Conformational restriction through Cai ? Cai cyclization: 1-aminocycloheptane-1-carboxylic acid (Ac7c)

Article abstract of DOI:10.1039/a701473b

A complete series of N- and C-blocked, monodispersed homo-oligopeptides to the pentamer level from 1-aminocycloheptane-1-carboxylic acid (Ac₇c), an α-amino acid conformationally restricted through C^a_i ? C^a_i cyclization, and three tripeptides with Ac₇c combined with Ala, Leu, and Val residues have been synthesized by solution methods and fully characterized. The solution conformational preferences have been determined by IR absorption and ¹H NMR spectroscopy. In addition, the molecular structures of three derivatives (Ac₇c hydantoin, ClCH₂CO-Ac₇c-OH, and Z-Ac₇c-OH; Z = benzyloxycarbonyl) and four peptides [the dipeptide Z-Ac₇c-L-Ala-OMe, the tripeptides Z-Ac₇c-(L-Ala)₂-OMe and Z-(Ac₇c)₃-OBu^t, the tetrapeptide Z-(Ac₇c)₄-OBu^t, and the pentapeptide Z-(Ac₇c)₅-OBu^t] have been assessed in the crystal state by X-ray diffraction. The results obtained confirm the tentative conclusions put forward on the basis of our previous preliminary study, namely that β-bends and 3₁₀-helices are preferentially adopted by Ac₇c-based peptides. A comparison with the structural tendencies extracted from published work on peptides from α-aminoisobutyric acid, the prototype of C^α,α-dialkylated glycines, and the other extensively investigated members of the class of 1-aminocycloalkane-1-carboxylic acids (Ac_nc, with n = 3-6, 8, 9) is made and the implications for the use of the Ac₇c residue in conformationally constrained analogues of bioactive peptides are briefly discussed.

Full text of DOI:10.1039/a701473b

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Conformational restriction through Cá ←→ Cá cyclization:
i
i
1-aminocycloheptane-1-carboxylic acid (Ac7c)
Ettore Benedetti,a Benedetto Di Blasio,a,b Rosa Iacovino,a,b Valeria Menchise,a
Michele Saviano,a Carlo Pedone,a Gian Maria Bonora,c Alessandro Ettorre,d
Laura Graci,e Fernando Formaggio,e Marco Crisma,e Giovanni Vallee and
Claudio Tonioloe
a
Biocrystallography Research Centre, CNR, Department of Chemistry, University of Naples
Federico II, Napoli, Italy
b Faculty of Environmental Sciences, 2nd University of Naples, 81100 Caserta, Italy
c Pharmaco-, Chemico-, Technological Department, University of Cagliari, 09124 Cagliari,
Italy
d Department of Chemistry, Menarini Ricerche Sud, 00040 Pomezia, Italy
e Biopolymer Research Centre, CNR, Department of Organic Chemistry, University of Padova,
35131 Padova, Italy
A complete series of N- and C-blocked, monodispersed homo-oligopeptides to the pentamer level from 1-
aminocycloheptane-1-carboxylic acid (Ac7c), an á-amino acid conformationally restricted through
Ciá ←→ Ciá cyclization, and three tripeptides with Ac7c combined with Ala, Leu, and Val residues have
been synthesized by solution methods and fully characterized. The solution conformational preferences
1
have been determined by IR absorption and H N M R spectroscopy. In addition, the molecular structures
of three derivatives (Ac7c hydantoin, ClCH 2CO-Ac7c-OH , and Z-Ac7c-OH ; Z = benzyloxycarbonyl) and
four peptides [the dipeptide Z-Ac7c-L-Ala-OM e, the tripeptides Z-Ac7c-(L-Ala)2-OM e and Z-(Ac7c)3-OBut,
the tetrapeptide Z-(Ac7c)4-OBut, and the pentapeptide Z-(Ac7c)5-OBut] have been assessed in the crystal
state by X-ray diffraction. The results obtained confirm the tentative conclusions put forward on the basis
of our previous preliminary study, namely that â-bends and 310-helices are preferentially adopted by Ac7c-
based peptides. A comparison with the structural tendencies extracted from published work on peptides
from á-aminoisobutyric acid, the prototype of Cá,á-dialkylated glycines, and the other extensively
investigated members of the class of 1-aminocycloalkane-1-carboxylic acids (Acnc, with n = 3–6, 8, 9) is
made and the implications for the use of the Ac7c residue in conformationally constrained analogues of
bioactive peptides are briefly discussed.
characterization and an extensive solution (IR absorption and
Introduction
1H NMR spectroscopy) conformational investigation of the
homo-oligomeric series Z-(Ac7c)n-OBut (Ac7c, 1-aminocyclo-
heptane-1-carboxylic acid; Z, benzyloxycarbonyl; OBut, tert-
butoxy) (n = 1–5) and three Ac7c-containing tripeptides. The
X-ray diffraction structures of three derivatives [Ac7c hydan-
toin, ClAc-Ac7c-OH (ClAc, monochloracetyl), and Z-Ac7c-
OH] and four peptides [the dipeptide Z-Ac7c--Ala-OMe
(OMe, methoxy), the tripeptides Z-Ac7c-(-Ala)2-OMe and
Z-(Ac7c)3-OBut, the tetrapeptide Z-(Ac7c)4-OBut, and the
pentapeptide Z-(Ac7c)5-OBut] are also reported.
In recent years conformationally constrained analogues of
bioactive peptides have acquired increasing popularity
among medicinal chemists in an effort to firmly establish 3D
structure–bioactivity relationships and to develop new
pharmaceutical agents with prolonged action and/or more
selective properties.1–5 In particular, conformational restric-
tion through Ciα ←→ Ciα cyclization generates the family of 1-
aminocycloalkane-1-carboxylic acid (Acnc) residues.6 Theor-
etical and experimental studies of the preferred conformations
of peptides characterized by the Acnc (n = 3–6, 8, 9) residues
have been the subject of recent review articles and papers.7–11 In
a close parallelism to the structural behaviour of Aib (α-
aminoisobutyric acid or Cα,α-dimethylglycine),7–9,12,13 the proto-
type of Cα,α-dialkylated glycines, it was shown that regular or
slightly distorted β-bend forms14–16 or 310-helical structures17
are adopted as a function of main-chain length and side-chain
size. The cyclopropyl-containing amino acid is the only residue
of this family known to strongly prefer the ‘bridge’ region
(φ = ±90Њ, ψ = 0Њ)18 of the conformational space. This anomal-
ous conformational propensity appears to be associated with a
distorted geometry, more specifically with the observed widen-
ing of the exocyclic τ(N᎐Cα᎐CЈ) bond angle to 116–117Њ.
