Phosphate-triggered self-assembly of N-[(uracil-5-yl)methyl]urea: A minimalistic urea-derived hydrogelator

Article abstract of DOI:10.1002/chem.201402916

N-[(Uracil-5-yl)methyl]urea is reported as a minimalistic low-molecular-weight hydrogelator (LMWHG). The unusual phosphate-induced assembly of this compound has been thoroughly investigated by IR, UV/Vis, and NMR spectroscopy, electron microscopy, and rheological experiments. This rare example of an anion-triggered urea-based LMWHG is the first example of a pyrimidine- and urea-containing molecule that can be forced into self-assembly in aqueous solution without additional aromatic or lipophilic groups. The gelator/phosphate ratio within the hydrogel was successfully determined by ³¹P MAS NMR spectroscopy. The hydrogel exhibits a very fast and repeatable self-healing property, and remarkable G' values. The viscoelastic properties of the hydrogel can easily be tuned by variation of the phosphate ratio. Minimalistic hydrogel: N-[(Uracil-5-yl)methyl]urea is reported as a minimalist low-molecular-weight hydrogelator (LMWHG), its assembly being induced by phosphate anions (see scheme). It is the first example of a pyrimidine- and urea-containing molecule that can be forced into self-assembly in aqueous solution without additional aromatic or lipophilic groups. The viscoelastic properties of the hydrogel can easily be tuned by variation of the phosphate ratio.

Full text of DOI:10.1002/chem.201402916

DOI: 10.1002/chem.201402916
Full Paper
&
Hydrogels
Phosphate-Triggered Self-Assembly of N-[(Uracil-5-yl)methyl]urea:
A Minimalistic Urea-Derived Hydrogelator
Alexander J. Kleinsmann, Nicole M. Weckenmann, and Boris J. Nachtsheim*[a]
Abstract: N-[(Uracil-5-yl)methyl]urea is reported as a minimal-
istic low-molecular-weight hydrogelator (LMWHG). The un-
usual phosphate-induced assembly of this compound has
been thoroughly investigated by IR, UV/Vis, and NMR spec-
troscopy, electron microscopy, and rheological experiments.
This rare example of an anion-triggered urea-based LMWHG
is the first example of a pyrimidine- and urea-containing
molecule that can be forced into self-assembly in aqueous
solution without additional aromatic or lipophilic groups.
The gelator/phosphate ratio within the hydrogel was suc-
cessfully determined by 31P MAS NMR spectroscopy. The hy-
drogel exhibits a very fast and repeatable self-healing prop-
erty, and remarkable G’ values. The viscoelastic properties of
the hydrogel can easily be tuned by variation of the phos-
phate ratio.
Introduction
Although it has so far been impossible to rationally predict
the gelation ability of a particular compound, ureas, nucleo-
bases, and peptides, or combinations thereof, have been inves-
tigated intensively in this regard and have been found to
show excellent hydrogelating properties because they are all
capable of excessive hydrogen bonding and therefore prone
to self-assembly. However, the hydrogelation of urea (Fig-
ure 1a)[10–12,15] and pyrimidine derivatives (Figure 1b) has only
been achieved so far by the inclusion of aromatic or lipophilic
groups to assist the formation of the hydrogen-bonding net-
work in aqueous solutions through p–p and/or van der Waals
interactions.[8e,13,14,16]
Gels are colloidal mixtures of self-assembled low-molecular-
weight compounds (low-molecular-weight gelators, LMWGs) or
polymers that are able to retain given solvent molecules by
physical effects such as surface tension.[1,2] Gels based on
LMWGs are constructed of individual molecules organized
through a self-complementary network of interactions such as
hydrogen bonding, which enables them to assemble into ex-
tended fibers and grants the corresponding gels particular ad-
vantages over covalently linked chemical gels: They are ther-
moreversible and their physical properties can be easily tuned
by external stimuli such as temperature, anions,[1,3] pH,[4] soni-
cation,[5] or UV light.[6] Therefore supramolecular gels have re-
cently received attention as versatile media for creating and
controlling attractive functions.[7]
Gelators that are capable of solidifying water to form the so-
called hydrogels are of particular interest because these inno-
vative materials play a vital role in biomedical applications
such as tissue engineering, retained drug delivery systems, and
implants.[8] The general structural requisites for low-molecular-
weight hydrogelators (LMWHGs) are appropriate hydrogen-
bond donor and acceptor moieties, which are responsible for
conferring sufficient solubility and strong self-assembly.[9] How-
ever, particularly in water, intermolecular hydrogen-bonding
networks are in general not sufficient to render a molecule an
efficient hydrogelator: Additional p–p stacking, metal coordi-
nation, or van der Waals interactions are necessary.
Figure 1. Typical examples of urea (I,[10] II,[11] III[12]) and nucleobase (IV,[13]
V
[14]) containing hydrogelators.
Our group is highly interested in minimalistic peptide- and
nucleobase-based LMWHGs,[17] and within this context we now
wish to introduce N-[(uracil-5-yl)methyl]urea (UMU, 2; Figure 2)
as the lowest-molecular-weight nucleobase-derived hydrogela-
tor to be described so far. We assumed that the combination
of both a urea and a pyrimidine motif would lead to a simple
[a] A. J. Kleinsmann, N. M. Weckenmann, Prof. B. J. Nachtsheim
Institut fꢀr Organische Chemie, Eberhard Karls Universitꢁt Tꢀbingen
Auf der Morgenstelle 18, 72076 Tꢀbingen (Germany)
E-mail: boris.nachtsheim@uni-tuebingen.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402916.
