Amide-triazole isosteric substitution for tuning self-assembly and incorporating new functions into soft supramolecular materials

Article abstract of DOI:10.1039/c4cc08593k

The proof-of-concept for the modular synthesis of new functional soft gel materials based on amide-triazole isosteric replacement has been demonstrated. A coassembly approach of isosteric amino acid-based hydrogelators was fruitfully applied for fine-tuning the release of entrapped drugs. This journal is

Full text of DOI:10.1039/c4cc08593k

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Amide–triazole isosteric substitution for tuning
self-assembly and incorporating new functions
into soft supramolecular materials†
Cite this:DOI: 10.1039/c4cc08593k
Received 30th October 2014,
Accepted 2nd December 2014
Jurgen Bachl,‡a Judith Mayr,‡a Francisco J. Sayago,b Carlos Cativielab and
¨
David Dıaz Dıaz*ac
´
´
DOI: 10.1039/c4cc08593k
www.rsc.org/chemcomm
The proof-of-concept for the modular synthesis of new functional
soft gel materials based on amide–triazole isosteric replacement
has been demonstrated. A coassembly approach of isosteric amino
acid-based hydrogelators was fruitfully applied for fine-tuning the
release of entrapped drugs.
Fig. 1 Main structural features of disubstituted trans-amides and 1,2,3-triazoles
for their consideration as bioisosteres.
Bioisosteric replacement is one of the most important tools in
medicinal chemistry for the rational design of new drugs.1,2 The
origins of isosterism can be traced back to the early twentieth
century to the pioneering work of Langmuir in which he coined 1,5-, respectively).11,12 Their planar structure exhibits the ability of
this concept to define atoms or molecules which possess the H-bonding due to the presence of both donor and acceptor groups
same number and/or arrangement of electrons.3 Thereafter, with similar relative positions. Although the distance R1–R2 in trans-
Grimm’s hydride displacement law4 and Erlenmeyer’s work5 amides is ca. 1 Å shorter than that in 1,4-disubstituted triazoles, the
paved the way to the definition of bioisosteres, largely spread overall dipolar moment of the latter (ca. 5.0 D) is slightly larger than
by Friedman6 and Thornber7 and later broadened by Burger8 as that of secondary amides (ca. 3.5–4.0 D). Moreover, the H-bonding
‘‘compounds or groups that possess near-equal molecular abilities (oxygen lone pair electrons = acceptor; N–H amide bond =
shapes and volumes, approximately the same distribution of donor) can be mimicked by the N-2/N-3 lone pairs (acceptor) and the
electrons, and which exhibit similar physical properties. . .’’.
triazole C–H bond (donor) (Fig. 1). Calculations in the gas phase have
Nowadays, bioisosteric replacement of amide bonds represents a also suggested that the lone pair on N-3 may be a better H-bond
key strategy in modern research due to its implications in peptide acceptor and metal coordination center than N-2. In addition, the
chemistry.9 Within this context, many promising approaches intro- potential b-turn geometry of 1,4-disubstituted triazoles has been
ducing acyclic esters, thioamides, ureas, carbamates, sulphonamides, supported by computational studies.13 A similar analysis can be done
oxadiazoles and triazoles have been successfully applied to improve between cis-amides and 1,5-disubstituted 1,2,3-triazoles (Fig. S1,
the activity of known drugs or to create new bioactive compounds ESI†). Besides the application of triazoles in peptidomimetics and
and materials.2,10 Among these examples, disubstituted 1,2,3-triazoles bioconjugation, they have also been proven to be useful structural
have been considered as powerful non-classical isosteres of amides as moieties for the fabrication of functional materials.14,15
they can mimic either a trans- or a cis- configuration of the amide
As part of our research program conceived to expand the
bond depending on the substitution pattern of the triazole (1,4- or conceptual toolbox for the development of advanced functional
materials, we report here the proof-of-concept demonstration
for the synthesis of novel supramolecular soft gel materials
based on amide–triazole isosteric substitution.
a
¨
¨ ¨
Universitat Regensburg, Fakultat fur Chemie und Pharmazie,
¨
¨
Institut fur Organische Chemie, Universitatsstr. 31, 93053 Regensburg, Germany.
