(S)-ABOC: A rigid bicyclic β-amino acid as turn inducer

Article abstract of DOI:10.1021/ol203406v

In order to investigate the ability of the (S)-aminobicyclo[2.2.2]octane-2- carboxylic acid 1 (H-(S)-ABOC-OH) to induce reverse turns into peptides, two model tripeptides, in which this bicyclic unit was incorporated into the second position, were synthes

Full text of DOI:10.1021/ol203406v

ORGANIC
LETTERS
2012
Vol. 14, No. 4
960–963
(S)-ABOC: A Rigid Bicyclic β-Amino Acid
as Turn Inducer
ꢀ ^,†
Christophe Andre, Baptiste Legrand,^,‡ Cheng Deng,‡ Claude Didierjean,§
Guillaume Pickaert,‡ Jean Martinez,† Marie Christine Averlant-Petit,‡
Muriel Amblard,*,† and Monique Calmes†
ꢀ
Institut des Biomolecules Max Mousseron (IBMM) UMR 5247 CNRS-UM1-UM2,
Faculte de pharmacie, 15 Avenue Charles Flahault, 34093 Montpellier, France,
ꢀ
ꢀ
Laboratoire de Chimie-Physique Macromoleculaire, UMR 7568 CNRS-Universite de
Lorraine, 1 rue Grandville, 54001 Nancy Cedex 1, France, and Laboratoire de
ꢀ
ꢀ
Cristallographie, Resonance Magnetique et Modelisation, UMR 7036 CNRS-Universite
de Lorraine, Boulevard des Aiguillettes, 54506 Vandoeuvre-Les-Nancy Cedex, France
ꢀ
ꢀ
ꢀ
ꢁ
muriel.amblard@univ-monpt1.fr
Received October 28, 2011
ABSTRACT
In order to investigate the ability of the (S)-aminobicyclo[2.2.2]octane-2-carboxylic acid 1 (H-(S)-ABOC-OH) to induce reverse turns into peptides, two
model tripeptides, in which this bicyclic unit was incorporated into the second position, were synthesized and analyzed by FT-IR, CD, NMR, and
X-ray studies.
The reverse-turn has long been recognized as a signifi-
cant secondary structure element in globular protein
architecture.1 It is involved in protein folding2 and con-
stitutes an important site of molecular recognition in many
biologically active peptides and proteins due to its frequent
localization in the exposed surface.3 Therefore, for more
than 30 years, this small secondary structural element has
been an attractive target to be mimicked by conformation-
ally constrained motifs that are able to induce secondary
structures found in peptides, proteins, or structural mimics
thereof.4 Most of these motifs are based on dipeptide
mimetics.4 However, β-amino acids that are mono- or di
substituted on their 2- or 3-position have also been shown
to induce turns in peptides. They were particularly applied
to an important field of research that consists in the
construction of small synthetic oligomers, called folda-
mers. Depending on the β-amino acid unit, various well-
defined folding secondary structures, including a 14-, 12-,
10- and 8-helix, or even sheets were obtained.5 With the
^ The authors contributed equally to this work.
†
ꢀ
Institut des Biomolecules Max Mousseron.
‡
§
ꢀ
ꢀ
Laboratoire de Chimie-Physique Macromoleculaire, Nancy Universite.
Laboratoire de Cristallographie, Resonance Magnetique et Modelisation,
ꢀ
ꢀ
ꢀ
ꢀ
Nancy Universite.
(1) Richardson, J. S. Adv. Protein Chem. 1981, 34, 167.
(2) Marcelino, A. M.; Gierasch, L. M. Biopolymers 2008, 89, 380.
(3) (a) Kessler, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 512. (b)
Rose, G. D.; Gierasch, L. M.; Smith, J. A. Adv. Protein Chem. 1985, 37,
1. (c) Chou, K. C. Anal. Biochem. 2000, 286, 1. (d) Yin, H.; Hamilton,
A. D. Angew. Chem., Int. Ed. 2005, 44, 4130.
(4) (a) Gillespie, P.; Cicariello, J.; Olson, G. L. Biopolymers 1997, 43,
191. (b) Hanessian, S.; McNaughton-Smith, G.; Lombart, H. G.; Lubell,
W. D. Tetrahedron 1997, 53, 12789. (c) Amblard, M.; Daffix, I.; Bedos,
P.; Berge, G.; Pruneau, D.; Paquet, J. L.; Luccarini, J. M.; Belichard, P.;
Dodey, P.; Martinez, J. J. Med. Chem. 1999, 42, 4185. (d) North, M.