Chem. Eur. J. 2014, 20, 9753 – 9761
9753
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
and effective hydrogelator based
on natural, cheap, and readily
available building blocks. In this
study we have found that 2 is not
Table 1. Gelation studies of 2 (5 wt%) in water and aqueous buffers.
Entry
Solvent, aq. buffer[a,b]
pH 7
pH 10[c]
1
2
3
4
5
6
7
H2O
insoluble
insoluble
insoluble
insoluble[d]
insoluble
insoluble
insoluble
P[b]
P[b]
P[b]
P
P[b]
GEL
P
only
a remarkable small-nucleo-
Na2SO4/NaHSO4
NaOAc/HOAc
Na2CO3/NaHCO3
NaCl
Na3PO4/NaH2PO4
K3PO4/KH2PO4
base-comprising hydrogelator, but
also a very rare example of a urea-
containing LMWHG, which can be
forced into self-assembly exclusive-
Figure 2. Chemical structure
of N-[(uracil-5-yl)methyl]urea
(UMU, 2).
ly by the addition of phosphate anions, which usually breaks
the urea a-tape motif responsible for supramolecular aggrega-
tion.[1,15f,m,18,19]
[a] Buffer concentration=1.0m. [b] The pH was additionally adjusted with
aq. NaOH. [c] P=precipitation; GEL=hydrogel formation. [d] The pH was
additionally adjusted with aq. HCl.
When we repeated the same experiment with 1m sodium
phosphate buffer, we were delighted to observe the formation
of a turbid hydrogel (Table 1, entry 6 and Figure 3, left). Reduc-
Results and Discussion
Synthesis of N-[(uracil-5-yl)methyl]urea 2
N-[(uracil-5-yl)methyl]urea (UMU, 2) was obtained in a straight-
forward two-step synthesis from uracil. The literature-known
synthesis was greatly improved by direct condensation of 5-hy-
droxymethyluracil (1) with urea under acidic conditions, which
gave 2 in a good isolated yield of 82% (Scheme 1).[20,21] This
new synthesis was achieved in water in a fewer number of
steps and in higher yield than that previously reported. Fur-
thermore, the reactants are naturally available compounds,
UMU precipitates as the only product, and can be used with-
out further purification.
Figure 3. Macroscopic appearance of hydrogels containing 2 at different
concentrations in 1.0m sodium phosphate buffer at pH 10.
tion of the hydrogelator concentration from 5 to 2.5 wt% re-
sulted in the formation of an opaque hydrogel (Figure 3, right).
On the basis of this observation, the molecular assembly of 2
must be significantly different from most of urea-based gels re-
ported in the literature, which often exhibit urea a-tapes.
These structural motifs are usually disrupted by phosphate
anions and therefore this is a very rare example of phosphate-
induced self-assembly of a urea-based hydrogelator.[15f] Howev-
er, it is worth mentioning that the cation must also play a cru-
cial role in the self-assembly process because changing the
buffer from sodium to potassium phosphate again disrupted
gel formation (Table 1, entry 7).
Scheme 1. Improved synthesis of N-[(uracil-5-yl)methyl]urea (2). Reagents
and conditions: i) paraformaldehyde, KOH, H2O, 558C, 72 h; ii) urea, HClaq
H2O, 808C, 4 h.
,
Initial hydrogelation studies
The hydrogelation of 2 was studied by using a simple “upside-
down tube” test. Initially, a defined amount (5 wt%) of 2 was
heated until boiling at pH 7 in pure water. We found that 2 is
insoluble at neutral pH and therefore decided to test the hy-
drogelation at pH 10 because 2 should deprotonate at this pH
to form a more soluble monoanionic species. Even though 2
completely dissolved in boiling water at pH 10, no hydrogela-
tion was observed after rapid cooling in an ice bath. Instead, 2
precipitated again as a white solid (Table 1, entry 1). A white
precipitate was also formed in aqueous sulfate-, acetate-, car-
bonate-, and chloride-containing buffer solutions at pH 10
(Table 1, entries 2–5). The finding that no hydrogelation was
observed in aqueous buffer solutions was not surprising be-
cause strongly coordinating anions usually prevent or at least
weaken the self-assembled state by disturbing the urea a-tape
motif, a key interaction that has been described extensively in
the aggregation of other urea-based hydrogelators.[1,15h,m,19,22]
Detailed studies on the hydrogelation ability of this system
were carried out by using different phosphate/gelator ratios at
constant pH. Figure 4 shows the minimum gelator concentra-
tion (MGC) of 2 as a function of phosphate equivalents added.
With mixtures containing less than 1 equiv of phosphate or
below a gelator concentration of 1.0 wt%, no hydrogel forma-
tion was observed. A blend with the minimal amount of phos-
phate exhibited a relatively high MGC of 5 wt%, which de-
creased with increasing amounts of phosphate. By adding
4 equiv of phosphate, the MGC dropped significantly to
1.5 wt%. Increasing the phosphate/gelator ratio further to 6:1
had no significant impact on the MGC. However, by adding
8 equiv of phosphate, a slight decrease in MGC to a minimal
value of 1 wt% was observed. Addition of more than 8 equiv
of phosphate did not have any further impact on the MGC.
The gel–sol transition temperature was determined by rheo-
logical temperature sweep experiments (Figure 5). We found
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9754
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 4. MGC of 2 at pH 10 with different ratios of sodium phosphate.
Figure 6. A–C) SEM and D) TEM images of the xerogel of 2 (5 wt% of 2 in
1.0m sodium phosphate) at pH 10.
