Remarkable solvent isotope dependence on gelation strength in low molecular weight hydro-gelators

Article abstract of DOI:10.1039/c7cc00017k

A delicate interplay of anisotropic hydrophobic/hydrophilic, π-π stacking, ionic and hydrogen bond interactions determine the strength of hydrogelators and are considered key factors in efforts to design potent small molecule hydrogelators. Here we show that solvent deuteration and electrolytic strength affect the strength of hydrogels formed from amino acid modified C₃-symmetric cyclohexane trisamides profoundly. Gels formed by self-assembly through heating/cooling of solutions or by pH switching show up to a 30 °C increase in their melting temperatures in D₂O compared to H₂O. The unusually large solvent isotope effect on gel formation and thermal properties indicates that, in contrast to expectations, hydrogen bonding is not the primary determinant of gel strength but instead that hydrophobic interactions between the gelator molecules and the terminal carboxylic acid units are of greater importance. A conclusion that is supported by a similarly large effect of electrolytes on gel strength.

Full text of DOI:10.1039/c7cc00017k

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. R. Canrinus, F.
J. Cerpentier, B. L. Feringa and W. Browne, Chem. Commun., 2017, DOI: 10.1039/C7CC00017K.
This is an Accepted Manuscript, which has been through the
Volume 52 Number
1 4 January 2016 Pages 1–216
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Accepted Manuscripts are published online shortly after
Chemical Communications
www.rsc.org/chemcomm
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
=
3
rsc.li/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C7CC00017K
ChemComm
COMMUNICATION
Remarkable solvent isotope dependence on gelation strength in
low molecular weight hydro-gelators
Tjalling R. Canrinus,a,b Florian J. R. Cerpentier,a Ben L. Feringab,* and Wesley R. Brownea,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
and coworkers reported a cyclohexane based C3 symmetric
hydrogelator modified with three amino acid side groups
),16–20 and Meijer and coworkers reported analogous
systems with a benzene core (
), Figure 1.12,21–24
A delicate interplay of anisotropic hydrophobic/hydrophilic, π-π
stacking, ionic and hydrogen bond formation interactions
determine the strength of hydrogelators and are considered key
factors in efforts to design potent small molecule hydrogelators.
Here we show that solvent deuteration and electrolytic strength
affect the strength of hydrogels formed from amino acid modified
C3-symmetric cyclohexane trisamides profoundly. Gels formed by
self-assembly through heating/cooling solutions or by pH
(
A
B
switching show up to
a 30 °C increase in their melting
temperatures in D2O compared to H2O. The unusually large
solvent isotope effect on gel formation and thermal properties
indicates that, in contrast to expectations, hydrogen bonding is
not the primary determinant of gel strength but instead that
hydrophobic interactions between the gelator molecules and the
terminal carboxylic acid units are of greater importance. A
Scheme 1. Hierarchical levels of interactions in the formation of gels from disc like low
molecular weight gelators: stack, fibre and intertwined network.
conclusion that is supported by
electrolytes on gel strength.
a similarly large effect of
More recently, Ulijn and coworkers showed that linear F-
moc protected peptide chains (
Tuttle and coworkers even small tripeptides (
hydrogels. The latter systems ( ) demonstrated the potential
of combined molecular dynamics/quantum chemistry
C
),25–28 and, together with
D
) can form
Hydrogels are applied widely as biocompatible materials in
medical, biological and the pharmaceutical applications and
their use has increased dramatically in recent years.1–7
Hydrogelators, either small molecule8 or polymer based,9,10
when present in minor amounts in water can form 3D
networks to form a solid like material consisting of > 95%
water by mass. Low molecular weight hydrogelators (LMWGs)
D
a
approach to rationally design gelators based on tripeptides.29
All systems structure show that they are reliable in driving
anisotropic fibre growth, Fig. 1, ascribed primarily to the triple
set of anisotropic amide amide H-bonds interactions.17,30 In all
systems, variation of the amino acid side chains, e.g., in
A,
are
a
subclass of hydrogelators, which aggregate
methionine and norleucine derivatives form hydrogels,
whereas those based on glycine do not. The end capping group
used has a pronounced effect on gelation confirming that H-
bond interactions are not the sole intermolecular forces
involved.
anisotropically to form fibers and then bundles of fibres and
finally the 3D network that holds water in place (Scheme 1).11–
15
The utility of amino acids as a structural motif in LMWGs
design is attractive not least because of their availability and
synthetic versatility. Over the last decade, Feringa, van Esch
X
X
O
R
O
R
R
H
O
R
O
O
O
O
H2
N
N
H
N
N
OH
O
X
HN
O
HN
O
H
N
R
O
R
H
O
R
O
O
O
C
D
H
H
N
N
O
R1
O
X
X
H
O
N
n
R
X
NH
O
O
R
R
X
NH
O
O
R
N
H
OH
O
NH
O
R2
A
B
E
F
Figure 2. Compounds discussed in the text. R = amino acid groups, X = OH, OMe or
ethylene glycol.
