Synthesis and characterisation of helical β-peptide architectures that contain (S)-β3-HDOPA(crown ether) derivatives

Article abstract of DOI:10.1002/chem.200701360

A new set of β-amino acids that carry various crown ether receptors on their side chains of the general formula (S)-β³-HDOPA(crown ether) (HDOPA: homo-3,4-dihydroxyphenylalanine; (crown ether): [15]crown-5 ([15-C-5]), [18]crown-6 ([18-C-6]), [21]crown-7 ([21-C-7]), 1,2-Benzo-[24]crown- 8 ([Benzo-24-C-8]) and (R)-Binol-[20]crown-6 ([(R)-Binol-20-C-6])) was prepared. Peptides that are based on these new crowned β-amino acids combined with (1S,2S)-ACHC (2-aminocyclohexanecarboxylic acid), which is known to be a potent 3₁₄-helix inducer, to the hexamer level, with two crowned residues at the i and i+3 posi_tions of the main-chain, were synthesized in solution by stepwise coupling using Boc-N^a-protection (Boc: tert-butoxycarbonyl) and the EDC/HOAt Cactivation method. Their conformational analysis was performed by using FTIR absorption, NMR and CD spectroscopy techniques. Our results are in full agreement with a 3₁₄-helix conformation.

Full text of DOI:10.1002/chem.200701360

DOI: 10.1002/chem.200701360
Synthesis and Characterisation of Helical b-Peptide Architectures that
Contain (S)-b3-HDOPA(Crown Ether) Derivatives**
Laurence Dutot,[a] Anne Gaucher,*[a] Khadidja Elkassimi,[a] Jeremy Drapeau,[a]
Michel Wakselman,[a] Jean-Paul Mazaleyrat,[a] Cristina Peggion,[b]
Fernando Formaggio,[b] and Claudio Toniolo*[b]
Abstract: A new set of b-amino acids
that carry various crown ether recep-
tors on their side chains of the general
formula (S)-b3-HDOPA(crown ether)
(HDOPA: homo-3,4-dihydroxypheny-
lalanine; (crown ether): [15]crown-5
based on these new crowned b-amino
acids combined with (1S,2S)-ACHC (2-
tions of the main-chain, were synthe-
sized in solution by stepwise coupling
using Boc-Na-protection (Boc: tert-bu-
toxycarbonyl) and the EDC/HOAt C-
activation method. Their conformation-
al analysis was performed by using
FTIR absorption, NMR and CD spec-
troscopy techniques. Our results are in
full agreement with a 314-helix confor-
mation.
aminocyclohexanecarboxylic
acid),
which is known to be a potent 314-helix
inducer, to the hexamer level, with two
crowned residues at the i and i+3 posi-
([15-C-5]),
[18]crown-6
([18-C-6]),
[21]crown-7 ([21-C-7]), 1,2-Benzo-
[24]crown-8 ([Benzo-24-C-8]) and (R)-
Keywords: amino acids · circular
dichroism
·
crown ethers
·
Binol-[20]crown-6
([(R)-Binol-20-C-
IR spectroscopy · peptides
6])) was prepared. Peptides that are
Introduction
et al. have discovered that the insertion of b-amino acids
with a constrained six-membered ring, such as trans-2-ami-
nocyclohexanecarboxylic acid (ACHC), into a peptide chain
dramatically enhances the 14-helix stability compared to b3-
amino acids.[5] Moreover, the recently reported properties of
b-peptides, such as resistance to proteolytic degradation,[6]
somastatin antagonism,[7] antimicrobial activity,[8] membrane
translocation[9] and disruption of protein–protein interac-
tions,[10] should lead to important therapeutic applications
for this class of “foldamers”.[3]
We envisioned that b-peptides could also be used for the
construction of molecular receptors and devices that are
based on peptide frameworks.[11] We were particularly im-
pressed by the pioneering work of Voyer et al.[11,12] who
studied l-DOPA a-amino acid derivatives that bear a crown
ether receptor on their side chain; these compounds could
be easily assembled in well-defined amphiphilic a-helical
nanostructures with multiple crown ethers that are aligned,
one on top of the other, on the same side of the peptide
backbone. Indeed, a 21-mer peptide was shown to act as an
artificial ion channel that was capable of facilitating the
transport of monovalent cations across bilayer membra-
nes.[12c,e,f,h,l,m] In contrast to the 21-mer, the 14-mer analogue
interacted as a powerful membrane-disrupting agent,[12i,n,o]
and the 7-mer analogue only behaved as an ion carrier.[12c]