J. Pept. Sci. 2000, 6, 301. (e) Cluzeau, J.; Lubell, W. D. Biopolymers 2005,
80, 98. (f) Robinson, J. A. Acc. Chem. Res. 2008, 41, 1278.
(5) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (b) Seebach, D.;
Abele, S.; Sifferlen, T.; Hanggi, M.; Gruner, S.; Seiler, P. Helv. Chim.
Acta 1998, 81, 2218. (c) Yang, D.; Qu, J.; Li, B.; Ng, F. F.; Wang, X. C.;
Cheung, K. K.; Wang, D. P.; Wu, Y. D. J. Am. Chem. Soc. 1999, 121,
589. (d) Srinivas, D.; Gonnade, R.; Ravindranathan, S.; Sanjayan, G. J.
J. Org. Chem. 2007, 72, 7022. (e) Wu, Y. D.; Han, W.; Wang, D. P.; Gao,
Y.; Zhao, Y. L. Acc. Chem. Res. 2008, 41, 1418.
r
10.1021/ol203406v
Published on Web 01/31/2012
2012 American Chemical Society

folding pattern being controlled by the monomeric motif
forming the β-peptides, the search for new scaffolds able to
modulate secondary structures appears to be of great
potential use.6 In this area, we have developed a research
program for the selection of conformationally constrained
motifs including β-turn mimics able to display well-defined
secondary structures by oligomerization.7 Among the
different motifs that have been selected, we have focused
on the (S)-aminobicyclo[2.2.2]octane-2-carboxylic acid 1
(H-(S)-ABOC-OH), a β2,3,3-trisubstituted cyclic β-amino
acid, as a potential turn inducer in peptides and a favorable
scaffold to induce structured oligomers.
significant differences were observed in NMR spectra in
CDCl3 or in CD3CN (Figure S3AÀL and Tables S4 and
S5). In addition, FT-IR and NMR data of 3 and 4 were
compared to those obtained for a control tripeptide Z-Ala-
β-Ala-Phe-NHiPr 10 containing an unconstrained β-ami-
no acid replacing the rigid ABOC residue.
Scheme 1. Synthesis of Tripeptides 3 and 4
We have recently reported the asymmetric synthesis of
this original rigid bicyclic β-amino acid 1 bearing an amino
group at the bridgehead (Figure 1).8 This compound is
particularly attractive because it combines both the struc-
tural properties of constrained cyclic compounds and
those of β-amino acids, which have been intensively used
as foldamer precursors.5a
The high level of constraint of the (S)-ABOC motif, with
all adjacent methylene protons eclipsed and its carboxylic
acid and amino groups in a synclinal conformation, pro-
vides a template with drastically reduced conformational
freedom. This feature should allow a high degree of con-
servation of ABOC angles when incorporated into peptide/
protein sequences or foldamer compounds. The crystal
structure of Boc derivative 2 is represented in Figure 1.
The extent of hydrogen bonding in 3 and 4 in CDCl3
was evaluated through FT-IR spectroscopy experiments
(Figure S1 AÀH), DMSO-titration NMR experiments,
and temperature coefficients of amide protons in the 283 to
313 K range (Figure S5).
First, the solvent accessibility of the NH proton resonances
was measured by a DMSO-titration experiment with gradual
addition of DMSO-d6 to the CDCl3 solution (Figure S4). It
can be assumed that low temperature coefficients in DMSO-
d6 (<5 ppb/K) are related to inaccessible protons to the
solvent and lower values in CDCl3 e 2.4 ppb/K are related
not only to shielded protons (which remained shielded over
the temperature range) but also to accessible ones. Accord-
ingly only temperature coefficients significantly larger than
2.4 ppb/K in CDCl3 can be unambiguously assigned to NH
protons initially shielded, which become exposed to the
solvent in the course of the temperature range.9
A similar behavior was observed for the tripeptides
3 and 4. First, the large NH(Ala) temperature coefficient
values for 3 and 4 (4.4 and 5.2 ppb/K) in CDCl3 associated
to weak chemical shift variations with the addition of
DMSO-d6 (0.87 and 0.45 ppm) suggested solvent accessi-
bility. In contrast, the NH(ABOC) and NH(Phe) exhibited
low temperature coefficients in CDCl3 (1.4 and 3.0 ppb/K
for 3 and 0.9 and 1.0 ppb/K for 4). The conformational
stability imposed by the ABOC motif should influence
these temperature coefficients.
