Synthesis and conformational studies of γ-aminoxy peptides

Article abstract of DOI:10.1021/ja0772750

We have synthesized a series of γ-aminoxy acids, including unsubstituted and γ⁴-Ph-, γ⁴-alkyl-, and γ^3,4-cyclohexyl-substituted systems. Coupling of these monomers to oligomers can be realized using EDCl/HOBt (or HOAt) as the coupling agent. γ-Aminoxy peptides can form 10-membered-ring intramolecular hydrogen bonds - so-called γ N-O turns - between adjacent residues, the extent of which is controlled by the nature of the side chain of each γ-aminoxy acid residue, increasing from the unsubstituted γ-aminoxy peptide to the γ⁴-alkyl aminoxy peptides to the γ⁴-phenyl- and γ^3,4-cyclohexyl-substituted aminoxy peptides. The presence of two consecutive homochiral 10-membered-ring intramolecular hydrogen bonds leads to the formation of a novel helical structure. Theoretical studies on a series of model peptides rationalize very well the experimentally observed conformational features of these γ-aminoxy peptides.

Full text of DOI:10.1021/ja0772750

Published on Web 12/19/2007
Synthesis and Conformational Studies of γ-Aminoxy
Peptides
Fei Chen,† Ke-Sheng Song,† Yun-Dong Wu,‡ and Dan Yang*,†
Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, People’s
Republic of China, and Department of Chemistry, Hong Kong UniVersity of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
Received September 20, 2007; E-mail: yangdan@hku.hk
Abstract: We have synthesized a series of γ-aminoxy acids, including unsubstituted and γ4-Ph-, γ4-alkyl-,
and γ3,4-cyclohexyl-substituted systems. Coupling of these monomers to oligomers can be realized using
EDCI/HOBt (or HOAt) as the coupling agent. γ-Aminoxy peptides can form 10-membered-ring intramo-
lecular hydrogen bondssso-called “γ N-O turns”sbetween adjacent residues, the extent of which is
controlled by the nature of the side chain of each γ-aminoxy acid residue, increasing from the unsubstituted
γ-aminoxy peptide to the γ4-alkyl aminoxy peptides to the γ4-phenyl- and γ3,4-cyclohexyl-substituted aminoxy
peptides. The presence of two consecutive homochiral 10-membered-ring intramolecular hydrogen bonds
leads to the formation of a novel helical structure. Theoretical studies on a series of model peptides rationalize
very well the experimentally observed conformational features of these γ-aminoxy peptides.
Chart 1
Introduction
Peptidomimetic foldamers,1,2 such as â-peptides,3-5 γ-pep-
tides,5,6 δ-peptides,7,8 and aminoxy peptides,9-11 attract much
attention because of their unusual conformations and interesting
bioactivities.12-15 In a search for new peptidomimetic foldamers,
we found that aminoxy peptides can feature strong intramo-
lecular hydrogen bonds between adjacent residues (Chart 1).
For example, peptides consisting of R-aminoxy acids can possess
eight-membered-ring intramolecular hydrogen bonds (R N-O
turns)9,16 and peptides consisting of â-aminoxy acids can possess
nine-membered-ring intramolecular hydrogen bonds (â N-O
turns).10,17 Oligomers of homochiral R- or â-aminoxy acids can
form helical structures consisting of consecutive N-O turns
(1.88 and 1.79 helices, respectively). Peptides containing R-ami-
noxy acids are also good receptors for anions because of the
acidity of their aminoxy amide protons.18,19 A compound derived
from an R-aminoxy acid has been used as an effective chemical
shift reagent for measuring the values of ee of carboxylic acids;20
another compound derived from an R-aminoxy acid can form
chloride channels to mediate chloride ion transportation across
cell membranes.21 To enrich the category of aminoxy peptides
and to test the ability of other aminoxy peptides to form local
intramolecular hydrogen bonds, we sought to synthesize γ-ami-
noxy acid-based peptides and explore their conformational
properties (Chart 1). Previously, we reported that γ4-Ph aminoxy
peptides can form turn and helix structures incorporating 10-
membered-ring intramolecular hydrogen bonds (γ N-O turns).22
Here we report the syntheses of a series of γ-aminoxy acid
peptidessincluding unsubstituted and γ4-Ph-, γ4-alkyl-, and γ3,4-
† The University of Hong Kong.
‡ Hong Kong University of Science and Technology.
(1) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180.
(2) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem.
ReV. 2001, 101, 3893-4011.
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3219-3232.
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Gellman, S. H. J. Am. Chem. Soc. 2001, 123, 11077-11078.
