A γ-turn induced by a highly constrained cyclopropane analogue of phenylalanine (c3diPhe) in the solid state

Article abstract of DOI:10.1016/S0040-4039(00)00841-8

In order to evaluate the possible influence of the side chain orientation on the backbone conformation we have synthesized the model dipeptides (t)BuCO-L-Pro-c₃diPhe-NH(i)Pr, where c₃diPhe represents (2S, 3S)-and (2R,3R)-1-amino-2,3-diphenylcyclopropanecarboxylic acid, two cyclopropane analogues of phenylalanine. In the solid state, the (2S,3S)c₃diPhe-containing compound adopts a classical βII-turn disposition. In contrast, the dipeptide incorporating the (2R,3R) enantiomer exhibits an open βII-turn structure that lacks the usual i+3 to i hydrogen bond, together with a γ-turn centred at the c₃diPhe residue. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00841-8

Tetrahedron Letters 41 (2000) 5353±5356
A g-turn induced by a highly constrained cyclopropane
analogue of phenylalanine (c3diPhe) in the solid state
Ana I. Jimenez,a Carlos Cativielaa,* and Michel Marraudb
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 2 May 2000; accepted 23 May 2000
Abstract
In order to evaluate the possible in¯uence of the side chain orientation on the backbone conformation
t
we have synthesized the model dipeptides BuCO-l-Pro-c3diPhe-NHiPr, where c3diPhe represents (2S,3S)-
and (2R,3R)-1-amino-2,3-diphenylcyclopropanecarboxylic acid, two cyclopropane analogues of phenyl-
alanine. In the solid state, the (2S,3S)c3diPhe-containing compound adopts a classical bII-turn disposition.
In contrast, the dipeptide incorporating the (2R,3R) enantiomer exhibits an open bII-turn structure that
lacks the usual i+3 to i hydrogen bond, together with a g-turn centred at the c3diPhe residue. # 2000
Elsevier Science Ltd. All rights reserved.
Keywords: phenylalanine cyclopropane analogues; X-ray di€raction; crystal structures; b-turn; g-turn.
Structure±activity relationship studies in bioactive peptides pursue the understanding of the
biological processes at the molecular level with the aim to establish their bioactive conformation
and thereafter to develop peptide analogues with improved pharmacological properties. In this
context, the side chain moieties deserve special attention since they are directly involved in peptide±
receptor recognition phenomena and determine biological speci®city. Moreover, some side sub-
stituents may modulate to a certain extent the backbone conformation due either to steric factors
or to speci®c side chain±main chain interactions.1 5
We have been working on the study of the conformational tendencies of 1-amino-2-phenylcyclo-
alkanecarboxylic acids (cnPhe, Fig. 1) when incorporated into model dipeptides RCO-l-Pro-
cnPhe-NHR0.6 9 In particular, we have reported6,7 that the four stereoisomers of the phenylala-
nine cyclopropane analogue (c3Phe, Fig. 1) exhibit di€erent preferences to occupy the i+2 posi-
tion of a bI- or a bII-turn, depending both on the side chain orientation and on the environment.
In order to further investigate the conformational behaviour of phenylalanine cyclopropane sur-
rogates, we have now undertaken the study of tBuCO-l-Pro-c3diPhe-NHiPr where c3diPhe stands
* Corresponding author. Fax: (34) 976761210; e-mail: cativiela@posta.unizar.es
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00841-8

