49.22, 168.95, 169.87, 172.45, 173.41,
˜
1627, 1560, 1449, 1238 cm–1. HRMS (ESI-TOF+): calcd. for
C28H53N5NaO5 [M + Na]+ 562.3939; found 562.3918 (∆ =
+
3.7 ppm).
174.08 ppm. IR: ν = 3291, 3089, 2930, 2859, 1694, 1626, 1545,
˜
1443, 1235 cm–1. HRMS (ESI-TOF+): calcd. for C19H35N5NaO5
+
N-Propyl-[NЈ-docecanoylglycyl-L-alanylglycyl]-L-alanine Amide (8):
[M + Na]+ 436.2530; found 436.2554 (∆ = 5.5 ppm), 849.5322 [2M
Method 1: White solid (0.27 g, 92% yield). M.p. 259–262 °C. 1H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.80 (m, 6 H), 1.191–
1.24 (m, 20 H), 1.37–1.49 (m, 6 H), 2.09 (t, J = 7.5 Hz, 2 H), 2.97
(s, 2 H) 3.68 (m, J = 4.5 Hz, 4 H), 4.20 (m, 2 H), 7.50 (s, 1 H),
7.58 (s, 1 H), 7.73 (s, 1 H), 7.83 (s, 1 H), 7.95 (s, 1 H) ppm. 13C
NMR (75 MHz, [D6]DMSO, 30 °C): δ = 11.97, 14.63, 18.56, 19.01,
22.77, 22.92, 25.84, 29.36, 29.38, 29.49, 29.60, 29.68, 29.70, 31.97,
35.82, 39.38, 42.62, 42.83, 48.90, 49.19, 168.93, 169.81, 172.50,
+ Na]+.
N-Propyl-[NЈ-benzoylglycyl-L-alanylglycyl]-L-alanine Amide (13):
Method 1: White powder (0.26 g, 85% yield). M.p. 233.5 °C. Rf
1
= 0.46 (MeOH/CH2Cl2, 10:90). H NMR (300 MHz, [D6]DMSO,
30 °C): δ = 0.76 (t, J = 7.2 Hz, 3 H), 1.11 (d, J = 7.2 Hz, 3 H), 1.17
(d, J = 7.2 Hz, 3 H), 1.35–1.42 (m, 2 H), 2.93 (t, J = 6.6 Hz, 2 H),
3.67 (m, 2 H), 3.90 (m, 3 H), 4.14–4.27 (m, 2 H), 7.42 (m, 2 H),
7.69 (t, J = 5.1 Hz, 1 H), 7.78 (m, 3 H), 8.21 (t, J = 6.6 Hz, 2 H),
8.69 (t, J = 5.1 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO,
30 °C): δ = 11.95, 18.55, 18.94, 22.91, 39.40, 42.87, 43.26, 48.91,
49.32, 127.97, 128.95, 132.02, 134.63, 167.25, 168.95, 169.78,
173.32,
173.34 ppm.
HRMS
(ESI-TOF+):
calcd.
for
C25H47N5NaO5 [M + Na]+ 520.3469; found 520.3478 (∆ =
+
1.7 ppm).
N-Dodecyl-[NЈ-hexanoylglycyl-L-alanylglycyl]-L-alanine Amide (9):
Method 1: White solid (0.29 g, 86% yield). M.p. 264–267 °C. 1H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.82 (t, J = 6.3 Hz, 3 H),
1.15–1.21 (m, 33 H), 1.45 (m, 2 H), 2.09 (t, J = 7.2 Hz, 2 H), 3.01
(m, 2 H), 3.66 (m, 4 H), 4.17 (m, 2 H), 7.60 (s, 1 H), 7.69 (d, J =
6.6 Hz, 1 H), 7.88 (s, 1 H), 8.01 (s, 1 H), 8.14 (s, 1 H) ppm. 13C
NMR (75 MHz, [D6]DMSO, 30 °C): δ = 14.39, 14.49, 18.46, 18.94,
22.47, 22.67, 25.11, 25.45, 26.97, 29.29, 29.36, 3C (29.61), 31.57,
31.91, 35.88, 39.21, 39.63, 42.92, 43.08, 49.03, 49.36, 168.95,
172.48, 173.40 ppm. IR: ν = 3283, 2969, 2854, 1696, 1622, 1542,
˜
+
1446, 1234 cm–1. HRMS (ESI-TOF+): calcd. for C20H29N5NaO5
[M + Na]+ 442.2061; found 442.2061 (∆ = 0.0 ppm).