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Am. Chem. Soc. 1996, 118, 9794-9795.
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(18) Yang, D.; Li, X.; Sha, Y.; Wu, Y. D. Chem. Eur. J. 2005, 11, 3005-3009.
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J. AM. CHEM. SOC. 2008, 130, 743-755
743

A R T I C L E S
Scheme 1
Chen et al.
Scheme 2 a
a Conditions: (a) DIAD, PPh3, PhthN-OH, THF, 1 h, 66% yield; (b) NaIO4, RuO2, CH3CN/H2O/EtOAc (15:10:3), 4 h, 80% yield.
Scheme 3a
a Conditions: (a) t-BuOH, DCC, DMAP, 12 h, CH2Cl2, 89% yield; (b) (-)-DIP-Cl, THF, -25 °C, 12 h, 95% yield, 97% ee; (c) PhthN-OH, PPh3,
DEAD, THF, 1 h, 80% yield.
Scheme 4a
cyclohexyl-substituted systemssand our detailed conformational
studies of those peptides using both experimental and theoretical
approaches.
Results and Discussion
Synthesis of γ-Aminoxy Peptides. Scheme 1 presents a
retrosynthetic analysis of a γ4-aminoxy acid monomer. Gener-
ally, the ONH2 and COOH groups of the aminoxy amide
monomer are protected as phthalimidoxy and tert-butyl ester
groups, respectively, which are readily removed using hydrazine
hydrate and trifluoroacetic acid, respectively.23 The phthalimi-
doxy group of the monomer can be readily introduced through
a Mitsunobu reaction24 between the γ-hydroxy ester and
N-hydroxyphthalimide (PhthN-OH), with inversion of the
configuration at the γ carbon atom. Previously, in syntheses of
â-aminoxy acids, we encountered the problem of R,â-elimina-
tion when attempting to introduce phthalimidoxy groups onto
â-hydroxy esters; avoiding this problem required a long
synthetic route.25 For syntheses of γ-aminoxy acids, no such
problem should be encountered, and a relatively short synthetic
route can be envisioned. Furthermore, optical active γ-hydroxy
esters can be prepared through asymmetric reduction of γ-keto
esters.
We synthesized the unsubstituted γ-aminoxy acid monomer
3 from 1,4-butanediol 1 (Scheme 2). The Mitsunobu reaction
of 1 with PhthN-OH afforded 2 in 66% yield. The oxidation of
2 with NaIO4, catalyzed by RuO2, gave the γ-aminoxy acid
monomer 3 in 80% yield.
γ4-Aminoxy acids present a side chain at the C-4 position.
We synthesized the γ4-Ph aminoxy acid monomer 7 from
3-benzoylpropionic acid (4) according to the method outlined
in Scheme 3. The carboxylic acid group of 4 was first protected
a Conditions: (a) t-BuOH, DCC, DMAP, CH2Cl2, 12 h, 46% yield; (b)
Baker’s yeast, sucrose, water, 3 d, 20% yield; (c) PhthN-OH, PPh3, DEAD,
THF, 1 h, 78% yield, 99% ee.
Scheme 5a
a Conditions: (a) (i) LDA, THF, -78 to -20 °C, 1 h; (ii) tert-butyl
R-bromoacetate, -78 °C to room temp, 12 h, 42% yield; (b) NaBH4, THF/
H2O, 1 h; (c) PhthN-OH, PPh3, DEAD, THF, 1 h, 56% yield, two steps.
as γ-keto tert-butyl ester 5, reduction of which using (-)-DIP-
Cl26 gave the optically active γ-hydroxy ester 6 in 95% yield
and 97% ee. The Mitsunobu reaction of 6 with PhthN-OH
resulted in inversion of configuration at the γ carbon atom,
affording chiral γ-aminoxy acid monomer 7 in 80% yield.
For the synthesis of alkyl substituted chiral γ-hydroxy esters,
the DIP-Cl reduction method was not applicable because it
requires the presence of a phenyl group on the γ-keto group to
achieve a high ee.26 Instead, we chose baker’s yeast as a
(23) Yang, D.; Li, B.; Ng, F. F.; Yan, Y. L.; Qu, J.; Wu, Y. D. J. Org. Chem.
2001, 66, 7303-7312.
(24) Mitsunobu, O. Synthesis 1981, 1-28.
(25) Yang, D.; Zhang, Y. H.; Li, B.; Zhang, D. W. J. Org. Chem. 2004, 69,
7577-7581.
(26) Ramachandran, P. V.; Pitre, S.; Brown, H. C. J. Org. Chem. 2002, 67,
5315-5319.
