J. Am. Chem. Soc. 2000, 122, 4821-4822
4821
Scheme 1
12-Helix Formation in Aqueous Solution with Short
â-Peptides Containing Pyrrolidine-Based Residues
Xifang Wang, Juan F. Espinosa, and Samuel H. Gellman*
Department of Chemistry, UniVersity of Wisconsin
Madison, Wisconsin 53706
ReceiVed January 7, 2000
Oligomers that display well-defined and predictable conforma-
tions (“foldamers”) have become subjects of widespread interest
in recent years.1,2 The foldamer strategy could be useful for
creating specific molecular shapes or specific arrangements of
functional groups, as required for many design goals. The
traditional approach to controlling shape and surface functionality
involves small, rigid skeletons, which are usually nonperiodic.
In contrast, oligomers display high backbone periodicity, with
functional group diversity arising from side chains. Use of
oligomeric scaffolds could prove advantageous relative to tradi-
tional scaffolds because there is no intrinsic size limit for a
periodic architecture, and because the combination of backbone
periodicity and side chain variation is conducive to optimization
of molecular properties via cycles of combinatorial variation and
selection. Most known oligomers, however, are very conforma-
tionally mobile, at least when short (<10 residues), and this
flexibility is disadvantageous in the context of many design goals.
hydrogen bonds), in organic solvents.9 Here we show that use of
pyrrolidine-based â-amino acid residue leads to formation of short
12-helices in aqueous solution.
To probe for 12-helical folding in water, we required a
hydrophilic â-amino acid residue with the proper conformational
constraint, as a complement to the hydrophobic (R,R)-trans-2-
aminocyclopentanecarboxylic acid (ACPC) residue used previ-
ously.9 Scheme 1 outlines the synthesis of enantiomerically pure
(R,S)-trans-3-aminopyrrolidine-4-carboxylic acid (APC) in a
protected form, from a known â-ketoester.10 The key step is
Michael addition of (R)-R-methylbenzylamine to the R,â-unsatur-
ated ester. The desired (R,S,R) isomer was readily isolated via
chromatography, and the absolute stereochemistry was confirmed
by X-ray crystallographic analysis of the hydrochloride salt. The
Fmoc/Boc-protected APC derivative was then used for solid-phase
synthesis of oligomers 1-3 on Rink amide resin with PyBOP as
We describe a set of â-amino acid oligomers (“â-peptides”1,3
)
that adopt a specific helical conformation in aqueous solution with
as few as four residues. Conformational stability in aqueous
solution is important with regard to biological applications.
Among conventional peptides (R-amino acid residues) confor-
mational stability is usually lower in water than in other common
solvents.4 Relatively few unnatural foldamers have been examined
in aqueous solution,5 and nearly all conformational analyses in
water have involved low resolution methods. Only one nonas-
sociated unnatural foldamer has been subjected to high-resolution
structural analysis in water, to our knowledge, a hexa-â-peptide
that adopts a 14-helical conformation in aqueous solution (i to i
- 2 CdO- -H-N hydrogen bonds).6-8 In this case, the foldamer’s
shape is enforced by cyclohexyl constraints at the residue level.
We have previously shown that cyclopentyl constraints enforce
a different â-peptide shape, the 12-helix (i to i - 3 CdO- -H-N
* To whom correspondence should be addressed. E-mail: gellman@
chem.wisc.edu.
(1) Reviews: Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. Kirshenbaum,
K.; Zuckermann, R. N.; Dill, K. A. Curr. Opin. Struct. Biol. 1999, 9, 530.
Stigers, K. D.; Soth, M. J.; Nowick, J. S. Curr. Opin. Chem. Biol. 1999, 3,
714. Barron, A. E.; Zuckermann, R. N. Curr. Opin. Chem. Biol. 1999, 3,
681.
(2) Recent examples: (a) Yang, D.; Qu, J.; Li, B.; Ng, F.; Wang, X.;
Cheung, K.; Wang, D.; Wu, Y. J. Am. Chem. Soc. 1999, 121, 589. (b) Nguyen,
J. Q.; Iverson, B. L. J. Am. Chem. Soc. 1999, 121, 2639. (c) Gin, M. S.;
Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 2643.