NMR studies
NMR titration experiments were conducted to investigate the
coordination affinity of phosphate to 2 and to verify the hydro-
gelator/anion stoichiometry. 13C NMR titration studies with
sodium phosphate were undertaken in D2O at pH 10, close to
the conditions of gelation. Strong shifts were observed for the
uracil carbons C-2, C-4, and C-6 (Figure 7), which suggests
Figure 5. Gel–sol transition temperature of 2 (5 wt%, pH 10) at different
ratios of phosphate.
that the thermal stability of the hydrogel increases with in-
creasing phosphate/2 ratio until 5 equiv of phosphate had
been added. However, further addition of phosphate reduced
the thermal stability again.
Electron microscopy
The morphology of the hydrogel was determined by scanning
electron microscopy (SEM) and transmission electron microsco-
py (TEM). The macroscopic morphology is roughly visible with-
out magnification, showing a parallel sheet structure (Fig-
ure 6A). Higher magnifications revealed dense lamellae with
a thickness of approximately 1–2 mm (Figure 6B,C) and a low
degree of cross-linking between the lamellae. A considerable
number of amorphous spheres in the range of 5–10 mm were
equally distributed over the sample (Figure 6A,B). Although the
lamellae appear to be rather robust and dense, TEM analysis
revealed their highly porous and sponge-like interior, with
pores as small as 10 nm in diameter (Figure 6D).
Figure 7. 13C NMR titration experiment of 2 with sodium phosphate at pH 10
in D2O.
a strong coordination of phosphate to the uracil ring, a shift of
the tautomer equilibrium (see the UV/Vis spectroscopy sec-
tion), or both. The most significant shifts were observed up to
the addition of 4 equiv of phosphate. A saturation effect can
be observed at higher concentrations of phosphate, with no
notable increase in the shifts detected.
The 1H NMR shifts of 2 in [D6]DMSO were also plotted
against equivalents of monobasic tetrabutylammonium phos-
phate added (Figure 8). Two urea signals, H8 and H10, as well
as the uracil signal of H6 were observed. All the signals were
shifted downfield upon addition of phosphate. However, the
strongest downfield shift of more than 1.2 ppm was observed
Surprisingly, higher magnifications did not show clearly de-
fined nanofibers, as in most supramolecular hydrogels.[8e,9,23]
However, other hydrogels containing high amounts of salt ad-
ditives showing particle-based and porous sponge-like mor-
phologies without observable nanofibers have already been
reported.[15f,24]
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9755
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 8. 1H NMR titration experiment of 2 with tetrabutylammonium dihy-
drogen phosphate in [D6]DMSO.
for 8-H, again with a saturation effect in excess of 4 equiv of
phosphate, which confirms the former trend.
For both titration experiments, a clear step-like increase in
the shifts occurred when 1 equiv phosphate was added, which
was previously described as the minimal amount of phosphate
additive needed to obtain a hydrogel. This result again implies
that a sudden self-assembly process is triggered at that partic-
ular phosphate ratio.
31P MAS NMR titration experiments of hydrogels containing
2 and different molar fractions of phosphate were performed
to quantify the phosphate/gelator stoichiometry within the gel
through a Job plot analysis. The spectra show two different
phosphorus states (Figure 9): A sharp signal at 2.84 ppm repre-
sents the amount of free phosphate in solution and a broad-
ened signal with spinning sidebands at about 0.40 ppm reflect
the phosphate incorporated into the hydrogel network.
Figure 10 displays the corresponding Job plot analysis. The
molarity of phosphate solidified within the hydrogel was deter-
mined by integration of the broadened signals. A maximum
was observed at a mole fraction c(phosphate) of 0.6, which
correlates to a 2/phosphate ratio of 1:1.5 within the hydrogel
network. This result confirms that the phosphate anion not
only coordinates to the urea groups and leads to urea–phos-
phate–urea dimers, but also plays a vital role in the self-assem-
bly process, presumably by additional coordination to the
uracil moieties, as already implied by the results of the
13C NMR titration experiments.
Figure 9. 31P MAS NMR titration experiments of hydrogels containing 2 at
pH 10 in D2O at different molar fractions of sodium phosphate: A) c=0.47,
B) c=0.50, C) c=0.60, D) c=0.67, E) c=0.8, and F) c=0.86. [a] 2.5 kHz
spinning frequency. [b] 4.0 kHz spinning frequency.
Figure 10. Job plot analysis of 2/phosphate hydrogels at pH 10.
FTIR spectroscopy
The IR of the hydrogel shows that the characteristic NH
stretching band shifts from 3314 to 3146 cmÀ1 due to strong
hydrogen bonding with C=O groups during the self-assem-
bly,[26] and the carbonyl stretching vibration band observed in
solution at 1632 cmÀ1 shifts to 1675 cmÀ1, which can also be
attributed to hydrogen-bonding interactions with the uracil
NÀH protons.[26]
FTIR spectroscopy is a powerful tool for the structural analysis
of a hydrogel network, and has been extensively used to inves-
tigate gelation because the IR spectrum of the gel state can
easily be compared with solution and solid states.
The FTIR spectrum of a 1.0 wt% supersaturated solution of 2
in 1.0m sodium phosphate buffer at pH 10 (Figure 11, dotted
line) shows absorption bands at n˜ =3314 (nNÀH), 1632 (nC O),
Furthermore, a strong increase in the intensities of the phos-
phate bands at 1070 and 982 cmÀ1 due to rigidification of the
phosphate clearly confirms the incorporation of phosphate
into the supramolecular assembly. Comparison of the IR spec-
tra of supersaturated solutions of 2 in sodium and potassium
=
1070 (nPÀO), and 982 cmÀ1 (nPÀO).[25] Subsequent cooling of this
solution for 10 min in an ice bath resulted in a metastable hy-
drogel, the IR spectrum of which was instantly recorded. The
spectrum of the gel reveals explicit changes (continuous line).