Figure 1. Crystal structure of the Boc-(S)-ABOC-OH 2.
To further explore its properties, we decided to synthe-
size and study the conformational behavior of two model
tripeptides 3 and 4 incorporating the constrained ABOC in
the central position, by means of CD, FT-IR, NMR
spectroscopies and XR diffraction. Their syntheses were
performed following the standard Boc protecting group
synthesis of peptides strategy in solution (Scheme 1).
We then investigated the structural features of 3 and 4 in
solution using several spectral analyses. Experiments were
carried out in CDCl3 or in CD3CN in accordance with the
peptide solubility and solvent transparency, i.e. CDCl3 for
infrared spectroscopy, CD3CN to record circular dichro-
ism spectra, and both solvents for NMR studies. No
(6) Rathore, N.; Gellman, S. H.; de Pablo, J. J. Biophys. J. 2006, 91,
3425.
(7) Raynal, N.; Averlant-Petit, M. C.; Berge, G.; Didierjean, C.;
Marraud, M.; Duru, C.; Martinez, J.; Amblard, M. Tetrahedron Lett.
2007, 48, 1787.
(8) Songis, O.; Didierjean, C.; Laurent, C.; Martinez, J.; Calmes, M.
Eur. J. Org. Chem. 2007, 3166.
(9) (a) Llinas, M.; Klein, M. P. J. Am. Chem. Soc. 1975, 97, 4731. (b)
Stevens, E. S.; Sugawara, N.; Bonora, G. M.; Toniolo, C. J. Am. Chem.
Soc. 1980, 102, 7048. (c) Medda, A. K.; Park, C. M.; Jeon, A.; Kim, H.;
Sohn, J. H.; Lee, H. S. Org. Lett. 2011, 13, 3486.
Org. Lett., Vol. 14, No. 4, 2012
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Despite border temperature coefficients in DMSO-d6,
small chemical shift variations (around 1 ppm) along the
DMSO-titration experiments indicated the participation
of these amide protons in an intramolecular hydrogen
bond. Finally, all measured parameters for the additional
NH(iPr) in 4, i.e. small temperature coefficient in CDCl3,
large value in DMSO-d6, and large chemical shift differ-
ence between CDCl3 and DMSO-d6 (ΔNH = 2.16 ppm),
indicated a solvent-accessible NH proton.
To gain further information on the hydrogen bond
network, FT-IR spectroscopy studies were undertaken.
As previously mentioned, frequencies of amide A and
amide I bands corresponding to NH and CO stretching
vibrations, respectively, depend on the involvement of
vibrators in hydrogen bonds.10 Typically, the frequencies
of stretching vibrations decrease with hydrogen bonding
formation. Those of the hydrogen bonded NH and CO
amide rise below 3400 and 1680 cm,À1 respectively, and
overlaps between “bound” and “free” bands are common
in the latter case. Moreover, with the time scale of infrared
spectroscopy being faster than that of NMR, an infrared
spectrum can contain the spectral contributions of all
components of a pool of conformers.11 Thus, for the same
CO or NH function, it should be possible to see both the
bonded and nonbonded forms.
band at a lower frequency (3295 cmÀ1) than 10, which
showed a major band at 3343 cmÀ1 and a shoulder at
3300 cmÀ1. Moreover, the ratio of the integrated intensity
of the hydrogen-bonded NH bands to free NH bands
(AH/AF)13 was much higher in the case of the ABOC
containing tripeptide (AH/AF = 1.94) than in the case of
the β-Ala analog (AH/AF = 0.89). This suggested a higher
involvement of NH groups in intramolecular interactions
in 4 compared to 10. Finally, in the amide I region (Figure
S1H), a free CO band at 1675 cmÀ1 was only observed for
10, which confirmed the previous observations of a higher
structuration level for 4.
