Conformational studies of phe-rich foldamers by VCD spectroscopy and ab initio calculations

Article abstract of DOI:10.1021/jo300688n

Employing VCD spectroscopy, we demonstrate that the structural behavior of the oligomers Boc-(l-Phe-l-Oxd)_n-OBn is similar from n = 2 to n = 6; ab initio calculations for the n = 1 case provide physical insight into the conformational properties. Further information is gained by IR, ¹H NMR, and ECD spectroscopies. ECD spectra suggest the presence of different conformations between n = 1 on one side and longer chain foldamers on the other side. VCD and absorption IR spectra in methanol solutions can be interpreted as indicative of a PPII structure. In the case of Boc-l-Phe-l-Oxd-OBn, VCD spectra in CCl₄ and detailed DFT computational analysis allow one to demonstrate that the most populated conformers exhibit backbone dihedral angles similar to those of a PPII geometry. This is a remarkable outcome, as we had previously demonstrated that the Boc-(l-Ala-d-Oxd)_n-OBn series folds in a β-band ribbon spiral that is a subtype of the 3₁₀ helix.

Full text of DOI:10.1021/jo300688n

Article
pubs.acs.org/joc
Conformational Studies of Phe-Rich Foldamers by VCD Spectroscopy
and ab Initio Calculations
Giovanna Longhi,*,† Sergio Abbate,† France Lebon,† Nicola Castellucci,‡ Piera Sabatino,*,‡
and Claudia Tomasini*,‡
†Dipartimento di Scienze Biomediche e Biotecnologie, Universita
̀
degli Studi di Brescia, Viale Europa 11, 25123 Brescia, Italy, and
CNISM, Consorzio Interuniversitario Scienze Fisiche della Materia, Via della Vasca Navale 84, 00146 Roma, Italy
‡Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum Universita
̀
di Bologna, Via Selmi 2, I-40126 Bologna, Italy
S
* Supporting Information
ABSTRACT: Employing VCD spectroscopy, we demonstrate
that the structural behavior of the oligomers Boc-(L-Phe-L-
Oxd)n-OBn is similar from n = 2 to n = 6; ab initio calculations
for the n = 1 case provide physical insight into the
conformational properties. Further information is gained by
1
IR, H NMR, and ECD spectroscopies. ECD spectra suggest
the presence of different conformations between n = 1 on one
side and longer chain foldamers on the other side. VCD and
absorption IR spectra in methanol solutions can be interpreted
as indicative of a PPII structure. In the case of Boc-L-Phe-L-Oxd-OBn, VCD spectra in CCl4 and detailed DFT computational
analysis allow one to demonstrate that the most populated conformers exhibit backbone dihedral angles similar to those of a PPII
geometry. This is a remarkable outcome, as we had previously demonstrated that the Boc-(L-Ala-D-Oxd)n-OBn series folds in a β-
band ribbon spiral that is a subtype of the 310 helix.
We have recently reported hybrid foldamers, where the Oxd
moiety is alternated with an α- or a β-amino acid.6 The relative
configuration of the Oxd and the alternated amino acid is very
important, since the L-Ala-D-Oxd series tends to form β-bend
ribbon spirals, while the L-Ala-L-Oxd series does not.
Furthermore, the insertion of the L-Phe residue alternated
with the D-Oxd moiety allows us to obtain interesting
INTRODUCTION
■
Foldamers are oligomers that adopt specific and stable
conformations similar to those typical of proteins and nucleic
acids. This neologism means “folding molecules” and refers
mainly to medium-sized molecules (about 500−5000 amu) that
fold into definite secondary structures (i.e., helices, turns, and
sheets), thus being able to mimic biomacromolecules despite
their smaller size.1 The essential requirement of a foldamer is to
possess a well-defined, repetitive secondary structure, imparted
by conformational restrictions of the monomeric unit.2 Since
1996 when the neologism was coined, this research field has
blossomed; many groups have explored oligomers with a wide
backbone variety as potential foldamers, and several reviews
have been published in this field.3
Our group has extensively studied the conformational
behavior of oxazolidin-2-one (Oxd) containing foldamers.4
On acylation of the Oxd units, imides are obtained: these
functions behave like rigid spacers so that the two carbonyls lie
apart from one another, and a trans conformation is imparted to
the adjacent peptide bond (Figure 1).5
compounds that behave as supramolecular materials.7
In this paper, we report the synthesis and the conformational
analysis in solution of a series of hybrid oligomers that have the
general formula Boc-(L-Phe-L-Oxd)n-OBn (with n = 1, 2, 3, 4,
6). We are interested in these compounds as they are possible
models for the study of amyloid fibers8 that are constituted by
peptides and are responsible for several neurodegenerative
diseases, like Alzheimer's and Parkinson's disease.9 These
peptides are often rich in Phe residues, have poor solubility, and
tend to lie in a very stable β-sheet structure.10
1
The conformational analysis has been carried out by H
NMR, IR, ECD (electronic circular dichroism), and VCD
(vibrational circular dichroism) spectroscopy, accompanied by
ab initio calculations. In particular, the latter two techniques can
be very useful in the determination of the preferred secondary
structure of this class of foldamers.
Received: April 16, 2012
Published: June 20, 2012
Figure 1. Preferred conformation of the imidic bond.
© 2012 American Chemical Society
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anhydrous trifluoracetic acid (TFA) in dichloromethane
(Scheme 1).
RESULTS AND DISCUSSION
■
1. Synthesis. Boc-L-Phe-L-Oxd-OBn 1 can be prepared in
high yield by addition of Boc-L-Phe-OH to L-Oxd-OBn in the
presence of N-[(1H-benzotriazolyl)(dimethylamino)-
methylene]-N-methylmethane-iminium hexafluorophosphate
N-oxide (HBTU) and triethylamine (TEA) in dry acetonitrile,
as previously reported (Scheme 1). The L-Oxd-OBn moiety
(Oxd = 4-methyl-5-carboxyoxazolidin-2-one, Bn = benzylox-
ycarbonyl) can easily be synthesized in multigram scale starting
from L-Thr.11
Then, 2 and 3 were coupled using HBTU and TEA in dry
acetonitrile in an inert atmosphere providing 4 in satisfactory
yield. By deprotection of the carbobenzoxy group with H2 in
methanol in the presence of Pd/C (5%), the corresponding
acid 5 was prepared. Repetition of these two steps produces
Boc-(L-Phe-L-Oxd)3-OBn 6 in good yield. The longer
oligomers Boc-(L-Phe-L-Oxd)4-OBn 8 and Boc-(L-Phe-L-
Oxd)6-OBn 10 have been obtained by coupling Boc-(L-Phe-L-
Oxd)2-OH 5 and Boc-(L-Phe-L-Oxd)4-OH 9, respectively, with
H-(L-Phe-D-Oxd)2-OBn·CF3CO2H 7 using HBTU and TEA in
dry acetonitrile.
a
Scheme 1
All the deprotection steps were performed with excellent
yields, while the coupling step yields were between 68 and 84%.