The shorter versions of this series of compounds also selec-
b-Peptides have received considerable attention in recent
years because of their remarkable ability to fold into well-
defined and predictable secondary structures.[1,2] The most
extensively studied b-peptide secondary structure is the 14-
helix (also named 314- or 31-helix), which is defined by a 14-
À
membered ring (i) N H···O=C (i+2) hydrogen bond be-
tween backbone amide groups.[2,3] In particular, Seebach
and co-workers have shown that b3-peptides adopt the 14-
helix conformation in organic solvents,[4] whereas Gellman
[a] Dr. L. Dutot, Dr. A. Gaucher, K. Elkassimi, J. Drapeau,
Dr. M. Wakselman, Dr. J.-P. Mazaleyrat
ILV, UMR CNRS 8180, University of Versailles
78035 Versailles (France)
Fax : (+33)1-3925-4452
E-mail: anne.gaucher@chimie.uvsq.fr
[b] Dr. C. Peggion, Prof. F. Formaggio, Prof. C. Toniolo
Institute of Biomolecular Chemistry
Padova Unit, CNR, Department of Chemistry
University of Padova, via Marzolo 1
35131 Padova (Italy)
Fax : (+39)049-827-5247
E-mail: claudio.toniolo@unipd.it
[**] HDOPA: homo-3,4-dihydroxyphenylalanine.
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
tively complexed ammonium[12b] and diammonium[12g] chains
of various lengths. Moreover, depending on the peptideꢁs
secondary structure (helix or sheet), chiral recognition of
either l- or d-amino acids could be induced as a result of
the relative orientation of the crown ether receptors.[11a,c,d]
We believe that extension of these properties to crowned
14-helical b-peptides could offer the same advantages that
were found for the a-helical peptides,[12j] namely, ease of
changing the size of the crown ether ring for modulation of
the ion selectivity, ease of changing the amino acid compo-
nents and/or peptide length by chemical synthesis, post-syn-
thetic modifications and end-group engineering for tuning of
the biological activity. Furthermore, because the 14-helix
structurally differs from the a-helix in many respects, that is,
it has a slightly wider radius and shorter rise for a given
main-chain length than the a-helix, a net dipole that is op-
posite to that of the a-helix, and a 3-residue repeat with the
side chains of the i and i+3 residues that are directly aligned
atop one another along the same face of the peptide back-
bone instead of the 3.6-residue repeat for the a-helix, it is
our view that the influence of these parameters could be
worth examining in comparison with Voyerꢁs results.
report the preparation of a new set of (S)-b3-HDOPA(crown
ether) amino acid derivatives. We also describe the synthesis
of peptides that are based on these new crowned b-amino
acids combined with (1S,2S)-ACHC (which is a hydrophobic
314-helix inducer) to the hexamer level, with two crowned
residues at the i and i+3 positions of the main chain (for
alignment of the crown ether rings atop one another). Final-
ly, we discuss the results of their conformational analysis by
using FTIR absorption, NMR and CD spectroscopy.
Results and Discussion
Crown ether amino acid synthesis: The crown ether deriva-
tives of (S)-b3-HDOPA were prepared from the terminally
protected derivative Boc-(S)-b3-HDOPA-OMe (Boc, tert-bu-
toxycarbonyl) according to the procedure of Voyer and co-
workers[12j] at relatively high dilution (ca 0.1m) with cesium
carbonate (1.1 equiv) in DMF at 608C. Either pentaethylene
glycol di-p-toluenesulfonate, hexaethyleneglycol dibromi-
de,[12j] 1,2-bis(8-tosyloxy-3,6-dioxa-1-octyloxy)benzene,[15a] or
(R)-2,2’-bis(5-tosyloxy-3-oxa-1-pentyloxy)-1,1’-binaph-
thyl[15b,16] were used as the alkylating agent (1.0 equiv) to
afford 1a (39%), 1b (22%), 1c (36%) and 1d (28%), re-
spectively, in reasonable yields after chromatography on
silica gel (Scheme 1). Saponification of the ester function of
1a–d was performed in methanol by using aqueous 1m
NaOH at room temperature for 24h to give the correspond-
ing N-protected amino acids: 1’a (98%), 1’b (75%), 1’c
(94%) and 1’d (40%) (see Scheme 1 for structures of 1a–d
and 1’a–d).