Only scant information is available in the literature on bio-
activity and conformational preferences of Ac7c, and its deriv-
atives and peptides. The tripeptide HCO--Met-Ac7c--Phe-
OMe shows excellent activity in the release of histamine and
lysosomal enzymes.19,20 The free amino acid itself is bitter 21 as it
is the aspartame analogue H--Asp-Ac7c-OMe.22 An Ac7c-
based dihydroimidazol-4-one is a potent nonpeptide AT1 angi-
otensin II receptor antagonist.23 The crystal structures of H-
Ac7c-OH hydrobromide monohydrate24,25 and the symmetrical
anhydride (Z-Ac7c)2O 26 have been described. The synthesis and
β-bend forming tendency of the dipeptide amides and tripep-
tide esters Z-Ile-Ac7c-NHAr,27,28 Boc--Ala-Ac7c--Ala-OMe
(Boc, tert-butoxycarbonyl),29 Boc-Aib-Ac7c-NHMe (NHMe,
methylamino),30 Boc--Pro-Ac7c--Ala-OMe,30 and HCO--
Met-Ac7c--Phe-OMe20 have been published. Preliminary
accounts of a limited part of this work have been reported.31,32
With the aim of further contributing to our knowledge of the
geometrical and structural preferences of the medium-ring res-
idues of this family, in this work we describe the synthesis,
J. Chem. Soc., Perkin Trans. 2, 1997
2023

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Table 1 Physical properties and analytical data for the Ac7c derivatives and peptides
TLC d
Yield
(%)
Mp a
/ЊC
Recrystallisation
solvent b
Compound
[α]D20 c
R(I)F R(II)F R(III)F
ν/cmϪ1 e
(a) Derivatives
Ac7c hydantoin
Z-Ac7c-OH
5(4H)-oxazolone from
Z-Ac7c-OH
38
40
97
215–216
117–118
Oil
MeOH/DE
AcOEt/LP
AcOEt/LP
—
—
—
0.60
0.65
0.95
—
0.95
—
—
0.40
0.85
3445, 3286, 1768, 1708
3304, 1704, 1526
1825, 1683
ClAc-Ac7c-OH
Z-Ac7c-OtBu
54
61
167–170
68–70
AcOEt/LP
AcOEt/LP
—
—
—
0.95
0.90
0.95
—
0.85
3392, 1734, 1632, 1532
3373, 1717, 1520
(b) Peptides
Z-(Ac7c)2-OBut
62
67
76
98
66
60
62
72
62
154–156
158–159
223–224
251–252
100–101
176–178
99–101
AcOEt/LP
AcOEt/LP
AcOEt/LP
Hot toluene
AcOEt/LP
Hot AcOEt
AcOEt/DE
AcOEt/LP
AcOEt/LP
—
—
—
—
Ϫ23.8
Ϫ47.7
Ϫ21.4
Ϫ14.4
Ϫ37.7
0.95
0.95
0.95
0.90
0.95
0.85
0.90
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.95
0.85
0.60
0.50
0.45
0.55
0.45
0.45
0.70
0.45
3401, 3291, 1719, 1650, 1529
3422, 3358, 1701, 1649, 1522
3429, 3352, 1705, 1675, 1527
3430, 3341, 1699, 1666, 1524
3321, 1733, 1688, 1651, 1537, 1519
3386, 3294, 1742, 1703, 1678, 1638
3427, 3348, 1759, 1681, 1650, 1584
3323, 1744, 1691, 1524
Z-(Ac7c)3-OBut
Z-(Ac7c)4-OBut
Z-(Ac7c)5-OBut
Z-Ac7c--Ala-OMe
Z-L-Ala-Ac7c--Ala-OMe
Z-(Ac7c)2--Ala-OMe
Z-Ac7c--Val-OMe
Boc--Leu-Ac7c--Val-OMe
97–98
150–151
3308, 1728, 1702, 1664, 1518
a
b
Determined on a Leitz Model Laborlux 12 apparatus (Wetzlar, Germany). DE, diethyl ether; AcOEt, ethyl acetate; LP, light petroleum.
c Determined on a Perkin-Elmer Model 241 polarimeter equipped with a Haake Model L thermostat (Karlsruhe, Germany); c = 0.5 (MeOH). d Silica
gel plates 60F-254 (Merck) using the following solvent systems: (I) chloroform–ethanol 9:1; (II) butan-1-ol–acetic acid–water 6:2:2; (III) toluene–
ethanol 7:1. The compounds were revealed either with the aid of a UV lamp or with the hypochloride–starch iodide chromatic reaction. A single
spot was observed in each case. e Determined on a Perkin-Elmer Model 580 B spectrophotometer equipped with a Perkin-Elmer Model 3600 IR data
station and a Model 660 printer. For the IR measurements the KBr disk technique was used.
Research Centre, at the University of Naples Federico II.
During all data collection, three reflections were measured
Experimental
Materials
every 120 min in order to check the stability of the crystals and
the electronics. The observed intensity decreases were within
Relevant physical properties and analytical data for the newly
synthesized Ac7c derivatives and peptides are listed in Table 1.
3%. The intensities were corrected for Lorentz and polarization
In addition, the results of the amino acid analyses [C. Erba
factors, but no absorption correction was applied. Unit cell
determinations were carried out for all crystals by least-squares
refinement of the setting angles of at least 25 high angle reflec-
tions accurately centred. Crystal data are listed in Tables 2 and
(Rodano, Milan, Italy) Model 3A 30 amino acid analyser] are
as follows: Z-Ac7c--Ala-OMe (Ala 0.98, Ac7c 1.01); Z--Ala-
Ac7c--Ala-OMe (Ala 1.95, Ac7c 1.05); Z-Ac7c-(-Ala)2-OMe
(Ala 1.98, Ac7c 1.02); Z-Ac7c--Val-OMe (Val 0.99, Ac7c 1.02);
3.
Boc--Leu-Ac7c--Val-OMe (Val 0.98, Leu 1.00, Ac7c 1.02).
The structures of the three Ac7c derivatives [Ac7c hydantoin,
ClAc-Ac7c-OH, and Z-Ac7c-OH] were solved by direct methods
and refined by the full-matrix least-squares procedure on F 2 (all
IR absorption spectra
Infrared absorption spectra were recorded with a Perkin-Elmer
(Norwalk, CT, USA) Model 1720X FTIR spectrophotometer,
nitrogen flushed, at 2 cmϪ1 nominal resolution, averaging 16
scans for 10 and 1.0 × 10Ϫ3 mol dmϪ3 sample concentrations or
64 scans for 0.1 × 10Ϫ3 mol dmϪ3 sample concentrations. Sol-
vent (baseline) spectra were recorded under the same condi-
tions. Cells with path lengths of 0.1, 1.0 and 10 mm (with CaF2
windows) were used. Spectrograde deuteriochloroform (99.8%
2H) was purchased from Merck (Darmstadt, Germany).
data) with anisotropic thermal factors for all non-hydrogen
atoms. Hydrogen atoms of Ac7c hydantoin were calculated, and
during the refinement were allowed to ride on the carrying
atoms, with Uiso set equal to 1.2 times the Ueqof the attached
atom. One carbon atom of the cycloheptane ring of ClAc-Ac7c-
OH (Cγ21) is disordered over two sites (A and B), which refined
with population parameters of 0.50. Hydrogen atoms of ClAc-
Ac7c-OH were calculated and treated as described above for
those of Ac7c hydantoin. The hydrogen atom of the two
independent molecules of Z-Ac7c-OH belonging to the carboxy
group was located on a ∆F map, while the positions of all other
hydrogen atoms were calculated. During the refinement all
hydrogen atoms were allowed to ride on their carrying atoms,
with Uiso set equal to 1.2 (or to 1.5 for the carboxylic acid
hydrogen atom) multiplied by the Ueq of the attached atom.
The structures of the other Ac7c compounds [Z-Ac7c--Ala-
OMe (with two crystallographically independent molecules, A
and B, in the asymmetric unit), Z-Ac7c-(-Ala)2-OMe, Z-
(Ac7c)3-OBut, Z-(Ac7c)4-OBut, and Z-(Ac7c)5-OBut] were solved
by direct methods and refined by full-matrix least-squares pro-
cedures on F 2 (all data) for the tri-, tetra- and penta-homo-
peptides and on F for the other compounds. As for Z-(Ac7c)5-
OBut, difference Fourier techniques revealed one methanol
molecule with statistical disorder on two sites, which were
refined with population parameters of 0.5. Two of the carbon
atoms of the cycloheptane ring in the B molecule of Z-Ac7c--
1H NMR spectra
1H NMR spectra were recorded with a Bruker (Karlsruhe,
Germany) Model AM 400 spectrometer. Measurements were
carried out in [2H]chloroform (99.96% 2H; Aldrich, Milwaukee,
WI, USA) and in [2H6]DMSO ([2H6]dimethyl sulfoxide)
2
(99.96% H6; Stohler, Waltham, MA, USA) with tetramethyl-
silane as the internal standard. The free radical TEMPO
(2,2,6,6-tetramethyl-1-piperidyloxyl) was purchased from
Sigma (St Louis, MO, USA).