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9756
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 13. Anionic tautomers of 2.
Figure 11. FTIR spectra of 2 (1 wt%) in 1m sodium phosphate at pH 10
(dotted line=supersaturated solution; continuous line=hydrogel).
gated the tautomeric equilibria of 2 by UV/Vis spectroscopy.
First, we were interested in the influence of the added phos-
phate salt. UV/Vis spectra were recorded in 1.0m sodium phos-
phate and 1.0m potassium phosphate buffers, as well as in
sodium hydroxide solutions at pH 10. In aqueous sodium hy-
droxide solution at pH 10, both tautomers are present in about
equal proportions.[28] However, the extinction coefficient of 2b
is higher than that of 2a. Therefore the observed total spec-
trum of 2, even though as a 1:1 mixture, shows a lmax near
285 nm with a small shoulder at 265 nm (Figure 14, dotted
line).[28,29]
In contrast, the UV spectrum of 2 in sodium phosphate
buffer (Figure 14, continuous line) shows the opposite with
a lmax at 265 nm and a small shoulder at 285 nm, which indi-
cates a significant shift of the equilibrium towards the 3-H-de-
protonated species 2a. This observation is in good agreement
with Shapiro and Kang, who also reported that the 3-H-depro-
tonated uracil tautomer is favored in concentrated basic phos-
phate solutions.[28,29] When potassium phosphate buffer was
used, the equilibrium shifts even more towards 2a (Figure 14,
dashed line). Because no hydrogels are formed under these
conditions, this could imply that no single tautomer is impor-
tant for the supramolecular aggregation, but the ratio between
2a and 2b. However, there was no evidence for the formation
of 2a’’ or 2b’’.
Figure 12. FTIR spectra of a supersaturated solution of 2 (1 wt%) at pH 10 in
1m sodium phosphate (dotted line) and 1m potassium phosphate (continu-
ous line).
phosphate buffers reveals a significant difference in the inten-
sities of the phosphate bands (Figure 12); the phosphate
bands in potassium phosphate buffer turn out to be much
weaker than in sodium phosphate buffer, which indicates that
potassium significantly disturbs the self-assembly.
UV/Vis spectroscopy
Many reported LMWHGs depend on aromatic moiet-
ies such as phenylalanine or those with conjugated
p systems such as fluorenyl, naphthyl, or pyrene
groups.[23a,27] Because the gelation of 2 can only be
observed under strongly basic conditions above the
pKa1 of uracil (9.5),[28] it is likely that a single depro-
tonated species is necessary for decent self-aggrega-
tion. Some of the possible anionic tautomers of 2
(Figure 13) exhibit a more extensive p system (2a’’
and 2b’’), which has often been observed to be im-
portant in the assembly of supramolecular hydrogels.
According to the literature, 2a (lmax =260 nm) and
2b (lmax =285 nm) show different absorption
maxima, equally to the corresponding tautomers of
Figure 14. UV/Vis spectra of 2 at pH 10 in 0.1 mm sodium hydroxide (dotted line), 1.0m
sodium phosphate buffer (continuous line), and 1.0m potassium phosphate buffer
unsubstituted uracil, and their ratio can be approxi-
mately estimated.[28,29] Therefore, we further investi- (dashed line).
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org 9757
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Rheology
significant influence on rheological parameters when more
than 8 equiv of phosphate were added.
The mechanical gel strength was characterized by rheological
strain sweep and frequency sweep experiments with different
equivalents of phosphate. Figure 15 shows a strain sweep ex-
periment in which the storage modulus G’ and the loss modu-
lus G’’ are plotted against deformation. G’ is typically three to
ten times higher than G’’, which demonstrates the viscoelastic
The hydrogel also showed impressive regeneration behavior;
Figure 17 displays the results of the thixotropy/regeneration
experiment. The hydrogel was allowed to mature for 30 min
(until constant G’ and G’’ values were reached), then sheared
for 30 s at 100% deformation to destroy the supramolecular
network, and finally allowed to regenerate.
Figure 17. Rheological thixotropy/regeneration experiment performed on
a hydrogel containing 5 wt% of 2 and 5 equiv of sodium phosphate at
pH 10.
Figure 15. Rheological strain sweep experiment of a hydrogel of 2
(5 wt%+4 equiv sodium phosphate) at pH 10.
behavior of the hydrogel.[27a,30] Furthermore, the high values of
the storage modulus, more than 106 Pa, indicate a hydrogel
with superior strength. We also investigated the influence of
the phosphate ratio on gel strength (Figure 16). Addition of
the minimal amount of 1 equiv of phosphate resulted in a rela-
tively weak hydrogel with a G’ of 102 Pa. By increasing the
amount of phosphate to 1.5 equiv, which is the optimal ratio
for the hydrogel, as identified by NMR titration experiments, G’
increased dramatically up to 105 Pa. Further increases in phos-
phate led to an almost continuous increase in G’. The highest
G’/G’’ ratio is reached at 4 equiv of phosphate. This is in good
agreement with our previous findings concerning a sudden de-
crease in MGC and a saturation effect at this ratio. In agree-
ment with the MGC and titration experiments, there was no
After only 1 min, complete regeneration of the hydrogel was
observed. This process was repeated 10 times without any ma-
terial fatigue, on the contrary, an increase in gel strength was
observed, although the regeneration time increased slightly.
pH dependency
Because gelation could be triggered in the pH range from 10
to 12, we were also interested in the influence of pH on the
fundamental material properties of the hydrogel. Therefore we
conducted different rheological experiments to determine the
dependence of gel strength, sol–gel transition temperature,
and regeneration behavior of the hydrogel on pH (Table 2). We
initially observed that the hydrogel becomes weaker as the pH
is increased from 10 to 11, as reflected by a slight decrease in
the storage modulus G’. Furthermore, the regeneration time
after destruction of the gel by shear force increased by
a factor of 10. In addition, the gel–sol transition temperature
decreases with increasing pH.