Therefore, we decided to apply the Arndt–Eistert homo-
logation procedure of a-amino acids, which was previously
improved by Seebach and co-workers,[13] to the synthesis of
(S)-b3-HDOPA (HDOPA: homo-3,4-dihydroxyphenylala-
nine), from which the first crown ether derivative of a b-
amino acid, (S)-b3-HDOPA-[18]crown-6 ((S)-b3-HDOPA-
[18-C-6]) was obtained.[14] In the present paper, we wish to
Peptide synthesis: We synthesized four hexapeptides, 6a–d,
that contained these new crowned b-amino acids combined
with (1S,2S)-ACHC, in which the two crowned residues
were introduced at the i and i+3 positions of the main chain
Scheme 1. Synthesis of the crown ether derivatives 1a–d and 1’a–d from Boc-(S)-b3-HDOPA-OMe. i) Cs2CO3, DMF, 608C; ii) 1m aq. NaOH/MeOH, RT.
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C. Toniolo, A. Gaucher et al.
by a stepwise elongation strategy (Scheme 2). We also pre-
pared the two model b-hexapeptides 6e and 6 f, in which the
absence of the crown ether carriers was expected to consid-
erably simplify the NMR spectra.
All coupling steps were performed in solution by using
the EDC/HOAt (EDC: N-ethyl,N’-(3-dimethylaminopro-
pyl)carbodiimide; HOAt: 7-aza-1-hydroxy-1,2,3-benzotria-
zole) methodology.[17] To avoid epimerisation problems that
Scheme 2. Synthesis of the crowned b-hexapeptides 6a–6 f. i) TFA/CH2Cl2 1:1, 08C to RT; ii) SOCl2, MeOH, 08C to RT; iii) 1’a, b, c or d or Boc-(S)-b3-
HPhe-OH or Boc-(S)-b3-HAla-OH, EDC, HOAt, NMM, CH2Cl2/THF; iv) TFA/CH2Cl2 1:3, 08C to RT, or HCl/p-dioxane, 08C to RT; v) Boc-(1S,2S)-
ACHC-OH, EDC, HOAt, NMM, CH2Cl2/THF.
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b-Peptide Architectures with Crown Ether Derivatives
FULL PAPER
have been encountered with triethylamine, we used the
weaker base NMM (NMM: N-methyl morpholine).[4b] The
cleavage of the Boc protecting group was performed with
trifluoroacetic acid (TFA/CH2Cl2, 1:3) or HCl/p-dioxane.
The peptides were usually obtained in 48–88% yield after
purification either by column chromatography or by precipi-
tation (the latter methodology for the less-soluble com-
pounds). However, for the last coupling step, the yields of
hexapeptides 6b (15%), 6d (29%) and 6e (13%) were con-
siderably lower, possibly for solubility reasons.
FTIR absorption analysis: The FTIR absorption spectra of
the fully protected b-di- to b-hexapeptides 2a–6a, 2b–6b,
2c–6c, 2d–6d and 2e–6e were recorded at room tempera-
ture in CDCl3 by using diluted solutions to directly detect
intramolecularly hydrogen-bonded and non-bonded NH
À
Figure 3. FTIR absorption spectra in CDCl3 solution in the N H stretch-
ing region for peptides 2c–6c.
À
groups by analysis of the conformationally sensitive N H
stretching (amide A) region (Figures 1–5).
The low-frequency band at about 3300 cmÀ1 indicates the
formation of hydrogen-bonded NH groups, whereas the
high-frequency band at about 3430 cmÀ1 arises from free,
solvated NH groups.[18] We also observed an increasing
amount of hydrogen-bonded NH groups when the length of
À
Figure 4. FTIR absorption spectra in CDCl3 solution in the N H stretch-
ing region for peptides 2d–6d.
À
Figure 1. FTIR absorption spectra in CDCl3 solution in the N H stretch-
ing region for peptides 2a–6a.
À
Figure 5. FTIR absorption spectra in CDCl3 solution in the N H stretch-
ing region for peptides 2e–6e.
the peptide chain was increased. Also, the low-frequency
band of the b-hexapeptides 6a–6d is of much higher intensi-
ty than the high-frequency band. However, we noticed the
À
Figure 2. FTIR absorption spectra in CDCl3 solution in the N H stretch-
ing region for peptides 2b–6b.