X-Ray diffraction analysis
Colourless single crystals of the Ac7c hydantoin, ClAc-Ac7c-
OH, Z-Ac7c-OH, Z-Ac7c--Ala-OMe, Z-Ac7c-(-Ala)2-OMe,
Z-(Ac7c)3-OBut, Z-(Ac7c)4-OBut, and Z-(Ac7c)5-OBut were
grown by slow evaporation at room temp. from the solvents
reported in Tables 2 and 3. The X-ray data for the three Ac7c
derivatives were collected on a Philips PW1100 diffractometer
(Eindhoven, The Netherlands), while the data for the five pep-
tides were obtained using an Enraf-Nonius CAD4 diffract-
ometer (Delft, The Netherlands) of the Biocrystallography
Ala-OMe (Cγ1 and Cγ21) are disordered each over two sites (A
1
and B), both of which were refined with population parameters
of 0.60 and 0.40. One of the carbon atoms of the cycloheptane
ring of Z-Ac7c-(-Ala)2-OMe (Cγ11) is disordered over two sites
2024
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
Table 2 Crystal data for the Ac7c derivatives and the dipeptide
Parameter
Ac7c hydantoin
ClAc-Ac7c-OH
Z-Ac7c-OH
Z-Ac7c--Ala-OMe
Molecular formula
Formula mass
Crystallization solvent
Crystal size/mm
Crystal system
Space group
Z, molecules/unit cell
a/Å
C9H14N2O2
182.2
C10H16ClNO3
233.7
C16H21N2O4
291.3
C20H28N2O5
362.5
Hot EtOH–H2O a (1:1)
0.6 × 0.5 × 0.4
Monoclinic
P21/c
Acetone–LP a
0.5 × 0.3 × 0.1
Monoclinic
P21/c
AcOEt–LP a
0.6 × 0.3 × 0.3
Monoclinic
I 1 2/a 1 (No. 15)
16
CHCl3–MeOH a
0.3 × 0.5 × 0.4
Monoclinic
P21
4
4
4
6.980(1)
6.969(1)
19.061(3)
95.5(1)
992.9(2)
9.827(1)
9.652(1)
12.553(2)
107.4(1)
1136.2(7)
1.37
20.729(3)
14.375(2)
21.521(3)
97.2(1)
6362(2)
1.22
9.553(2)
19.02(1)
11.587(5)
103.32(2)
2049(2)
1.22
b/Å
c/Å
β/Њ
V/Å3
Dc/g cmϪ3
1.31
Independent reflections
Observed reflections
Radiation/Å
Data collection method
θ range
2219
2737
7465
4023
1646 [F > 4σ(F)]
Mo-Kα(0.71073)
θ–2θ
1148 [F > 4σ(F)]
Mo-Kα(0.71073)
θ–2θ
1261 [F > 4σ(F)]
Mo-Kα(0.71073)
θ–2θ
3474 [I > 3σ(I)]
Cu-Kα(1.54178)
ω–2θ
2.2–28.0
2.2–28.0
2.2–28.0
1–70
Temperature
Solved by
Refined by
R value
Rw value
w
Ambient
Ambient
Ambient
Ambient
SIR 92g
SDP h
SHELXS 86b
SHELXL 93c
0.052 (R1, on F )
0.144 (wR2, on F 2, all data)
d
SHELXS 86b
SHELXL 93c
0.042 (R1, on F )
0.128 (wR2, on F 2, all data)
e
SHELXS 86b
SHELXL 93c
0.04261(R1, on F )
0.211 (wR2, on F 2, all data)
f
0.072
0.064
1/σ(F 2)
0.891
S
0.997
0.375
Ϫ0.224
0.835
0.275
Ϫ0.225
0.637
0.336
Ϫ0.200
(∆p)max/e ÅϪ3
(∆p)min /e ÅϪ3
0.394
Ϫ0.086
2
2
2
a LP, light petroleum; AcOEt, ethyl acetate. b Ref. 33. c Ref. 34. d w = 1/[σ2 (F0 ) ϩ (0.1143 P)2] where P = (F02 ϩ 2Fc )/3. e w = 1/[σ2 (F0 ) ϩ (0.0577 P)2]
where P = (F02 ϩ 2Fc )/3. f w = 1/[σ2 (F0 ) ϩ (0.0878 P)2] where P = (F02 ϩ 2Fc )/3. g Ref. 35. h Ref. 36.
2
2
2
Table 3 Crystal data for the Ac7c tri-, tetra-, and penta-peptides
Parameter
Z-Ac7c-(-Ala)2-OMe
Z-(Ac7c)3-OBut
Z-(Ac7c)4-OBut
Z-(Ac7c)5-OBut
Molecular formula
Formula mass
Crystallization solvent
Crystal size/mm
Crystal system
Space group
Z, molecules/unit cell
a/Å
C23H33N3O6
447.5
C36H55N3O6
625.9
CHCl3
0.4 × 0.5 × 0.4
Monoclinic
P21/c
C43H68N4O7
753.0
C52H81N5O8CH3OH
936.3
Toluene
0.3 × 0.5 × 0.5
Monoclinic
P21/n
AcOEt–LP a
0.5 × 0.3 × 0.3
Monoclinic
P21
AcOEt a
0.4 × 0.5 × 0.3
Monoclinic
P21/n
2
4
4
4
11.434(3)
11.624(2)
9.524(3)
105.46(2)
1220.1(6)
1.22
11.904(4)
19.867(5)
16.355(6)
111.2(1)
3605(2)
1.15
19.105(7)
22.663(9)
10.576(4)
101.4(1)
4489(3)
1.11
12.722(2)
20.501(7)
21.937(7)
104.9(1)
5530(3)
b/Å
c/Å
β/Њ
V/Å3
Dc/g cmϪ3
1.12
Independent reflections
Observed reflections
Radiation/Å
Data collection method
θ range
2431
6823
8502
10468
1994 [I > 3σ(I)]
Cu-Kα(1.54178)
ω–2θ
2238 [I > 3σ(I)]
Cu-Kα(1.54178)
ω–2θ
5497 [I > 2σ(I)]
Cu-Kα(1.54178)
ω–2θ
7566 [I > 2σ(I)]
Cu-Kα(1.54178)
ω–2θ
1–70
1–70
1–70
1–70
Temperature
Solved by
Refined by
R value
Rw value
w
Ambient
SIR 92b
SDP c
Ambient
SIR 92b
SDP c
Ambient
SIR 92b
SHELXL 93d
0.081e
Ambient
SIR 92b
SHELXL 93d
0.086e
0.076
0.076
0.070
0.076
0.161f
0.260d
1/σ(F 2)
1/σ(F 2)
2.394
g
g
S
0.768
0.409
Ϫ0.697
1.693
0.540
Ϫ0.315
0.919
0.636
Ϫ0.225
(∆p)max/e ÅϪ3
(∆p)min /e ÅϪ3
0.300
Ϫ0.14
a AcOEt, ethyl acetate; LP, light petroleum. b Ref. 35. c Ref. 36. d Ref. 34. e R1 (on F ). f wR2 (on F 2, all data). g w = [σ2 F02 ϩ 0.3625 P2 ϩ 3.0739 P] where
2
P = (F02 ϩ 2Fc )/3.
(A and B), which were refined with population parameters of
0.80 and 0.20. Statistical disorder over two sites (A and B) was
with anisotropic temperature factors. Positional parameters of
the hydrogen atoms for Z-Ac7c--Ala-OMe and Z-Ac7c-(-
Ala)2-OMe were stereochemically determined and introduced
in the calculations with isotropic thermal parameters equal to
the isotropic thermal factor of the corresponding carrier atom,
but not refined. Hydrogen atoms of Ac7c homo-tri-, tetra- and
penta-peptides were calculated and during the refinement
were allowed to ride on their carrying atoms, with Uiso set equal
to 1.2 times the Ueq of the attached atom.
also found for residue 1 of Z-(Ac7c)3-OBut-(Cγ2 with popul-
1
ation parameters of 0.50), for residues 1 and 4 of Z-(Ac7c)4-
OBut- (Cγ1 and Cγ21, both with population parameters of 0.60
1
and 0.40, and Cγ2 with population parameters of 0.80 and
4
0.20), and for residues 1,2 and 4 of Z-(Ac7c)5-OBut-(Cγ2 with
1
population parameters of 0.60 and 0.40, Cγ1 with population
2
parameters of 0.80 and 0.20, and Cγ1 with population param-
4
eters of 0.50). In all cases the non-hydrogen atoms were refined
Complete lists of bond lengths, bond angles and torsion
J. Chem. Soc., Perkin Trans. 2, 1997
2025

View Article Online
Fig. 1 IR absorption spectra (3500–3250 cmϪ1 region) of the Z-
(Ac7c)n-OBut (n = 1–5) homopeptide series in CDCl3 solution (conc.:
1 × 10Ϫ3 mol dmϪ3). Numbers refer to peptide main-chain length.