Interestingly, when the pH was further increased to pH 12,
at which complete deprotonation can be expected, the hydro-
gel becomes stronger than at pH 10, as indicated by the
Table 2. Variation of hydrogel parameters with pH.
pH
G’ [106 Pa]
Tgs [8C]
Reg. time [min]
10
11
12
3.64
1.03
3.91
34.3
30.6
23.2
1.2
11.2
7.0
Figure 16. G’ (black squares) and G’’ (black diamonds) of a hydrogel contain-
ing 5 wt% of 2 and different equivalents of phosphate (left y axis) and G’/G’’
ratio (grey triangles, right y axis) at 88C.
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9758
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
higher value of G’. In addition, the regeneration time de-
creased with respect to pH 11.
self-healing ability with a remarkably fast regeneration time
and a high storage modulus of up to 107 Pa. Its stiffness,
elasticity, gel–sol transition temperature, morphology, and
regeneration ability are dramatically affected by simply varying
the amount of phosphate salt added or the pH. Even though
UV experiments revealed the importance of enol tautomerism
for self-assembly and the ideal ratio for self-assembly between
2 and phosphate was found to be 1:1.5, as revealed by Job
plot analysis, we have no detailed knowledge of the molecular
structure of the hydrogel. This will form a part of future
investigations.
The change in the viscoelastic properties is also reflected in
the macroscopic appearance and morphology of the corre-
sponding xerogels. At pH 10 a turbid hydrogel is formed,
whereas at pH 11 and 12 the hydrogels appear to be opaque.
On a nanoscale, the almost parallel lamellae of the xerogel at
pH 10 (Figure 18A) changed to a honeycomb-like morphology
at pH 11 (Figure 18B). The xerogel at pH 12 (Figure 18C), on
the other hand, shows a highly porous morphology with disor-
dered lamella regions (see the Supporting Information for dif-
ferent magnifications).
Experimental Section
Materials
Unless otherwise stated, all chemicals were used as received from
a commercial supplier. The water used for the preparation for the
hydrogels and buffers was of Millipore Milli-Q grade.
Scanning electron microscopy
The hydrogel samples were prepared on aluminum sheets, lyophi-
lized, and coated with a thin layer of platinum by using a Balzers
SCD 050 sputter coater. A Zeiss DSM 940 scanning electron micro-
scope was used to record the images of the xerogels with an accel-
erating voltage of 10 kV. For higher magnifications, the samples
were prepared without metal coating and images were recorded
on an Hitachi SU8030 scanning electron microscope using an ac-
celerating voltage of 1 kV.
Transmission electron microscopy
TEM images were recorded with an Hitachi SU8030 scanning elec-
tron microscope in STEM mode at an accelerating voltage of 30 kV.
The hydrogel samples were lyophilized and distributed onto a TEM
grid (200 mesh copper grid) that was coated with carbon film.
NMR spectroscopy
1
1H and 13C NMR spectra of compounds 1 and 2 and H NMR titra-
tion experiments were recorded on a Bruker Advance 400 MHz in-
strument in [D6]DMSO. The 1H chemical shifts are reported as d
(parts per million) relative to the quintet signal of DMSO at
2.50 ppm. The 13C chemical shifts are reported as d (parts per mil-
lion) relative to the DMSO septet at 39.43 ppm. The following ab-
breviations have been used to describe splitting patterns: br=
broad, s=singlet, d=doublet, t=triplet, q=quartet, qi=quintet,
m=multiplet. Coupling constants J are given in Hz. 13C NMR titra-
tion experiments were performed on a Bruker DRX-250 instrument
in D2O. Chemical shifts are reported as d (parts per million) relative
to the singlet signal arising from [2,2,3,3-D4]3-(trimethylsilyl)pro-
pionic acid sodium salt at 0.0 ppm. 31P MAS NMR spectra were re-
corded at 202 MHz on a Bruker AVII+500 NMR spectrometer with
a 4 mm triple resonance HR/MAS probehead. The hydrogel sam-
ples were measured at 88C at spinning frequencies of 2.5 or
4.0 kHz.
Figure 18. SEM images of xerogels of 2 (5 wt% of 2+5 equiv phosphate) at
A) pH 10, B) pH 11, and C) pH 12.
Conclusion
N-[(Uracil-5-yl)methyl]urea (2) represents not only a minimalistic
pyrimidine-containing hydrogelator, but is also a rare example
of a urea-based molecule, the self-assembly of which can be
triggered by phosphate anions. Accordingly, this very simple
hydrogelator is the lowest-molecular-weight hydrogelator
comprising a nucleobase. In addition, 2 shows an impressive
FTIR spectroscopy
IR spectra were recorded with a Jasco FT/IR-4100 spectrometer.
The corresponding buffers were used for background measure-
ments of the hydrogels and supersaturated solutions.
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9759
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
UV/Vis spectroscopy
Synthesis of N-[(Uracil-5-yl)methyl]urea (2)
UV/Vis spectra were recorded with a Perkin–Elmer Lambda 2 UV/
Vis spectrometer. The concentration of the samples was 0.08 mm
and of the buffers was 1.0m.