(d) Claridge, T. D. W.; Long, D. D.; Hungerford, N. L.; Aplin, R. T.; Smith,
M. D.; Marquess, D. G.; Fleet, G. W. J. Tetrahedron Lett. 1999, 40, 2199. (e)
Hanessian, S.; Luo, X.; Schaum, R. Tetrahedron Lett. 1999, 40, 4925. (f)
Beier, M.; Reck, F.; Wagner, T.; Krishnamurthy, R.; Eschenmoser, A. Science
1999, 283, 699.
(3) Reviews: Seebach, D.; Matthews, J. L. J. Chem. Soc., Chem. Commun.
1997, 2015-2022. DeGrado, W. F.; Schneider, J. P.; Hamuro, Y. J. Pept.
Res. 1999, 54, 206. Gademann, K.; Hintermann, T.; Schreiber, J. V. Curr.
Med. Chem. 1999, 6, 905.
the coupling agent. After reverse-phase HPLC purification, the
oligomers were isolated as trifluoroacetate salts (one TFA
counterion per APC residue, as determined via 19F NMR).
Alternation of hydrophobic ACPC and hydrophilic APC residues
should distribute the cationic groups around the circumference
of the 12-helix, which has ∼2.5 residues per turn,9 thereby
minimizing the prospect of aggregation.
Initial studies involved conformational analysis of hexamer 2
in CD3OH (10 mM). â-Peptide conformational propensity is very
sensitive to residue substitution pattern,3 and it was therefore
(7) CD data indicate that some â-peptides composed of acyclic residues
display modest 14-helical populations in aqueous solution: (a) Abele, S.;
Guichard, G.; Seebach, D. HelV. Chim. Acta 1998, 81, 2141. (b) Gung, B.
W.; Zou, D.; Stalcup, A. M.; Cottrell, C. E. J. Org. Chem. 1999, 64, 2176.
(c) Hamuro, Y.; Schneider, J. P.; DeGrado, W. F. J. Am. Chem. Soc. 1999,
121, 12200.
(4) For leading references, see: Goodman, M.; Verdini, A. S.; Toniolo,
C.; Phillips, W. D.; Bovey, F. A. Proc. Natl. Acad. Sci. U.S.A. 1969, 64, 444.
Walgers, R.; Lee, T. C.; Cammers-Goodwin, A. J. Am. Chem. Soc. 1998,
120, 5073.
(8) High-resolution structural data are available for unnatural nucleic acid
analogues in duplex form: (a) Brown, S. C.; Thomson, S. A.; Veal, J. M.;
Davis, D. G. Science 1994, 265, 777. (b) Schlo¨nvogt, I.; Pitsch, S.; Lesueur,
C.; Eschenmoser, A.; Jaun, B.; Wolf, R. M. HelV. Chim. Acta 1996, 79, 2316.
(c) Rasmussen, H.; Kastrup, J. S.; Nielsen, J. N.; Nielsen, J. M.; Nielsen, P.
E. Nat. Struct. Biol. 1997, 4, 98.
(9) (a) Appella, D. H.; Christianson, L. A.; Klein, D. A.; Powell, D. R.;
Huang, X.; Barchi, J. J.; Gellman, S. H. Nature 1997, 387, 381. (b) Appella,
D. H.; Christianson, L. A.; Klein, D. A.; Richards, M. R.; Powell, D. R.;
Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 7574.
(10) Blake, J.; Willson, C. D.; Rapoport, H. J. Am. Chem. Soc. 1964, 86,
5293.
(5) Lokey, R. S.; Iverson, B. L. Nature 1995, 375, 303. Bolli, M.; Micura,
R.; Eschenmoser, A. Chem. Biol. 1997, 4, 309. Szabo, L.; Smith, B. L.;
McReynolds, K. D.; Parrill, A. L.; Morris, E. R.; Gervay, J. J. Org. Chem.
1998, 63, 1074. Kirshenbaum, K.; Barron, A. E.; Goldsmith, R. A.; Armand,
P.; Bradley, E. K.; Truong, K. T.; Dill, K. A.; Cohen, F. E.; Zuckermann, R.
N. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4303.
(6) Appella, D. H.; Barchi, J. J.; Durell, S.; Gellman, S. H. J. Am. Chem.
Soc. 1999, 121, 2309.
10.1021/ja000093k CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/02/2000