Fig. 2 IR absorption spectra (3500–3200 cmϪ1 region) of the tri-
peptides Z-Ac7c-(-Ala)2-OMe (A), Z--Ala-Ac7c--Ala-OMe (B), Boc-
-Leu-Ac7c--Val-OMe (C), and Z-(Ac7c)3-OBut(D) in CDCl3 solution
(conc.: 1 × 10Ϫ3 mol dmϪ3
)
angles, final positional parameters for all non-hydrogen atoms
along with their thermal factors, have been deposited and are
available from the Cambridge Crystallographic Data Centre
(CCDC). See ‘Instructions for Authors’, J. Chem. Soc., Perkin
Trans. 2, 1997, Issue 1. Any request to the CCDC for this
material should quote the full literature citation and reference
number 188/90.
curves of the tripeptide and the higher oligomers are character-
ized by two bands, at about 3428 cmϪ1 (free, solvated NH
groups) and at 3377–3354 cmϪ1 (strongly H-bonded NH
groups), respectively.41,42 The intensity of the low-frequency
band relative to the high-frequency band increases as main-
chain length increases. Concomitantly, the absorption max-
imum of the low-frequency band shifts markedly to lower
wavenumbers. An inspection of the spectrum of the homo-
tripeptide, compared to those of the Ac7c-containing tri-
peptides Z-Ac7c-(-Ala)2-OMe, Z--Ala-Ac7c--Ala-OMe and
Boc--Leu-Ac7c--Val-OMe (Fig. 2) allows us to conclude that
the 3377–3351 cmϪ1 band is much higher (relative to the 3438–
3429 cmϪ1 band) in the homo-tripeptide. Furthermore, in the
co-peptides the low-frequency band is more intense when the
Ac7c is incorporated at position 1 than at position 2. We have
also been able to demonstrate that, even at 10Ϫ2 mol dmϪ3 con-
centration, there are only marginal changes in the various pep-
tides (not shown). Therefore, the observed band at 3377–3351
cmϪ1 should be interpreted as arising almost exclusively from
Results and Discussion
Synthesis and characterization
Ac7c hydantoin 22,29,37 was prepared by treatment of cyclo-
heptanone with sodium cyanide and excess of ammonium
carbonate in a 1:1 water–ethanol mixture under reflux for 6 h.
Alkaline hydrolysis (with a 3  NaOH solution) of the
hydantoin, followed by acidification, afforded the free amino
acid.21,29,38–40
The Z-protected Ac7c derivative was obtained by reacting the
free amino acid with N-(benzyloxycarbonyloxy)succinimide.
Treatment of Z-Ac7c-OH with one equivalent of N-ethyl,NЈ-
(3-dimethylaminopropyl)carbodiimide (EDC) gave the 5(4H)-
oxazolone from Z-Ac7c-OH. The subsequent reaction of
Z-Ac7c-OH with the above mentioned oxazolone in an equi-
molar amount afforded the symmetrical anhydride (Z-
Ac7c)2O.26 Z-Ac7c-OBut was obtained by esterification of the N-
protected amino acid with isobutene in the presence of a
catalytic amount of sulfuric acid. ClAc-Ac7c-OH was syn-
thesized by reacting ClAc-Cl with the free amino acid in an
aqueous solution at alkaline pH.
intramolecular N᎐H ؒ ؒ ؒ O᎐C interactions. The present IR
᎐
absorption study has provided convincing evidence that main-
chain length dependent intramolecular H-bonding is an essen-
tial factor influencing the conformation of the N- and C-
protected Ac7c-rich peptides in CDCl3 solution. Our results
also support the view that Ac7c is a stronger inducer of
intramolecularly H-bonded structures than the protein amino
acids Ala and Leu.
To get more detailed information on the preferred conform-
ation of these peptides in CDCl3 solution we carried out a
400 MHz 1H NMR study. The delineation of inaccessible
Ac7c-Ac7c, -Ala-Ac7c and Ac7c--Ala [the latter in the
-Ac7c-(-Ala)2-tripeptide] peptide bond formation was achieved
by the symmetrical anhydride method. On the other hand,
Ac7c--Ala (in the dipeptide), Ac7c--Val, and -Leu-Ac7c
peptide bond formation was achieved using the EDC–HOBt (1-
hydroxy-1,2,3-benzotriazole) method. Removal of the Z-group
was performed by catalytic hydrogenation. The various pep-
tides were characterized by melting point determination,
optical rotatory power, TLC (in three solvent systems) and solid-
state IR absorption spectroscopy (Table 1), amino acid analysis
1
(or intramolecularly H-bonded) NH groups by H NMR was
carried out using: (i) solvent dependence of NH chemical shifts,
by adding increasing amounts of the strong H-bonding
acceptor solvent DMSO 43,44 to the CDCl3 solution and (ii) free-
radical (TEMPO) induced line broadening of NH resonances.45
As a typical example, Fig. 3 illustrates the behaviour of the NH
resonances of the homo-pentamer upon addition of DMSO
and TEMPO. The upfield resonance in CDCl3 solution is
unequivocally assigned to the N(1)H urethane group.46 The
second upfield resonance is assigned to the N(2)H proton by
analogy with the chemical shifts in the same halohydrocarbon
and the spectroscopic behaviour upon addition of the same
1
(Experimental section), and H NMR spectroscopy (data not
reported).
Solution conformational analysis
The conformational preferences adopted by the N- and C-
protected Ac7c-rich peptides were determined in the structure
supporting solvent CDCl3 by IR absorption spectroscopy and
1H NMR spectroscopy as a function of concentration (over the
range 10–0.1 × 10Ϫ3 mol dmϪ3).
Fig. 1 shows the IR absorption spectra (N᎐H stretching
region) of the Ac7c homo-peptides series (from monomer
through to pentamer) at 1 × 10Ϫ3 mol dmϪ3 concentration. The
perturbing agents of peptides from different types of Cα,α
-
dialkylated glycines.46–48 In one case a complete assignment of
the NH protons was achieved from the COSY and ROESY
spectra.48 From an analysis of the spectra as a function of con-
centration (10–1 × 10Ϫ3 mol dmϪ3) in CDCl3 solution (results
not shown), we have been able to conclude that dilution induces
a negligible shift to higher fields of the NH resonances of all the
2026
J. Chem. Soc., Perkin Trans. 2, 1997

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1
Fig. 3 (A) Plot of NH chemical shifts in the H NMR spectrum of
Z-(Ac7c)5-OBut as a function of increasing percentages of DMSO
added to the CDCl3 solution (v/v). (B) Plot of the bandwidth of the NH
protons of the same peptide as a function of increasing percentages of
TEMPO (mass/vol) in CDCl3. Peptide concentration: 1 mol dm3.
Fig. 5 X-Ray diffraction structure of ClAc-Ac7c-OH with numbering
of the atoms. The intramolecular H-bond is represented by a dashed
line.
Fig. 4 X-Ray diffraction structure of Ac7c hydantoin with numbering
of the atoms
peptides investigated. In particular, the most sensitive N(1)H
proton of the homo-pentapeptide shifts only by 0.05 ppm.
In the Ac7c peptides examined in the CDCl3–DMSO solvent
mixtures and in the presence of the paramagnetic perturbing
agent TEMPO at 1 × 10Ϫ3 mol dmϪ3 peptide concentration, two
classes of NH protons were observed. Class (i) [N(1)H and
N(2)H protons] includes protons whose chemical shifts are
extremely sensitive to the addition of DMSO and whose reson-
ances broaden significantly upon addition of TEMPO. Interest-
ingly, the sensitivity of the N(1)H proton is higher than that of
the N(2)H proton. Class (ii) [N(3)H to N(5)H protons] includes
those displaying behaviour characteristic of shielded protons
(relative insensitivity of chemical shifts to solvent composition
and of linewidths to the presence of TEMPO).