Concentrated hydrochloric acid (2.89 mL, 35.18 mmol) was added
to a solution of 1-hydroxymethyluracil (1; 1.00 g, 7.04 mmol) in
water (25 mL) and the reaction mixture was slowly added dropwise
to a stirred solution of urea (2.11 g, 35.18 mmol) in water (10 mL)
at 808C. After complete addition, the mixture was stirred for 4 h at
808C and subsequently cooled in an ice bath. The precipitate was
filtered, washed with water, and dried under vacuum to yield the
pure product (1.06 g, 5.77 mmol, 82%). In some cases it was neces-
sary to suspend the solid again in boiling water, filter whilst hot,
and evaporate to yield the pure product. 1H NMR (400 MHz,
[D6]DMSO): d=11.12 (brs, 1H, CONHCO), 10.71 (brs, 1H, CHNH),
Rheology
Rheological measurements were carried out by using an Anton
Paar Physica MCR 501 instrument. The geometry was plate–plate
with a diameter of 25 mm and a 0.2 mm gap between the plates.
The hydrogel samples were heated until complete dissolution, ap-
plied to the rheometer plate, and subsequently cooled to 88C after
moving the stamp into the measuring position. For comparing
strain sweep experiments with different amounts of phosphate,
the samples were matured for about 30 min at 88C, until G’ and
G’’ reached a constant level, and measured at 88C. For other ex-
periments, the hydrogel samples were cooled to 88C for 10 min,
subsequently heated to 208C, matured for another 20 min until G’
and G’’ reached a constant level, and measured at 208C. Strain
sweep experiments were conducted with a constant angular fre-
quency of 10 radsÀ1 and frequency sweep experiments were un-
dertaken with a constant deformation of 0.005%. The thixotropy/
regeneration experiments were carried out by application of a con-
tinuous deformation of 0.005% for 5 min, followed by rupture of
the gel at 100% deformation for 30 s. After complete rupture of
the gel, as denoted by G’’>G’, gel recovery was observed at a con-
stant deformation of 0.005%. The entire study was performed at
3
7.19 (s, 1H, CH), 6.11 (t, JHH =6.0 Hz, 1H, CH2NH), 5.52 (s, 2H, NH2),
3.74 ppm (d, 3JHH =6.0 Hz, 2H, CH2); 13C{1H} NMR (100 MHz,
[D6]DMSO): d=164.2 (NHCOC), 158.4 (NHCONH2), 151.1 (NHCNH),
138.5 (CH), 110.6 (CHC), 35.6 ppm (CH2); FTIR (neat): 3445.2 (m),
3414.4 (m), 3013.2 (m), 2832.0 (s), 1708.6 (s), 1630.5 (vs), 1590.0
(vs), 1538.9 (vs), 1452.1 (s), 1436.7 (s), 1322.9 (m), 1245.8 (s), 1212.0
(s), 1123.3 (w), 1009.6 (w), 888.1 (s), 857.2 (s), 823.5 (s), 756.9 (s).
643.1 cmÀ1 (w); MS (HR-FTICR, +): m/z calcd for [M+Na]+
207.048861; found: 207.049027.
Acknowledgements
Financial support by the Baden-Wꢁrttemberg Stiftung (Junior
Professor Programm) and the Fonds der Chemischen Industrie
(Sachkostenzuschuss and Ph. D. stipend to N.M.W.) is gratefully
acknowledged. Elke Nadler and Prof. Tomas Chassꢂ (Institut fꢁr
Physikalische Chemie, Tꢁbingen) are acknowledged for assis-
tance with electron microscopy. Prof. Rolf Daniels (Institut fꢁr
Pharmazeutische Technologie, Tꢁbingen) is acknowledged for
assistance with rheological measurements.
a constant angular frequency of 10 radsÀ1
.
Mass spectrometry
Mass spectra were recorded on a Finnigan MAT95 spectrometer.
High-resolution mass spectra were recorded by using ESI method
with a Bruker Daltonics Apex II FT-ICR mass analyzer.
Keywords: gels
assembly · urea
· hydrogen bonds · nucleobases · self-
Minimal gelator concentration (MGC)
[1] J. W. Steed, Chem. Soc. Rev. 2010, 39, 3686–3699.
MGCs were determined by means of the inverted vial test. Screw
cap vials with a volume of 4.0 mL and sample volumes of 0.5 mL
were used. The sample was heated until a clear solution was
obtained and cooled for 10 min in an ice bath. The sample was
defined as a hydrogel if no gravitational flow was observed upon
inversion.
[2] a) L. Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P. Sijbesma, Chem. Rev.
2001, 101, 4071–4097; b) O. Gronwald, E. Snip, S. Shinkai, Curr. Opin.
Colloid Interface Sci. 2002, 7, 148–156; c) A. R. Hirst, D. K. Smith, Chem.
Eur. J. 2005, 11, 5496–5508; d) J. A. Foster, M.-O. M. Piepenbrock, G. O.
Lloyd, N. Clarke, J. A. K. Howard, J. W. Steed, Nat. Chem. 2010, 2, 1037–
1043; e) J. W. Steed, Chem. Commun. 2011, 47, 1379–1383.
[3] a) T. Becker, C. Yong Goh, F. Jones, M. J. McIldowie, M. Mocerino, M. I.
Ogden, Chem. Commun. 2008, 3900–3902; b) M.-O. M. Piepenbrock,
G. O. Lloyd, N. Clarke, J. W. Steed, Chem. Rev. 2010, 110, 1960–2004;
c) J.-S. Shen, Q.-G. Cai, Y.-B. Jiang, H.-W. Zhang, Chem. Commun. 2010,
46, 6786–6788.