The purification by flash chromatography of the longer
oligomers proved to be difficult due to the low solubility of
Boc-(L-Phe-L-Oxd)3-OBn 6, Boc-(L-Phe-L-Oxd)4-OBn 8, and
Boc-(L-Phe-L-Oxd)6-OBn 10 in any solvent, as all the products
were blocked in the silica gel and the yields dramatically
decreased. This obstacle was overcome by performing the
purification with the help of an ultrasound bath. The reaction
crude was dissolved in ethyl acetate and washed with water to
eliminate the water-soluble byproduct (HOBt in part,
unreacted amines and acids, etc.), and then it was concentrated
and the solvent replaced with cyclohexane. This mixture was
sonicated so that the byproduct, tetramethylurea, and the
remaining part of HOBt dissolved in the apolar solvent, and the
desired product could be recovered pure after filtration.
Following this procedure, the oligomers 6, 8, and 10 were
obtained pure in satisfactory yields.
2. Conformational Analysis. Information on the preferred
conformation of the oligomers in solution was obtained by the
analysis of FT-IR, 1H NMR, ECD, and VCD absorption spectra
and by DFT (density functional theory) quantum chemical
calculations on Boc-L-Phe-L-Oxd-OBn.
a
(i) HBTU (1.1 equiv), Et3N (2 equiv), dry CH3CN, tr = 1 h; (ii) H2,
Pd/C (10%), MeOH, tr = 16 h; (iii) TFA (18 equiv), dry CH2CI2, tr =
4 h; (iv) HBTU (1.1 equiv), Et3N (3 equiv), dry CH3CN, tr = 1 h.
In particular, FT-IR and 1H NMR spectroscopy helped us to
understand if any intramolecular N−H···OC hydrogen
bonds are formed, while CD spectroscopy provided us with
some interesting information on the preferred secondary
structure. Finally, DFT calculations on 1 are presented with a
detailed conformational analysis validated by comparison with
VCD data in an apolar solvent like CCl4.
The oligomer series Boc-(L-Phe-L-Oxd)n-OBn (n = 2, 3, 4, 6)
4−10 have been synthesized in solution. Boc-(L-Phe-D-Oxd)-
OH 2 has been obtained by selective deprotection of the C-
terminal benzyl ester with H2 in methanol in the presence of
Pd/C (5%), and H-L-Phe-D-Oxd-OBn·CF3CO2H 3 was
prepared by cleavage of the N-terminal Boc moiety with
Figure 2. (a) N−H stretching regions of the IR absorption spectra for 3 mM samples of oligomers 1, 4, 6, 8, and 10 in CH2Cl2 at room temperature.
(b) N−H stretching regions of the IR absorption spectra for 3 mM samples of oligomers 1, 4, 6, 8, and 10 in CCl4 at room temperature.
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2.1. IR Spectroscopy. The analysis of the N−H stretching
regions helps one to detect if intramolecular N−H···OC
hydrogen bonds are formed, since nonhydrogen-bonded amide
NH bonds exhibit a stretching signal above 3400 cm−1, while
hydrogen-bonded amide NH bonds12 produce a stretching
band below 3400 cm−1.
calculations may provide interesting information on the
preferred conformations of the oligomers.
2.3. ECD Spectroscopy. Electronic circular dichroism
spectroscopy was used in a preliminary conformational analysis
of the homochiral oligomers Boc-(L-Phe-L-Oxd)n-OBn (n = 1,
2, 3, 4, 6), compounds 1, 4, 6, 8, and 10, respectively, based on
their electronic transitions. ECD measurements were recorded
for 1 mM methanol solutions at room temperature for all
compounds. The overlap of the spectra of 1, 4, 6, 8, and 10,
normalized per residue, is reported in Figure 3 and exhibits, for
The FT-IR spectra were recorded for 3 mM solutions in both
methylene chloride (aprotic polar solvent) and carbon
tetrachloride (apolar solvent). This low concentration was
chosen in order to avoid possible self-aggregation and due to
the low solubility of the longer oligomers 6, 8, and 10. Figure 2
shows the FT-IR absorption spectra of the oligomers 1, 4, 6, 8,
and 10 in methylene chloride and carbon tetrachloride.
For all compounds in methylene chloride, a strong band
above 3400 cm−1 is noticed indicating that no hydrogen bond is
formed (Figure 2a). A shoulder centered below 3400 cm−1
(about 3340 cm−1) appears for 4, 6, 8, and 10: it becomes
stronger with increasing foldamer chain length but never
becomes the main stretching band. The formation of this
shoulder suggests that equilibrium takes place among different
conformations. In an apolar solvent, like carbon tetrachloride,
the results are quite different: while for compound 1, only one
band centered at 3446 cm−1 is observed, for longer oligomers, a
broad band centered at about 3300 cm−1 appears (Figure 2b).
This effect may be due to equilibrium between structures, or it
simply could be ascribed to the formation of intermolecular
hydrogen bonds due to the self-aggregation of these
compounds in apolar solvents.
Figure 3. Normalized per-residue ECD spectra of the Boc-(L-Phe-L-
Oxd)n-OBn (n = 1−6) series in MeOH solution (1 mM).
2.2. 1H NMR Spectroscopy. The occurrence of intra-
molecular CO···H−N hydrogen bonds was checked also
by investigating the dependence of the NH proton chemical
shifts on an increasing percentage (up to 10%) of DMSO-d6 in
a 3 mM CDCl3 solution. DMSO is a strong hydrogen-bond
acceptor, and if DMSO is bound to a free NH proton, a
considerable downfield shift of the proton signal can be
expected.13
The Δδ values (difference of NH proton chemical shifts for
the spectra recorded in pure CDCl3 and in 9:1 CDCl3/DMSO-
d6) for NH hydrogens of oligomers Boc-(L-Phe-L-Oxd)n-OBn
6, 8, and 10 are summarized in Table 1 (see Figure S1 of the
all foldamers, a positive band centered at 195 nm, due to either
the peptide transition or the strong B transitions of Phe that are
usually located around 188 nm.
The ECD spectra of compound 1 display a negative signal at
ca. 215 nm and a strong positive band at 195 nm, while at 190
nm, a negative ellipticity value is shown (see Figure S3 of the
Supporting Information). In addition, a broad positive
unfeatured band between 270 and 220 nm (crossover points)
is displayed and may be attributed to the aromatic contribution
of the Phe residue, a strongly absorbing chromophore. Phe
chromophore contributions, La, are reported to fall in the
peptide region at 208 nm,15 or at 225−230 nm.16 The Lb
phenylalanine transition, on the other hand, occurs at 257 nm.
