Intramolecular conformational control in a cyclic peptide composed of alternating L-proline and substituted 3-aminobenzoic acid subunits

Article abstract of DOI:10.1039/b000568l

Methoxycarbonyl groups in the 4-position of the aromatic subunits strongly influence the conformational behaviour and the receptor properties of cyclic peptides composed of alternating 3-aminobenzoic acid and L-proline.

Full text of DOI:10.1039/b000568l

Intramolecular conformational control in a cyclic peptide composed of
alternating -proline and substituted 3-aminobenzoic acid subunits†
L
Stefan Kubik*a and Richard Goddardb
a Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität, Universitätsstr. 1,
D-40225 Düsseldorf, Germany. E-mail: kubik@uni-duesseldorf.de
b Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim/Ruhr, Germany
Received (in Liverpool, UK) 19th January 2000, Accepted 28th January 2000
Methoxycarbonyl groups in the 4-position of the aromatic
subunits strongly influence the conformational behaviour
and the receptor properties of cyclic peptides composed of
region 2–0.2 mM, an intermolecular association of the peptide
at these concentrations is unlikely. Instead, the spectrum more
probably represents a non-symmetrical conformation of 2 or
different slowly interconverting peptide conformers. Tem-
perature-dependent 1H NMR spectroscopy of 2 in C2D2Cl4
shows that at 120 °C the flexibility of the peptide is still
somewhat restricted. The rigidity of 2 is certainly caused by the
effects of the additional methoxycarbonyl substituents. The
alternating 3-aminobenzoic acid and -proline
L
We recently showed that cyclic hexapeptides composed of
alternating natural amino acids and 3-aminobenzoic acid
subunits bind cations and anions with high affinity.1,2 Cation–p
interactions with the aromatic subunits of such cyclopeptides
generally result in inclusion of cations into the shallow dish-
shaped receptor cavity.3 By contrast, anions are only bound
when they are able to form hydrogen bonds to the peptide amide
groups. This latter interaction induces a receptor conformation
in which all NH groups of the peptide point towards the cavity
centre.4 In the absence of suitable anions, the amide groups are
able to rotate more freely. We speculated that suitable
substituents on the peptide subunits would also restrict amide
rotation. Such an approach may have the additional advantage
that the anion complexation could be completely prevented by
locking the NH groups in an orientation unsuitable for
interactions with these guests. Here we report our first results in
this direction.
1
unusually large downfield shift of the NH protons in the H
NMR spectrum of 2 in CDCl3 at d 11.1 [for 1 d(NH) 9.2] and the
strong N–H vibration band at 3303 cm21 in the FTIR spectrum
in CDCl3 indicate that these groups are involved in hydrogen
bonds. The crystal structure of 2 monohydrate (Fig. 2) shows
that, as predicted, these hydrogen bonds are formed between the
amide NH groups and the neighbouring methoxycarbonyl
groups.‡ The overall peptide conformation in this structure is
non-symmetrical, presumably caused by the water molecule,
with one aromatic subunit tilted away from the others. As a
result, one would expect a weak cation affinity of 2.
Nevertheless, a significant upfield shift of the guest protons is
observed upon addition of quaternary ammonium salts such as
n-butyltrimethylammonium picrate (BTMA+ picrate) to solu-
tions of 2 in CDCl3. This shift is generally interpreted in terms
of an inclusion of the cation into a receptor cavity which brings
the guest protons in close proximity to the aromatic subunits.3
The spectrum of the peptide is also affected in the presence of
the cation. On increasing the guest concentration, the spectrum
becomes simpler until, after addition of 4 equivalents of
BTMA+ picrate to a 2 mM solution of 2 in CDCl3, it represents
a symmetrical conformation [Fig. 1(b)]. Complex formation
Inspired by the work of Hamilton and coworkers who have
shown that methoxycarbonyl groups can be used for the
conformational control in oligoanthranilamides,5 we introduced
these substituents at the 4-position of the aromatic subunits of 1.
Methoxycarbonyl groups are able to form hydrogen bonds to
adjacent NH protons and thus induce an amide orientation
parallel to the aromatic rings. They also cause the NH protons to
point away from the cavity centre and make the amides less
available for anion complexation.
Peptide 2 was synthesised from the commercially available
2-aminoterephthalic acid 1-methyl ester by following a proce-
dure similar to that used for 1.1 Whereas 1 possesses a simple 1H
NMR spectrum that represents an averaged C3-symmetrical
structure, the spectrum of 2 is more complicated [Fig. 1(a) and
(c)]. Identical protons of 2 give two signals in most cases, and
even the methyl ester signal is split. Since the spectrum is not
significantly affected by varying the concentration of 2 in the
† Electronic supplementary information (ESI) available: synthesis, IR and
NMR data for 2. See http://www.rsc.org/suppdata/cc/b0/b000568l/
Fig. 1 1H NMR spectra of 2 (a) in C2D2Cl4 (60 °C), (c) in CDCl3 (25 °C)
and (b) after addition of 4 equiv. of BTMA+ picrate in CDCl3 (25 °C).
DOI: 10.1039/b000568l
Chem. Commun., 2000, 633–634
This journal is © The Royal Society of Chemistry 2000
633

Table 1 BTMA+ complex stabilities in CDCl3 at 298 K (Ka = stability
constant in M21, error limits of Ka < 20%; Ddmax = maximum chemical
shift in ppm; DGH
=
Gibbs free energy of hydration of the anions in kJ
mol21
)
1
2
Anion
Ka
2Ddmax
Ka
2Ddmax
2DGH
Picrate
Iodide
Tosylate
1260
21100
5050000
0.70
1.11
1.16
10800
3310
740
0.54
0.59
0.54
197
283
318
complex stabilities of 2 decrease when going from picrate to
iodide and tosylate. A similar anion effect has also been
reported for cation complexes of calixarenes.7 Bartsch and
coworkers have shown that the extraction efficiency of certain
crown ether salt complexes correlates inversely with the
hydration enthalpy of the anion.8 In accordance with these
findings, the stabilities of the BTMA+ complexes of 2 decreases
with increasing Gibbs free energy of hydration of the anion.9
The dependence of the complex stabilities on the type of anion
can therefore be attributed to an intrinsic property of the salts
and not to possible peptide–anion interactions. Indeed, the FTIR
spectrum of 2 is unaffected by the different anions, not even
those that bind very strongly to 1.
In summary, we have shown that the NH groups of 2 can be
locked in a defined orientation by hydrogen bonds to methoxy-
carbonyl groups on the aromatic subunits. This results in a
reduction of the conformational freedom of the cyclopeptide
and in improved cation affinity as well as a complete loss of
anion binding ability. Currently, we are investigating effects of
other substituents. The fact that the conformation and hence the
binding properties of these peptides can be influenced by non-
covalent intramolecular interactions give them important ad-
vantages over many other artificial receptors.
Fig. 2 Molecular structure of 2·H2O. Selected interatomic distances (Å):
N1···O7 2.677(2), N3···O9 2.656(2), N5···O11 2.903(3).
obviously causes a shift of the conformational equilibrium of 2.
The NOESY NMR spectrum shows strong NOE effects
between NH and H(a), which confirms that the NH groups are
oriented towards the methoxycarbonyl substituents. As yet, we
have not been able to obtain crystals of the 2·BTMA+ complex.
However, 2 crystallises from acetone with one solvent molecule
per peptide unit. X-Ray crystallography reveals that the solvent
molecule is located inside the peptide cavity (Fig. 3). Moreover,
the peptide conformation in this structure is more symmetrical
than in 2·H2O. The three aromatic subunits are all tilted into the
same direction with all hydrogen bonds between NH and the
methoxycarbonyl substituents retained. These results indicate
that suitable guest molecules can induce a symmetrical peptide
conformation well suited for guest binding when they are
included into the cavity of 2. The NMR spectroscopic results
demonstrate that certain cations induce a similar conformation
in solution. This mechanism of complex formation is therefore
consistent with an ‘induced-fit’.
S. K. thanks Professor G. Wulff, to whom this paper is
