Cyclopropane analogue of valine: Influence of side chain orientation on peptide folding

Article abstract of DOI:10.1016/S0040-4039(03)00514-8

The cyclopropane analogue of valine (1-amino-2,2-dimethylcyclopropanecarboxylic acid, c₃Val) has been synthesised and incorporated into the model peptides ^tBuCO-L-Pro-L-c₃Val-NHⁱPr and ^tBuCO-L-Pro-D-c₃Val-NHⁱPr. In the solid state, both dipeptides accommodate a type II β-turn stabilised by an NHⁱPr to ^tBuCO hydrogen bond. Remarkably, the peptide incorporating L-c₃Val also exhibits a distorted γ-turn around the cyclopropane residue, with Pro-CO and NHⁱPr intramolecularly hydrogen-bonded.

Full text of DOI:10.1016/S0040-4039(03)00514-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3147–3150
Cyclopropane analogue of valine: influence of side chain
orientation on peptide folding
Ana I. Jime´nez,a,* Michel Marraudb and Carlos Cativielaa
aDepartment of Organic Chemistry, ICMA, University of Zaragoza-CSIC, 50009 Zaragoza, Spain
bLaboratory of Macromolecular Physical Chemistry, UMR CNRS-INPL 7568, ENSIC, BP 451, 54001 Nancy, France
Received 5 February 2003; revised 20 February 2003; accepted 20 February 2003
Abstract—The cyclopropane analogue of valine (1-amino-2,2-dimethylcyclopropanecarboxylic acid, c3Val) has been synthesised
t
t
and incorporated into the model peptides BuCO-L-Pro-L L-Pro-D
-c3Val-NHiPr and BuCO- -c3Val-NHiPr. In the solid state, both
dipeptides accommodate a type II b-turn stabilised by an NHiPr to tBuCO hydrogen bond. Remarkably, the peptide incorporating
L
-c3Val also exhibits a distorted g-turn around the cyclopropane residue, with Pro-CO and NHiPr intramolecularly hydrogen-
bonded. © 2003 Elsevier Science Ltd. All rights reserved.
The introduction of conformational constraints consti-
tutes a widely used approach in the search of peptide
analogues with more favourable pharmacological prop-
erties. In this context, extensive efforts have been
devoted to the design of peptides with well-defined
backbone conformations,1–3 while the geometry of the
side chain moieties has received much less attention.
Side chains are, however, directly involved in the
molecular recognition processes and their three-dimen-
sional arrangement is essential for adequate peptide–
receptor interaction.4 Moreover, the conformation of
the backbone may be modulated, to a certain extent, by
interactions with the side chains either of steric or
electronic nature.5,6 The dependence of the main chain
conformation on the side chain disposition becomes
evident from the statistical analysis of protein crys-
talline structures.6
Cyclopropane a-amino acids are a unique class of side
chain constrained residues. The orientation of the sub-
stituents is fixed by the stereochemistry of the rigid
1
three-membered ring ( can adopt values only in the
0° and 140° regions), and this offers the possibility to
evaluate accurately the influence of the side chain orien-
tation on the conformation adopted by the peptide
backbone. The great potential of the cyclopropane sys-
tem for this kind of structural studies has actually been
evidenced in model7–9 as well as in biologically active10
peptides that incorporate cyclopropane analogues of
phenylalanine. However, very little work has been
Scheme 1. Synthesis of the cyclopropane analogue of valine (c3Val). Abbreviations: Boc, tert-butyloxycarbonyl; DMAP,
4-(dimethylamino)pyridine.
Keywords: 2,3-methanovaline; constrained amino acid; cyclopropane amino acid; X-ray diffraction; crystal structure; peptide conformation;
peptide structure; peptide turn; beta-turn; gamma-turn.
* Corresponding author. Fax: (34) 976761210; e-mail: anisjim@posta.unizar.es
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00514-8