Fig. 2 shows the structure of the compounds synthesized for this
study. N-Stearoyl-L-glutamic acid (C18-Glu) is an amphiphilic com-
pound that has been reported to self-assemble into nanofibers in
chloroform and nanodisc-like structures in 1 : 1 mixed EtOH–H2O.16
We envisioned that this molecule could also undergo self-assembly
in many other solvents at suitable concentrations. In addition, it
provided us with a unique opportunity to assess the transplantation
E-mail: David.Diaz@chemie.uni-regensburg.de
b
´
´
´
´
Instituto de Sıntesis Quımica y Catalisis Homogenea (ISQCH),
CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain
c IQAC-CSIC, Jordi Girona 18-26, Barcelona 08034, Spain
† Electronic supplementary information (ESI) available: Full experimental
details, tabular data, expanded discussion, additional plots, figures and expanded
literature. See DOI: 10.1039/c4cc08593k
‡ These two authors are equal main contributors.
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Fig. 2 Model compounds C18-Glu and click-Glu derived from L-glutamic
acid used in this work.
of the isosteric replacement paradigm from medicinal chemistry
to soft materials. Thus, the analogue click-Glu, in which the
1,4-disubstituted 1,2,3-triazole unit has formally replaced the amide
moiety in C18-Glu keeping the total number of C atoms intact, was
easily synthesized via click chemistry17 by the copper(I)-catalyzed
azide–alkyne cycloaddition. The synthesis of this compound required
prior preparation of (S)-2-azidopentanedioic acid from the corres-
ponding L-glutamic acid using imidazole-1-sulfonyl azide hydro-
chloride as the diazo transfer reagent (Section S3.2, ESI†).
The gelation ability of C18-Glu and click-Glu was systematically
investigated by applying a heating–cooling cycle in H2O and 25
organic solvents (including non-polar, polar protic and polar aprotic
solvents). Materials that did not exhibit gravitational flow upon
turning the vial upside-down were preliminary classified as stable
gels. No visible changes were observed in model examples upon one-
year of aging, and their gel nature was further confirmed by dynamic
rheological measurements (vide infra). In general, more than 10
different gels could be obtained with both gelators in oxygenated,
hydrocarbon (including aromatic and aliphatic) and halogenated
solvents. However, some exceptions were also found in most types of
solvents leading to the formation of precipitates, clear solutions or
partial gels after cooling down the corresponding isotropic solutions
(e.g., 1-hexanol, DMF, THF, EtOAc, 1,4-dioxane, acetone, chloro-
benzene, benzonitrile, and silicon oil). Interestingly, only click-Glu
was able to form stable gels in DMSO, EtOH and i-PrOH, expanding
the gelation scope in comparison to C18-Glu (Table S1, ESI†). The
remarkable gelation tendency of click-Glu was also revealed by fast
in situ gelation of DMSO–H2O mixtures during its synthesis (Fig. S5,
ESI†). In agreement with the possible formation of particle aggre-
gates larger than the wavelength of visible light (ca. 350–750 nm),
most gels exhibited an opaque white appearance except for those
obtained in glycerol, which were translucent (Fig. 3).
Fig. 4 Comparative graphs of the (A) CGC, (B) gelation time and (C) Tgel of
gels prepared from C18-Glu and click-Glu. Time and Tgel values were
measured at the same gelator concentration defined by the higher CGC in
each case (see the ESI† for details and additional data).
thermal stability and gelation kinetics were different in each case
(Fig. 4; Table S1, ESI†). CGC values were established in the range of
10–200 g Lꢀ1 in most cases. Nevertheless, click-Glu generally
exhibited ca. 25–55% lower CGC values than C18-Glu when polar
protic and polar aprotic solvents were used (e.g., H2O, CH3OH,
glycerol, and CH3CN). On the other hand, an opposite behavior
(ca. 10–75% higher CGC) was usually observed in non-polar
solvents (e.g., Et2O, CH2Cl2, n-hexane, and toluene) (Fig. 4A). The
above results already constituted a clear indication of the signifi-
cant impact that the amide–triazole isosteric replacement could
have on the self-assembly properties of the gelators.
Despite the fact that many solvents could be gelled by either
C18-Glu or click-Glu, the critical gelation concentration (CGC),
Although most gels were obtained within minutes regardless of
the gelator structure, the gelation kinetics was found to be signifi-
cantly slower in polar solvents. Taking H2O and CHCl3 as polar and
non-polar model solvents for further discussion, Ln–Ln plots of
gelation-time vs. gelator-concentration revealed that for an equivalent
increment in the concentration with respect to the CGC, the gelation
rate was nearly identical either in water or CHCl3 for both gelators.