Synthesis of 5-(Hydroxymethyl)-2,4(1H,3H)-
pyrimidinedione (1)
[4] S. R. Haines, R. G. Harrison, Chem. Commun. 2002, 2846–2847.
[5] a) T. Naota, H. Koori, J. Am. Chem. Soc. 2005, 127, 9324–9325; b) K. Iso-
zaki, H. Takaya, T. Naota, Angew. Chem. 2007, 119, 2913–2915; Angew.
Chem. Int. Ed. 2007, 46, 2855–2857.
[6] a) S. Kume, K. Kuroiwa, N. Kimizuka, Chem. Commun. 2006, 2442–2444;
b) J. H. Kim, M. Seo, Y. J. Kim, S. Y. Kim, Langmuir 2009, 25, 1761–1766.
[7] a) M. George, R. G. Weiss, Acc. Chem. Res. 2006, 39, 489–497; b) A. R.
Hirst, B. Escuder, J. F. Miravet, D. K. Smith, Angew. Chem. 2008, 120,
8122–8139; Angew. Chem. Int. Ed. 2008, 47, 8002–8018; c) A. Ajaya-
ghosh, V. K. Praveen, C. Vijayakumar, Chem. Soc. Rev. 2008, 37, 109–
122; d) S. Banerjee, R. K. Das, U. Maitra, J. Mater. Chem. 2009, 19, 6649–
6687.
Uracil (10.00g, 89.20mmol) and paraformaldehyde (3.35g,
111.50mmol) were added to a 0.54m aqueous solution of KOH
(100mL) and the reaction mixture was stirred at 558C for 72h. The
solvent was then removed under reduced pressure to one third
volume, and the same volume of acetone was added. The precipi-
tate was filtered, washed with acetone, and dried under vacuum.
The pure product was obtained by recrystallization from water
1
(10.78g, 75.86mmol, 85% yield). H NMR (400 MHz, [D6]DMSO): d=
11.04 (brs, 1H, CONHCO), 10.70 (brs, 1H, CHNH), 7.24 (s, 1H, CH),
4.84 (brs, 1H, OH), 4.10 ppm (s, 2H, CH2); 13C{1H} NMR (100 MHz,
[D6]DMSO): d=163.7 (NHCOC), 151.3 (NHCNH), 138.0 (CH), 112.6
(CHC), 55.7 ppm (CH2); MS (FAB, +): m/z: 142.0 [M+H]+. The ana-
lytical data is in agreement with literature.[31]
[8] a) K. Y. Lee, D. J. Mooney, Chem. Rev. 2001, 101, 1869–1880; b) T. Miyata,
T. Uragami, K. Nakamae, Adv. Drug Delivery Rev. 2002, 54, 79–98; c) J. C.
Tiller, Angew. Chem. 2003, 115, 3180–2183; Angew. Chem. Int. Ed. 2003,
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9760
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
42, 3072–3075; d) S. Zhang, Nat. Mater. 2004, 3, 7–8; e) Y. Tang, C. L.
Heaysman, S. Willis, A. L. Lewis, Expert Opin. Drug Delivery 2011, 8,
1141–1159.
J. Shi, Y. Kuang, B. Xu, Soft Matter 2012, 8, 7402–7407; d) R. Iwaura, M.
Ohnishi-Kameyama, Chem. Commun. 2012, 48, 6633–6635.
[17] A. J. Kleinsmann, B. J. Nachtsheim, Chem. Commun. 2013, 49, 7818–
7820.
[18] M. D. Segarra-Maset, V. J. Nebot, J. F. Miravet, B. Escuder, Chem. Soc. Rev.
2013, 42, 7086–7098.
[19] G. O. Lloyd, M.-O. M. Piepenbrock, J. A. Foster, N. Clarke, J. W. Steed, Soft
Matter 2012, 8, 204–216.
[20] T. P. Johnston, G. S. McCaleb, J. A. Montgomery, J. Med. Chem. 1963, 6,
669–681.
[9] a) L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201–1216; b) T.
Park, S. C. Zimmerman, J. Am. Chem. Soc. 2006, 128, 11582–11590; c) C.
Yan, D. J. Pochan, Chem. Soc. Rev. 2010, 39, 3528–3540; d) E. K. John-
son, D. J. Adams, P. J. Cameron, J. Mater. Chem. 2011, 21, 2024–2027;
e) L. E. Buerkle, S. J. Rowan, Chem. Soc. Rev. 2012, 41, 6089–6102; f) A.
Noro, M. Hayashi, Y. Matsushita, Soft Matter 2012, 8, 6416–6429; g) H.
Wang, Z. Yang, Nanoscale 2012, 4, 5259–5267; h) H. Wang, Z. Yang,
D. J. Adams, Mater. Today 2012, 15, 500–507; i) J. Raeburn, A. Zamith
Cardoso, D. J. Adams, Chem. Soc. Rev. 2013, 42, 5143–5156.
[10] P. K. Vemula, G. John, Chem. Commun. 2006, 2218–2220.
[11] D. K. Kumar, D. A. Jose, A. Das, P. Dastidar, Chem. Commun. 2005, 4059–
4061.
[12] J.-U. Kim, D. Schollmeyer, M. Brehmer, R. Zentel, J. Colloid Interface Sci.
2011, 357, 428–433.
[21] R. E. Cline, R. M. Fink, K. Fink, J. Am. Chem. Soc. 1959, 81, 2521–2527.