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A. I. Jime´nez et al. / Tetrahedron Letters 44 (2003) 3147–3150
carried out with cyclopropane derivatives of other pro-
tein-ogenic amino acids11,12 (obviously, the unsubstituted
1-aminocyclopropanecarboxylic acid, Ac3c, analogue of
alanine, has no application in this field). We report here
the synthesis and structural study in the crystalline state
amine was then achieved via formation of the intermedi-
ate isobutyl chloroformate mixed anhydride.15
After Boc removal by acidolysis (Scheme 2), the resulting
amino terminus was coupled to Piv-
oyl-
L-Pro-OH (N-pival-
of the model dipeptides Piv-
Piv- -Pro-
-c3Val-NHiPr, incorporating the cyclo-
propane analogue of - and -valine, respectively.
L
-Pro-
L
-c3Val-NHiPr and
L
-proline) by the mixed anhydride method.15 The
L
D
diastereomeric dipeptides formed were separated by
column chromatography on silica gel and isolated in
optically pure form. Both 1† and 2‡ yielded single crystals,
which were subjected to X-ray diffraction analysis.§
L
D
Racemic c3Val was prepared starting from 4-isopropyli-
dene-2-phenyl-5(4H)-oxazolone13 (Scheme 1). Its exo-
cyclic double bond was reacted with diazomethane to
afford the corresponding spirocyclopropane derivative.
Methanolysis of the oxazolone ring followed by amide/
urethane exchange14 and saponification of the methyl
ester group provided the N-Boc-protected amino acid in
high overall yield. Condensation with isopropyl-
Taking
establish the absolute configuration of the c3Val residue
as R (or ) in 1 and S (or ) in 2.
L-proline as a reference, this allowed us to
D
L
Model dipeptides RCO-Xaa-Ybb-NHR% constitute the
minimum sequence able to adopt the b-turn structure.16
When position i+1 is occupied by
L-proline (Xaa=L-
Scheme 2. Synthesis of the diastereoisomeric dipeptides 1 and 2. Abbreviations: Boc, tert-butyloxycarbonyl; c3Val, 1-amino-2,2-
dimethylcyclopropanecarboxylic acid; Piv, pivaloyl (tert-butylcarbonyl); Pro, proline.
Figure 1. Stereoview of the crystal molecular structure of Piv-L-Pro-D
-c3Val-NHiPr (1) showing a bII-turn conformation (torsion
angles: Pro-ƒ,„=−61°, 143°; c3Val-ƒ,„=75°, 9°). The intramolecular hydrogen bond is represented as a dashed line. Most
hydrogen atoms are omitted for clarity.
† Single crystals of 1 were grown by slow evaporation from a dichloromethane/diisopropyl ether solution. Orthorhombic, P21212; a=15.9654(12),
b=20.9507(16), c=5.9824(5) A; Z=4; Dcalcd=1.167 g cm−3; 12218 reflections collected, 4584 unique (Rint=0.034); final R indices [3994 observed
,
reflections, I>2|(I)]: R1=0.038, wR2=0.071; final R indices (all data): R1=0.044, wR2=0.073.
‡ Single crystals of 2 were grown by slow evaporation from a dichloromethane/diisopropyl ether solution. Orthorhombic, P212121; a=9.0027(7),
b=12.3632(9), c=18.7698(14) A; Z=4; Dcalcd=1.118 g cm−3; 11567 reflections collected, 3693 unique (Rint=0.039); final R indices [2729
,
observed reflections, I>2s(I)]: R1=0.043, wR2=0.089; final R indices (all data): R1=0.059, wR2=0.095.
§ Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC-201547 (1) and CCDC-201548 (2). Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44(0)-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk].