5354
for (2S,3S)- and (2R,3R)-1-amino-2,3-diphenylcyclopropanecarboxylic acid (Fig. 1). Recently,
both enantiomers of this amino acid have been shown to impart very di€erent backbone con-
formations to a midsize peptide.10
Figure 1. Structure of cyclic constrained analogues of phenylalanine
Racemic c3diPhe was prepared by 1,3-dipolar cycloaddition of phenyldiazomethane to (Z)-2-
phenyl-4-benzylidene-5(4H)-oxazolone.11 Oxazolone ring opening with methanol and further
transformations of the benzamido and methyl ester groups provided 1 in good yield. Racemic 1
was coupled to Boc-Pro-OH (N-tert-butyloxycarbonyl-l-proline) by the mixed anhydride method
using isobutyl chloroformate as the coupling agent12 (Fig. 2). Diastereomeric dipeptides 2a and 2b
were separated by column chromatography on silica gel. The Boc group was removed with tri-
¯uoroacetic acid and the proline amino group was acylated with pivaloyl chloride (pivaloyl=tert-
butylcarbonyl, Piv) to a€ord enantiomerically pure 3a and 3b.
Figure 2. (i) BuOCOCl/N-methylmorpholine/THF/^15ꢀC; (ii) column chromatography CH2Cl2:AcOEt, 9:1; (iii)
i
CF3COOH/CH2Cl2/rt; (iv) BuCOCl/N-methylmorpholine/CHCl3/0ꢀC
t

5355
Both 3a and 3b yielded single crystals, which were subjected to X-ray di€raction analysis.y In
this way, and taking l-proline as a reference, the absolute con®guration of the c3diPhe residue
was established as (2S,3S) in 3a and (2R,3R) in 3b (Fig. 3).
Figure 3. Crystal molecular structures of 3a (bII-turn) and 3b (open bII-turn and g-turn). Most hydrogen atoms are
omitted for clarity. The intramolecular hydrogen bonds are represented as dashed lines
In the crystal state, dipeptide 3a assumes a b-folded conformation13 stabilized by an intra-
molecular NH(iPr) to Piv-C0O hydrogen bond, closing a ten-membered pseudocycle (Fig. 3). The
14
...
N O distance of 3.37 A is at the upper limit for hydrogen bonding. The orientation of the
middle amide group, with the proline CˆO and Ca±H bonds in an anti disposition,{ corresponds
to the type II b-turn.13 This result was not unexpected since, in the solid state, the bII form has
proven to be the preferred conformation for most l-Pro-Xaa sequences.5,7,13,15
In contrast, the crystal structure of the (2R,3R)c3diPhe-containing dipeptide exhibits some
striking features (Fig. 3). Thus, 3b adopts a distorted g-turn13,16 around the c3diPhe residue, with
0
i
...
Pro-C O and NH( Pr) intramolecularly hydrogen-bonded (N O distance=3.25 A). The g-turn or
the C7 structure is less widely distributed than the b-turn disposition and has only rarely been
observed in crystallized small linear peptides.13,15,17 19 The molecule is globally b-folded, the b-turn
being of the II type,x but the i+3 to i hydrogen bond generally observed in b-folded structures is
absent. The distance between the pivaloyl O and the isopropylamide N of 3.74 A is indeed inap-
propriate to allow hydrogen bonding, as is the unfavourable alignment of the CˆO and N±H
bonds. In spite of this, the distance between the extreme Cꢀi and Ciꢀ‡3 carbons (6.20 A) is inferior
to 7 A, thus satisfying the requirement generally accepted to be considered as a b-turn.20,21 It may
be assumed that the global b-folded structure observed for 3b is due to the proline residue, which
is known to adopt with high preference the disposition found in position i+1 of a b-turn.22
y
Single crystals of 3a were grown by slow evaporation of a methanol solution: monoclinic, P21; a=5.995(3) A,
b=10.763(4) A, c=21.832(10) A, ꢁ=92.13(3)ꢀ; Z=2; dcalc.=1.12 g/cm^3; 5162 unique re¯ections; R=0.0621. Single
crystals of 3b were grown by slow evaporation of a dichloromethane/ethyl acetate solution: orthorhombic, P212121;
a=12.085(6) A, b=13.827(7) A, c=17.024(10) A; Z=4; dcalc.=1.11 g/cm^3; 5293 unique re¯ections; R=0.0663.
Torsion angles: Pro-ꢂ; =^59ꢀ, 140ꢀ; c3diPhe-ꢂ; =67ꢀ, 25ꢀ.
{
Torsion angles: Pro-ꢂ; =^60ꢀ, 154ꢀ; c3diPhe-ꢂ; =86ꢀ, ^20ꢀ.
x