Fig. 6 X-Ray diffraction structure of the two independent molecules
(A and B) in the asymmetric unit of Z-Ac7c-OH
In summary, these 1H NMR results allow us to conclude that,
in CDCl3 solution at a concentration lower than 10Ϫ2 mol dmϪ3
,
expected, the imidazolidine-2,5-dione ring is nearly planar with
mean deviations of 0.002 Å from the average plane. All geo-
metrical and conformational parameters (bond lengths and
bond angles, torsion angles) are in good agreement with the
the N(3)H to N(5)H protons of all the Ac7c peptides studied are
almost inaccessible to perturbing agents and are, therefore,
most probably, intramolecularly H-bonded. In view of these IR
1
absorption and H NMR observations, it is reasonable to con-
corresponding average values obtained from
a statistical
clude that the most populated structures adopted in CDCl3
solution by the N- and C-protected tri-, tetra- and penta-
peptides are the β-turn, two consecutive β-turns, and the 310-
helix, respectively.
analysis of the crystal-state hydantoin-containing compounds
reported in the Cambridge Crystallographic Data Bank. Inter-
molecular H-bonding involves all available donors and
acceptors (Table 6), with N ؒ ؒ ؒ O distances within the average
Crystal-state conformational analysis
values for N᎐H ؒ ؒ ؒ O᎐C separations.49–51
᎐
The structure of the Ac7c hydantoin is represented in Fig. 4. As
The molecular structures of ClAc-Ac7c-OH, Z-Ac7c-OH,
J. Chem. Soc., Perkin Trans. 2, 1997
2027

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Fig. 9 X-Ray diffraction structure of Z-(Ac7c)3-OBut with numbering
of the atoms. The intramolecular H-bond is represented by a dashed
line.
Fig. 7 X-Ray diffraction structure of the two independent molecules
(A and B) in the asymmetric unit of Z-Ac7c--Ala-OMe with number-
ing of the atoms
Fig. 10 X-Ray diffraction structure of Z-(Ac7c)4-OBut with number-
ing of the atoms. The two intramolecular H-bonds are represented by
dashed lines.
previously reported values for the geometry of the benzyl-
oxycarbonylamino 53 and monochloroacetamido 54 moieties, the
ester groups,55 and the peptide unit.56,57 We have also calculated
the average geometry for the Ac7c residue. The average C᎐C
bond length for the cycloheptane ring is 1.52 Å (with average
lengths of 1.53 and 1.54 Å for the Cα᎐Cβ bonds, 1.50 and 1.51 Å
for the Cβ᎐Cγ bonds, 1.52 and 1.55 Å for the Cγ᎐Cδ bonds, and
1.49 Å for the Cδ1᎐Cδ2 bond), in excellent agreement with the
literature average value of 1.52 Å for the ᎐CH2᎐CH2᎐ dis-
tance.58 The values for the N᎐Cα, Cα᎐CЈ, and CЈ᎐O bond
᎐
lengths fit nicely with the corresponding values for peptides
based on protein amino acids.56 The average value for the bond
angles internal to the seven-membered ring is 115.1Њ, definitely
larger than the regular tetrahedral value (109.5Њ). The bond
angles actually vary from 112.1(5)Њ at Cγ2 to 117.6(5)Њ at Cβ2.
The average geometrical parameters for the seven-membered
ring of Ac7c reported here compare well with those published
for other Ac7c residues and cycloheptane-containing
compounds.24–26,30,59 In addition, the bond angles indicate an
asymmetric geometry for the Cα atom. This observation is
common also to Aib- and Acnc- (n = 3–6, 8, 9) rich peptides.7–11
The value for the conformationally sensitive N᎐Cα᎐CЈ (τ) bond
angle, external to the cyclic system, is 110.1(3)Њ, comparable to
Fig. 8 X-Ray diffraction structure of Z-Ac7c-(-Ala)2-OMe with
numbering of the atoms. The intramolecular H-bond is represented by
a dashed line.
Z-Ac7c--Ala-OMe, Z-Ac7c-(-Ala)2-OMe, Z-(Ac7c)3-OBut, Z-
(Ac7c)4-OBut, and Z-(Ac7c)5-OBut with the atomic numbering
schemes are shown in Figs. 5–11. Relevant backbone and side-
chain torsion angles52 are presented in Tables 4 and 5. In Table
6 the intra- and inter-molecular H-bond parameters are listed,
while the average bond distances and bond angles character-
izing the seven-membered ring system of the Ac7c residue are
given in Table 7.
Bond lengths and bond angles are in general agreement with
2028
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 4 Selected torsion angles (Њ) for the Ac7c derivatives and the dipeptide
Z-Ac7c-OH
Z-Ac7c--Ala-OMe
Torsion angle
Ac7c hydantoin
ClAc-Ac7c-OH
Mol. A
Mol. B
Mol. A
Mol. B
178.4(5)
56.7(7)
44.0(7)
175.3(5)
Ϫ107.2(7)
28.3(8)
Ϫ179.5(6)
ω0 [Ou᎐CЈ0᎐N1᎐Cα
]
0.3(2)a
Ϫ0.2(2)b
Ϫ0.1(1)
0.3(2)
178.4(2)c
Ϫ50.9(3)
178.3(4)
Ϫ54.5(6)
159.3(4)d
169.3(4)
Ϫ49.1(6)
Ϫ31.8(6)d
179.9(5)
Ϫ50.7(7)
Ϫ45.7(7)
177.0(5)
Ϫ119.6(7)
162.0(6)
175.0(8)
82.9(7)
1
φ1 [CЈ0᎐N1᎐Cα1᎐CЈ1]
ψ1 [N1᎐Cα1᎐CЈ1᎐N2]
Ϫ44.4(3)d
ω1 [Cα1᎐CЈ1᎐N2᎐Cα
]
2
φ2 [CЈ1᎐N2᎐Cα2᎐CЈ2]
ψ2 [N2᎐Cα2᎐CЈ2᎐OT]
ω2 [Cα2᎐CЈ2᎐OT᎐CT]
χ11,2 [N1᎐Cα1᎐Cβ21᎐Cγ2
]
161.2(1)
Ϫ85.7(2)
72.7(2)
74.9(4) [144.7(3)]e
77.3(5) [Ϫ81.4(4)]e
Ϫ74.5(5) [75.8(5)]e
9.2(5) [Ϫ53.7(4)]e
74.8(5)
Ϫ90.5(6)
72.8(7)
85.7(5)
88.3(6)
Ϫ74.2(7)
53.3(7)
Ϫ81.9(8) [Ϫ152(1)]e
Ϫ88(1) [86(2)]e
77(1) [Ϫ86(2)]e
Ϫ59.1(8) [11(1)]e
[76(1)]e [61(1)]e
73.2(9) [Ϫ89(1)]e
Ϫ86.1(5) [91(1)]e
Ϫ164.1(5) [80.9(9)]e
1
χ12,2 [Cα1᎐Cβ21᎐Cγ21᎐Cδ2
]
88.7(9)
Ϫ75.6(9)
55(1)
1
χ13,2 [Cβ21᎐Cγ21᎐Cδ21᎐Cδ1
]
1
χ14 [Cγ11᎐Cδ11᎐Cδ21᎐Cγ2
]
53.5(2)
60.0(8)
1
χ13,1 [Cβ11᎐Cγ11᎐Cδ11᎐Cδ2
χ12,1 [Cα1᎐Cβ11᎐Cγ11᎐Cδ1
]
69.3(2)
Ϫ88.3(2)
Ϫ82.0(2)
65.3(4)
Ϫ86.8(3)
Ϫ70.1(3)
79.8(7)
Ϫ86.4(6)
149.5(4)
Ϫ69.8(6)
89.9(5)
Ϫ168.0(4)
Ϫ73.2(9)
89.1(8)
Ϫ163.9(6)
1
]
1
χ11,1 [N1᎐Cα1᎐Cβ11᎐Cγ1
]
1
a [N2᎐CЈ2᎐N1᎐Cα1]. b [CЈ2᎐N1᎐Cα1᎐CЈ1]. c [Cl᎐CЈ0᎐N1᎐Cα1]. d [N1᎐Cα1᎐CЈ1᎐OT]. e Values in parentheses refer to atoms which show statistical occupancy.