This journal is © The Royal Society of Chemistry 2016
ChemComm, 2016, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C7CC00017K
COMMUNICATION
ChemComm
Beyond structural modifications, changes in solvent properties chain length and increases with branching. Furthermore the
influence gel properties through the addition of co-solutes, pH lack of gelation by Gly and Ala indicate that hydrogen bonding
and isotope exchange. Indeed an ion specific influence (
∆Tm is not the sole determinant of gelation properties.
ca. 12 °C) on gel strength in the Fmoc protected dipeptides (
C
),
Table 1. Appearance in water after heating/cooling or pH jumping. S: Solution, P:
Precipitate, C: Crystal, G: Gel, value between bracket CGC in mg/mL
Fig. 1, that follows the Hofmeister series was reported.27,31,i
AFM studies indicated that the effect was due to changes in
fibre morphology and not salting out. Solvent isotope effects
(i.e. H2O/D2O) have received only limited attention to date
despite the opportunities it presents to disrupt hydrogen
bonding driven assemblies. The effects on gelation that have
been report are however modest with a number of reports on
Compound (amino acid)
02 (Gly)
Heat/cool
pH
S
S
03 (Ala)
S
S
04 (Val)
C
C
05 (Leu)
C
G (5.0)
poly(N-isopropylacrylamide) (
°C in gel melting point, and for
Here we report that for a series of cyclohexane core based
LMWGs ( , Fig. 2) both solvent deuteration and ionic strength
E
), that show an increase of 0.6
06 (Ile)
G (7.5)
G (0.6)
C
P
F
a 50% reduction in G’.32–36
07 (Met)
08 (Phe)
G (0.6)
C
A
09 (Trp)
C
C
have a profound influence on gel properties with as much as a
30 °C increase in melting temperature. Hence, although
anisotropic hydrogen bonding interactions between amides in
10 (Abu)
11 (Nva)
G (6.0)
G (5.0)
G (0.6)
G (7.5)
P
12 (Nle)
G (0.6)
the
C series of LMWGs has been focused upon to rationalize
The temperature dependence of the mechanical stability of
the gels was determined by dropping ball measurements. In
H2O, gels formed by Ile (7.5 mg/mL), Nva (5.0 mg/mL) and Nle
(2 mg/mL) do not melt below 130 °C.iv The Met and Abu gels
show an increase in melting point with an increase gelator
concentration, 50-110 °C (7.5 – 12 mg/mL) for Abu and 45-100
°C (2 – 7.5 mg/mL) for Met. As the other gelators melt only at
high temperatures further studies on the effect of salts and
D2O focused on Abu and Met.
their gelation properties,17 the unexpected and unprecedented
major increase in gel strength upon substitution of H2O with
D2O or the addition of electrolytes indicates that amide H-
bonding is in fact of minor importance.
Both Met and Abu gels prepared by pH jumping show an
45 °C increase in melting point compared with those prepared
by a heating/cooling cycle. However, a gel prepared thermally
from H2O containing 0.1 M NaCl(aq) (a neutral salt on the
Hofmeister series27,31) showed the same melting temperature
as the pH jump prepared samples confirming that the increase
was due to differences in electrolytic strength. Gels formed by
heat/cool cycling with 0.1 M of kosmotropic (Na2SO4, CaCl2)
and chaotropic (NaI, NH4Cl) salts as well as smaller and larger
alkali salts of chloride (RbCl, KCl, LiCl), showed that the
increase in melting temperature was approximately constant
at 40 °C for Abu and 20 °C for Met, regardless of the salt used.
The only exceptions were LiCl and RbCl, which yielded a gel
melting point 8 °C lower than with all other salts. These data
can be rationalized by a model in which the the cation
occupies the space between the fibres and stabilizes the
carboxylate groups. The concentration of salt used, however,
was 100 times higher than the concentration of Met and 8
times higher than Abu. This prompted us to assess the effect
where salt concentrations approached the gelator
concentration. With 0.33 eq. and 1 eq. of salt with respect to
the gelator, the melting point was the same as that of 0.1 M,
however, with 5 eq. the melting temperature decreased again,
in contrast to Abu which showed an increase as the amount of
salt added increased.
