A network of small molecules connected by cross-linked NH bonds

Article abstract of DOI:10.1021/cg900874f

The synthesis of the small pseudopeptide Boc-L-Phe-D-Imz-OBn (Imz = imidazolidin-2-one-4-carboxylate) is reported. Crystallization of this peptide from methanol, ethanol, and isopropanol leads to isostructural solvates when the solvent is methanol or etha

Full text of DOI:10.1021/cg900874f

DOI: 10.1021/cg900874f
A Network of Small Molecules Connected by Cross-Linked NH Bonds
2010, Vol. 10
244–251
Gaetano Angelici,† Nicola Castellucci,† Simone Contaldi,†
†
‡
Giuseppe Falini, Hans-Jorg Hofmann, Magda Monari,*,† and Claudia Tomasini*,†
€
†
ꢀ
Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum Universita di Bologna - Via Selmi 2,
I-40126 Bologna, Italy, and ‡Institute of Biochemistry, University of Leipzig - Bru€derstrasse 34,
D-04103 Leipzig, Germany
Received July 27, 2009; Revised Manuscript Received September 14, 2009
ABSTRACT: The synthesis of the small pseudopeptide Boc-L-Phe-D-Imz-OBn (Imz = imidazolidin-2-one-4-carboxylate) is
reported. Crystallization of this peptide from methanol, ethanol, and isopropanol leads to isostructural solvates when the
solvent is methanol or ethanol with a peptide/solvent ratio of 2:1 and to an unsolvated polymorph in the case of isopropanol. The
solvate peptide crystallizes forming infinite chains with the monomers in parallel orientation connected by a single hydrogen
bond. The chains are arranged in antiparallel direction and cross-linked through the NH group of the imidazolidin heterocycle
with formation of a stable two-dimensional (2D) network. Crystals from isopropanol form a different 2D network. The degree
of order in the crystal assembly decreases from methanol and ethanol solvates to the unsolvated pseudopeptide grown from
isopropanol. Quantum chemical calculations at the HF/6-31G* level of ab initio MO theory, carried out on the two different
packings, show a slight preference for the unsolvated packing.
Introduction
ethyl acetate.7 This compound forms fibers with the mono-
mers arranged in infinite chains connected only by a single
hydrogen bond.8 It may be tempting to look for other
molecules with such unusual properties. Thus, we synthesized
the pseudodipeptide Boc-L-Phe-D-Imz-OBn 1 (Imz = imida-
zolidin-2-one-4-carboxylate) and investigated its structure in
the solid state. This compound is similar to Boc-L-Phe-D-Oxd-
OBn but contains an additional NH group in the heterocyclic
ring oriented nearly orthogonal to the pre-existing one
(Figure 1). The presence of two NH hydrogen bonds that
are not oriented along the same axis should favor the forma-
tion of a two-dimensional arrangement. This effect should
drive the formation of layers instead of fibers, that were
obtained with Boc-L-Phe-D-Oxd-OBn, which can form only
one hydrogen bond.
Synthetic oligomers which are able to form stable second-
ary structures have been defined as “foldamers”.1 Foldamer
research was essentially stimulated by the investigation of
peptidic foldamers composed of unnatural and proteinogenic
amino acids or even completely of unnatural amino acids.2
These molecules are designed and prepared with the aim of
forming three-dimensional structures that mimic complex
natural structures, such as peptides and proteins. Sometimes,
these compounds exhibit interesting novel properties, as for
instance antibacterial or anticancer effects.3 Furthermore,
they might be applied against neurodegenerative diseases that
are characterized by the formation of amyloidal peptides,
resulting from the degradation of natural proteins.4 There-
fore, the study of foldamers, in particular representatives with
potential biological activity, is highly desirable.