The Ac7c--Ala and the N-terminal Ac7c-Ac7c sequences of the
two tripeptides are folded in a 1 ← 4 C᎐O ؒ ؒ ؒ H᎐N intra-
᎐
molecularly H-bonded β-bend conformation. The β-bend is
intermediate between type I (φ1 = Ϫ60Њ, ψ1 = Ϫ30Њ; φ2 = Ϫ90Њ,
ψ2 = 0Њ) and type III (φ1 = Ϫ60Њ, ψ1 = Ϫ30Њ; φ2 = Ϫ60Њ,
ψ2 = Ϫ30Њ)14–16 in the Ac7c/Ala tripeptides, whereas it is regular
type III in the homo-trimer. The 1–3 sequence of the Ac7c
homo-tetramer forms an incipient 310-helix (two consecutive
type-III β-turn conformations) stabilized by two 1 ← 4
C᎐O ؒ ؒ ؒ H᎐N intramolecular H-bonds. The backbone of the
᎐
homo-pentamer is folded in a regular right(left)-handed 310-
helix. Peptide groups N ᎐H to N ᎐H and CЈ ᎐O to CЈ ᎐O
᎐
᎐
2
3
5
0
0
2
participate in three consecutive 1 ← 4 C᎐O ؒ ؒ ؒ H᎐N intra-
᎐
molecular H-bonds. In all bent and helical peptides the N-
terminal intramolecular H-bond is weak.49–51
In the seven Ac7c linear derivatives and peptides few signifi-
cant deviations of the ω torsion angles from the ideal value of
the trans planar urethane, amide, peptide and ester units (180Њ)
are observed. In particular, the ω0 torsion angles of molecule B
of Z-Ac7c-OH and of the homo-tri- and homo-penta-mer, and
the ω3 and ω4 torsion angles of Z-(Ac7c)4-OBut differ by little
more than 10Њ from 180Њ. In ClAc-Ac7c-OH an intramolecular
Cl ؒ ؒ ؒ H᎐N1 interaction is seen, producing a C5 form,15,54 the
relevant θ1[Cl᎐C(1)᎐CЈ0᎐N1] torsion angle being 13.8(4)Њ. The
methyl and tert-butyl ester conformation with respect to the
preceding Cα᎐N bond is intermediate between the synperiplanar
and synclinal conformations in molecule A of Z-Ac7c--Ala-
OMe and in Z-Ac7c-(-Ala)2-OMe, while intermediate between
the anticlinal and antiperiplanar conformations in molecule B
of Z-Ac7c--Ala-OMe and in the three Ac7c homo-oligomers.61
Each of the ten seven-membered rings, in which statistically
occupied positions are not present, is found in the twist-chair
(TC) conformation,62–70 although a substantial degree of distor-
tion from this conformation is observed. The TC conformation,
with C2 symmetry, is that theoretically predicted as the min-
Fig. 11 X-Ray diffraction structure of Z-(Ac7c)5-OBut with number-
ing of the atoms. The three intramolecular H-bonds are represented by
dashed lines.
that exhibited by the Cα,α-dialkylated glycines forming regular
helices (110–111Њ).7–11,60
All of the Ac7c residues are found in the helical region A(A*)
of the conformational map,18 with the exception of that of
molecule A of Z-Ac7c-OH which is semi-extended. Each of the
Ac7c derivatives and homo-peptides, having no chiral atoms,
crystallizes in a centrosymmetric space group; thus, in each unit
cell molecules of both handedness occur simultaneously. The
average values for the φ, ψ backbone torsion angles of the Ac7c
residue completely involved in a bend or helical structure are
±58.8Њ, ±31.3Њ, close to those expected for a 310 helix (±57Њ,
±30Њ).17 Also the C-terminal Ac7c residues of the homo- tri- and
tetra-peptides adopt a conformation in the helical region, but
they have opposite handedness to that shown by the preceding
residues, a common observation for Aib- and Acnc- (n = 3–6, 8,
9) rich peptides.7–11,13 However, the same helical handedness
is maintained by the C-terminal residue of the homo-
pentapeptide. The major conformational difference between
molecules A and B of Z-Ac7c-OH is seen in the ψ backbone
torsion angle, extended for A while helical for B. Among the
various conformational differences distinguishing molecules A
and B of the chiral dipeptide Z-Ac7c--Ala-OMe the most
relevant is the opposite handedness of the helical Ac7c residue.
imum energy conformation for a cycloheptane ring.70
A
different behaviour is found for the nine Ac7c residues of the
molecules in which the statistically occupied positions for the
seven-membered ring atoms occur. For these residues the most
populated conformation is still the TC conformation, but a
second conformation, the chair conformation (C), with Cs
symmetry is found.70 From the analysis of the experimental
data it appears that the residues in the TC conformation have
average dihedral angles (χ4 = 51Њ, χ3,1 = Ϫ73Њ, χ3,2 = Ϫ68Њ,
χ2,1 = 87Њ, χ2,2 = 86Њ, δ1,1 = Ϫ32Њ and δ1,2 = Ϫ46Њ) similar to those
calculated for this conformation (χ4 = 54.8Њ, χ3 = Ϫ72.5Њ,
χ2 = 87.8Њ and δ1 = Ϫ39.0Њ).70 Also the analysis of the residues in
the C conformation shows average dihedral angles (χ4 = Ϫ14Њ,
χ3,1 = 73Њ, χ3,2 = Ϫ63Њ, χ2,1 = Ϫ78Њ, χ2,2 = 84Њ, δ1,1 = 49Њ and
J. Chem. Soc., Perkin Trans. 2, 1997
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Table 5 Selected torsion angles (Њ) for the Ac7c tri-, tetra-, and penta-peptides