Figure 3: Reagents and condiꢀons‡ i) SOCl2, Δ, 20h, 96% ii) a) Amino acid methyl ester
hydrochloride, DCM, trimethylamine, RT, 48h, 88%. b) Methanol, water, NaOH, RT,
20h, 62%. R: Amino acid side chain
Eleven LMWGs were prepared (Fig. 2) using both natural and
unnatural amino acids and their hydrogelation behavior
determined from both heat/cool and pH-jump formation
thermotropic properties, rheological properties and TEM
analysis (for synthesis and spectroscopic data see ESI).
The two approaches taken to form gels from
C were to
dissolve the LMWGs in water with heat or high pH (ca. 10) and
then form gels by cooling or pH jumpingii (to below the
isoelectric point, ca. pH 3), respectively (Table 1). Although
both methods lead to the formation of gels, for several Gly and
Ala based compounds solutions were obtained only and for
Valine, Phe and Trp based compounds crystallization was
observed instead of gelation. For Leu, gels formed only upon
pH jumping; cooling from hot solutions led to crystal growth.
By contrast Ile formed a precipitate with pH jumping but gels
upon heating/cooling, which indicates that heating and cooling
allows for anisotropic growth to take place whilst the sudden
pH switching results in flash precipitation. These variations in
behavior indicate that gel fibre growth may proceed differently
with heat/cool cycles than by pH jumping. Gels with Abu form
slower (min vs s) upon pH switching compared to the other
gelators. The differences in the CGC of Abu and Nle, and of Ile
and Leu indicate that CGCs decrease with an increase in alkyl
A priori it is expected that hydrogen bonding makes a
significant contribution to hydrogelation. Hence weakening the
hydrogen bonds with H-D exchange would be expected to
suppress the gels’ melting points. Surprisingly in 100 % D2O an
increase in melting point by ~50 °C for Met and Abu was
2 | ChemComm, 2016, 00, 1-3
This journal is © The Royal Society of Chemistry 2016
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 3 of 4
View Article Online
DOI: 10.1039/C7CC00017K
ChemComm
COMMUNICATION
observed. FTIR and Raman spectra of lyophilized gelators
confirmed that rapid exchange of the amide hydrogens
occurred when dissolved in D2O, manifested in a shift of the
amide bands. The ratio of the intensity of N-H and N-D bands
were used to quantify exchange, with the C-H stretch at 3000
cm-1 as an internal standard. A full exchange to N-D was
observed (Figure S61-63) and can be reversed by rehydrating
in H2O. TEM analysis shows that fibers formed in D2O are
similar to fibers formed in H2O.
Table 2. Melting points of Met and Abu LMWGs under various conditions
Melting point (°C)
Condition
Heat gel
Met (2 mg/mL)
Abu (8 mg/mL)
45
56
pH jump
65
99.5
105
25
0.1 M NaCl
0.33 eq. NaCl
1 eq. NaCl
5 eq. NaCl
0.1 M LiCl
0.1 M KCl
0.1 M RbCl
0.1 M NH4Cl
0.1 M CaCl2
0.1 M Na2SO4
0.1 M NaI
25 % D2O
64
65
63.5
50
27.5
38
50
88
64
90.5
76
55
61
97
64
95
61
90
58
94
79.5
86.5
92
79
50 % D2O
96
75 % D2O
101
103
100 % D2O
96
The storage (G’) and loss (G”) modulus for gels formed by
Met and Abu in H2O and D2O were essentially the same as
were the breaking points where G” becomes greater than G’.
These data indicate that the structure formed in both solvents
is in fact the same and that the change in hydrogen bonding
does not affect the network. It should be noted that although
the solvents differ in viscosity over the range of electrolyte
concentrations used, the solvent viscosity does not change
significantly37 and the 20% increase of viscosity in D2O vs H2O
does not contribute substantially to the rheology since the
viscosity increase due to the gel is much greater than the
increase due to D2O.
Figure 4. Representative TEM images of Met and Abu made by heating/cooling cycle in
H2O, pH jumping in H2O, heating/cooling cycle in D2O and heating/cooling in 0.1 M
NaCl(aq) in H2O. Scale bar 0.5 µm.