[22] M. Suzuki, M. Yumoto, H. Shirai, K. Hanabusa, Org. Biomol. Chem. 2005,
3, 3073–3078.
[23] a) Z. Yang, B. Xu, J. Mater. Chem. 2007, 17, 2385–2393; b) P. K. Sukul, S.
Malik, Soft Matter 2011, 7, 4234–4241; c) Y. Gao, M. J. C. Long, J. Shi, L.
Hedstrom, B. Xu, Chem. Commun. 2012, 48, 8404–8406.
[24] a) Y. Li, J. Liu, G. Du, H. Yan, H. Wang, H. Zhang, W. An, W. Zhao, T. Sun,
F. Xin, L. Kong, Y. Li, A. Hao, J. Hao, J. Phys. Chem. B 2010, 114, 10321–
10326; b) A. Tripathi, P. S. Pandey, Tetrahedron Lett. 2011, 52, 3558–
3560; c) I. Kapoor, E.-M. Schon, J. Bachl, D. Kuhbeck, C. Cativiela, S. Saha,
R. Banerjee, S. Roelens, J. J. Marrero-Tellado, D. D. Diaz, Soft Matter
2012, 8, 3446–3456.
[25] F. A. Miller, C. H. Wilkins, Anal. Chem. 1952, 24, 1253–1294.
[26] B. Roy, A. Saha, A. Esterrani, A. K. Nandi, Soft Matter 2010, 6, 3337–
3345.
[27] a) A. M. Smith, R. J. Williams, C. Tang, P. Coppo, R. F. Collins, M. L. Turner,
A. Saiani, R. V. Ulijn, Adv. Mater. 2008, 20, 37–41; b) T. Kar, S. K. Mandal,
P. K. Das, Chem. Eur. J. 2011, 17, 14952–14961; c) J. Nanda, A. Biswas, A.
Banerjee, Soft Matter 2013, 9, 4198–4208.
[28] R. Shapiro, S. Kang, Biochim. Biophys. Acta 1971, 232, 1–4.
[29] a) N. Iza, M. Gil, J. Morcillo, J. Mol. Struct. 1988, 175, 31–36; b) B. E. Bill-
inghurst, S. A. Oladepo, G. R. Loppnow, J. Phys. Chem. B 2009, 113,
7392–7397.
[30] a) B. Adhikari, J. Nanda, A. Banerjee, Soft Matter 2011, 7, 8913–8922;
b) J. Nanda, A. Banerjee, Soft Matter 2012, 8, 3380–3386.
[31] L. Bꢂrillon, R. Wagner, P. Knochel, J. Org. Chem. 1998, 63, 9117–9121.
[13] S. M. Park, Y. S. Lee, B. H. Kim, Chem. Commun. 2003, 2912–2913.
[14] X. Li, Y. Kuang, B. Xu, Soft Matter 2012, 8, 2801–2806.
[15] a) G. Wang, A. D. Hamilton, Chem. Commun. 2003, 310–311; b) L. A. Es-
troff, L. Addadi, S. Weiner, A. D. Hamilton, Org. Biomol. Chem. 2004, 2,
137–141; c) M. de Loos, A. Friggeri, J. van Esch, R. M. Kellogg, B. L. Fer-
inga, Org. Biomol. Chem. 2005, 3, 1631–1639; d) M. ꢃvalos, R. Babiano,
P. Cintas, A. Gꢄmez-Carretero, J. L. Jimꢂnez, M. Lozano, A. L. Ortiz, J. C.
Palacios, A. Pinazo, Chem. Eur. J. 2008, 14, 5656–5669; e) F. Rodrꢅguez-
Llansola, B. Escuder, J. F. Miravet, D. Hermida-Merino, I. W. Hamley, C. J.
Cardin, W. Hayes, Chem. Commun. 2010, 46, 7960–7962; f) G. O. Lloyd,
J. W. Steed, Soft Matter 2011, 7, 75–84; g) M. Yamanaka, N. Haraya, S.
Yamamichi, Chem. Asian J. 2011, 6, 1022–1025; h) F. Rodrꢅguez-Llansola,
D. Hermida-Merino, B. Nieto-Ortega, F. J. Ramirez, J. T. L. Navarrete, J.
Casado, I. W. Hamley, B. Escuder, W. Hayes, J. F. Miravet, Chem. Eur. J.
2012, 18, 14725–14731; i) Y. Jinno, M. Yamanaka, Chem. Asian J. 2012,
7, 1768–1771; j) D. Higashi, M. Yoshida, M. Yamanaka, Chem. Asian J.
2013, 8, 2584–2587; k) K. Liu, J. W. Steed, Soft Matter 2013, 9, 11699–
11705; l) M. Yamanaka, J. Inclusion Phenom. Macrocyclic Chem. 2013, 77,
33–48; m) J. A. Foster, R. M. Edkins, G. J. Cameron, N. Colgin, K. Fucke,
S. Ridgeway, A. G. Crawford, T. B. Marder, A. Beeby, S. L. Cobb, J. W.
Steed, Chem. Eur. J. 2014, 20, 279–291.
[16] a) R. Iwaura, K. Yoshida, M. Masuda, K. Yase, T. Shimizu, Chem. Mater.
2002, 14, 3047–3053; b) L. Moreau, P. Barthꢂlꢂmy, M. El Maataoui, M. W.
Grinstaff, J. Am. Chem. Soc. 2004, 126, 7533–7539; c) X. Li, X. Du, Y. Gao,
Received: April 2, 2014
Published online on July 7, 2014
Chem. Eur. J. 2014, 20, 9753 – 9761
www.chemeurj.org
9761
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim