other half with 20% piperidine/CH2Cl2, the resulting dipep-
tides, 7b and 7c, were coupled using HBTU in the presence
of DIEA, giving tetrapeptide 8a in 81% yield. Double
deprotection of 8a with piperidine followed by TFA, and
cyclization of the resulting peptide 8b, afforded the cyclic
peptide 4b in 51% yield from 8a.
Definitive evidence of the existence of dimers of 4c in
the solid state was obtained by X-ray crystallography of
The 1H NMR spectra of 4b in polar and nonpolar solvents
(CCl4, CDCl3, MeOH, DMSO) are well defined, reflect a
high degree of symmetry, and in CDCl3 show a JNH,HR
coupling constant of 10.1 Hz that is typical of the all-trans
backbone conformation required for flatness of the peptide
ring. However, neither these spectra nor others run in 9:1
CCl4/CDCl3 show any signs of intermolecular hydrogen
bonding, the N-H resonance of γ-Ach remaining at the same
position (δ ) 5.86 ppm) [regardless of concentration]. In
the FT-IR spectrum,10 amide I and amide IIII bands at 1621
and 1522 cm-1, respectively, suggest the expected flatness
of the peptide ring, but the position of the amide A band
[ν(NH)], 3366 cm-1, is that of an amide proton that is not
involved in any hydrogen bond.
To study (R-R) bonding, we prepared cyclo[(D-Phe-L-
MeN-γ-Ach)2-] (4c) using the same approach as that for 4b.
Its 1H NMR spectra in polar and nonpolar solvents once again
indicate a flat, all-trans configuration, but in this case, unlike
that of 4b, dimerization through intermolecular hydrogen
bonds in nonpolar solvents is reflected by the fact that the
Phe N-H signal shifts increasingly downfield as concentra-
tion is increased, from 5.9 ppm at a concentration of 1 mM
to 7.7 ppm at 200 mM. This concentration dependence
allowed the dimerization constant, Ka, in chloroform at 298
K to be determined as 15 M-1,11 and a series of experiments
carried out at different temperatures in the range of 233-
313 K allowed the corresponding thermodynamic parameters
∆H°298 ) -30.8 kJ mol-1 and ∆S°298 ) -82.0 J K-1 mol-1
to be extracted from van’t Hoff plots. The negative enthalpy
and entropy, together with the observed fall in Ka with
increasing solvent polarity, support the idea that the formation
of dimers of 4c is an enthalpy-driven, entropy-opposed12
process brought about principally by intermolecular hydrogen
bonding. Furthermore, the â-sheetlike nature of this bonding10
is supported by FT-IR spectra recorded in chloroform, which
not only show amide I and amide IIII bands (at 1627 and
1522 cm-1, respectively), but also amide A bands near 3300
cm-1, while a band at 3411 cm-1 that mainly appears in the
most dilute solutions may be due to the N-H vibration of
the monomer.
Figure 2. Compound 4c spontaneously forms dimers in nonpolar
solvents, compound 4b does not.
colorless prismatic single crystals formed by vapor-phase
equilibration of a tetrachloroethane solution of 4c with
hexane. These crystals consist of dimers in which the two
CPs are stacked with antiparallel orientation and linked by
a â-sheetlike set of four hydrogen bonds N‚‚‚O distances of
2.91-3.08 Å (Figure 3). The fact that all the hydrogen
Figure 3. Crystal structure of dimeric 4c: (a) top view; (b) side
view.
bonding amide groups are slightly tilted toward the center
of the dimer (Figure 3b) may explain the low enthalpic
contribution to the dimerization process.
(10) (a) P. I. Haris, D. Chapman, Biopolymers (Peptide Sci.) 1995, 37,
251-263. (b) Krimm, S.; Bandekar, J. In AdVances in Protein Chemistry;
Anfinsen, C. B., Edsall, J. T., Richards, F. M., Eds.; Academic Press:
Orlando, FL, 1986; pp 181-364. (c) Bandekar, J. Biochim. Biophys. Acta
1992, 1120, 123-143. (d) Kubelka, J.; Keiderling, T. A. J. Am. Chem.
Soc. 2001, 123, 12048-12058.
(11) The chemical shift of the NH group was measured at concentrations
ranging from 5.9 ×10-3 to 5.0 × 10-2 M, and the association constant was
estimated using a nonlinear regression program to fit the δNH versus
concentration data with the equation δobs ) δdimer + (δmonomer - δdimer)[(1
+ 8KaC)1/2 - 1]/(4KaC), where δobs is the shift observed at concentration,
C, and the shifts of the monomer and dimer (δmonomer and δdimer) are adjusted
together with the association constant Ka.
Stable (R-R)-bonded dimers were also formed when cis-
3-aminocyclopentanecarboxylic acid (γ-Acp) was used as
the γ-Aca of the R,γ-CP, instead of γ-Ach.4d The cyclo[(L-
Leu-D-MeN-γ-Acp)2-] (5c) was prepared using the same
approach as for 4b and 4c, but starting from D-Boc-MeN-γ-
Acp-OH (Scheme 2),4d and its flat, all-trans conformation
1
was indicated by the characteristics of its H NMR spectra
in polar and nonpolar solvents (JNH,Hγ ) 9.0-9.5 Hz). As
in the case of 4c, the formation of intermolecular hydrogen
bonds was indicated by the progressive downfield shift of
(12) (a) Searle, M. S.; Westwell, M. S.; Williams, D. H. J. Chem. Soc.,
Perkin Trans. 2 1995, 141-151. (b) Dunitz, J. D. Chem. Biol. 1995, 2,
709-712.
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