However, click-Glu showed faster absolute gelation kinetics in water
than C18-Glu at the corresponding CGC (Table S1, ESI†). Similarly,
shorter gelation times were generally observed for C18-Glu in non-
polar solvents (Fig. 4B).
Independent of the nature of the solvent or the gelator, all gels
exhibited full thermo-reversibility with gel-to-sol transition tem-
peratures (Tgel) in the range of ca. 40–70 1C as indicated by DSC
among other techniques (Fig. S7, ESI†). Remarkably, some Tgel
values exceeded the boiling points of the corresponding solvents
Fig. 3 Selected digital photographs of upside-down vials containing stable
gels prepared from C18-Glu and click-Glu at their corresponding critical
gelation concentrations (see the ESI† for additional pictures and details).
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(e.g., n-hexane, bp = 69 1C; Tgel (C18-Glu) = 84 ꢁ 2 1C; Tgel (click-Glu) = values at break. However, an exact opposite behavior could be
72 ꢁ 2 1C). Similar to the trends observed for CGC-values, materials observed in polar protic solvents like water, where click-Glu exhibited
derived from click-Glu generally exhibited, at comparable concentra- superior properties (Table 1; Fig. S13, ESI†).
tions, higher Tgel values in polar solvents than those obtained from
Electron microscopy imaging was used in order to gain visual
C18-Glu, which were superior in non-polar solvents (Fig. 4C). Up to insight into the morphologies of model gels (Fig. 5; Fig. S11, ESI†). In
40% increase of Tgel could be observed when the gelator and solvent good agreement with previous observations,16 FESEM images of the
polarities matched.
xerogels based on C18-Glu in a polar-protic environment revealed
As expected, Tgel increased with the increasing gelator the formation of nanoalmond-like structures with diameters in the
concentration regardless of the solvent or the gelator, which is in range of 1–5 mm, whereas layered lamellar assemblies ca. 5–10 mm in
agreement with the gradual formation of denser packed supra- width and 15–50 mm in length were observed in CHCl3. Fused
molecular networks. The concentration of C18-Glu could be nanofibrillar structures with diameters in the range of 30–50 nm
increased up to 500 g Lꢀ1 in both model solvents affording homo- were identified as the smallest features in these aggregates.
geneous gels, whereas the concentration of isosteric click-Glu could A practically opposite behavior was observed for materials based
be increased even up to 700 g Lꢀ1 in CHCl3 and 900 g Lꢀ1 in water. on the isosteric click-Glu. Thus, wrinkled lamellar structures few mm
Obvious plateau regions for all examples (DTgel = 20–33 1C) were in length and 100–500 nm in width were observed in water, whereas
reached before the gels collapsed. Ln–Ln plots before the plateau almond crunch-like structures with diameters between hundreds of
regions showed a clear linear relationship between the increment in nm up to few mm were revealed in CHCl3. In general, dense fibrillar
the gelator concentration and the consequent increment in the Tgel networks of high aspect ratios were also observed by TEM imaging.
with respect to the initial values at the CGC. Within the end limits When click-Glu was used as the gelator, the density of the fibrillar
defined by the CGCs and the maximum Tgel values, the slopes of bundles was much higher than observed in the case of C18-Glu.
these straight lines indicated that a 1.3-fold higher percentage In good agreement, gas-adsorption measurements revealed that
increment of Tgel could be obtained either in water or CHCl3 when materials derived from C18-Glu possess a 2.4-fold higher porosity
C18-Glu or click-Glu are used as gelators, respectively (Fig. S9, ESI†). than those obtained from the analogue click-Glu (53.4 m2 gꢀ1 and
In terms of gel stability towards chemical additives, the 22.6 m2 gꢀ1, respectively) (Fig. S12, ESI†).
hydrogels made from click-Glu at the CGC remained stable
In agreement with previous observations,16 the formation of dis-
under neutral and acidic conditions, whereas irreversible gel- tinctive nanostructures could be explained by different H-bonding
to-sol phase transitions could be observed by mechanical patterns caused by either polar protic or non-polar environments.