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744 J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008

γ
-Aminoxy Peptides
A R T I C L E S
Scheme 6a
a Conditions: (a) (i) LDA, THF, -78 to -20 °C, 1 h; (ii) tert-butyl R-bromoacetate, -78 °C to room temp, 12 h, 80% yield; (b) Baker’s yeast, sucrose,
water, 3 d, 26% yield; (c) PhthN-OH, PPh3, DEAD, THF, 1 h, 40% yield, 98% ee.
Scheme 7a
a Conditions: (a) EDCI, HOAt, isobutylamine, CH2Cl2; (b) NH2NH2, MeOH/CH2Cl2; (c) EDCI, HOAt, isobutyric acid, CH2Cl2.
Scheme 8a
a Conditions: (a) NH2NH2, MeOH/CH2Cl2; (b) EDCI, HOBt, isobutyric acid, CH2Cl2; (c) TFA, CH2Cl2; (d) EDCI, HOBt, cyclohexylamine, CH2Cl2.
biocatalyst because it is a common microorganism for the
asymmetric reduction of carbonyl compounds.27
of 14 with PhthN-OH afforded 15 in 60% yield. Reduction of
13 with baker’s yeast gave the chiral γ-hydroxy ester 14 and
the chiral monomer 15, but the ee of 15 was a disappointing
55%. Therefore, we used NaBH4 to reduce 13 to 14, and
employed the racemic sample of the γ4-Et aminoxy amide for
our conformational studies.
We synthesized the γ4-Me aminoxy acid monomer 11 from
4-oxopentanoic acid 8 (Scheme 4). The carboxylic group of 8
was protected in the form of the γ-keto tert-butyl ester 9, which
we reduced using baker’s yeast to afford the chiral γ-hydroxy
ester 10 in 20% yield. The relatively low yield of this step was
probably due to the incompatibility of t-Bu group with baker’s
yeast or the poor solubility of substrate 9. The Mitsunobu
reaction of 10 with PhthN-OH yielded the γ4-Me aminoxy acid
monomer 11 in 78% yield and 99% ee.
We synthesized the chiral γ3,4-cyclohexyl aminoxy acid
monomer 19 from cyclohexanone (Scheme 6). Cyclohexanone
(16) was first deprotonated using LDA and then reacted with
tert-butyl R-bromoacetate to afford the γ-keto ester 17 in 80%
yield. Asymmetric reduction of 17 using baker’s yeast gave the
optically active γ-hydroxy ester 18 in 26% yield.28 After a
Mitsunobu reaction, the final chiral product 19 was obtained in
40% yield and 98% ee.
We synthesized the γ4-Et aminoxy acid monomer following
the route illustrated in Scheme 5. Butanone was first deproto-
nated using LDA and then reacted with tert-butyl R-bromoac-
etate to afford the γ-keto ester 13, reduction of which using
NaBH4 gave the γ-hydroxy ester 14. The Mitsunobu reaction
Following our previously reported protocol23 for R-aminoxy
peptide synthesis using EDCI/HOBt (or HOAt) as the coupling
(27) Ema, T.; Sugiyama, Y.; Fukumoto, M.; Moriya, H.; Cui, J. N.; Sakai, T.;
(28) Ganaha, M.; Funabiki, Y.; Motoki, M.; Yamauchi, S.; Kinoshita, Y. Biosci.
Biotechnol. Biochem. 1998, 62, 181-184.
Utaka, M. J. Org. Chem. 1998, 63, 4996-5000.
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A R T I C L E S
Scheme 9a
Chen et al.
a Conditions: (a) TFA, CH2Cl2; (b) EDCI, HOBt, isobutylamine, CH2Cl2; (c) NH2NH2, MeOH, CH2Cl2; (d) EDCI, HOBt, CH2Cl2.
Scheme 10a
1
relatively large chemical shift changes in both the H NMR
spectroscopic dilution (∆δ ) 0.70-1.37 ppm) and DMSO-d6
addition (∆δ ) 1.66-2.30 ppm) studies, suggesting that no
strong intramolecular hydrogen bonds were formed using these
protons. Similarly, the chemical shift of the NHb proton of 21
at 0.78 mM was located quite far upfield (6.20 ppm), with
1
relatively large chemical shift changes in both the H NMR
spectroscopic dilution (∆δ ) 0.56 ppm) and DMSO-d6 addition
(∆δ ) 0.91 ppm) studies, again suggesting that this proton was
not involved in any strong intramolecular hydrogen bonds.