A. I. Jime´nez et al. / Tetrahedron Letters 44 (2003) 3147–3150
3149
Figure 2. Stereoview of the crystal molecular structure of Piv-L-Pro-L
-c3Val-NHiPr (2) showing a bII-turn and a distorted g-turn
(torsion angles: Pro-ƒ,„=−57°, 141°; c3Val-ƒ,„=82°, −19°). The intramolecular hydrogen bonds are represented as dashed lines.
Most hydrogen atoms are omitted for clarity.
Pro) b-turns of types I or II are accommodated depend-
ing both on the nature of the i+2 residue and on the
environment.16–18 In the solid state, the bII-turn is
largely preferred, and type I b-turns are adopted almost
bond. It should be noted that
(2R,3R)c3diPhe share a substituent situated in an
L-c3Val and
L
position and cis with respect to the carbonyl function.
The close proximity of this substituent (a methyl and a
phenyl group, respectively) to the cyclopropane car-
bonyl oxygen is responsible for the deviation of the „
torsion angle to negative values, thus allowing the
formation of the hydrogen bond that stabilises the C7
conformation around the cyclopropane residue.
exclusively when Ybb is an
polar side chain.
L-residue with a highly
Figures 1 and 2 show the crystalline structures of 1 and
2, respectively. As expected, both molecules adopt a
b-folded conformation, with the anti disposition of the
proline CꢀO and CaꢁH bonds corresponding to the
type II b-turn.16 In both cases, the folded structure is
stabilised by an intramolecular hydrogen bond between
the Piv-C%O and NH(iPr) terminal groups (dipeptide 1:
The present results constitute an experimental probe of
the influence that the side chain orientation can exert
on the conformation accommodated by the peptide
backbone, and evidences the potential value of cyclo-
propane amino acids as g-turn inductors.
,
N···O distance=3.26 A, NꢁH···O angle=158°; dipep-
,
tide 2: N···O distance=3.07 A, NꢁH···O angle=153°),
that closes a ten-membered cycle.
Acknowledgements
In addition to the classical bII-turn arrangement, a
remarkable structural feature is observed in Figure 2.
The NH(iPr) group in dipeptide 2 is not only involved
in the i+3“i hydrogen bond typical of the b-turn
structure, but is also intramolecularly hydrogen-bonded
The authors thank the Centro de Excelencia Bruker-
ICMA for collection and preliminary treatment of the
X-ray diffraction data. Financial support from FEDER
and Ministerio de Ciencia y Tecnolog´ıa (projects
PPQ2001-1834 and PPQ2002-819; Ramo´n y Cajal con-
tract for A.I.J.) is gratefully acknowledged.
,
to the proline carbonyl (N···O distance=3.25 A,
NꢁH···O angle=126°). The latter interaction closes a
seven-membered ring, stabilising a g-turn16,19 around
the L-c3Val residue. This result is noteworthy since the
g-turn (also called C7 structure) is very infrequent in
crystalline small linear peptides.2,16,18,20,21 Moreover,
this finding confirms that the g-turn conformation
References
observed in the crystalline structure of Piv-L-Pro-
1. Venkatraman, J.; Shankaramma, S. C.; Balaram, P.
Chem. Rev. 2001, 101, 3131–3152.
2. Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C.
(2R,3R)c3diPhe-NHiPr9 (c3diPhe stands for 1-amino-
2,3-diphenylcyclopropanecarboxylic acid) was not a
consequence of the crystal packing forces but an intrin-
sic characteristic of this phenylalanine cyclopropane
analogue.
Biopolymers 2001, 60, 396–419.
3. Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.;
Moore, J. S. Chem. Rev. 2001, 101, 3893–4011.
4. Hruby, V. J. Acc. Chem. Res. 2001, 34, 389–397.
5. Hruby, V. J.; Li, G.; Haskell-Luevano, C.; Shenderovich,
M. Biopolymers 1997, 43, 219–266.
6. Chakrabarti, P.; Pal, D. Prog. Biophys. Mol. Biol. 2001,
76, 1–102.
7. Jime´nez, A. I.; Vanderesse, R.; Marraud, M.; Aubry, A.;
Cativiela, C. Tetrahedron Lett. 1997, 38, 7559–7562.
8. Jime´nez, A. I.; Cativiela, C.; Aubry, A.; Marraud, M. J.
Am. Chem. Soc. 1998, 120, 9452–9459.
The „ angle assumed by the
L
-c3Val residue in 2
(„=−19°) deviates significantly from the value expected
for an ideal g-turn („:−70°).16,19 However, it is almost
identical to that adopted by (2R,3R)c3diPhe in the
dipeptide sequence aforementioned („=−20°)9 and also
shows an excellent agreement with the small „ value
predicted theoretically for the C7 minimum energy
conformation of cyclopropane residues („:−30°)22–24
and that has been ascribed to hyperconjugative effects
between the three-membered ring and the adjacent CꢀO
9. Jime´nez, A. I.; Cativiela, C.; Marraud, M. Tetrahedron
Lett. 2000, 41, 5353–5356.

3150
A. I. Jime´nez et al. / Tetrahedron Letters 44 (2003) 3147–3150
10. Moye-Sherman, D.; Jin, S.; Ham, I.; Lim, D.; Scholtz, J.
M.; Burgess, K. J. Am. Chem. Soc. 1998, 120, 9435–9443.
11. Burgess, K.; Ho, K.-K.; Pal, B. J. Am. Chem. Soc. 1995,
117, 3808–3819.
1985, 107, 7640–7647.
18. Marraud, M.; Aubry, A. Biopolymers 1996, 40, 45–83.
19. Ne´methy, G.; Printz, M. P. Macromolecules 1972, 5,
755–758.
20. Benedetti, E. Biopolymers 1996, 40, 3–44.
21. Rizo, J.; Dhingra, M. M.; Gierasch, L. M. In Molecular
Conformation and Biological Interactions; Balaram, P.;
Ramaseshan, S., Eds.; Indian Academy of Sciences: Ban-
galore, 1991; pp. 469–496.
22. Alema´n, C.; Jime´nez, A. I.; Cativiela, C.; Pe´rez, J. J.;
Casanovas, J. J. Phys. Chem. B 2002, 106, 11849–11858.
23. Alema´n, C.; Casanovas, J.; Galembeck, S. E. J. Comput.
Aided Mol. Design 1998, 12, 259–273.
12. Lim, D.; Burgess, K. J. Am. Chem. Soc. 1997, 119,
9632–9640.
13. Ramage, G. R.; Simonsen, J. L. J. Chem. Soc. 1935,
532–535.
14. Burk, M. J.; Allen, J. G. J. Org. Chem. 1997, 62, 7054–
7057.
15. Bodanszky, M.; Bodanszky, A. The Practice of Peptide
Synthesis; Springer-Verlag: Berlin, 1994.
16. Rose, G. D.; Gierasch, L. M.; Smith, J. A. Adv. Protein
Chem. 1985, 37, 1–109.
24. Go´mez-Catala´n, J.; Alema´n, C.; Pe´rez, J. J. Theor. Chem.
Acc. 2000, 103, 380–389.
17. Aubry, A.; Cung, M. T.; Marraud, M. J. Am. Chem. Soc.