Torsion angle
Z-Ac7c-(-Ala)2-OBut
Z-(Ac7c)3-OBut
Z-(Ac7c)4-OBut
Z-(Ac7c)5-OBut
ω0 [Ou᎐CЈ0᎐N1᎐Cα
]
178.4(5)
Ϫ 58.6(7)
Ϫ164.9(7)
Ϫ62(1)
Ϫ33(1)
Ϫ174.4(7)
Ϫ57(1)
Ϫ174.0(3)
Ϫ62.9(4)
Ϫ26.8(3)
Ϫ176.5(2)
Ϫ52.7(3)
Ϫ31.9(3)
Ϫ173.9(2)
Ϫ59.2(3)
Ϫ28.2(3)
Ϫ169.4(2)
54.7(4)
Ϫ166.9(3)
Ϫ59.4(4)
Ϫ36.1(3)
Ϫ171.6(2)
Ϫ54.5(3)
Ϫ32.4(3)
Ϫ174.6(2)
Ϫ53.6(3)
Ϫ36.3(3)
Ϫ173.9(2)
Ϫ67.6(3)
Ϫ25.7(3)
Ϫ174.9(2)
Ϫ50.2(3)
Ϫ49.8(3)
Ϫ176.2(3)
60.0(4)
86.6(5)
1
φ1 [CЈ0᎐N1Cα1᎐CЈ1]
ψ1 [N1᎐Cα1᎐CЈ1᎐N2]
Ϫ 29.9(7)
Ϫ 176.3(5)
Ϫ 72.6(8)
Ϫ 20.8(9)
179.8(6)
Ϫ 69.6(8)
157.8(5)a
179.7(6)b
ω1 [Cα1᎐CЈ1᎐N2᎐Cα
]
2
φ2 [CЈ1᎐N2᎐Cα2᎐CЈ2]
ψ2 [N2᎐Cα2᎐CЈ2᎐N3]
Ϫ32.7(9)
179.1(6)
49.8(9)
ω2 [Cα2᎐CЈ2᎐N3᎐Cα
]
3
φ3 [CЈ2᎐N3᎐Cα3᎐CЈ3]
ψ3 [N3᎐Cα3᎐CЈ3᎐N4]
47.9(8)a
174.8(6)b
ω3 [Cα3᎐CЈ3᎐N4᎐Cα
]
4
φ4 [CЈ3᎐N4᎐Cα4᎐CЈ4]
ψ4 [N4᎐Cα4᎐CЈ4᎐N5]
45.4(4)d
ω4 [Cα4᎐CЈ4᎐N5᎐Cα
]
167.0(4)e
5
φ5 [CЈ4᎐N5᎐Cα5᎐CЈ5]
ψ5 [N5᎐Cα5᎐CЈ5᎐OT]
ω5 [Cα5᎐CЈ5᎐OT᎐CT]
χ11,1 [N1᎐Cα1᎐Cβ11᎐Cγ1
]
158.1(7) [Ϫ78.2(7)]c
94(1) [Ϫ84.7(7)]c
Ϫ 81(1) [80.4(8)]c
47(1) [Ϫ22(1)]c
Ϫ 53(2)
65(1)
84(1)
Ϫ54(2)
62.0(5) [140.9(6)]c
89.1(7) [Ϫ87(1)]c
Ϫ66(1) [96(1)]c
Ϫ6(1) [Ϫ15(1)]c [Ϫ75(1)]c [54(1)]c
68.(1) [Ϫ80(1)]c
Ϫ71.3(7) [79(1)]c
Ϫ74.6(5) [Ϫ136.1(7)]c
77.8(4)
86.3(5)
Ϫ68.9(6)
53.8(8)
Ϫ72.5(6)
89.1(5)
Ϫ161.4(3)
77.3(3)
91.1(4)
Ϫ70.1(5)
49.3(5)
Ϫ68.7(4)
88.1(4)
1
χ12,1 [Cα1᎐Cβ11᎐Cγ11᎐Cδ1
]
1
χ13,1 [Cβ11᎐Cγ11᎐Cδ11᎐Cδ2
]
Ϫ49.9(8)
1
χ14 [Cγ11᎐Cδ11᎐Cδ21᎐Cγ2
]
Ϫ21(2) [41(2)]c
79(1) [Ϫ78(2)]c
Ϫ75(2) [90(2)]c
Ϫ75(1) [Ϫ145(1)]c
87.6(9)
Ϫ24.2(9) [35(1)]c
76.6(7) [Ϫ69(1)]c
Ϫ70.2(6) [79(1)]c
Ϫ75.5(4) [Ϫ142.9(8)]c
80.9(5) [153(1)]c
84.7(6) [Ϫ85(2)]c
Ϫ67.2(8) [92(2)]c
48(1) [Ϫ16(1)]c
Ϫ65.4(8)
1
χ13,2 [Cβ21᎐Cγ21᎐Cδ21᎐Cδ1
]
1
χ12,2 [Cα1᎐Cβ21᎐Cγ21᎐Cδ2
]
71(2)
86(1)
1
χ11,2 [N1᎐Cα1᎐Cβ21᎐Cγ2
χ21,1 [N2᎐Cα2᎐Cβ12᎐Cγ1
]
]
1
2
χ22,1 [Cα2᎐Cβ12᎐Cγ12᎐Cδ1
]
81(1)
Ϫ70(2)
51(1)
Ϫ67(1)
2
χ23,1 [Cβ12᎐Cγ12᎐Cδ12᎐Cδ2
]
2
χ24 [Cγ12᎐Cδ12᎐Cδ22᎐Cγ2
]
2
χ23,2 [Cβ22᎐Cγ22᎐Cδ22᎐Cδ1
]
2
χ22,2 [Cα2᎐Cβ22᎐Cγ22᎐Cδ2
]
87(1)
87.5(5)
Ϫ166.3(3)
155.2(3)
Ϫ85.5(4)
72.7(5)
Ϫ56.3(6)
73.0(5)
Ϫ90.4(4)
Ϫ78.1(3)
2
χ21,2 [N2᎐Cα2᎐Cβ22᎐Cγ2
χ31,1 [N3᎐Cα3᎐Cβ13᎐Cγ1
]
]
Ϫ165.1(7)
Ϫ86.0(8)
Ϫ84.6(9)
71(1)
Ϫ53(1)
67(1)
Ϫ88.2(8)
167.7(6)
Ϫ157.6(3)
2
3
χ32,1 [Cα3᎐Cβ13᎐Cγ13᎐Cδ1
]
3
χ33,1 [Cβ13᎐Cγ13᎐Cδ13᎐Cδ2
]
3
χ34 [Cγ13᎐Cδ13᎐Cδ23᎐Cγ2
]
3
χ33,2 [Cβ23᎐Cγ23᎐Cδ23᎐Cδ1
]
3
χ32,2 [Cα3᎐Cβ23᎐Cγ23᎐Cδ2
]
3
χ31,2 [N3᎐Cα3᎐Cβ23᎐Cγ2
χ41,1 [N4᎐Cα4᎐Cβ14᎐Cγ1
]
]
Ϫ159.4(2)
3
4
64.6(5) [140.2(4)]c
84.9(5)
χ42,1 [Cα4᎐Cβ14᎐Cγ14᎐Cδ1
]
93.7(7) [Ϫ77.6(7)]c
Ϫ78.7(9) [79.0(8)]c
Ϫ0.8(9) [Ϫ66.0(8)]c
72.2(8)
Ϫ82.1(6)
Ϫ71.5(3)
92.9(4)
81.0(5)
Ϫ71.3(7)
49.4(8)
4
χ43,1 [Cβ14᎐Cγ14᎐Cδ14᎐Cδ2
]
Ϫ71.3(6)
4
χ44 [Cγ14᎐Cδ14᎐Cδ24᎐Cγ2
]
57.0(7) [Ϫ13.7(7)]c
Ϫ77.0(6) [88(1)]c
90.5(4) [Ϫ86(1)]c
83.7(3) [159.6(6)]c
4
χ43,2 [Cβ24᎐Cγ24᎐Cδ24᎐Cδ1
]
4
χ42,2 [Cα4᎐Cβ24᎐Cγ24᎐Cδ2
]
4
χ41,2 [N4᎐Cα4᎐Cβ24᎐Cγ2
χ51,1 [N5᎐Cα5᎐Cβ15᎐Cγ1
]
]
4
5
χ52,1 [Cα5᎐Cβ15᎐Cγ15᎐Cδ1
]
5
χ53,1 [Cβ15᎐Cγ15᎐Cδ15᎐Cδ2
]
5
χ54 [Cγ15᎐Cδ15᎐Cδ25᎐Cγ2
]
5
χ53,2 [Cβ25᎐Cγ25᎐Cδ25᎐Cδ1
]
Ϫ64.2(6)
91.0(4)
Ϫ174.4(3)
5
χ52,2 [Cα5᎐Cβ25᎐Cγ25᎐Cδ2
]
5
χ51,2 [N5᎐Cα5᎐Cβ25᎐Cγ2
]
5
a [N3᎐Cα3᎐CЈ3᎐OT]. b [Cα3᎐CЈ3᎐OT᎐CT]. c Values in parentheses refer to atoms which show statistical occupancy. d [N4᎐Cα4᎐CЈ4᎐OT]. e [Cα4᎐CЈ4᎐OT᎐CT].
δ1,2 = Ϫ52Њ) similar to those calculated for this conformation
(χ4 = 1.2Њ, χ3,1 = 67.2Њ, χ3,2 = Ϫ67.2Њ, χ2,1 = Ϫ85.6Њ, χ2,2 = 85.6Њ,
δ1,1 = 64.6Њ and δ1,2 = Ϫ64.6Њ).70
In addition, it is noteworthy that for residues in the TC con-
formation the χ1,1 and χ1,2 side-chain torsion angles are in the (t,
gϩ) and (gϪ, t) conformations for right-handed and left-handed
Ac7c residues, respectively, while for residues in the C conform-
ation they are in the (gϪ, gϩ) and (gϩ, gϪ) conformations for
right-handed and left-handed Ac7c residues, respectively.
The packing mode of the ClAc-Ac7c-OH molecules is char-
The two independent molecules (A and B) of Z-Ac7c--Ala-
OMe pack together along the a direction, producing rows of
molecules stabilized by four N᎐H ؒ ؒ ؒ O᎐C intermolecular
᎐
H-bonds [N1A᎐H ؒ ؒ ؒ O0B᎐CЈ , N2A᎐H ؒ ؒ ؒ O1B᎐CЈ , N1B᎐H ؒ ؒ ؒ
᎐ ᎐
0B 1B
O0A᎐CЈ and N2B᎐H ؒ ؒ ؒ O1A᎐CЈ1A]. Then, van der Waals inter-
᎐
᎐
0A
actions link together rows of peptide molecules running in the
b and c directions.
The Z-Ac7c-(-Ala)2-OMe molecules pack together along the
c
direction, producing rows of molecules stabilized by
(urethane) N᎐H ؒ ؒ ؒ O᎐C (peptide) intermolecular H-bonds
᎐
acterized by (carboxylic acid) O ᎐H ؒ ؒ ؒ O ᎐CЈ (amide) inter-
(N ᎐H ؒ ؒ ؒ O ᎐CЈ ). In addition, the crystal structure is stabil-
᎐
1 2 2
᎐
T
0
0
molecular H-bonds, forming rows along the b direction.54 The
geometrical parameters of this O᎐H ؒ ؒ ؒ O H-bond are in the
ranges expected for such interactions.71,72
ized by van der Waals interactions in the ab plane.