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C. Toniolo, A. Gaucher et al.
presence of an additional band (shoulder) between 3325 and
3370 cmÀ1, which suggests that at least two different kinds of
hydrogen-bonded NH groups are present. Table 1 summariz-
À
es the FTIR absorption data in the N H stretching region
that was observed for the hexapeptides of each series.
À
Table 1. FTIR absorption data in the N H stretching region for hexa-
peptides 6a–6e in CDCl3 solution.
b-Hexapeptides
Non-hydrogen-
Shoulders
[cmÀ1
Hydrogen-
À
À
bonded N H
]
bonded N H
[cmÀ1
]
[cmÀ1
]
6a [18-C-6]
6b [21-C-7]
6c [Benzo-24-C-8]
6d [(R)-Binol-20-C-6]
6e b3-HPhe
3427.2
3428.7
3429.9
3426.6
3429.3
3360.3
3359.7
3326.8
3350.8
3324.9
3295.8
3296.6
3296.4
3292.7
3296.4
In conclusion, the results of our FTIR absorption analysis
clearly indicate the presence of two different classes of
(non-bonded and hydrogen-bonded) NH groups for the
highest oligomers. This information is consistent with well-
organised b-peptide secondary structures.
Circular dichroism analysis: The peptides 2a–6a, 2b–6b,
2c–6c and 2 f–6 f were also studied by circular dichroism
(CD), which is an extensively used technique for the 3D-
structural investigation of peptides in solution.[4b,18d,19] As ex-
pected, we were not able to get any information on peptides
2d–6d because their CD spectra were entirely dominated by
contributions of the binaphthyl chromophore.[20] Experi-
ments that were run at a 10À4 m concentration in 2,2,2-tri-
fluoroethanol (TFE), which is a well-known solvent for sta-
bilising peptide helical structures[21] gave satisfying results
(Figure 6). Table 2 summarizes the characteristic parameters
of the positive (l1) and negative (l2) Cotton effects that
were observed for the longest members of each series.
While no significant CD ellipticities were observed for the
b-di- and b-tripeptides, a negative band near 215 nm and a
positive band at about 192 nm were clearly seen in the spec-
tra of the b-penta- and b-hexapeptides. These spectra show
a typical pattern that is assigned to the (M)-314 helix struc-
ture.[2] It should also be pointed out that the high intensity
for the l2 value of the b-hexapeptide 6a (À112.5) is proba-
bly related to a particularly good stability of the 314-helix.
1H NMR spectroscopic analysis: NMR spectroscopy is one
of the most widely used techniques for determination of the
solution structure of b-peptides.[1a,22] The convention that is
used throughout this study is the accepted labelling of CaH
and CbH for b-amino acids relative to standard a-amino
acids, in which Ca bears the carbonyl group and Cb bears the
nitrogen atom.[1a,2] The [(R)-Binol-20-C-6] series (2d–6d)
1
could not be studied by H NMR because of an overlapping
of NH protons and aromatic CH protons from the binaphth-
1
yl nucleus. In contrast, the one-dimensional H spectra of b-
Figure 6. CD spectra for peptides 2a–6a (A), 2b–6b (B), 2c–6c (C) and
2 f–6 f (D) in TFE solution (peptide conc. 0.1 mm).
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b-Peptide Architectures with Crown Ether Derivatives
FULL PAPER
Table 2. l1/l2 (nm) Values for the b-penta- and hexapeptides in TFE so-
lution (the intensity ”10À3 of l2 is given in parentheses for each com-
pound).
b-Pentapeptides 5
b-Hexapeptides 6
A
191.3/216.3 (À41.6)
191.3/218.2 (À55.0)
201.7/218.5 (À54.3)
192.8/216.4( À44.7)
189.2/214.6 (À112.5)
195.3/215.7 (À68.2)
194.6/216.4 (À60.4)
189.3/213.3 (À73.4)
ACHTREUNG
[Benzo-24-C-8] series (c)
(S)-b3-HAla series (f)
hexapeptides 6a–c and 6e in [D6]DMSO solution revealed
well-defined signals for the backbone, which suggests high-
populations of a single well-organised conformation for each
oligomer. At room temperature, the NH protons of each
residue exhibit coupling constants JACHTRE(UNG NH,Hb) in the range of
7 to 10 Hz (Table 3). Those high values correspond to an an-
À
À
tiperiplanar arrangement of the N H(i) and Cb Hb(i) bonds
for each residue, which are typically encountered with a 14-
helix conformation.[1a]
Table 3. Coupling constant J(NH(i)/Hb(i)) values [Hz] for the b-hexapepti-
des 6a–c, and e in [D6]DMSO solution.
Residue i
1
2
3
4
5
6
6a
6b
6c
6e
9.1
9.2
–
9.0
9.0
9.2
8.47.8
7.1
7.3
8.7
7.7
7.7
10.48.7
9.6
8.7
8.6
8.7
9.6
7.9
8.1
10.2
6.6
Because an NMR spectroscopic characterisation was only
possible in [D6]DMSO, as a result of the poor solubility of
these compounds, we were not able to perform
a
[D6]DMSO titration in CDCl3 solution to evaluate the pres-
ence of intramolecular hydrogen bonds.[23] Alternatively, the
extent of the intramolecular hydrogen bonding in the b-hex-
apeptides 6a–c and 6e was evaluated by temperature-de-
pendent 1H NMR spectroscopic measurements,[24] over a
range of 30 K (Figure 7). The graphs reveal a generally
higher variation of chemical shifts for the NH5 and NH6 pro-
tons than for the NH1–4 protons. Some of the NH1 and/or
NH2 chemical shifts could not be assigned because of their
juxtaposition with aromatic CH signals. The temperature co-
efficients (DdDTÀ1) (Table 4) range from À3.7 to
À4.6 ppbKÀ1 for NH1–4. Such relatively low values indicate
poor solvent accessible and intramolecular hydrogen-
bonded amide protons. However, the DdDTÀ1 values fall be-
tween À5.3 to À6.7 ppbKÀ1 for NH5 and NH6, which sug-
Figure 7. Plots and histograms showing the variations of the NH proton
chemical shifts in the 1H NMR spectra ([D6]DMSO) of the b-hexapepti-
des 6a–c and 6e as a function of increasing temperature; A) b-hexapep-
tide 6a; B) b-hexapeptide 6b; C) b-hexapeptide 6c; D) b-hexapeptide
6e. The NH2 proton of 6e is overlapped by the aromatic CH protons.
Table 4. Temperature coefficients (DdDTÀ1
) ] determined in
[ppbKÀ1
[D6]DMSO solution between 298 and 328 K for the b-hexapeptides 6a–c
and e.
À
gests the onset of (DMSO) S=O···H N (amide) hydrogen
bonding interactions.[25] Therefore, these results provide evi-
dence that the NH5 and NH6 protons of b-hexapeptides 6a–
c and 6e are more solvent exposed than their NH1–4 proton
counterparts, as expected for 314-helical structures.
Residue-specific assignments of the b-hexapeptides 6a–c
and 6e were based on a combination of COSY, DQFCOSY,
TOCSY and ROESY experiments. We performed a “NOE
walk” along the backbone, and assigned the NH, Hb and Ha
for each residue. Intraresidue assignments were confirmed
NH1
NH2
NH3
NH4
NH5
NH6
6a
6b
6c
6e
–
–
–
À4.6
À4.0
À4.3
–
À4.3
À4.0
À4.0
À3.7
À4.3
À4.0
À3.7
À3.7
À6.7
À6.0
À5.7
À6.0
À6.0
À5.3
À6.0
À6.0
À4.6
from 2D COSY and TOCSY data. The crown ether free
hexapeptide 6e was chosen as a model system because no
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C. Toniolo, A. Gaucher et al.
1
signal overlap was observed in the Hb region of its H NMR
spectrum. A list of chemical shifts for 6e is given in Table 5.
Table 5. 1H NMR chemical shifts (d/ppm) of the b-hexapeptide 6e in
[D6]DMSO solution at room temperature.
Residues
Ha
Hb
NH
ACHC1
b3-HPhe2
ACHC3
ACHC4
b3-HPhe5
ACHC6
2.36
2.32
2.35
3.42
4.18
3.77
3.87
4.61
3.96
6.91
7.64
7.42
7.08
7.88
8.07
2.12
2.17 and 2.20
2.44
The NH/Ha, NH/Hb and Ha/Hb regions of the ROESY
spectra for 6e are shown in Figure 8. In the NH/Ha region,
correlations NH(i)–Ha(i) and NH(i)–Ha(iÀ1) were observed be-
tween all possible pairs, such as ACHC1NH–ACHC1Ha,
ACHC1Ha–Phe2NH, Phe2NH–Phe2Ha, Phe2Ha–ACHC3NH,
ACHC3NH–ACHC3Ha,
ACHC4NH–ACHC4Ha,
ACHC3Ha–ACHC4NH,
Phe5NH–
ACHC4Ha–Phe5NH,
Phe5Ha, Phe5Ha–ACHC6NH and ACHC6NH–ACHC6Ha
(Figure 8A).
In the NH/Hb region, we noted correlations NH(i)–Hb(i+2)
between
the
residue
pairs
ACHC1NH–ACHC3Hb,
ACHC3NH–Phe5Hb and ACHC4NH–ACHC6Hb. Correla-
tions NH(i)–Hb(i+3) were also obtained between the residue
pairs ACHC3NH–ACHC6Hb (Figure 8B). Finally, in the Ha/
Hb region, Ha(i)–Hb(i+3) and Ha(i)–Hb(iÀ1) cross-peaks, respec-
tively, are present between the residue pairs ACHC1Ha–
ACHC4Hb, Phe2Ha–Phe5Hb, ACHC3Ha–ACHC6Hb and
ACHC6Ha–ACHC5Hb (Figure 8C).
Integration of the signals in the ROESY spectrum al-
lowed us to determine intramolecular distances. Calibration
of the spectrum was elaborated from a reference length that
was calculated with the WebLab Viewer Pro 3.7 program.
For this study, this length was 3.09 Š, which corresponds to
the distance between the Ha and Hb protons of the ACHC4
residue. From that calibration, distance (d) values were ob-
tained. These data are directly correlated with the observed
NOE strengths and classified into four categories, d<2.5 Š
(strong interactions), 2.5<d<3.0 Š (medium to strong in-
teractions), 3.0<d<3.5 Š (weak to medium interactions)
and d>3.5 Š (weak interactions) (Figure 9).
2D COSY and TOCSY experiments were also performed
on the crown ether b-hexapeptides 6a–c to assign the chemi-
cal shifts for the Ha, Hb and NH protons of all residues of
these hexapeptides (Table 6).
Then, each region of the ROESY spectra of 6a–c was an-
alysed to evaluate the observed NOEs for the backbone
atoms of these b-hexapeptides. In the NH/Ha region, corre-
lations NH(i)–Ha(i) and NH(i)–Ha(iÀ1) were seen between all
possible pairs for 6a–c. As for the NH/Hb region, we ob-
served correlations NH(i)–Hb(i+2) between the residue pairs
ACHC1NH–ACHC3Hb for 6a and 6b and between
ACHC4NH–ACHC6Hb for 6c. Correlations NH(i)–Hb(i+3)
were obtained between the residue pairs ACHC3NH–
Figure 8. ROESY spectrum of the b-hexapeptide 6e in the A) NH/Ha
region, B) NH/Hb region and C) Ha/Hb region ([D6]DMSO solution).
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b-Peptide Architectures with Crown Ether Derivatives
FULL PAPER
ACHC3Ha–ACHC6Hb for 6b,
and ACHC1Ha–ACHC4Hb and
ACHC3Ha–ACHC6Hb for 6a.
Integration of the signals in
the ROESY spectra allowed us
to determine intramolecular
distances between protons that
are bound to the backbone
carbon atoms and evaluate the
strength of their observed
NOEs. Calibration of the spec-
tra was based on the well-
known interaction between the
two geminal protons Ha of b-
DOPA2 for 6a and 6b, and be-
tween the Ha–Hb protons of
ACHC4 for 6c. The resulting
NOE cross-peaks are presented
in Figure 9. In conclusion, the
overall patterns of NOEs ob-
served with 6a–c and 6e are in
full agreement with a 314-helix
conformation.
Conclusion
We have prepared a new series
of b-amino acids 1a–d that
carry various crown ether hosts
on their side chains. Subse-
quently, four b-hexapeptides
6a–d that contain two crowned
b-amino residues at the i and
i+3 positions of the main
Figure 9. Weak, medium and strong NOEs observed in the ROESY spectra for 6e (A), 6a (B), 6b (C) and
6c (D) in [D6]DMSO solution.
ACHC6Hb for 6b and b-DOPA2NH–b-DOPA5Hb and
ACHC3NH–ACHC6Hb for 6a. The correlation NH(i)–Hb(iÀ1)
chain, combined with (1S,2S)-ACHC, and two b-hexapepti-
des 6e and 6 f, which were used as models, were synthesised
by a stepwise elongation strategy. FTIR absorption spectra
was
observed
between
ACHC3NH–b-DOPA2Hb,
ACHC4NH–b-DOPA3Hb and b-DOPA5NH–ACHC4Hb for
6b and b-DOPA2NH–ACHC1Hb, ACHC3NH–b-DOPA2Hb,
ACHC4NH–b-DOPA3Hb and ACHC6NH–DOPA5Hb for 6a.
Finally, in the Ha/Hb region, Ha(i)–Hb(i+3) and Ha(i)–Hb(iÀ1)
cross-peaks appeared between the residue pairs ACHC1Ha–
ACHC4Hb, b-DOPA2Ha–b-DOPA5Hb and ACHC3Ha–
in CDCl3 solution in the N H stretching (amide A) region
for the series of b-di- to b-hexapeptides show a low-frequen-
À
cy band at about 3300 cmÀ1 which indicates the occurrence
of hydrogen-bonded N H groups. The CD spectra of the
highest oligomers of all the series of peptides are character-
ized by an intense negative Cotton effect that is centred
near 215 nm, followed by a positive Cotton effect at about
À
ACHC6Hb
for
6c,
b-DOPA2Ha–b-DOPA5Hb
and
Table 6. 1H NMR chemical shifts (d/ppm) of the Ha, Hb and NH protons for the b-hexapeptides 6a–c in [D6]DMSO solution at room temperature.
d [ppm]
6a
6b
6c
Ha
Hb
NH
Ha
Hb
NH
Ha
Hb
NH
ACHC1
DOPA2
ACHC3
ACHC4
DOPA5
ACHC6
2.38
1.97, 2.28
2.35
2.12
2.13, 2.40
2.41
3.40
4.18
3.76
3.88
4.53
3.95
6.89
7.56
7.39
7.08
7.81
8.01
2.37
1.97, 2.28
2.33
2.13
2.12, 2.39
2.40
3.41
4.19
3.75
3.88
4.54
3.94
6.89
7.55
7.39
7.08
7.81
8.01
2.38
2.31
2.35
2.12
2.17, 2.42
2.41
3.42
4.19
3.77
3.88
4.54
3.95
6.89
7.56
7.39
7.09
7.84
8.05
Chem. Eur. J. 2008, 14, 3154– 3163
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3161

C. Toniolo, A. Gaucher et al.
1
The synthesis and characterisation of all compounds (1a–6a, 1b–6b, 1c–
6c, 1d–6d, 1e–6e and 1 f–6 f) are reported in the Supporting Informa-
tion.
200 nm. H NMR spectroscopy results from temperature-de-
pendent DMSO titrations provide evidence that the NH5
and NH6 protons of b-hexapeptides are more solvent-ex-
posed than their NH1–4 proton counterparts. At room tem-
perature, the typically recorded high J(NH,Hb) values are in
favour of an antiperiplanar arrangement for the NH and
Hb(i) protons of each residue. Intraresidue assignments were
confirmed by 2D COSY and TOCSY experiments. A “NOE
walk” along the backbone allowed us to assign the NH, Ha
and Hb(i) protons for each residue of the b-hexapeptides. Al-
together, our data sets are in full agreement with a 14-helix
structure for all of the crowned b-hexapeptides; the posi-
tioning of the (S)-b3-HDOPA(crown ether) residues at the i
and i+3 positions of the main-chain should allow for a par-
allel orientation of their side-chain receptors with the oppor-
tunity for a cooperative cation binding. This structural study
represents one of the initial steps towards functional uses of
this new class of amphiphilic, either 14-helical or 310-heli-
cal,[26] crowned peptides as potential ion channels and/or cy-
tolytic agents, as well as potential peptide-based catalysts[27]
for enantioselectively catalysed reactions,[28] which will now
be explored by our groups.
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Experimental Section
General methods: Melting points were determined by using either a Met-
tler FP61 apparatus with a temperature rise of 38CminÀ1 or a Büchi melt-
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Published online: February 12, 2008
Chem. Eur. J. 2008, 14, 3154– 3163
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3163