Some spectral features of longer oligomers are probably due
to a mixture of conformers present in solution. It is clear from
Figure 3 that the negative feature at 215 nm in 1 becomes
weaker and blue shifted with increasing chain length (in 10 it
falls at 205 nm). A similar blue shift is shown by the positive
broad band from 238.5 nm in 1 to 234.0 nm in 10 (see Figures
S3 and S4 of the Supporting Information). These spectral
features seem to rule out the possibility of a self-assembly of the
oligomer, driven by the stacking interaction of aromatic units,17
while a PPII conformation could be taken into account
supported by the fact that the broad band due to the aromatic
chromophore contributions (with its maximum at 234 nm)
could hide the characteristic positive peak around 220 nm.
2.4. VCD Spectroscopy. In order to gain further information
regarding the preferred conformations of these systems, we also
recorded VCD spectra in the mid-IR region. VCD has been
around since 1975,18,19 for almost the same amount of time as
ECD, but has become a handy tool for absolute-configuration
determination as well as for conformational analysis, after
extension of the accessible frequency range to mid-IR and after
the use of ab initio calculations has allowed impeccable and
a
Table 1. Δδ (ppm) for NH Titration for Oligomers Boc-(L-
Phe-L-Oxd)n-OBn with n = 3, 4, and 6
compound
NH-Boc
NH1
NH2
NH3
NH4
NH5
6
0.19
0.10
0.08
1.98
1.98
1.85
2.08
2.05
1.99
8
2.07
1.96
10
1.97
2.16
a
Difference of NH proton chemical shifts between the spectrum
recorded in pure CDCl3 and in 9:1 CDCl3/DMSO-d6.
Supporting Information). All the hydrogen amides have Δδ
values above 1.8 ppm: these values are very high and account
for non-hydrogen-bonded NH amide protons. In any case, it is
worth mentioning that the NH-Boc hydrogens have small Δδ
values that become smaller as the foldamer becomes longer.14
Unfortunately, these compounds are either waxy or
amorphous solids, so no crystal suitable for X-ray diffraction
analysis could be grown to allow a direct determination of the
preferred conformation of this class of oligomers (see Figure S2
of the Supporting Information). For this reason, we resorted to
ECD (electronic circular dichroism) and VCD (vibrational
circular dichroism) measurements, which together with DFT
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Figure 4. IR and VCD spectra of the Boc-(L-Phe-L-Oxd)n-OBn (n = 1, 2, 3, 4, 6) series in methanol-d4. The ε and Δε are presented in residue units
(see text).
Figure 5. IR (top) and VCD (botton) spectra of Boc-L-Phe-L-Oxd-OBn 1 (pink trace) and of the enantiomer Boc-D-Phe-D-Oxd-OBn (blue trace) in
CCl4 (20 mM solution). Numbers are introduced to facilitate correspondence between IR and VCD.
easy interpretation of VCD spectra in the mid-IR range, in
frequency, sign, and intensity.20−25 Besides, as mentioned
above, this technique is particularly adequate for the systems
under study since they contain aromatic amino acids. As
explained above, aromatic ECD chromophores interfere with
the signals characteristic of the amino groups, which are usually
associated with backbone conformations of peptides and
proteins.
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Figure 6. Atom numbers and definition of dihedral angles (in degrees) for compound 1. ϕ1: 23, 7, 24, 40. ψ1: 7, 24, 40, 9. τPro(trans−cis Pro): 44, 9,
40, 24. ψ2: 9, 44, 49, 51. χ1Phe1: 7, 24, 26. χ2Phe1: 24, 26, 29, 30. τCOOBn: 44, 49, 51, 52. χ1Bn: 49, 51, 52, 55. χ2Bn: 51, 52, 55, 56. ϕ2 is not reported since
it has a fixed value as explained in the text.
Figure 7. Comparison of experimental IR (top) and VCD (bottom) spectra of 1 with the corresponding Boltzmann average calculated spectra.
VCD measurements were recorded at room temperature for
20 mM solutions in carbon tetrachloride for 1; for longer
oligomers the solubility in this solvent is too low to obtain
VCD spectra. For this reason all Boc-(L-Phe-L-Oxd)n-OBn
compounds (n = 1, 2, 3, 4, 6) were considered in 20 mM
CD3OD solutions. The concentrations needed in VCD
spectroscopy are actually quite large, and this should be kept
in mind when comparing with the previous analysis based on
the ECD and IR of dilute solutions.
A qualitative comparison of the VCD spectra recorded in
methanol, with help from the literature data on peptides, can be
carried out. In Figure 4 we report the IR and VCD spectra for
all oligomers in the L form. To provide evidence for the
differences in the behavior of different-sized foldamers, we
normalized IR and VCD spectra, dividing ε and Δε by n, for
each Boc-(L-Phe-L-Oxd)n-OBn considered. The more tradi-
tional comparison in the original ε and Δε scales is given in
Figure S5 of the Supporting Information. We distinguish the
two regions: 900−1600 cm−1 on the right side and 1600−1800
cm−1 on the left side (the latter contains the carbonyl stretching
modes coupled with the NH in-plane bending modes, defining
the amide I modes, which are so often used in the study of
peptides and proteins26). The most prominent VCD bands are
observed at 1215 cm−1 having a positive sign (+), 1280 (−)
(1300 cm−1 for 1), 1390 (+), 1675 (−), and 1710 cm−1 (+).
Regarding the amide I bands, the negative feature at 1675 cm−1,
dominating over the positive component at higher frequency,
may be suggestive of β structures for compounds 4, 6, 8, and
10. However, in usual amino-acidic peptides, this band appears
at wavenumbers lower than 1650 cm−1;22,27,28 the copresence
of a positive component at higher energy is compatible with
PPII-type conformations, which in the case of peptides present
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precisely a couplet. The fact that observed wavenumbers do not
correspond to the values usually observed for peptides can be
attributed to the Oxd unit.
Both IR and VCD spectra of compound 1 are somewhat
different from those of the longer foldamers, in agreement with
ECD findings.
To gain further information on these systems and to verify
the latter observation, we recorded VCD spectra also in CCl4
for compound 1, which can be dissolved in sufficient amount.
The VCD spectra of the two enantiomers are reported in the
bottom part of Figure 5, while in the top part we provide the
corresponding IR spectra; still we distinguish the two regions
1600−900 and 1600−1800 cm−1.
The shape of the IR spectrum presents differences with
respect to the one in methanol; in particular the band at 1690
cm−1 is not present. This suggests that, as expected, the
preferential conformations are strongly dependent on the
solvent nature.
values (ϕ1 ≈ −70°, ψ1 ≈ 150°), green for β-structure-like
angles (ϕ1 ≈ −150°, ψ1 ≈ 150°), and blue for angle values
appropriate to a helix (ϕ1 ≈ −100°, ψ1 ≈ −50°). The most
populated conformers exhibit VCD and IR spectra resembling
the experimental ones; this is important since it means that the
calculated geometries and populations adequately represent the
situation in CCl4 solution.
From Table S1 and Figure S6 of the Supporting Information,
we may conclude that the two most-populated conformers A
and B (red spectra) exhibit values for the ϕ1 and ψ1 angles
typical of a PPII helix29 (the PPII and β structures have similar
ψ but different ϕ values). All conformers with similar PPII-type
ϕ1 and ψ1 values give the negative, observed intense band at
about 1710 cm−1. Other structures whose population is non-
negligible (C and D in the table with green spectra in Figure S6
of the Supporting Information) show calculated spectra which
do not exhibit the negative band at ca. 1710 cm−1 (clearly
observed as #10 in Figure 5) but present all correct features
observed in the range 1200−1300 cm−1. The former band at
1710 cm−1 can be attributed to the amide I mode, which is
normally taken as a marker of peptide secondary structure;30 it
has the aspect of a strong negative band at lower frequencies
followed by a positive feature at higher frequencies. In
agreement with literature data,28,31 we may recognize the
signature of a PPII structure. We notice, however, that the
frequency is much higher than the standard amide I frequency
(ca. 1650 cm−1); this may be due to both the short foldamer
length and the presence of the non-amino-acidic group, Oxd.
For all populated conformers the assignment of the bands
observed between 1700 and 1800 cm−1 is the following (for the
band numbering see Figure 5): 10 and 11, amide I vibrations of
the two peptidic carbonyls; 12, carbonyl stretching adjacent to
the terminal OBn; and 13, the carbonyl stretching of the Oxd
ring. We remark that this spectroscopic region is not perturbed
by phenyl signals. The calculated frequencies of aromatic ring
vibrations are lower than 1700 cm−1, and their calculated
rotational strengths are very small in this region.
The good correspondence between calculated and exper-
imental IR and VCD spectra can be checked in Figure 7, where
the averages of spectra weighted with ΔG and ΔE Boltzmann
factors are reported. In conclusion, considering the overall
spectrum, we maintain that the observed VCD signals are
compatible with the lower-energy conformers whose backbone
angles are consistent with a PPII or a β geometry, but different
from higher-energy conformers with ϕ1 and ψ1 angles
appropriate for α or 310 helices. We reiterate, though, that
the description given is just indicative of the backbone dihedral
angles assumed by Boc-L-Phe-L-Oxd-OBn 1, which is too short
to show a real peptide secondary structure.
2.5. DFT Calculations. In order to understand the preferred
conformation adopted by monomer 1 in an apolar solvent,
DFT calculations have been performed to simulate its VCD
spectrum. The data from CCl4 can be considered representative
of the isolated molecule, i.e., interactions with solvent
molecules are expected to be small. For this reason, such data
are the easiest ones to simulate and calculations in vacuo can be
considered a good approximation.21 We performed DFT
calculations on compound 1 by first optimizing geometries
and then by calculating vibrational frequencies, normal modes,
and infrared and VCD intensities (dipole and rotational
strengths), allowing us to finally simulate spectra for due
comparison with experimental data (based on the assumption
of Lorentzian bandshapes with a 12 cm−1 width). The relevant
conformational degrees of freedom for compound 1 are shown
in Figure 6 where the backbone dihedral angles ϕ1, ψ1, and ψ2
have been systematically varied. The dihedral angle, ϕ2,
assumes values of ∼70°, and the Oxd group is approximately
in the trans position (we checked that the cis conformation,
which is possible for proline, is quite improbable due to
repulsion between two carbonyl groups). Also, the two
torsional angles, χ1 and χ2, for each of the two phenyl groups
have been varied. The conformational search has been
conducted at the PM3 level and at the B3LYP/6-31+G**
level. Vibrational mode analysis ensures that the optimized
geometries are local minima and permit evaluation also of
Gibbs free energies for the various conformers. In Table S1 of
the Supporting Information, we report the list of the 15 lowest-
energy conformers with relative populations obtained, by
considering energy (second column), Gibbs free energy values
(third column), and the values for all relevant dihedral angles.
The complete set of calculated IR and VCD spectra
corresponding to the geometrical structures of Table 1 are
reported in Figure S6 of the Supporting Information, while in
Figure 7 we show only the comparison of the Boltzmann
weighted averages of the calculated spectra with the
experimental data. The calculated spectra (absorption and
VCD) of Figure S6 of the Supporting Information are reported
in order of decreasing population factors based on ΔG from
bottom to top, compared to the experimental one in CCl4
(bottom trace). In the figure, when calculated spectra refer to
conformations with similar ϕ1 and ψ1 angles, the same color has
been used. In fact, one can observe that similar ϕ1 and ψ1
backbone angles are related to similar, yet nonidentical, spectra.
We have identified three instances: red for PPII-like angle
On the same conformers examined by VCD, we also
calculated ECD spectra via the TD-DFT methodology, using
the functional CAM-B3LYP and 6-31+G** basis set. We
present these calculations in Figure S7 of the Supporting
Information and report in Figure 8 the averages of the
calculated spectra weighted with ΔG and ΔE Boltzmann
factors. It is reassuring to notice how the most populated
conformers show the positive band attributable to the Phe
transitions as experimentally observed. In any case we are
conscious that the comparison of in vacuo calculations with
experiments carried out in methanol is not fully appropriate:
solvent effect is quite important as was also concluded from the
fact that the amide I VCD signals recorded in CCl4 (Figure 5)
are not observed in methanol (Figure 4).
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solution cell and a FT-IR spectrometer. All spectra were obtained in 3
mM solutions in dry CH2Cl2 at 297 K. All compounds were dried in
vacuo, and all the sample preparations were performed in a nitrogen
atmosphere. Routine NMR spectra were recorded with spectrometers
at 400 MHz (1H NMR) and 100 MHz (13C NMR). The
measurements were carried out in CD3OD and in CDCl3. The
proton signals were assigned by gCOSY spectra. Chemical shifts are
reported in δ values relative to the solvent (CD3OD or CDCl3) peak.
Boc-L-Phe-L-Oxd-OBn 1. A solution of Boc-L-Phe-OH (0.50 g,
1.89 mmol) and HBTU (0.72 g, 1.89 mmol) in dry acetonitrile (40
mL) was stirred in a nitrogen atmosphere for 10 min at room
temperature. Then L-Oxd-OBn (0.44 g, 1.89 mmol) in dry acetonitrile
(10 mL) was added dropwise at room temperature, followed by Et3N
(3.78 mmol, 0.52 mL). The solution was stirred for 50 min in a
nitrogen atmosphere, and then acetonitrile was removed under
reduced pressure and replaced with ethyl acetate. The mixture was
washed with brine (1 × 30 mL), 1 N aqueous HCl (1 × 30 mL), and a
concentrated solution of NaHCO3 (1 × 30 mL), dried over sodium
sulfate, and concentrated in vacuo. The product was obtained pure
after silica gel chromatography (90:10 c-Hex/ethyl acetate → 70:30 c-
Hex/ethyl acetate as eluant) in 84% (0.77 g) overall yield as a waxy
solid. [α]D: −2.1 (c 1.0, CHCl3). IR (CH2Cl2, 3 mM): ν = 3442, 1799,
1761, 1738, 1717 cm−1. IR (solid, 1% in KBr): ν 3354, 3436, 3354,
1791, 1739, 1725, 1690 cm−1. 1H NMR (CDCl3): δ 1.25 (s, 9H, t-Bu),
1.48 (d, 3H, Me, J = 6.4 Hz), 2.50 (dd, 1H, J = 9.6, 13.6 Hz, CHH-
Ph), 3.22 (dd, 1H, J = 3.3, 13.6 Hz, CHH-Ph), 4.50−4.58 (m, 2H,
CHN-Oxd + CHO-Oxd), 4.84 (d, 1H, J = 8.8 Hz, NH), 5.16 (AB, 2H,
J = 12.4 Hz), 5.57 (dt, 1H, J = 4.0, 9.6 Hz), 7.12−7.33 (m, 10H, 2 ×
Ph). 13C NMR (CDCl3): δ 28.0, 37.6, 53.9, 67.7, 67.8, 73.3, 73.6, 79.6,
126.6, 126.8, 128.1, 128.2, 128.3, 128.6, 129.3, 134.3, 135.8, 151.3,
155.0, 167.5. Anal. Calcd for C26H30N2O7: C, 64.72; H, 6.27; N, 5.81.
Found: C, 64.69; H, 6.25; N, 5.78.
Boc-L-Phe-L-Oxd-OH 2. Compound 1 (0.72 mmol, 0.48 g) was
dissolved in MeOH (30 mL) under nitrogen. C/Pd (35 mg, 10% w/
w) was added under nitrogen. A vacuum was created inside the flask
using the vacuum line. The flask was then filled with hydrogen using a
balloon (1 atm). The solution was stirred for 16 h under a hydrogen
atmosphere. The product was obtained pure in 98% yield (0.28 g) as
an oil, after filtration through filter paper and concentration in vacuo.
1H NMR (CD3OD): δ 1.37 (s, 9H, t-Bu), 1.62 (d, 3H, Me, J = 6.4
Hz), 2.71 (dd, 1H, J = 10.8, 14.0 Hz, CHH-Ph), 3.35 (m, 1H, CHH-
Ph), 4.65 (d, 1H, J = 4.4 Hz, CHN-Oxd), 4.80 (m, 1H, CHO-Oxd),
5.59 (dd, 1H, J = 3.2, 10.8 Hz), 7.21−7.44 (m, 5H, Ph). 13C NMR
(CD3OD): δ 21.2, 28.6, 38.2, 56.2, 62.9, 76.0, 80.7, 127.7, 129.3, 130.4,
138.5, 153.8, 158.0, 171.3, 174.3. Anal. Calcd for C19H24N2O7: C,
58.16; H, 6.16; N, 7.14. Found: C, 58.13; H, 6.19; N, 7.12.
Figure 8. Comparison of experimental ECD (bottom) spectra of 1
with the corresponding Boltzmann average calculated spectra.
CONCLUSIONS
■
The oligomers of the Boc-(L-Phe-L-Oxd)n-OBn series have
been synthesized in solution in high yield and have been
purified by means of ultrasound irradiation. This technique
allowed us to avoid the purification through flash chromatog-
raphy that strongly reduces the yields. Then, employing several
experimental techniques we demonstrated that the structural
behavior of the oligomers Boc-(L-Phe-L-Oxd)n-OBn are similar
from n = 2 to n = 6. An analysis of ECD spectra of the
oligomers recorded in methanol, although not leading to a
clear-cut conclusion with respect to foldamer conformation
being perturbed by aromatic contributions, suggests the
presence of different conformations for 1, on one side, and
longer chain foldamers, on the other side, as clearly shown by
the significant blue shift of both the negative and the positive
ECD bands with increasing chain length.
VCD and absorption IR spectra in methanol solutions
suggest the formation of PPII structure, particularly due to the
presence of a positive signal above 1700 cm−1 and a negative
one at lower wavenumbers. This indication is given in analogy
to what was observed for peptides: the fact that the band
wavenumbers observed here do not correspond to literature
data for real peptides is due to the presence of the Oxd groups.
In any case, since we are not dealing with peptides,
assignment and interpretation of the spectra must rely also
on calculations. Detailed DFT computational analysis has been
conducted for Boc-L-Phe-L-Oxd-OBn. This short model
presents a low number of possible conformers and can be
treated theoretically as a molecule in vacuo since it can be
dissolved in CCl4 in which the VCD experiments are run. The
most populated conformers have been identified; they exhibit
backbone dihedral angles in the same range as those of a PPII
geometry. Oxd−carbonyl presents the highest stretching
frequency, but also vibrations involving backbone CO
stretchings show quite a high wavenumber value. The amide
I region of the absorption and VCD spectra is not perturbed by
aromatic contributions.
H-L-Phe-L-Oxd-OBn·CF3CO2H 3. A solution of 1 (0.52 mmol,
0.25 g) and TFA (9.36 mmol, 0.720 mL) in dry methylene chloride
(20 mL) was stirred at room temperature for 4 h; then the volatiles
were removed under reduced pressure, and the corresponding amine
salt was obtained pure in 99% yield (0.26 g) as a waxy solid, without
1
further purification. H NMR (CDCl3): δ 1.52 (d, 3H, Me, J = 3.96
Hz), 2.77 (dd, 1H, J = 9.2, 14.4 Hz, CHH-Ph), 3.37 (dd, 1H, J = 3.8,
14.4 Hz, CHH-Ph), 4.51−4.64 (m, 2H, CHN-Oxd + CHO-Oxd), 5.15
(AB, 2H, J = 11.6 Hz), 5.57 (m, 1H, NH), 7.08−7.40 (m, 10H, 2 ×
Ph). 13C NMR (CDCl3): δ 19.6, 19.8, 35.2, 53.8, 60.1, 60.3, 65.0, 67.6,
67.7, 73.5, 73.9, 127.4, 127.5, 127.6, 127.8, 128.0, 128.3, 128.4, 128.5,
131.5, 133.2, 150.4, 159.6 (q, CF3), 166.0, 167.8. Anal. Calcd for
C23H23F3N2O7: C, 55.65; H, 4.67; N, 5.64. Found: C, 55.68; H, 4.68;
N, 5.61.
Boc-(L-Phe-L-Oxd)2-OBn 4. A solution of 2 (0.20 g, 0.52 mmol)
and HBTU (0.20 g, 0.52 mmol) in dry acetonitrile (20 mL) was
stirred in a nitrogen atmosphere for 10 min at room temperature.
Then a mixture of 3 (0.52 mmol) and Et3N (1.56 mmol, 0.22 mL) in
dry acetonitrile (10 mL) was added dropwise at room temperature.
The solution was stirred for 50 min in a nitrogen atmosphere, and
then acetonitrile was removed under reduced pressure and replaced
with ethyl acetate. The mixture was washed with brine (1 × 30 mL), 1
N aqueous HCl (1 × 30 mL), and a concentrated solution of NaHCO3
(1 × 30 mL), dried over sodium sulfate, and concentrated in vacuo.
The above conformational assignment is a remarkable
outcome as we had demonstrated in the past that the Boc-(L-
Ala-D-Oxd)n-OBn series folds in a β-band ribbon spiral, that is a
subtype of the 310 helix. This finding has been predicated
mainly on the basis of VCD, which has been applied here for
the first time to study these foldamer systems.
EXPERIMENTAL SECTION
Synthesis. The melting points of the compounds were determined
in open capillaries and are uncorrected. High-quality infrared spectra
(64 scans) were obtained at 2 cm−1 resolution using a 1 mm NaCl
■
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The Journal of Organic Chemistry
Article
The product was obtained pure after silica gel chromatography (90:10
c-Hex/ethyl acetate → 70:30 c-Hex/ethyl acetate as eluant) in 80%
yield (0.32 g) as a white solid. Mp: 81−82 °C. [α]D20: −26 (c 1.0,
CHCl3). IR (CH2Cl2, 3 mM): ν 3427, 1789, 1752, 1711 cm−1. IR
(solid, 1% in KBr): ν 3362, 1785, 1752, 1707 cm−1. 1H NMR
(CDCl3): δ 1.22−1.30 (m, 15H, 2 × Me + t-Bu), 2.46−2.59 (m, 2H, 2
× CHH-Ph), 3.08−3.46 (m, 2H, 2 × CHH-Ph), 4.12−4.68 (m, 4H, 2
× CHN-Oxd + 2 × CHO-Oxd), 4.89 (bs, 1H, NH), 5.06−5.24 (m,
2H, OCH2Ph), 5.44−5.60 (m, 1H, CHN-CH2Ph), 5.86−5.93 (m, 1H,
CHN-CH2Ph), 6.63 (d, 1H, J = 8.4 Hz, NH), 6.89−7.47 (m, 15H, 3 ×
Ph). 13C NMR (CDCl3): δ 14.2, 20.4, 21.0, 21.2, 26.9, 27.8, 28.2, 37.9,
53.0, 54.2, 55.4, 60.4, 61.2, 61.5, 62.6, 68.3, 73.9, 74.2, 79.8, 80.2,
126.7, 126.8, 127.0, 127.1, 127.2, 128.4, 128.5, 128.8, 129.5, 130.1,
134.4, 135.7, 136.1, 151.4, 151.6, 155.2, 158.7, 163.8, 166.7, 166.9,
167.5, 167.7, 171.2, 171.3, 171.7, 171.9, 172.7, 173.4. Anal. Calcd for
C40H44N4O11: C, 63.48; H, 5.86; N, 7.40. Found: C, 63.49; H, 5.88; N,
7.43.
54.3, 55.5, 61.5, 62.2, 68.4, 74.4, 127.1, 127.3, 128.5, 128.7, 128.8,
129.0, 129.3, 129.5, 129.6, 134.3, 134.8, 151.7, 159.4 (q, CF3), 167.5,
171.1. Anal. Calcd for C37H37F3N4O11: C, 57.66; H, 4.84; N, 7.27.
Found: C, 57.68; H, 4.83; N, 7.30.
Boc-(L-Phe-L-Oxd)4-OBn 8. A solution of 5 (0.16 g, 0.24 mmol)
and HBTU (0.091 g, 0.24 mmol) in dry acetonitrile (20 mL) was
stirred in a nitrogen atmosphere for 10 min at room temperature.
Then a mixture of 3 (0.24 mmol) and Et3N (0.72 mmol, 0.100 mL) in
dry acetonitrile (10 mL) was added dropwise at room temperature.
The solution was stirred for 50 min in a nitrogen atmosphere, and
then acetonitrile was removed under reduced pressure and replaced
with ethyl acetate. The mixture was washed with brine (1 × 30 mL), 1
N aqueous HCl (1 × 30 mL), and a concentrated solution of NaHCO3
(1 × 30 mL), dried over sodium sulfate, and concentrated in vacuo.
For the purification from tetramethylurea and the byproduct, the
residue was suspended in cyclohexane and sonicated for 15 min to
dissolve the byproduct. Compound 8 was filtered, dried in vacuo, and
obtained pure in 70% yield (0.22 g) as a white solid. Mp: 87−88 °C.
[α]D20: −28.0 (c 1.0, CHCl3). IR (CH2Cl2, 3 mM): ν 3428, 1789,
1707 cm−1. IR (solid, 1% in KBr): ν 3361, 1791, 1749, 1714 cm−1. 1H
NMR (CDCl3): δ 1.10−1.38 (m, 21H, 4 × Me + t-Bu), 2.71−3.30 (m,
8H, 4 × CH2-Ph), 4.07−4.64 (m, 8H, 4 × CHN-Oxd + 4 × CHO-
Oxd), 4.79−5.17 (m, 3H, OCH2Ph + NH), 5.40−6.13 (m, 4H, 4 ×
CHN-CH2Ph), 6.98−7.33 (m, 28H, 5 × Ph + 3 × NH), 7.47 (bs, 1H,
NH). 13C NMR (CDCl3): δ 20.3, 20.6, 20.9, 28.3, 37.0, 37.8, 38.4,
53.1, 53.7, 54.1, 61.7, 62.7, 68.3, 73.9, 74.5, 74.9, 79.9, 126.8, 126.9,
127.2, 127.4, 128.4, 128.5, 128.8, 129.7, 134.5, 135.5, 136.0, 151.3,
151.8, 152.1, 155.2, 167.3, 168.0, 171.3. Anal. Calcd for C68H72N8O19:
C, 62.57; H, 5.56; N, 8.58. Found: C, 62.55; H, 5.54; N, 8.61.
Boc-(L-Phe-L-Oxd)4-OH 9. Compound 8 (0.30 mmol, 0.36 g) was
dissolved in MeOH (30 mL) under nitrogen. C/Pd (35 mg, 10% w/
w) was added under nitrogen. A vacuum was created inside the flask
using the vacuum line. The flask was then filled with hydrogen using a
balloon (1 atm). The solution was stirred for 16 h under a hydrogen
atmosphere. The product was obtained pure in 98% yield (0.28 g) as a
white solid, after filtration through filter paper and concentration in
vacuo. Mp: 76 °C. 1H NMR (CDCl3): δ 1.20−1.61 (m, 21H, 4 × Me
+ t-Bu), 2.52−2.68 (m, 1H, 1 × CHH-Ph), 2.72−3.02 (m, 3H, 1 ×
CHH-Ph, 2 × CHH-Ph), 4.46−4.83 (m, 6H, 3 × CHN-Oxd + 3 ×
CHO-Oxd), 5.44−5.94 (m, 4H, 4 × CHN-CH2Ph), 7.12−7.56 (m,
20H, 4 × Ph). 13C NMR (CD3OD): δ 19.2, 19.9, 35.8, 36.0, 36.6, 53.0,
54.8, 61.8, 62.0, 75.1, 76.3, 79.3, 126.4, 127.9, 128.0, 129.0, 136.6,
137.2, 152.2, 152.6, 169.1, 171.4, 171.6, 172.8. Anal. Calcd for
C61H66N8O19: C, 60.29; H, 5.47; N, 9.22. Found: C, 60.32; H, 5.46; N,
9.20.
Boc-(L-Phe-L-Oxd)6-OBn 10. A solution of 9 (0.13 g, 0.11 mmol)
and HBTU (0.09 g, 0.11 mmol) in dry acetonitrile (20 mL) was
stirred in a nitrogen atmosphere for 10 min at room temperature.
Then a mixture of 7 (0.09 g, 0.11 mmol) and Et3N (0.33 mmol, 0.05
mL) in dry acetonitrile (15 mL) was added dropwise at room
temperature. The solution was stirred for 50 min in a nitrogen
atmosphere, and then acetonitrile was removed under reduced
pressure and replaced with ethyl acetate. The mixture was washed
with brine (1 × 30 mL), 1 N aqueous HCl (1 × 30 mL), and a
concentrated solution of NaHCO3 (1 × 30 mL), dried over sodium
sulfate, and concentrated in vacuo. For the purification from
tetramethylurea and the byproduct, the residue was suspended in
cyclohexane and sonicated for 15 min to dissolve the byproduct.
Compound 9 was filtered, dried in vacuo, and obtained pure as a white
solid in 68% (0.14 g) yield. Mp: 92−93 °C. [α]D20: −30 (c 1.0,
CHCl3). IR (CH2Cl2, 3 mM): ν 3411, 3328, 1789, 1733, 1699 cm−1.
IR (solid, 1% in KBr): ν 3330, 1790, 1747, 1701, 1650 cm−1. 1H NMR
(CDCl3): δ 0.97−1.64 (m, 27H, 6 × Me + t-Bu), 2.69−3.42 (m, 12H,
8 × CH2-Ph), 4.11−4.75 (m, 12H, 6 × CHN-Oxd + 6 × CHO-Oxd),
5.01−5.77 (m, 3H, OCH2Ph + NH), 5.39−6.11 (m, 6H, 6 × CHN-
CH2Ph), 6.67 (bs, 1H, NH), 6.93−7.45 (m, 39H, 7 × Ph + 4 × NH).
13C NMR (CDCl3): δ 13.1, 19.3, 19.7, 20.0, 27.7, 28.6, 35.7, 36.7, 37.3,
37.7, 51.9, 52.3, 52.6, 53.0, 60.7, 61.5, 61.6, 67.2, 72.8, 73.2, 74.0, 78.9,
125.8, 126.3, 127.5, 127.6, 127.7, 128.4, 128.6, 133.6, 134.4, 134.5,
134.8, 150.2, 150.9, 165.8, 166.3, 166.8, 170.2, 170.4, 170.6. Anal.
Boc-(L-Phe-L-Oxd)2-OH 5. Compound 4 (0.40 mmol, 0.30 g) was
dissolved in MeOH (30 mL) under nitrogen. C/Pd (35 mg, 10% w/
w) was added under nitrogen. A vacuum was created inside the flask
using the vacuum line. The flask was then filled with hydrogen using a
balloon (1 atm). The solution was stirred for 16 h in a hydrogen
atmosphere. The product was obtained pure in 97% yield (0.26 g) as a
waxy solid, after filtration through filter paper and concentration in
1
vacuo. H NMR (CD3OD): δ 1.15−1.30 (m, 9H, t-Bu), 1.37−1.48
(m, 6H, 2 × Me), 2.48 (dd, 1H, J = 10.8, 14.0 Hz, CHH-Ph), 2.77 (dd,
1H, J = 10.4, 14.4 Hz, CHH-Ph), 3.10−3.33 (m, 2H, 2 × CHH-Ph),
4.44−4.52 (m, 2H, 2 × CHN-Oxd), 4.61−4.69 (m, 2H, 2 × CHO-
Oxd), 5.32−5.39 (m, 1H, NH), 5.65−5.74 (m, 1H, NH), 7.05−7.33
(m, 10H, 2 × Ph). 13C NMR (CD3OD): δ 20.6, 28.6, 37.4, 38.0, 55.3,
56.1, 63.0, 63.3, 76.2, 76.4, 80.6, 127.6, 127.8, 129.2, 130.4, 137.8,
138.5, 163.8, 154.1, 158.0, 170.1, 171.4, 173.0, 174.1. Anal. Calcd for
C33H38N4O11: C, 59.45; H, 5.75; N, 8.40. Found: C, 59.41; H, 5.74; N,
8.38.
Boc-(L-Phe-L-Oxd)3-OBn 6. A solution of 5 (0.168 g, 0.25 mmol)
and HBTU (0.095 g, 0.25 mmol) in dry acetonitrile (20 mL) was
stirred in a nitrogen atmosphere for 10 min at room temperature.
Then a mixture of 3 (0.25 mmol) and Et3N (0.75 mmol, 0.104 mL) in
dry acetonitrile (10 mL) was added dropwise at room temperature.
The solution was stirred for 50 min in a nitrogen atmosphere, and
then acetonitrile was removed under reduced pressure and replaced
with ethyl acetate. The mixture was washed with brine (1 × 30 mL), 1
N aqueous HCl (1 × 30 mL), and a concentrated solution of NaHCO3
(1 × 30 mL), dried over sodium sulfate, and concentrated in vacuo.
For the purification from tetramethylurea and the byproduct, the
residue was suspended in cyclohexane and sonicated for 15 min to
dissolve the byproduct. Compound 6 was filtered, dried in vacuo, and
obtained pure in 73% yield (0.18 g) as a white solid. Mp: 79−80 °C.
[α]D20: −30 (c 0.3, CHCl3). IR (CH2Cl2, 3 mM): ν 3442, 1789, 1752,
1718 cm−1. IR (solid, 1% in KBr): ν 3371, 1788, 1754, 1714 cm−1. 1H
NMR (CDCl3): δ 1.05−1.68 (m, 18H, 3 × Me + t-Bu), 2.58−2.94 (m,
3H, 3 × CHH-Ph), 3.27−3.51 (m, 3H, 3 × CHH-Ph), 4.20−4.87 (m,
6H, 3 × CHN-Oxd + 3 × CHO-Oxd), 4.94 (bs, 1H, NH), 5.05−5.18
(m, 2H, OCH2Ph), 5.82−6.07 (m, 3H, 3 × CHN-CH2Ph), 6.75 (bs,
1H, NH), 6.89−7.47 (m, 25H, 5 × Ph), 7.17 (bs, 2H, 2 × NH), 7.34
(bs, 1H, NH). 13C NMR (CDCl3): δ 20.3, 21.1, 27.8, 28.2, 37.2, 37.7,
53.1, 54.3, 60.3, 61.7, 62.7, 63.0, 68.2, 74.0, 74.2, 74.4, 80.0, 126.9,
127.1, 128.4, 128.8, 129.2, 129.6, 129.9, 134.5, 135.9, 151.0, 151.7,
155.2, 155.6, 167.0, 167.6, 171.1, 171.4. Anal. Calcd for C54H58N6O15:
C, 62.90; H, 5.67; N, 8.15. Found: C, 62.87; H, 5.65; N, 8.18.
H-(L-Phe-L-Oxd)2-OBn·CF3CO2H 7. A solution of 2 (0.52 mmol,
0.39 g) and TFA (9.36 mmol, 0.720 mL) in dry methylene chloride
(20 mL) was stirred at room temperature for 4 h; then the volatiles
were removed under reduced pressure, and the corresponding amine
salt was obtained pure in 96% yield (0.38 g) as a waxy solid. 1H NMR
(CDCl3): δ 1.34−1.52 (m, 6H, 2 × Me), 2.49−2.90 (m, 2H, 2 ×
CHH-Ph), 3.01−3.35 (m, 2H, 2 × CHH-Ph), 4.19−4.68 (m, 4H, 2 ×
CHN-Oxd + 2 × CHO-Oxd), 4.89−5.18 (m, 1H, NH), 5.30−5.81 (m,
2H, 2 × CHN-CH2Ph), 7.05−7.35 (m, 15H, 3 × Ph). 13C NMR
(CDCl3): δ 20.5, 20.9, 27.7, 28.1, 36.1, 36.6, 37.7, 38.8, 53.2, 53.7,
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Article
Calcd for C96H100N12O27: C, 62.20; H, 5.44; N, 9.07. Found: C, 62.22;
H, 5.46; N, 9.11.
computational facilities and Fondazione del Monte di Bologna
e di Ravenna for financial support.
ECD Spectra. The CD spectra were recorded at room temperature
on a spectropolarimeter. Quartz cuvettes of 0.01, 0.1, 1, and 2 cm path
length were employed. Foldamers were dissolved in methanol yielding
clear approximately 1 mM solutions. The spectra were scanned
between 190 and 300 nm for the far-UV and near-UV regions, with 0.2
nm resolution. Thirty-six scans were collected with a scanning speed of
50 nm min−1 and a time constant of 1 s, and the solvent baseline was
subtracted from the averaged spectra. Final spectra are presented in
molar ellipticity. Spectral analysis was performed by fitting the
measured spectra in the far-UV spectra with reference spectra based on
the CD curves of different model peptides with varying amounts of α-
helix, β-sheet, β-turn, and random coil conformations. Reference
spectra were described by J. Reed and T. A. Reed for peptide
analysis.32
IR and VCD Spectra. IR and VCD spectra of Figure 5 and 7 were
measured at a resolution of 4 cm−1 in the mid-IR region from about
900 to 1850 cm−1. Foldamers (monomers to hexamers) were dissolved
in methanol-d4 and in CCl4 (monomers) at a concentration of 0.02 M
in an IR liquid cell with a path length of 0.2 mm. All the VCD spectra
reported were measured with a collection time of 1 h. In all cases, the
spectrum of the solvent was subtracted from the IR and VCD spectra.
For n = 1 in CCl4, both enantiomers have been measured to check the
VCD data reliability.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Variation of the NH proton chemical shifts (ppm) of 6, 8, and
10; X-ray powder diffraction (XRD) patterns for 4, 6, 8, and
10; ECD spectra of 1, 4, 6, 8, and 10 in MeOH solution; IR
and VCD spectra of 1, 4, 6, 8, and 10 in CD3OD;
characteristics of the conformers obtained for compound 1;
calculated IR and VCD spectra of 1, for the most populated
conformers; calculated ECD spectra of 1 for the most
populated conformers; Cartesian coordinates of optimized
structures of 1 at the B3LYP/6-31+g(d,p) level for the four
1
most populated conformers; copies of the H and 13C NMR
(10) (a) Colletier, J.-P.; Laganowsky, A.; Landau Meytal, M.; Zhao,
M. L.; Soriaga, A. B.; Goldschmidt, L.; Flot, D.; Cascio, D.; Sawaya, M.
R.; Eisenberg, D. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 16938−16943.
(b) Makin, O. S.; Serpell, L. C. FEBS J. 2005, 272, 5950−5961.
(c) McLaurin, J.; Yang, D. S.; Yip, C. M.; Fraser, P. E. J. Struct. Biol.
2000, 130, 259−270.
spectra of oligomers 1−10. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
(11) Tomasini, C.; Trigari, V.; Lucarini, S.; Bernardi, F.; Garavelli,
M.; Peggion, C.; Formaggio, F.; Toniolo, C. Eur. J. Org. Chem. 2003,
259−267.
*Email: longhi@med.unibs.it, piera.sabatino@unibo.it, claudia.
tomasini@unibo.it.
Notes
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Benedetti, E. Crit. Rev. Biochem. 1980, 9, 1−44. (e) Imperiali, B.;
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4087−4095. (b) Martin, D.; Hauthal, G. G. In Dimethyl Sulphoxide;
Van Nostrand-Reinhold: Wokingham, U.K., 1975.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to thank “Ministero dell’Istruzione, dell’Uni-
versita
̀
e della Ricerca” (MIUR) for financial support, under the
auspices of the programs PRIN 2008LYSESR and PRIN
2008J4YNJY. We also thank CILEA (Consorzio Interuniversi-
tario Lombardo Elaborazione Automatica) for access to their
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