In order to assign the positions of free NH and CO
vibrators of tripeptides 3 and 4, we first studied precursors
6, 7, 8, and 9 (Tables S1 and S2 and Figure S1AÀD). For
these precursors, we found no evidence of strong bonded
NH absorption bands as could be observed in the intra-
molecular reverse turn or in intermolecular interactions.12
However, the (S)-ABOC motif led to a new band at 3398
cmÀ1 in precursor 8, suggesting a certain contribution of
H-bonded species (Figure S1A). In the case of 4 a broad
band at 3295 cmÀ1 suggested the existence of a stronger
hydrogen-bonding interaction and therefore a higher level
of structuration (Figure S1A). No distinguishable band
around 3300 cmÀ1 was observed for 3, but an enlargement
ofthe signalsuggested anemergenthydrogen bond (Figure
S1B). Nevertheless for this tripeptide, a shoulder around
1655 cmÀ1 corresponding to a hydrogen bonded CO
stretch was present (Figure S1D). The study of CO stretch-
ing vibrations of all compounds showed either a global
shift of vibrators to lower frequencies or shoulders, both
related to bonded carbonyls. Both FT-IR amide A and
amide I band analyses of 3 and 4 confirmed the involve-
ment of NH and CO groups in hydrogen-bonding inter-
actions. In addition, comparison of 10 with 4 showed a
higher level of structuration for the latter. Indeed, in the
amide A region (Figure S1G), 4 revealed a NH bonded
Figure 2. ROE long-range distances (red arrows) in CDCl3 at
298 K for 3 and 4.
To gain more insight into the conformation of com-
pounds 3 and 4 in solution, we recorded CD spectra in
CD3CN and performed NMR studies based on ROESY
spectra (10 mM, 298 K) in CDCl3 and CD3CN. As no
signal could be detected for 2, the similar CD spectra of
3 and 4 with a maximum around 220 nm could indicate the
presence of comparable structuration (Figure S2). For
both compounds, ROE derived distances as upper bound
distances were used for structural calculations (Table S7).
The ROE long-range distances in CDCl3 are reported in
Figure 2. For the structure calculations using AMBER10,
21 and 19 distance restraints for 3 and 22 and 20 distance
restraints for 4 in CDCl3 and CD3CN respectively were
introduced. As expected, the set of ROEs observed on the
ROESY spectra were very close for 3 and 4 in both
solvents. The characteristic ROE long-range distances
between protons on each side of the ABOC residue sug-
gested a reverse turn scaffold for both compounds. The 15
lowest energy structures of 3 and 4 in CDCl3 and CD3CN
are reported in Figure S6. As indicated, the root-mean-
square deviations (rmsd) fit on backbone heavy atoms
were very low in the four calculated sets of structures and
decreased significantly when the terminal flexible moieties
were omitted (rmsd around 0.2). Indeed, we noticed that
the backbone trace of Ala-(S)-ABOC-Phe was particularly
(10) (a) Byler, D. M.; Susi, H. Biopolymers 1986, 25, 469. (b)
Arrondo, J. L. R.; Muga, A.; Castresana, J.; Goni, F. M. Prog. Biophys.
Mol. Biol. 1993, 59, 23. (c) Vass, E.; Hollosi, M.; Besson, F.; Buchet, R.
Chem. Rev. 2003, 103, 1917.
(11) Vass, E.; Kurz, M.; Konat, R. K.; Hollosi, M. Spectrochim.
Acta, Part A 1998, 54, 773.
(12) (a) Chung, Y. J.; Huck, B. R.; Christianson, L. A.; Stanger,
H. E.; Krauthauser, S.; Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc.
2000, 122, 3995. (b) Allix, F.; Curcio, P.; Quoc, N. P.; Pickaert, G.;
Jamart-Gregoire, B. Langmuir 2010, 26, 16818.
(13) (a) Benedetti, E.; Bavoso, A.; Di Baso, B.; Pavone, V.; Pedone,
C.; Crisma, M.; Bonora, G. M.; Toniolo, C. J. Am. Chem. Soc. 1982, 104,
2437. (b) Bonora, G. M.; Mapelli, C.; Toniolo, C.; Wilkening, R. R.;
Stevens, E. S. Int. J. Biol. Macromol. 1984, 6, 179.
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Org. Lett., Vol. 14, No. 4, 2012

Figure 3. Overlay of 3 (magenta) and 4 (green) average NMR
solution structures in CDCl3 and representation of the (S)-
ABOC-driven reverse turn. H-bonds are shown as dashed lines.
Figure 4. Overlay of crystal structure (yellow) and average
NMR solution structure (green) in CDCl3 of 4. H-bonds are
shown as dashed lines.
well-defined in a reverse turn conformation in CDCl3 and
CD3CN. Figure 3 showed that 3 and 4 share similar
conformations retained in both solvents. For compound
10, 21 distance restraints were used for structure calculation
but only two long-range NOEs could be detected in the
ROESY spectrum in CDCl3. Indeed while, for 4, typical
NOEs involved the NH(Ala) and the NH, CH, or CH3 of the
C-terminal NHiPr group, none of them could be detected for
10. As a result, the rmsd fit on backbone heavy atoms for this
compound was higher than that of 3 and 4 (0.429 for 10 vs
0.160 and 0.131 for 3 and 4 respectively) indicating signifi-
cant structural discrepancies among the NMR lowest energy
structures despite a global turn shape (Figure S6E). The
significant standard deviation on the dihedral angle values
also underlined the higher conformational freedom of the
tripeptide 10 in comparison to 3 and 4 (Table S8).
For compounds 3 and 4 the turn was stabilized by two
hydrogen bonds, between the NH(Phe) and CO(Ala) and
the NH(Ala) and CO(Phe), which formed a “C8” and a
“C12” pseudocycle respectively. In addition, “C7 lateral
pseudocycles” were stabilized by hydrogen bonds between
the NH(ABOC) and CO(N-ter) for 3 and 4 and the NH-
(iPr) and CO(ABOC) for 4.
with the long-range ROEs previously described in solution
(especially between the NH(Ala) and the isopropyl group).
To further compare the (S)-ABOC turn in solution and
in the solid state, the backbone torsion angles φ, ψ, and
θ were determined (Table S8). As expected, backbone
torsion angles of compounds 3 and 4 were similar. The
characteristic θ angle value of the (S)-ABOC β-residue
was ∼50ꢀÀ60ꢀ. The variation of the benzyl urethane group
position in solution and in the solid state was associated
to rotations of the alanine φ and ψ dihedral angles (i.e.,
À88.7ꢀ vs À48.9ꢀ and 31.1ꢀ vs 142.2ꢀ in solution and in the
crystal, respectively). The preferred conformation in the
crystal was stabilized by the CO(Z)-NH(Phe) hydrogen
bond. Nevertheless, in tripeptide 4, despite discrepancies in
the hydrogen bond network, other angles exhibited rela-
tively close values in solution and in the crystal.
In this paper, we developed a novel reverse turn scaffold
based on the β-residue (S)-ABOC conformationally re-
strained motif. We described its ability to induce a highly
stable turn in solution. Thus, it constitutes a new building
block to design original larger ABOC-based oligomers
providing original secondary structures (e.g., helices).
The crystals obtained in CH2Cl2 by slow evaporation
allowed us to obtain the solid state structure of the tripeptide
4 (Figure 4). We noticed that this peptide also adopted a turn
in the crystal but with a different hydrogen bond network. In
the crystal, two hydrogen bonds were present between the
NH(ABOC) andtheCO(Phe), andtheNH(Phe) andCO(Z)
forming a “C9” and a “C11” pseudocycle. However, as
observed in Figure 4, the reverse turn global conformation
in the solid state and solution were almost superimposed.
The main difference was the position of the benzyl moiety
rotated toward the phenylalanine side chain in the solid
state (Figure S7), which was, however, incompatible
Acknowledgment. The authors thank the CNRS, MESR,
and ANR (ANR-08-BLAN-0066-01) for financial support,
the SCBIM and service commun de cristallographie of
ꢀ
Universite de Lorraine for NMR and XRD facilities, and
Mr E. Wenger for RX experiments.
Supporting Information Available. Experimental pro-
cedure, FT-IR, CD spectra, NMR structural calculations
and crystal data. This material is available free of charge
via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 4, 2012
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