Recently, it was shown5 that there is no need of very long
molecules to form fiber-like materials. Formation of fibrils
might even be prevented in longer molecules by the formation
of intramolecular hydrogen bonds instead of the required
intermolecular interactions. In contrast, small molecules
could be good candidates to form fiber- and fibril-like materi-
als, provided thatthey are rigidenoughtoassumewell-defined
conformations. The formation of unconventional “poly-
mers”,6 where the chains are not only formed by covalent
bonds, but also by weaker interactions, such as hydrogen
bonds, electrostatic interactions, π-stacking interactions, and
noncovalent interactions, is possible. If all these weaker
interactions cooperate, the formation of peptide-based nano-
materials can be observed.
Experimental Section
Synthesis. The melting points of the compounds were determined
in open capillaries and are not precise (see Thermal Analyses). High
quality infrared spectra (64 scans) were obtained at 2 cm-1 resolu-
tion using a 1 mm NaCl solution cell and a Nicolet 210 FT-infrared
spectrometer. The spectra were obtained in 3 mM solutions in dry
CH2Cl2 at 297 K or as a 1% mixture in dry KBr. All compounds
were dried in a vacuum, and all the sample preparations were
performed in a nitrogen atmosphere. NMR spectra were recorded
with spectrometers at 400 MHz (1H NMR) and at 100 MHz (13
C
NMR). The proton signals were assigned by gCOSY spectra.
Chemical shifts are reported in δ values relative to the solvent peak.
Boc-L-Phe-D-Asn-OBn was prepared by coupling of Boc-L-Phe-OH
and H-Asn-OBn in solution utilizing conventional methods.
Boc-L-Phe-D-Imz-OBn 1. DBU (0.127 mL, 0.85 mmol) was added
to a stirred solution of Boc-L-Phe-D-Asn-OBn (0.2 g, 0.43 mmol) in
THF (20 mL). The mixture was stirred for 10 min. Then, PhI(OAc)2
(274 mg, 0.85 mmol) was added and the mixture was stirred at room
temperature for 45 min. After this time, a few drops (<1 mL) of
H2O were added, and the mixture was stirred for another 15 min.
The reaction mixture was gently concentrated. The resulting yellow
liquid was dissolved in ethyl acetate and washed with H2O, dried
over Na2SO4, filtered, concentrated, and dried under a vacuum. The
crude was purified by flash chromatography (cyclohexane/ethyl
We have recently described the formation of a fiber-like
material obtained by the slow evaporation of a solution of the
dipeptide Boc-L-Phe-D-Oxd-OBn (Boc = tert-butoxycarbo-
nyl; Phe=phenylalanine; Oxd=4-methyl-5-carboxy oxazo-
lidin-2-one; Bn = benzyl) in a 1:1 mixture of cyclohexane/
*To whom correspondence should be addressed. E-mail: claudia.tomasini@
unibo.it (C.T.), magda.monari@unibo.it (M.M.).
r
pubs.acs.org/crystal
Published on Web 10/05/2009
2009 American Chemical Society

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 245
Figure 1. Schematic structural comparison of Boc-L-Phe-D-Oxd-OBn and Boc-L-Phe-D-Imz-OBn (red and blue arrows show the hydrogen
bond directions).
acetate 1:1 as eluant) and 1 was obtained pure in 65% yield. Mp=
68 ꢀC; [R]D þ0.081 (c 1.0, CH2Cl2); IR (CH2Cl2, 10 mM): ν=3444,
1759, 1716, 1689 cm-1; IR (1% in dry KBr): ν = 3391, 1761, 1687
cm-1; 1H NMR (CDCl3, 300 MHz): δ 1.42 (s, 9H, t-Bu), 2.95 (dd,
1H, J=7.5, 13.8 Hz, CHN-CHH-Ph), 3.2 (dd, 1H, J=4.8, 13.5 Hz,
CHN-CHH-Ph), 3.4 (dd, 1H, J = 3.0, 9.3 Hz, NCHH), 3.7 (t, 1H,
J=9.9 Hz, NCH), 4.8 (m, 1H, NCHH), 5.2-5.4 (m, 3H, OCH2Ph þ
Boc-NH), 5.85 (bs, 1H, NH), 5.95 (m, 1H, CHN-CHH-Ph), 7.2-7.4
(m, 10H, 2 ꢀ Ph); 13C NMR (CDCl3, 75 MHz): δ 28.5, 39.9, 40.6,
54.1, 55.4, 68.0, 127.0, 128.6, 129.0, 129.9, 135.2, 136.6, 154.7, 155.0,
169.1, 172.7.