In conclusion fibres formed by the C3-symmetric
A series
TEM analysis of gels formed with Met and Abu by
heat/cool cycling and pH jumping, Figure 4, indicates that for
Met, pH jumping provides for a decrease in order compared to
gels formed by heating and cooling. Notably, Abu gels formed
by either method do not show differences in morphology,
which reflects the slower formation of gel fibers compared to
Met. The interlocking network of Met shows more crosslinks in
the pH gel than in the heat gel which, considering the increase
in melting point with the former method, suggests that gel
stability is a result of the interlocking fibers. In the TEM images
of gels formed in the presence of NaCl spherical objects are
observed attached to the fibers in Met, which are most likely
salt crystals.
hydrogelators show a decrease in CGC with longer linear alkyl
chains, however, branched alkyl chains disrupt aggregation.
The effect of ionic strength on gelation by
A-type LMWGs is
equally pronounced, however, the magnitude of the effect and
the lack of specificity contrast sharply with observations made
by Ulijn et al. for gels of
C
-type LMWGs.27 The lack of a
dependence of the effect on the electrolyte used, with the
notable exceptions of LiCl and RbCl, indicates that a reduction
in carboxylate-carboxylate repulsion may play a role. Taken
together with the remarkable and unprecedented increase in
gel strength upon solvent deuteration, the data indicate that
amide H-bonding is not the dominant interaction in driving
anisotropic fibre growth but instead side chain hydrophobic
interactions dominate. These observations and the differences
This journal is © The Royal Society of Chemistry 2016
ChemComm, 2016, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C7CC00017K
COMMUNICATION
ChemComm
in effects observed for
A and C-type hydrogelators hold 17 K. J. C. Van Bommel, C. Van Der Pol, I. Muizebelt, A. Friggeri, A.
considerable implications in regard to efforts to develop
general rules for the rational design of low molecular weight
hydrogelators.
Heeres, A. Meetsma, B. L. Feringa and J. Van Esch, Angew.
Chemie - Int. Ed., 2004, 43, 1663–1667.
18 A. Friggeri, C. Van Der Pol, K. J. C. Van Bommel, A. Heeres, M. C.
A. Stuart, B. L. Feringa and J. Van Esch, Chem. - A Eur. J., 2005,
11, 5353–5361.
Acknowledgements
19 WO 2007/073180, 2007.
20 A. Brizard, M. Stuart, K. Van Bommel, A. Friggeri, M. De Jong
and J. Van Esch, Angew. Chemie - Int. Ed., 2008, 47, 2063–2066.
21 M. M. J. Smulders, A. P. H. J. Schenning and E. W. Meijer, J. Am.
Chem. Soc., 2008, 130, 606–611.
The Ministry of Education, Culture and Science of the
Netherlands (Gravity program 024.001.035) are acknowledged
for financial support.
22 K. P. Van Den Hout, R. Martín-Rapún, J. A. J. M. Vekemans and
E. W. Meijer, Chem. - A Eur. J., 2007, 13, 8111–8123.
23 P. J. M. Stals, M. M. J. Smulders, R. Martín-Rapún, A. R. A.
Palmans and E. W. Meijer, Chem. - A Eur. J., 2009, 15, 2071–
2080.
Notes and references
i
The salts examined range from chaotropic (water structure
breaking) to kosmotropic (water structure making) salts.
ii Gelation of the series LMWGs based gelators is pH
C
dependent by virtue of the terminal acid groups, which disrupt
aggregation when deprotonated through charge repulsion.
iii Gel preparation on the plates of the rheometer preclude
24 P. Besenius, K. P. Van Den Hout, H. M. H. G. Albers, T. F. A. De
Greef, L. L. C. Olijve, T. M. Hermans, B. F. M. De Waal, P. H. H.
Bomans, N. A. J. M. Sommerdijk, G. Portale, A. R. A. Palmans, M.
H. P. Van Genderen, J. A. J. M. Vekemans and E. W. Meijer,
Chem. - A Eur. J., 2011, 17, 5193–5203.
use of heating and cooling to form gels for these experiments.
iv The thermal stability of CH-Ile, CH-Nva and CH-Nle is
greater than previously highest reported by Friggeri et al.18
25 C. Tang, R. V. Ulijn and A. Saiani, Langmuir, 2011, 27, 14438–
14449.
1
N. Annabi, A. Tamayol, J. A. Uquillas, M. Akbari, L. E. Bertassoni,
C. Cha, G. Camci-Unal, M. R. Dokmeci, N. a. Peppas and A.
Khademhosseini, Adv. Mater., 2014, 26, 85–124.
N. P. Murphy, K. J. Lampe and N. P. Murphy, J. Mater. Chem. B,
2015, 3, 7867–7880.
26 M. Hughes, P. W. J. M. Frederix, J. Raeburn, L. S. Birchall, J.
Sadownik, F. C. Coomer, I.-H. Lin, E. J. Cussen, N. T. Hunt, T.
Tuttle, S. J. Webb, D. J. Adams and R. V Ulijn, Soft Matter, 2012,
8, 5595–5602.
2
3
4
5
6
7
8
9
C. Ghobril, E. K. Rodriguez, A. Nazarian and M. W. Grinstaff,
Biomacromolecules, 2016, 17, 1235–1252.
Y. L. Wu, X. Chen, W. Wang and X. J. Loh, Macromol. Chem.
Phys., 2016, 217, 175–188.
27 S. Roy, N. Javid, P. W. J. M. Frederix, D. A. Lamprou, A. J.
Urquhart, N. T. Hunt, P. J. Halling and R. V. Ulijn, Chem. - A Eur.
J., 2012, 18, 11723–11731.
28 C. Tang, R. V. Ulijn and A. Saiani, Eur. Phys. J. E, 2013, 36.
29 P. W. J. M. Frederix, G. G. Scott, Y. M. Abul-Haija, D. Kalafatovic,
C. G. Pappas, N. Javid, N. T. Hunt, R. V. Ulijn and T. Tuttle, Nat.
Chem., 2014, 7, 30–37.
K. Ulbrich, K. Holá, V. Šubr, A. Bakandritsos, J. Tuček and R.
Zbořil, Chem. Rev., 2016, 116, 5338–5431.
B. O. Okesola and D. K. Smith, Chem. Commun., 2013, 49,
11164–11166.
30 Z.-T. Li and L.-Z. Wu, Hydrogen Bonded Supramolecular
Materials, 2015.
E. J. Howe, B. O. Okesola and D. K. Smith, Chem. Commun.,
2015, 51, 7451–7454.
31 F. Hofmeister, Arch. für Exp. Pathol. und Pharmakologie, 1888,
24, 247–260.
X. Du, J. Zhou, J. Shi and B. Xu, Chem. Rev., 2015, 115, 13165–
13307.
32 P. Kujawa and F. M. Winnik, Macromolecules, 2001, 34, 4130–
4135.
S. J. Buwalda, K. W. M. Boere, P. J. Dijkstra, J. Feijen, T.
Vermonden and W. E. Hennink, J. Control. Release, 2014, 190,
254–273.
33 H. Mao, C. Li, Y. Zhang, S. Furyk, P. S. Cremer and D. E.
Bergbreiter, Macromolecules, 2004, 37, 1031–1036.
34 K. A. Houton, K. L. Morris, L. Chen, M. Schmidtmann, J. T. A.
Jones, L. C. Serpell, G. O. Lloyd and D. J. Adams, Langmuir, 2012,
28, 9797–9806.
10 E. S. Dragan, Chem. Eng. J., 2014, 243, 572–590.
11 F. Wang, M. A. J. Gillissen, P. J. M. Stals, A. R. A. Palmans and E.
W. Meijer, Chem. - A Eur. J., 2012, 18, 11761–11770.
12 P. Besenius, G. Portale, P. H. H. Boman, H. M. Janssen, A. R. A.
Palmans and E. W. Meijer, PNAS, 2010, 107, 17888–17893.
13 M. A. J. Gillissen, M. M. E. Koenigs, J. J. H. Spiering, J. A. J. M.
Vekemans, A. R. A. Palmans, I. K. Voets and E. W. Meijer, J. Am.
Chem. Soc., 2014, 136, 336–343.
35 C. Colquhoun, E. R. Draper, E. G. B. Eden, B. N. Cattoz, K. L.
Morris, L. Chen, T. O. McDonald, A. E. Terry, P. C. Griffiths, L. C.
Serpell and D. J. Adams, Nanoscale, 2014, 6, 13719–13725.
36 L. Chen, S. Revel, K. Morris, L. C. Serpell and D. J. Adams,
Langmuir, 2010, 26, 13466–13471.
14 D. Zhao and J. S. Moore, Org. Biomol. Chem., 2003, 1, 3471–
3491.
37 Z. Hai-Lang and H. Shi-Jun, J. Chem. Eng. Data, 1996, 41, 516–
520.
15 E. W. De Greef, F.A., Smulders, M.J., Wolffs, M., Schenning,
A.P..H.J., Sijbesma, R.P., Meijers, Chem. Rev., 2009, 109, 5687–
5754.
16 A. Heeres, C. Van Der Pol, M. Stuart, A. Friggeri, B. L. Feringa
and J. Van Esch, J. Am. Chem. Soc., 2003, 125, 14252–14253.
4 | ChemComm, 2016, 00, 1-3
This journal is © The Royal Society of Chemistry 2016
Please do not adjust margins