agitation or addition of NaOH, electrolytes, buffers or certain Thus, protic environments apparently favor the assembly of C18-Glu in
organic solvents. Organogels made from click-Glu and organo-/ molecular multilayers resembling nanoalmond crunch-like structures
hydrogels made from C18-Glu behaved similarly, albeit they mainly due to intermolecular H-bonding between the amide NH-bond
showed a considerable stability in the presence of electrolytes and the CO-group of the acid moiety next to the chiral centre, whereas
and phosphate buffered saline (PBS) solutions (Fig. S10, ESI†). non-polar solvents lead to the formation of nanofibers in which both
Oscillatory rheological measurements (i.e., DFS, DSS and DTS intermolecular H-bonding and intramolecular H-bonding are nearly
experiments) of representative gels unequivocally confirmed their equally important.16 The opposite tendency was observed for click-Glu,
viscoelastic nature. Within the linearity limits of deformation, the suggesting that the isosteric replacement can be used to trigger the
storage modulus G0 was approximately one order of magnitude formation of desired nanostructures in specific solvents (Fig. S14,
higher than the loss modulus G0 0 (Table 1). Destruction of the gels at ESI†). The evidence of such divergent assembly mechanisms of the
low frequency and below 10% strain indicated the brittle nature of two isosteres associated to different H-bonding arrangements depend-
the materials. DTS measurements at 0.1% strain and 1 Hz frequency ing on the solvent nature was also obtained from FT-IR measurements
confirmed the stability of the gel materials as a function of the using H2O and CHCl3 as model solvents (Section S7, ESI†).
ageing time at room temperature. In general, the tan d (G0 0/G0) values
At this point, hydrogels prepared from C18-Glu and click-Glu
among random measurements of the same material were reprodu- were preliminary evaluated as carriers for tuning the kinetics of
cible and decreased with the concentration of the gelator, suggesting drug release. Vancomycin, a useful glycopeptidic antibiotic for
an enhancement of the mechanical damping properties. Well
aligned to the impact of the gelator structure on both the CGC
and Tgel (vide supra), C18-Glu provided gel materials with higher
mechanical strength in aprotic solvents like CHCl3, as indicated by
higher absolute G0-values, lower tan d and higher maximum strain
Table 1 Summary of rheological properties of model gelsa
Gelator
Solvent G0 (kPa)
G0 0 (kPa)
tan d
g (%)
C18-Glu
C18-Glu
click-Glu H2O
H2O
CHCl3
15 ꢁ 1.8
778 ꢁ 6.0
102 ꢁ 10.6
109 ꢁ 1.7
4 ꢁ 0.2 0.30 ꢁ 0.01 6 ꢁ 1.3
101 ꢁ 1.9 0.13 ꢁ 0.01 4 ꢁ 0.8
19 ꢁ 0.4 0.19 ꢁ 0.02 9 ꢁ 1.4
45 ꢁ 4.0 0.41 ꢁ 0.03 2 ꢁ 0.7
Fig. 5 FESEM images of xerogels prepared by freeze drying the corres-
ponding gels. (A) C18-Glu in H2O (25 g Lꢀ1); (B) C18-Glu in CHCl3 (100 g Lꢀ1);
(C) click-Glu in H2O (25 g Lꢀ1); (D) click-Glu in CHCl3 (100 g Lꢀ1).
click-Glu CHCl3
a
Gelator concentrations: 25 g Lꢀ1 (H2O), 100 g Lꢀ1 (CHCl3). See ESI.
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In conclusion, the isosteric replacement paradigm from medicinal
chemistry can be successfully applied to the synthesis of novel soft
functional materials. As a proof-of-concept, isosteric gelators C18-Glu
and click-Glu afforded the preparation of a variety of physical
hydrogels and organogels with different thermal, mechanical,
morphological and diffusional properties. In general, click-Glu
revealed superior features with respect to the CGC, Tgel and mechan-
ical stabilities in polar protic solvents, whereas C18-Glu exhibited
improved properties in non-polar solvents. Moreover, the coassembly
of both isosteres was successfully applied for fine-tuning the release
of the antibiotic vancomycin. Studies involving the 1,5-disubstituted
triazole-based gelators19 as well as computational calculations to gain
deeper insights into the gelation mechanisms and gel properties are
currently underway in our lab. Overall, these results open up many
exciting opportunities for the development of new functional materi-
Fig. 6 (A) Release profile of vancomycin in a PBS buffer from hydrogels based
on click-Glu, C18-Glu and coassembled click-Glu :C18-Glu (9 : 1, w/w). (B) The
zone of inhibition test against Staphylococcus aureus for vancomycin-
containing hydrogel samples and their corresponding controls.
the treatment of infections mostly caused by Gram-positive bacteria, als with unique properties for different applications beyond the field
was selected as hydrophilic model drug for this study.18 Stable of gels.