Plate-Like Crystal Precipitation. The purified compound was
Figure 2. Suggested mechanism of the modified Hofmann rearran-
gement for the synthesis of 4-substituted imidazolidin-2-one.
crystallized by slow evaporation of a solution of 1 (35 mg) in metha-
nol, ethanol, or isopropanol (1 mL in each case). Crystals called 1a
(crystallized from methanol), 1b (crystallized from ethanol), and 1c
(crystallized from isopropanol) were prepared.
were performed by using high purity standards (n-decane, benzene,
and indium). The material (about 1.5 mg of sample) was sealed in
aluminum pans. Heating was carried out at 3 ꢀC min-1 in the
temperature range 20-200 ꢀC. Transition, denaturation, or deso-
lvation temperatures (TD) were determined as the maximum peak
value of the corresponding endothermic phenomena. Weight loss
during heating was evaluated by thermogravimetric analysis (TGA)
using a calorimeter DSC-Q100 from TA Instruments Waters
(USA). The temperature range was from 23 to 350 ꢀC at a heating
rate of 5 ꢀC/min under dry nitrogen atmosphere.
Calculations. A detailed conformational analysis of Boc-L-Phe-D-
Imz-OBn and calculations on the various crystal packages were
performed at the HF/6-31G* level of ab initio MO theory. The
Gaussian03 software package was employed in all cases.13 The
details are given in the Supporting Information.
Imaging. The solid material was observed by optical microscopy
(OM) and scanning electron microscopy (SEM). The OM images
were collected using a Leica optical microscope equipped with a
CCD camera. Samples SEM images were collected on glass cover-
slip after coating with gold and observed using Philips 515 SEM.
The images were recorded using a CCD digital camera.
Single Crystal X-ray Diffraction for 1a, 1b, and 1c. The X-ray
intensity data for 1a, 1b, and 1c were measured on a Bruker SMART
Apex II CCD area detector diffractometer. Cell dimensions and the
orientation matrix were initially determined from a least-squares
refinement on reflections measured in three sets of 20 exposures,
collected in three different ω regions, and eventually refined against
all data. A full sphere of reciprocal space was scanned by 0.3ꢀ ω steps.
The software SMART9 was used for collecting frames of data, index-
ing reflections, and determination of lattice parameters. The collected
frames were then processed for integration by the SAINT program,10
and an empirical absorption correction was applied using SADABS.11
The structures were solved by direct methods (SIR 97)11 and subse-
quent Fourier analysis and refined by full-matrix least-squares on
F2 (SHELXTL),12 using anisotropic thermal parameters for all non-
hydrogen atoms. All hydrogen atoms, except the N-H protons and
methine hydrogens, were added in calculated positions, included in the
final stage of refinement with isotropic thermal parameters, U(H) =
1.2Ueq(C)[U(H)=1.5Ueq(C-Me)], and allowed to ride on their carrier
atoms. The absolute structure configuration was not determined from
X-ray data but was known from the synthetic route. Crystal data and
experimental details for 1a, 1b, and 1c are reported in Table S1,
Supporting Information.
Results and Discussion
The dipeptide Boc-L-Phe-D-Imz-OBn 1 was prepared in a
single step starting from the readily available Boc-L-Phe-D-
Asn-OBn (Asn=asparagine) utilizing a modified methodol-
ogy for the Hofmann rearrangement that we have recently
described.14 This method was necessary because the prepara-
tion of an imidazolidin-2-one-4-carboxylate could not be
performed by reaction of 1,2-diaminopropanoic acid (DAP)
with triphosgene, due to the low availability of DAP and the
poor reaction yield.