N-Propyl-[NЈ-(4-nitrobenzoyl)glycyl-L-alanylglycyl]-L-alanine Amide
(14): Method 2: White powder (0.19 g, 77% yield). M.p. 249–
253 °C. Rf = 0.43 (MeOH/CH2Cl2, 10:90). 1H NMR (300 MHz,
[D6]DMSO, 30 °C): δ = 0.75 (t, J = 7.2 Hz, 3 H), 1.11 (d, J =
6.9 Hz, 3 H), 1.22 (d, J = 6.9 Hz, 3 H), 1.32 (m, 2 H), 2.93 (d, J =
6.9 Hz, 2 H), 3.63 (m, 2 H), 3.88 (m, 2 H), 4.13 (m, 1 H), 4.22 (m,
1 H), 7.69 (t, J = 5.4 Hz, 1 H), 7.72 (d, J = 7.2 Hz, 1 H), 8.05 (d,
J = 8.7 Hz, 2 H), 8.18 (t, J = 5.4 Hz, 1 H), 8.28 (t, J = 8.1 Hz, 3
H), 9.02 (t, J = 5.4 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO,
30 °C): δ = 11.90, 18.43, 18.87, 22.86, 39.24, 42.84, 43.26, 48.96,
49.41, 124.20, 129.50, 140.27, 149.80, 165.77, 169.04, 169.42,
169.88, 172.47, 173.35, 173.38 ppm. IR: ν = 3293, 2925, 2854, 1696,
˜
1622, 1560, 1448, 1235 cm–1. HRMS (ESI-TOF+): calcd. for
+
C28H53N5NaO5 [M + Na]+ 562.3939; found 562.3923 (∆ =
2.8 ppm).
N-Propyl-[NЈ-hexanoylglycyl-L-alanylglycyl]-L-alanine Amide (10):
Method 1: White solid (0.26 g, 87% yield). M.p. 244–245 °C. 1H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.78 (m, 6 H), 1.16–1.23
(m, 10 H), 1.34–1.51 (m, 4 H), 2.06 (t, J = 7.5 Hz, 2 H), 2.94 (q, J
= 6.9 Hz, 2 H), 3.64 (m, 4 H), 4.16 (m, 2 H), 7.72 (t, J = 5.4 Hz,
1 H), 7.81 (d, J = 7.5 Hz, 1 H), 7.97 (t, J = 5.4 Hz, 1 H), 8.09 (d,
J = 6.6 Hz, 1 H), 8.18 (t, J = 5.4 Hz, 1 H) ppm. 13C NMR
(75 MHz, [D6]DMSO, 30 °C): δ = 11.96, 14.25, 18.58, 19.01, 22.56,
22.92, 25.52, 31.55, 35.78, 42.63, 42.83, 48.90, 49.18, 49.27, 168.94,
172.60, 173.47 ppm. IR: ν = 3280, 2922, 2854, 1696, 16226, 1545,
˜
1448, 1349 cm–1. HRMS (ESI-TOF+): calcd. for C20H28N6NaO7
+
[M + Na]+ 487.1912; found 487.1914 (∆ = 0.4 ppm).
N-Hexyl-[NЈ-benzoylglycyl-L-alanylglycyl]-L-alanine Amide (15):
Method 1: White powder (0.24 g, 80% yield). M.p. 238 °C. Rf =
0.58 (MeOH/CH2Cl2, 10:90). 1H NMR (300 MHz, [D6]DMSO,
30 °C): δ = 0.81 (t, J = 6.9 Hz, 3 H), 1.13 (d, J = 7.2 Hz, 3 H),
1.14–1.35 (m, 11 H), 1.34–1.42 (m, 2 H), 2.98 (d, J = 5.7 Hz, 2 H),
3.66 (m, 2 H), 3.89 (m, 2 H), 4.16–4.25 (m, 2 H), 7.42 (m, 3 H),
7.68 (s, 1 H), 7.76 (d, J = 7.8 Hz, 1 H), 7.83 (d, J = 8.4 Hz, 1 H),
8.18 (t, J = 6.9 Hz, 1 H), 8.69 (t, J = 6.9 Hz, 1 H) ppm. 13C NMR
(75 MHz, [D6]DMSO, 30 °C): δ = 14.54, 18.42, 18.84, 22.67, 26.56,
29.55, 31.58, 39.28, 42.88, 43.26, 48.93, 49.42, 127.95, 128.96,
169.81, 172.50, 173.34, 173.37 ppm. IR: ν = 3293, 2961, 2854, 1696,
˜
1622, 1542, 1444, 1248 cm–1. HRMS (ESI-TOF+): calcd. for
+
C19H35N5NaO5 [M + Na]+ 436.253; found 436.2520 (∆ =
2.2 ppm), 413.2654 [M + H]+.
N-Dodecyl-[NЈ-propanoylglycyl-
L-alanylglycyl]-
L-alanine
Amide
(11): Method 2: White solid (0.31 g, 98% yield). M.p. 234–238 °C.
1H NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.81 (t, J = 6.9 Hz,
3 H), 0.85 (t, J = 7.5 Hz, 3 H), 1.15–1.22 (m, 11 H), 1.35 (s, 2 H),
2.07 (q, J = 7.5 Hz, 2 H), 2.99 (s, 2 H), 3.68 (m, 4 H), 4.17 (m, 2
H), 7.74 (s, 1 H), 7.82 (d, J = 6 Hz, 1 H), 7.99 (m, 1 H), 8.15 (m,
2 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ = 10.45,
14.59, 18.57, 19.04, 19.19, 33.73, 26.64, 28.94, 29.65, 31.64, 32.96,
39.21, 42.67, 42.86, 48.81, 48.91, 49.22, 168.94, 169.86, 172.44,
132.06, 134.54, 167.35, 169.01, 169.89, 172.49, 173.48 ppm. IR: ν
˜
= 3283, 2929, 2857, 1696, 1622, 1542, 1448, 1235 cm–1. HRMS
(ESI-TOF+): calcd. for C23H35N5NaO5+ [M + Na]+ 484.253; found
484.2521 (∆ = 1.8 ppm), 945.3710 [2M + Na]+.
N-Hexyl-[NЈ-(4-nitrobenzoyl)glycyl-L-alanylglycyl]-L-alanine Amide
(16): Method 1: White powder (0.25 g, 81% yield). M.p. 258.4 °C.
Rf = 0.34 (MeOH/CH2Cl2, 10:90). 1H NMR (300 MHz, [D6]-
DMSO, 30 °C): δ = 0.86 (t, J = 6.6 Hz, 3 H), 1.11 (d, J = 7.2 Hz,
3 H), 1.22 (m, 9 H), 1.34 (t, J = 6.3 Hz, 2 H), 2.97 (m, 2 H), 3.68
(t, J = 6.3 Hz, 2 H), 3.94 (d, J = 6.3 Hz, 2 H), 4.14–4.28 (m, 2 H),
7.69 (t, J = 6.6 Hz, 1 H), 7.78 (d, J = 7.5 Hz, 1 H), 8.07 (d, J =
9.0 Hz, 2 H), 8.19 (t, J = 6.3 Hz, 1 H), 8.27 (m, 3 H), 9.03 (t, J =
5.1 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO, 30 °C): δ =
14.36, 18.49, 18.54, 22.66, 26.56, 29.55, 31.57, 39.67, 43.21, 43.23,
48.82, 49.28, 124.04, 129.37, 140.26, 149.76, 165.62, 168.90, 169.29,
173.39, 174.08 ppm. IR: ν = 3290, 3084, 2925, 2854, 1694, 1626,
˜
1545, 1444, 1234 cm–1. HRMS (ESI-TOF+): calcd. for
C22H34N6NaO5 [M + Na]+ 520.3469; found 520.3458 (∆ =
+
1.9 ppm).
N-Hexyl-[NЈ-propanoylglycyl-L-alanylglycyl]-L-alanine Amide (12):
Method 2: White solid (0.31 g, 78% yield). M.p. 269–271 °C. 1H
NMR (300 MHz, [D6]DMSO, 30 °C): δ = 0.81 (t, J = 6.3 Hz, 3 H),
0.95 (t, J = 6.3 Hz, 3 H), 1.15–1.21 (m, 12 H), 1.35 (q, J = 7.8 Hz,
4588
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4580–4590

Self-Assembly of Tetrapeptides into Fibrillar Networks and Organogels
172.41, 173.33 ppm. IR: ν = 3300, 2930, 2854, 1696, 1629, 1528,
Supporting Information (see footnote on the first page of this arti-
cle): CD, FTIR, powder diffraction, and NMR spectroscopic data
for all the described compounds.
˜
1443, 1238 cm–1. HRMS (ESI-TOF+): calcd. for C23H34N6NaO7
+
[M + Na]+ 529.2381; found 529.2352 (∆ = 5.4 ppm).
N-Dodecyl-[NЈ-benzoylglycyl-L-alanylglycyl]-L-alanine Amide (17):
Method 1: White powder (0.17 g, 80% yield). M.p. 277.2 °C. Rf
= 0.58 (MeOH/CH2Cl2, 10:90). H NMR (300 MHz, [D6]DMSO,
Acknowledgments
1
30 °C): δ = 080 (t, J = 6.6 Hz, 3 H), 1.10 (d, J = 7.2 Hz, 3 H),
1.20–1.33 (m, 23 H), 2.97 (m, 2 H), 3.65 (m, 2 H), 3.88 (m, 2 H),
4.10–4.25 (m, 2 H), 7.44–7.51 (m, 3 H), 7.68 (t, J = 5 Hz, 1 H),
7.77 (d, J = 6 Hz, 1 H), 7.83 (dd, J = 6.9, 1.5 Hz, 2 H), 8.19 (m, 2
H), 8.70 (t, J = 5 Hz, 1 H) ppm. 13C NMR (75 MHz, [D6]DMSO,
30 °C): δ = 14.60, 18.45, 18.88, 22.74, 26.92, 3C (29.35), 5C (29.64),
29.69, 31.95, 39.19, 42.87, 43.26, 48.91, 49.37, 127.95, 128.96,
The authors thank the Spanish Ministry of Science and Innovation
(Grant CTQ2006-14984) and Universitat Jaume I (Grant P1
1A2006–01) for funding. S. I. thanks Universitat Jaume I – Ban-
caixa for a fellowship. Technical assistance by SCIC is also ac-
knowledged.
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(ESI-TOF+): calcd. for C29H47N5NaO5 [M + Na]+ 568.3469;
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(18): Method 1: White powder (0.27 g, 89% yield). M.p. 247.1 °C.
Rf = 0.57 (MeOH/CH2Cl2, 10:90). 1H NMR (300 MHz, [D6]-
DMSO, 30 °C): δ = 0.83 (t, J = 6.3 Hz, 3 H), 1.11–1.24 (m, 26 H),
2.98 (m, 2 H), 3.66 (m, 2 H), 3.93 (m, 2 H), 4.11–4.23 (m, 2 H),
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IR: ν = 3299, 2921, 2851, 1696, 1639, 1560, 1451, 1234, 1109 cm–1.
˜
+
HRMS (ESI-TOF+): calcd. for C29H46N6NaO7 [M + Na]+
613.3320; found 613.3358 (∆ = 6.1 ppm).
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www.eurjoc.org
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S. Iqbal, J. F. Miravet, B. Escuder
FULL PAPER
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Received: June 5, 2008
Published Online: July 29, 2008
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4580–4590