The packing modes of Z-(Ac7c)3-OBut, Z-(Ac7c)4-OBut and
Z-(Ac7c)5-OBut molecules are similar and characterized by one
intermolecular H-bond between the (urethane) N᎐H of the
In the crystal of Z-Ac7c-OH the molecules are connected
through a complex network of intermolecular H-bonds, in
which OT᎐H ؒ ؒ ؒ O1 H-bonds are formed between molecules of
the same type (A or B), whereas N1᎐H ؒ ؒ ؒ O0 H-bonds link A to
B molecules.
first residue and the CЈ᎐O (peptide) of the n Ϫ 1 residue
᎐
(N1᎐H ؒ ؒ ؒ OnϪ1᎐CЈnϪ1). These intermolecular H-bonds are
᎐
established along the a direction for Z-(Ac7c)3-OBut and Z-
(Ac7c)4-OBut and the c direction for Z-(Ac7c)5-OBut molecules,
2030
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
Table 6 Intra- and inter-molecular H-bond parameters for the Ac7c derivatives and peptides
Donor
D᎐H
Acceptor
A
Symmetry
equiv. of A
Distance/Å
D ؒ ؒ ؒ A
Distance/Å
H ؒ ؒ ؒ A
Angle (Њ)
D᎐H ؒ ؒ ؒ A
Peptide
1
2
1
–
–
Ac7c hydantoin
N1᎐H
N2᎐H
O2
O1
Ϫx, ϩy, Ϫz
2.905(2)
2.896(2)
2.082(2)
2.068(2)
159.1(1)
161.9(1)
2
Ϫx, Ϫy, 1Ϫz
ClAc-Ac7c-OH
Z-Ac7c-OH
N1᎐H
OT᎐H
Cl
O0
x, y, z
2Ϫx, yϪ2, Ϫ2Ϫz
2.973(3)
2.630(3)
2.513(9)
1.840(3)
114.3(2)
161.3(2)
1
–
1
–
N1B᎐H
OTA᎐H
N1A᎐H
OTB᎐H
O0A
O1A
O0B
O1B
x, y, z
2.944(5)
2.726(5)
2.890(5)
2.606(5)
2.239(5)
1.912(5)
2.069(5)
1.794(5)
139.2(5)
171.8(4)
159.3(4)
170.6(4)
3
3
2
3
–
–
–
2Ϫx, Ϫy, Ϫz
2
1
3
–
–
2Ϫx, yϪ2, Ϫz
2Ϫx, 2Ϫy,22Ϫz
Z-Ac7c--Ala-OMe
N1B᎐H
N2B᎐H
N1A᎐H
N2A᎐H
O0A
O1A
O0B
O1B
x, y, z
x, y, z
xϩ1, y, z
xϩ1, y, z
2.905(6)
2.891(6)
3.018(6)
2.973(6)
1.941(4)
2.065(4)
2.050(4)
2.099(4)
165.9(3)
141.1(3)
168.5(3)
147.9(4)
Z-Ac7c-(-Ala)2-OMe
Z-(Ac7c)3-OBut
N3᎐H
N1᎐H
O0
O2
x, y, z
3.117(8)
2.861(7)
2.220(5)
2.066(6)
154.8(4)
139.6(4)
1
–
Ϫx, ϩy, 1Ϫz
2
N3᎐H
N1᎐H
O0
O2
x, y, z
1Ϫx, ϪyϪ2, Ϫzϩ2
3.144(8)
2.930(9)
2.270(5)
1.982(6)
150.9(4)
172.4(4)
1
–
1
–
Z-(Ac7c)4-OBut
N3᎐H
N4᎐H
N1᎐H
O0
O1
O3
x, y, z
x, y, z
3.204(3)
3.032(3)
2.885(3)
2.368(3)
2.187(3)
2.066(3)
164.08(8)
167.43(8)
158.87(9)
1
1
1
–
–
–
xϩ2, Ϫyϩ2, zϩ2
Z-(Ac7c)5-OBut
N3᎐H
N4᎐H
O0
O1
x, y, z
x, y, z
3.205(3)
3.070(3)
2.338(3)
2.261(3)
158.89(8)
156.68(7)
N5᎐H
N1᎐H
O2
O4
x, y, z
3.021(3)
2.888(3)
2.226(3)
2.035(3)
153.85(7)
167.02(7)
1
3
1
–
–
–
xϪ2, Ϫyϩ2, zϪ2
n = 4–6, 8, 9) residues, but all of these Cα,α-dialkylated glycines
exhibit strictly comparable, strong conformational bias to bend-
ing and helix formation.7–13 This remarkable lack of flexibility
of Ac7c is also reflected in the strictly comparable conform-
ations found for its N- and C-protected tri-, tetra-, and penta-
peptides in solution and in the crystal state.
Considerable recent interest has been focused on the
development of conformationally constrained analogues of
bioactive peptides.1–5 The availability of highly active, structur-
ally restricted agonists and antagonists is of great value in
delineating the nature of receptor-bound conformations. It
seems reasonable to foresee that future investigations on
analogues of biologically relevant peptides, incorporating Aib
and Acnc (with n = 4–9) residues at selected positions, will be
rewarding.
Table 7 Average bond distances and bond angles for the Ac7c residue
Bond distance/Å
Bond angle (Њ)
N᎐Cα
1.465(5)
N᎐Cα᎐CЈ
110.1(3)
113.7(4)
116.6(5)
113.7(6)
116.7(6)
115.5(6)
112.1(5)
117.6(5)
110.6(4)
109.3(4)
106.3(4)
108.9(4)
Cα᎐CЈ
1.537(6)
1.220(6)
1.534(4)
1.496(8)
1.518(9)
1.49(1)
1.55(1)
1.508(8)
1.536(6)
Cβ1᎐Cα᎐Cβ2
Cα᎐Cβ1᎐Cγ1
Cβ1᎐Cγ1᎐Cδ1
Cγ1᎐Cδ1᎐Cδ2
Cδ1᎐Cδ2᎐Cγ2
Cδ2᎐Cγ2᎐Cβ2
Cγ2᎐Cβ2᎐Cα
N᎐Cα᎐Cβ1
N᎐Cα᎐Cβ2
CЈ᎐Cα᎐Cβ1
CЈ᎐Cα᎐Cβ2
CЈ᎐O
Cα᎐Cβ1
Cβ1᎐Cγ1
Cγ1᎐Cδ1
Cδ1᎐Cδ2
Cδ2᎐Cγ2
Cγ2᎐Cβ2
Cβ2᎐Cα
Interestingly, Cα,α-di-n-propylglycine (Dpg),20,28,29,73–77 with
the same number of side-chain carbon atoms as Ac7c, has been
shown to favour fully extended (C5) conformations (φ, ψ Х 180,
180Њ).15,78 Comparison of the conformational preferences of
Dpg and Ac7c serves to highlight the effect of side-chain
cyclization on conformation.
producing long rows of H-bonded peptide molecules. The
crystal structures are further stabilized by van der Waals
interactions along the other crystallographic directions. The
statistical methanol molecule in the Z-(Ac7c)5-OBut structure is
not intermolecularly H-bonded, and is located in a solvophobic
region in the crystallographic packing.
Acknowledgements
Conclusions
The authors gratefully acknowledge MURST, the Ministry of
University and of Scientific and Technological Research, and
the National Council of Research (CNR) of Italy, for their
continuous and generous support of this research. The authors
thank Dr Gabriella De Vita for technical assistance.
The experimental results described in this paper conclusively
reconfirm the few and scattered data reported in previous
studies.20,27–30 strongly supporting the view that the medium-
ring alicyclic Ac7c residue imparts considerable restriction to
the peptide backbone and is forced to adopt conformations in
the 310/α-helical region of the φ, ψ space. Thus, the Ac7c residue
can be easily accommodated in either position i ϩ 1 or i ϩ 2 of
type III(IIIЈ) β-bend and at the position i ϩ 1 of type I(IЈ) β-
bend. It may also be accommodated, although with some devi-
ation from the expected φ, ψ values, at the position i ϩ 2 of type
I(IЈ) or type II(IIЈ) β-bends. In summary, Ac7c has a different
effective volume and hydrophobicity to Aib and Acnc (with
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Paper 7/01473B
Received 3rd March 1997
Accepted 17th June 1997
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