dedicated on the occasion of his 65th birthday, for his generous
support and Mrs D. Kubik for the preparative work.
Notes and references
The upfield shift of the BTMA+ protons in the presence of 2
can be used to quantitatively determine the complex stability by
NMR titrations.6 When the shifts of the cation protons of
BTMA+ picrate were followed in the titration, a stability
constant Ka was obtained that is almost an order of magnitude
larger than that of the corresponding complex of 1 (Table 1).
This significant increase of cation complex stability can be
attributed to the conformational rigidity of 2.
‡ Crystal data: 2·H2O: C42H42N6O12·H2O, Mr = 840.83, colourless prism,
crystal size 0.44 3 0.54 3 0.58 mm, a = 13.2214(6), b = 17.0958(8), c =
18.0413(8) Å, U = 4077.9(3) Å3, T = 100 K, orthorhombic, space group
P212121 (no. 19), Z = 4, Dc = 1.37 g cm23, m = 0.10 mm21. Siemens
SMART diffractometer, l = 0.71073 Å. 44640 measured reflections,
15293 unique, 8260 with I > 2.0s(Fo2). The structure was solved by direct
methods and refined by full-matrix least squares on F2 for all data with
Chebyshev weights to R = 0.0595 [I > 2s(Fo2)], wR = 0.144 (all data),
553 parameters.
Whereas a dramatic increase of the cation complex stability
was observed for 1 with iodide or tosylate anions,1 the BTMA+
2·Me2CO: C42H42N6O12·C3H6O, Mr = 880.89, colourless prism, crystal
size 0.17 3 0.28 30.64 mm, a = 10.5607(6), b = 17.6047(10), c =
22.7991(13) Å, U = 4238.8(4) Å3, T = 100 K, orthorhombic, space group
P212121 (no. 19), Z = 4, Dc = 1.38 g cm23, m = 0.10 mm21. Siemens
SMART diffractometer, l = 0.71073 Å. 48668 measured reflections,
16532 unique, 9124 with I > 2.0s(Fo2). Structure solution and refinement
as above, R
= 0.073 [I > = 0.172 (all data), 582
2s(Fo2)], wR
parameters.
CCDC 182/1564. See http://www.rsc.org/suppdata/cc/b0/b000568l/ for
crystallographic files in .cif format.
1 S. Kubik and R. Goddard, J. Org. Chem., 1999, 64, 9475.
2 S. Kubik, J. Am. Chem. Soc., 1999, 121, 5846.
3 J. C. Ma and D. A. Dougherty, Chem. Rev., 1997, 97, 1303; P. Lhotak and
S. Shinkai, J. Phys. Org. Chem., 1997, 10, 273.
4 See also: H. Ishida, M. Suga, K. Donowaki and K. Ohkubo, J. Org.
Chem., 1995, 60, 5374.
5 Y. Hamuro, S. J. Geib and A. D. Hamilton, J. Am. Chem. Soc., 1996, 118,
7529.
6 K. A. Connors, Binding Constants, Wiley, New York, 1987; R. S.
Macomber, J. Chem. Educ., 1992, 69, 375.
7 R. Arnecke, V. Böhmer, R. Cacciapaglia, A. Dalla Cort and L.
Mandolini, Tetrahedron, 1997, 53, 4901.
8 U. Olsher, M. G. Hankins Y. D. Kim and R. A. Bartsch, J. Am. Chem.
Soc., 1993, 115, 3370.
9 Y. Marcus, Ion Properties, Marcel Dekker, New York, 1997.
Fig. 3 Molecular structure of 2·Me2CO projected onto a plane through the
three amide N atoms of 2 (tilt angles of the aromatic rings to this plane (°):
at N1, +35; at N3, +57; at N5, +31). Selected interatomic distances (Å):
N1···O7 2.668(3), N3···O9 2.692(3), N5···O11 2.678(2).
Communication b000568l
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Chem. Commun., 2000, 633–634