The rearrangement is a very general reaction and can be
applied both to protected asparagine and to asparagine-
containing protected polypeptides. It is promoted by the
readily available PhI(OAc)2 in the presence of DBU (DBU =
1,8-diazabicyclo[5,4,0]undec-7-ene) and provides the desired
heterocycle in high yield (Figure 2).
After preparation and purification with conventional meth-
ods, Boc-L-Phe-D-Asn-OBn was treated with PhI(OAc)2 and
DBU in tetrahydrofuran for 1 h to furnish the desired com-
pound 1 in 65% yield after flash chromatography (Scheme 1).
X-ray Powder Diffraction Analysis. Powder X-ray diffraction
patterns were collected using a PanAnalytical X’Pert Pro equipped
with X’Celerator detector powder diffractometer using Cu KR
radiation generated at 40 kV and 40 mA. The instrument was
configured with a 1/32ꢀ divergence and 1/32ꢀ antiscattering slits.
A standard quartz sample holder 1 mm deep, 20 mm high, and
15 mm wide was used. The diffraction patterns were collected within
the 2θ range from 3ꢀ to 40ꢀ with a step size (Δ 2θ) of 0.02ꢀ and a
counting time of 300 s.
Thermal Analysis. Calorimetric measurements were performed
using a Perkin-Elmer DSC-7. Temperature and enthalphy calibrations

246 Crystal Growth & Design, Vol. 10, No. 1, 2010
Angelici et al.
Figure 3. FT-IR absorption spectra (a) in the N-H stretching region (3600-3200 cm-1) and (b) in the CdO stretching region (1850-
1600 cm-1) for 3 mM samples of 1 in pure CH2Cl2 (black line) and as a 1% solid mixture with KBr (red line) at room temperature.
Scheme 1. Synthesis of Boc-L-Phe-D-Imz-OBn 1
The conformation of compound 1 was analyzed in solution
and in the solid state. A first analysis was performed, compar-
ing the IR spectrum of a 3 mM solution of 1 in dichloro-
methane with the spectrum of a 1% mixture of 1 in KBr. The
spectral regions concerning the NH and CO stretching bands
are reported in Figure 3.
While hydrogen bonding is clearly present in the solid state
as shown by two peaks (3391 and 3371 cm-1) belonging to a
broadband, no hydrogen bonding is present in the diluted
solution (3444 cm-1), and thus intramolecular NH hydrogen
bonds are absent. A strong variation of the spectrum in the
CO region confirms this result.
Several attempts were made to obtain macroscopic ordered
structures in the solid state by slow evaporation of solutions of
1 in different solvents. The formation of crystal aggregates of
1 was observed by slow evaporation of a 1:1 mixture of
cyclohexane/ethylacetate, whichwas used for the evaporation
of Boc-L-Phe-D-Oxd-OBn. This material was highly crystal-
line, as shown by the powderX-ray diffraction pattern(Figure
S3, Supporting Information).
used as solvents, as shown by the reduced birefringence of
crystals 1b and 1c (insets in Figure 4c,e). A mismatch in lateral
aggregation is present in 1b (insets in Figure 4d) and becomes
remarkable in 1c, where the crystals show a low birefringence
(inset in Figure 4e). However, all these aggregates are highly
crystalline, as shown by the X-ray diffraction pattern (Figure
S3, Supporting Information). Crystals suitable for single
crystal X-ray diffraction were obtained from isopropanol
solutions in a crystallization trial using seed crystals from
the homogeneous precipitation in the same solvent (Figure 4f).
This material is the same polymorph of that obtained from
homogeneous precipitation from isopropanol (Figure S3,
Supporting Information). The observation of crystals by
SEM (Figure 4b,d,f) shows that each crystal is made of several
layers which are cast on the top of each other. This super-
structure organization is also present in the single crystals
obtained from isopropanol by seeding (inset of Figure 4f).