5356
These results suggest that (2R,3R)c3diPhe may have a strong tendency to induce g-turn con-
formations. Studies in solution are in progress in order to discern whether the behaviour observed
in the solid state is an intrinsic characteristic of this residue or rather a consequence of the crystal
packing.
Acknowledgements
The authors thank Dr. J. A. Galvez for collecting the X-ray data. Financial support from the
Ministerio de Educacion y Cultura (project PB97-0998) and Diputacion General de Aragon
(project P22/98) is gratefully acknowledged.
References
1. Baker, E. N.; Hubbard, R. E. Prog. Biophys. Mol. Biol. 1984, 44, 97±180.
2. Wolf, W. M.; Stasiak, M.; Leplawy, M. T.; Bianco, A.; Formaggio, F.; Crisma, M.; Toniolo, C. J. Am. Chem. Soc.
1998, 120, 11558±11566.
3. Abbadi, A.; Mchar®, M.; Aubry, A.; Premilat, S.; Boussard, G.; Marraud, M. J. Am. Chem. Soc. 1991, 113, 2729±
2735.
4. Aubry, A.; Vlassi, M.; Marraud, M. Int. J. Pept. Protein Res. 1986, 28, 637±648.
5. Aubry, A.; Cung, M. T.; Marraud, M. J. Am. Chem. Soc. 1985, 107, 7640±7647.
6. Jimenez, A. I.; Vanderesse, R.; Marraud, M.; Aubry, A.; Cativiela, C. Tetrahedron Lett. 1997, 38, 7559±7562.
7. Jimenez, A. I.; Cativiela, C.; Aubry, A.; Marraud, M. J. Am. Chem. Soc. 1998, 120, 9452±9459.
8. Jimenez, A. I.; Cativiela, C.; Parõs, M.; Peregrina, J. M.; Avenoza, A.; Aubry, A.; Marraud, M. Tetrahedron Lett.
1998, 39, 7841±7844.
9. Jimenez, A. I.; Cativiela, C.; Gomez-Catalan, J.; Perez, J.-J.; Aubry, A.; Parõs, M.; Marraud, M. J. Am. Chem.
Soc. 2000, 122, in press.
10. Moye-Sherman, D.; Jin, S.; Ham, I.; Lim, D.; Scholtz, J. M.; Burgess, K. J. Am. Chem. Soc. 1998, 120, 9435±9443.
11. Jimenez, A. I.; Lopez, P.; Cativiela, C. manuscript in preparation.
12. Bodanszky, M.; Bodanszky, A. The Practice of Peptide Synthesis; Springer-Verlag: Berlin, 1994.
13. Rose, G. D.; Gierasch, L. M.; Smith, J. A. Adv. Protein Chem. 1985, 37, 1±109.
14. Gorbitz, C. H. Acta Cryst., Sect. B 1989, 45, 390±395.
15. Marraud, M.; Aubry, A. Biopolymers 1996, 40, 45±83.
16. Nemethy, G.; Printz, M. P. Macromolecules 1972, 5, 755±758.
17. Benedetti, E. Biopolymers 1996, 40, 3±44.
18. Rizo, J.; Dhingra, M. M.; Gierasch, L. M. In Molecular Conformation and Biological Interactions; Balaram, P.;
Ramaseshan, S., Eds.; Indian Academy of Sciences: Bangalore, 1991; pp. 469±496.
19. Perczel, A.; Hollosi, M. In Circular Dichroism and the Conformational Analysis of Biomolecules; Fasman, G. D.,
Ed.; Plenum Press: New York, 1996; pp. 285±380.
20. Chou, P. Y.; Fasman, G. D. J. Mol. Biol. 1977, 115, 135±175.
21. Hutchinson, E. G.; Thornton, J. M. Protein Sci. 1994, 3, 2207±2216.
22. MacArthur, M. W.; Thornton, J. M. J. Mol. Biol. 1991, 218, 397±412.