¨
hydrogels towards a PBS buffer could be obtained using a gelator
Financial support from DFG (DI 1748/3-1), Universitat
concentration of 60 g Lꢀ1 (Sections S2.2 and S14, ESI†). Thus, we Regensburg, Ministerio de Economıa y Competitividad-FEDER
´
used these conditions for preliminary studies of the release kinetics (grants CTQ2010-17436, CTQ2013-40855-R) and Gobierno de
´
of entrapped vancomycin (initial drug concentration in the gel Aragon-FSE (research group E40) is gratefully acknowledged. We
phase = 1.38 ꢂ 10ꢀ3 M). Up to 90% drug release was observed thank Dr R. Banerjee and S. Saha (NCL, Pune) for gas-adsorption
within 13 days in the case of click-Glu, whereas C18-Glu showed measurements and Prof. Minghua Liu (ICCAS, Beijing) for his
much slower kinetics (ca. 56% within the same period). In general, valuable advice on the isolation of C18-Glu. We are indebted to Prof.
release kinetics obeyed a first-order integrated rate law reasonably J. Schlossmann, Prof. F. Kees and Astrid Seefeld (Department of
¨
well (kobs [C18-Glu] = 1.5 ꢁ 0.2 ꢂ 10ꢀ2
h
ꢀ1; kobs [click-Glu] = 4.3 ꢁ Pharmacology and Toxicology, Universitat Regensburg) for their
0.3 ꢂ 10ꢀ2
h
ꢀ1) (Fig. 6). These results can be associated with generous assistance with the antimicrobial studies. D.D.D. thanks
different diffusion properties of the drug through hydrogels with DFG for the Heisenberg Professorship Award.
diverse nano-morphologies, which are ultimately governed by the
isosteric gelator structure (obviously, the interaction patterns
between solvent–gelator, gelator–gelator and gelator/fibril-drug mole-
Notes and references
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3 I. Langmuir, J. Am. Chem. Soc., 1919, 41, 1543.
4 H. G. Grimm, Naturwissenschaften, 1929, 17, 557.
5 H. Erlenmeyer and M. Leo, Helv. Chim. Acta, 1932, 15, 1171.
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7 C. W. Thornber, Chem. Soc. Rev., 1979, 8, 563.
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release rate was also observed upon decreasing the gelator concen-
tration due to the decrease of the crosslink density of the network
(Fig. S16A, ESI†). Remarkably, the complementary structure of the
isosteric gelators allowed the preparation of stable hydrogels via
coassembly of both gelators at different ratios, offering an alternative
tuning option for fine-tuning the drug release (Fig. 6A). However, the
change in the kinetics upon coassembly was not proportional to the
molar ratio of the gelator (Fig. S16B, ESI†), suggesting the formation
of complex interpenetrated networks and release kinetics that will be
a subject of future studies. The antimicrobial activities of the
hydrogels with and without vancomycin against the Gram-positive
´
´
´
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test (Fig. 6B). Large inhibition zones (21.5 mm and 18 mm for drug-
loaded click-Glu and C18-Glu, respectively) were observed. The results
were in good agreement with the faster release kinetics of click-Glu
hydrogels vs. C18-Glu. No inhibition zones were evident with control
gel samples. Maintenance of antimicrobial activity of the released
drug was also confirmed (Fig. S18, ESI†).
Finally, it is worth mentioning that the amphiphilic character of
the gelators also allowed the entrapment of other drugs with a
completely different hydrophilic/hydrophobic balance (i.e., anti-
cancer drugs methotrexate, camptothecin and flutamide) and their
further release at different rates depending on the isostere used to
prepare the gels (Fig. S17, ESI†).
47, 8740, and references therein.
´
´
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19 Several examples of triazole-based gels are given in the ESI.† For
representative examples, see: (a) H.-F. Chow, C.-M. Lo and Y. Chen,
Top. Heterocycl. Chem., 2012, 28, 137, and references therein;
(b) A. Hemamalini and T. M. Das, New J. Chem., 2013, 37, 2419.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014