Thus, inallthesecrystals a plywoodstructureispresent, witha
varying degree of order as a function of the precipitation
conditions.
In contrast, slow evaporation from protonated solvents
such as methanol, ethanol, and isopropanol provided crystals,
which were useful for single crystal X-ray diffraction studies.
Interestingly, the nature of the alcohol has an influence on the
preferential elongation and macroscopic side-aggregation of
the crystals (Figure 4), which show a common tendency to
aggregate laterally along their main axis (Figure 4a,c,e). In the
aggregates of 1a, the lateral association extends over several
hundred micrometers. Their image under the optical micro-
scope with cross polars shows them preferentially iso-oriented
(insets in Figure 4a). The degree of order in crystal association
continuously decreased when ethanol and isopropanol were
The formation of these plate-like crystals only by slow
evaporation of the protonated solvents suggests the idea that
the solvents may play an active role in crystal formation.8 The
obtained single crystals were analyzed by X-ray diffraction
(XRD) and thermal analysis. Crystals of 1 grown from
methanol and ethanol, respectively, show the formation of
methanol (1a) and ethanol solvates (1b). In addition to the
solvent, two independent molecules of 1 are present in the unit
cell (Figure 5).
The two peptide molecules in structure 1a have approxi-
mately the same backbone conformation (Table 1, Figure 5).
The twoconformersdiffer only in the orientationof the benzyl

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 247
Figure 4. Optical micrographs of crystals of 1 precipitated by slow evaporation from solutions of 1 in (a) 1a from methanol, (c) 1b from
ethanol, and (e) 1c from isopropanol. The top insets in (a, c, e) show the same images under cross polars. The bottom inset (c) shows an
aggregate of plate-like crystals precipitated from an ethanol solution. In the left insets in (f) optical micrographs of a crystal of 1c are shown
obtained in a seeding crystallization trial from an isopropanol solution. Scanning electron microscopy images of crystals of 1a, 1b, and 1c are
shown in (b), (d), and (f), respectively. The plywood structure is evident for 1a (b) and 1b (d). The insets in (b) and (d), and that on the right in (f),
show that all crystals are made of several layers.
ring of the OBn moiety (see the superimposition in Figure S4,
Supporting Information). In the unit cell, each conformer
belongs to a separate infinite chain of peptide molecules,
which are arranged in parallel orientation. Two consecutive
leads to an interconnection of two peptide chains via the
bifurcated NH bond involving the Imz NH group of a peptide
molecule of one chain and two carbonyls of the other chain.
One carbonyl group comes from an L-Phe residue, and the
other from the D-Imz constituent of the other chain. Although
residues in the chains are connected by only one NH OdC
3 3 3
˚
hydrogen bond between the amidic NH proton of the L-Phe
residue of the one peptide molecule and the carbonyl function
of the t-Boc group of the other one (Table 2).
the NH OC distances are larger (2.35 and 2.52 A) than
3 3 3
those found in typical hydrogen bonds, the general electro-
static effect in this arrangement favors the interconnection. In
comparison to the D-Oxd crystals, the packing is more com-
plex because of the presence of one methanol molecule that
acts as a linker between the two chains establishing two
H-bonds using both the oxygen and hydrogen atoms
The infinite chains are arranged in an antiparallel orienta-
tion along the a axis (Figure 6a). The phenyl rings are in a
displaced stacked arrangement, but the offset of the phenyls
prevents π-π stacking. The conformation of the monomers
and the arrangement of the chains is in close correspondence
to that found in Boc-L-Phe-D-Oxd-OBn.7 However, the back-
bone torsion angle ψ of the D-Imz constituents is -163.2ꢀ and
-150.8ꢀ, respectively, different from that in D-Oxd, which is
23.4ꢀ. According to the quantum chemical calculations, these
two conformers can be realized both in D-Oxd and in D-Imz.
They are the most stable conformers and close together in
energy (see Supporting Information). The presence of the NH
unit in D-Imz, replacing the oxygen atom in the D-Oxd ring, allows
the formation of further hydrogen bonds. This possibility
(O13-H80 O8, O13 H1-N1) of the solvent molecule
3 3 3 3 3 3
(Figure 6b).
Asa result of theseinterchaininteractionsinvolvingalso the
methanol molecule, the solid-state arrangement of 1a can be
described as a 2D network made of layers, which are parallel
to the ab plane (Figure 6a).
The single crystal X-ray diffraction study carried out on
crystals of 1 grown from ethanol shows the formation of an
ethanol solvate 1b (Tables S2 and S3, Supporting Informa-
tion, Figure 5b) isostructural with the methanol solvate 1a.

248 Crystal Growth & Design, Vol. 10, No. 1, 2010
Angelici et al.
Figure 5. View of the content of the unit cell of 1 crystallized with (a) methanol (structure 1a) and (b) ethanol (structure 1b).
crystals of the peptide Boc-L-Phe-D-Oxd-OBn.7 In the crystal
packing of 1c, two different types of intermolecular hydrogen
bonds arepresent (Figure 7). One hydrogen bondconnectsthe
amidic NH hydrogen of the L-Phe unit with the carbonyl
oxygen of the CO group of the D-Imz moiety belonging to
another peptide molecule. The other involves the NH proton
of the D-Imz ring and the carbonyl oxygen of a t-Boc group of
another peptide molecule. These weak interactions generate
also a 2D-network parallel to the ab plane, which is different
from that observed in the packing of 1a and 1b, respectively
(Tables 3 and 4). Quantum chemical calculations on the two
different crystal packings with six peptide molecules but
without solvent molecules show a slight preference for the
packing of 1c by 8.3 kJ/mol atthe HF/6-31G* level of ab initio
MO theory. By interaction with an additional solvent mole-
cule, the methanol/ethanol solvate packings can obviously
compensate for this disadvantage.
In order to investigate the effect of temperature on the
structure of the plate-like crystals of 1a, 1b, and 1c, calori-
metric and X-ray powder diffraction analyses were carried out
(Figure 8). The heating profile of single crystals of 1a
(Figure8a) shows abroadendothermic peakat46ꢀCfollowed
by two sharp endothermic peaks, a strong one at 89 ꢀC and a
weak one at 141 ꢀC. For a better understanding, the structural
reorganization of 1a associated with the thermal treatment
was followed by X-ray powder diffraction (Figure 8b). The
experimental X-ray diffraction pattern of 1a collected at room
temperature (Figure 8b, polymorph 1a-I) shows all the peaks
present in the diffraction pattern calculated from the single
crystal structure of 1a (Figure 8b, polymorph 1a-C) and some
additional peaks belonging to the diffraction pattern of 1a
after the thermal treatment just above the temperature first
endothermic peak (Figure 8b, polymorph 1a-II). The 1a
material TGA profile analysis indicates a weight loss of 3.8%
in the temperature range between 23 and 68 ꢀC (Figure S8,
Supporting Information). Thus, the first endothermic peak
Table 1. Comparison of the Backbone Torsion Angles for the Two
Independent Molecules of 1a (Methanol Solvate)
torsion anglesa
molecule A
molecule B
C22-N3-C14-C3 (φ1)
N3-C14-C3-N2 (ψ1)
N2-C4-C6-O4 (φ2)
C37-N6-C29-C28 (φ1)
N6-C29-C28-N5 (ψ1)
N5-C43-C44-O12 (φ2)
-116.9(4)
159.7(4)
163.2(5)
-124.8(4)
169.6(4)
-150.8(4)
a For atom numbering, see Figure 5.
Table 2. Hydrogen Bond Parameters for 1aa
Ab Ab
D-H
A
H
D
—D-H Ac
3 3 3
3 3 3
3 3 3
3 3 3
N3-H3N O5(1)
2.232
1.879
2.259
2.348
2.518
2.226
3.058(5)
2.788(7)
3.037(5)
2.949(6)
3.453(8)
2.836(6)
164.19
152.87
149.49
117.73
155.64
131.40
3 3 3
N1-H1 O13
3 3 3
N6-H6N O9(2)
3 3 3
N4-H4N O1(3)
3 3 3
N4-H4N O3(4)
3 3 3
O13-H80 O8
3 3 3
a For atom numbering, see Figure 5; symmetry operations: (1) x - 1,
y, z; (2) x þ 1, y, z; (3) x, y þ 1, z; (4) x - 1, y þ 1, z. b In A. c In degrees.
˚
And, therefore, the crystal packing is almost identical to that
described for 1a (see Figures S5-S6, Supporting Information)
and needs no further comments.
A crystal suitable for single crystal X-ray diffraction studies
could also be obtained from isopropanol. In this case, no
solvent molecules are involved in the crystal structure of 1c,
probably due to the larger size of the isopropanol molecule,
that can no longer be accommodated in the cavity between
two molecules of 1 (Figure S7, Supporting Information). The
peptide conformation is different from that in 1a and 1b. The
torsion angle ψ of the D-Imz ring is 41.5ꢀ now in comparison
to -163.2ꢀ/-150.8ꢀ and -165.5ꢀ/-151.7ꢀ for the two peptide
molecules in the unit cells of1a and 1b, respectively. This value
corresponds to that observed for the D-Oxd moiety in the

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 249
Figure 6. (a) View down the c axis showing the chains of the two different conformers running along the a axis; (b) view down the a axis showing
the H-bond interactions among chains of the two different conformers of 1a. Dark dotted lines indicate hydrogen bonds; color code: C gray
(in one conformer) and green (in the second conformer), O red, N blue.
Figure 7. (a) View down the c axis and (b) view down the a axis of the crystal packing of 1c (hydrogen atoms omitted for clarity).
Table 3. Backbone Torsion Angles in the Peptide Molecule of 1c
temperature of 100 ꢀC after the strong endothermic peak at
89 ꢀC, the polymorph of 1a-III is formed, which melts at a
temperature of 141 ꢀC giving an amorphous material
(Figure 8b, polymorph 1a-IV). The heating profile of single
crystals of 1b (Figure 8c) shows two overlapping endothermic
peaks at 95 and 109 ꢀC followed by a very weak one at 144 ꢀC.
Also in this case the structural reorganization of 1b asso-
ciated with the thermal treatment was followed by X-ray
powder diffraction (Figure 8d). The experimental X-ray
diffraction pattern of 1b-I collected at room temperature
shows all the peaks present in the diffraction pattern calcu-
lated from the single crystal structure of 1b-C. This indicates
that the material precipitating at room temperature is a pure
phase. Ata temperatureof 97ꢀC afterthe endothermicpeak at
95ꢀC, a secondpolymorph of1b-II isformed. This polymorph
melts at a temperature of 109 ꢀC giving the amorphous
material 1b-III. The material 1b TGA profile indicates a
weight loss of 5.8% in the range of temperature between 65
and 120 ꢀC (Figure S8, Supporting Information). This value is
close tothe stoichiometric content ofsolvent in1b(5.8%w/w).
Finally, single crystals of 1c (Figure 8e) show a heating
profile, in which only one strong endothermic peak at 150 ꢀC
torsion anglea
deg
C22-N3-C14-C3 (φ1)
N3-C14-C3-N2 (ψ1)
N2-C4-C6-O4 (φ2)
-108.5(1)
158.7(1)
41.5(2)
a For atom numbering, see Figure 5.
Table 4. Hydrogen Bond Parameters for 1ca
D-H
A
H
Ab
D
Ab
—D-H Acc
3 3 3
3 3 3
3 3 3
3 3 3
N3-H3N O3(1)
2.25(2)
2.14(2)
3.072(1)
2.899(2)
155(2)
148(2)
3 3 3
3 3 3
N1-H1N O5(2)
a For atom numbering, see Figure 5; symmetry operation: (1) -x,
y - 0.5, -z. (2) x þ 1, y þ 0.5, -z. b In A. c In degrees.
˚
could be associated with the loss of methanol molecules from
the crystal lattice (stoichiometric content of 3.3% w/w). These
observations suggest that the material precipitating at room
temperature is a mixture of crystals containing solvent and
solvent-free crystals. No differences in the morphology of
these two species of crystals have been observed. Above a

250 Crystal Growth & Design, Vol. 10, No. 1, 2010
Angelici et al.
Figure 8. Differential scanning calorimetry profiles of crystals of 1 precipitated from a solution of (a) methanol (1a), (c) ethanol (1b), and (e)
isopropanol (1c). (b) X-ray powder diffraction patterns of single crystals of 1a after thermal treatment up to the temperature I-IV. (d) X-ray
powder diffraction patterns of single crystals of 1b after thermal treatment up to the temperature I-III. (f) X-ray powder diffraction patterns of
single crystals of 1c after thermal treatment up to the temperature I and II. In panels b, d, and f, the calculated powder diffraction pattern from
the single crystal structure (c) is reported for comparison with I. In panel b, the asterisks in I indicate the diffraction peaks, which belong to the
desolvated form II.
is present. The material 1c is a pure phase, as shown by the
correspondence among all the peaks present in the experi-
mental X-ray powder diffraction pattern (Figure 8f, 1c-I) and
the one calculated from the single crystal structure (1c-C).
Moreover, this compound melts at 150 ꢀC, giving the amor-
phous material 1c-II. This material does not host solvent in its
crystal lattice, as confirmed by the absence of peaks in the first
derivative of the TGA profile below the decomposition
temperature (Figure S8, Supporting Information). All these
materials start to decompose at temperatures above 180 ꢀC
(see TGA profiles in Figure S8, Supporting Information).
and 1b with similar unit cell parameters. In both cases, the
peptide crystallizes in infinite chains with the peptide mono-
mers in a parallel arrangement connected by a single hydrogen
bond. The chains are in antiparallel orientation and cross-
linked by hydrogen bonds employing the heterocyclic NH
group of the D-Imz ring. Thus, a 2D network made of layers
parallel to the ab plane results producing a plywood structure.
Crystallization of the peptide from isopropanol provides also
a 2D network by hydrogen bond cross-linking. However, this
network does not involve solvent molecules and has a struc-
ture completely different from the networks crystallizing with
methanol and ethanol. The degree of order in crystal assembly
is continuously decreasing from methanol to ethanol and
isopropanol.
Conclusions
The pseudopeptide Boc-L-Phe-D-Imz-OBn 1 has been de-
signed and prepared to obtain a molecule, able to form cross-
linked hydrogen bonds. Crystallization of this compound
from methanol and ethanol provides solvate structures 1a
Acknowledgment. G.A., S.C., G.F., M.M., and C.T. are
ꢀ
grateful to Ministero dell’Universita e della Ricerca Scientifica

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 251
ꢀ
(PRIN 2006), Universita di Bologna (Funds for selected
topics), and Fondazione del Monte di Bologna e di Ravenna
for financial support. H.-J.H. thanks Deutsche Forschungs-
gemeinschaft (SFB 610, HO 2346/1-3) for funding.
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(7) Angelici, G.; Falini, G.; Hofmann, H.-J.; Huster, D.; Monari, M.;
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(9) SMART & SAINT Software Reference Manuals, version 5.051
(Windows NT Version); Bruker Analytical X-ray Instruments Inc.:
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Supporting Information Available: Crystallographic data (cif
files) for 1a, 1b and 1c. Supplemental crystallographic Figures of
1a, 1b and 1c. This material is available free of charge via the
Internet at http://pubs.acs.org.
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