The chemical shifts of the NHb protons of 23, 25, 27, 29, 31,
and 33 and the NHc proton of 33 were located rather downfield
(7.05, 6.45, 6.39, 7.02, 10.57, 10.43, and 6.90 ppm, respectively)
and underwent relatively small changes in the 1H NMR
spectroscopic dilution (∆δ < 0.4 ppm) and DMSO-d6 addition
(∆δ < 0.9 ppm) studies, revealing that these amide protons were
parts of 10-membered-ring intramolecular hydrogen bonds
between CdOi and NHi+2 groups, that is, γ N-O turns. We
noted that the chemical shifts of the NHb protons in 25 (6.45
ppm) and 27 (6.39 ppm) were located less downfield than those
in 23 (7.05 ppm) and 29 (7.02 ppm), suggesting that the NHb
protons in 25 and 27 formed weaker intramolecular hydrogen
bonds than did those in 23 and 29. This situation arose because
one of the driving forces for the formation of a 10-membered-
ring intramolecular hydrogen bond is the gauche orientation of
the Cγ-O bond relative to the CR-Câ bond. For 23, the phenyl
group at the γ position favors an anti orientation relative to the
CR-Câ bond, which results in a gauche orientation of the Cγ-O
bond relative to the CR-Câ bond. For 29, the cyclohexane ring
has already restricted the Cγ-O bond to be positioned gauche
to the CR-Câ bond. For 25 and 27, however, the alkyl
substituent at the γ position is relatively small; it cannot control
very well the gauche orientation of the Cγ-O bond relative to
the CR-Câ bond and, thus, weaker 10-membered-ring intramo-
lecular hydrogen bonds exist in 25 and 27 relative to those in
23 and 29.
a Conditions: (a) NH2NH2, MeOH, CH2Cl2; (b) EDCI, HOBt, isobutyric
acid, CH2Cl2; (c) TFA, CH2Cl2; (d) EDCI, HOBt, CH2Cl2.
agent, we coupled the monomers 3, 7, 11, 15, and 19 to form
the diamides 21 (Scheme 7), 23, 25, 27, and 29 (Scheme 8)
and the triamides 31 (Scheme 9) and 33 (Scheme 10). Below,
we discuss our detailed conformational studies of these com-
pounds; we have previously communicated some conformational
studies of 21, 23, and 31 (Chart 2).22
Two 1H NMR spectroscopic methods, dilution29 and DMSO-
d6 addition,3030 are applied to probe the formation of intramo-
lecular hydrogen bonds in the oligomers of γ-aminoxy acids
(Chart 2). The 1H NMR dilution study is to monitor the chemical
shift changes of amide protons with respect to peptide concen-
tration, and the 1H NMR DMSO-d6 addition study is to follow
the chemical shift changes of amide protons with respect to the
content of DMSO-d6 added. Data for the 1H NMR dilution study
of amide proton NHc of 31 and the 1H NMR DMSO-d6 addition
studies of amide protons NHb of 23 and NHc of 31 are not
presented here because the chemical shift values of these protons
overlapped with the chemical shift values of phenyl protons in
these molecules.
Table 1 summarizes the chemical shifts of the amide protons
of 21, 23, 25, 27, 29, 31, and 33 and their chemical shift changes
The chemical shift of the NHc proton in 33 was located further
downfield (6.90 ppm) than that of the NHb proton in 25 (6.45
ppm), which indicates that the intramolecular hydrogen bond
in 33 is stronger than that in 25, suggesting that the intramo-
lecular hydrogen bond of the NHc proton was strengthened after
elongation of the peptide chain from the diamide 25 to the
triamide 33; that is, the formation of consecutive 10-membered-
ring intramolecular hydrogen bonds is a cooperative process.
1
(Figure 1) in H NMR spectroscopic dilution and DMSO-d6
addition studies at room temperature. The chemical shifts of
the N-oxy amide protons NHa of these compounds at 0.78 mM
were located considerably upfield (7.81-8.62 ppm), with
(29) Haque, T. S.; Little, J. C.; Gellman, S. H. J. Am. Chem. Soc. 1994, 116,
4105-4106.
(30) Copeland, G. T.; Jarvo, E. R.; Miller, S. J. J. Org. Chem. 1998, 63, 6784-
6785.
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-Aminoxy Peptides
A R T I C L E S
Chart 2. 1H NMR Spectroscopic Studies of γ-Aminoxy Peptides
Table 1. Chemical Shifts (δNH) and Chemical Shift Changes
(∆δNH) of Amide Protons in 1H NMR Spectroscopic Dilution (dilu.)
and DMSO-d6 Addition (DMSO) Studies of Peptides 21, 23, 25,
27, 29, 31, and 33 at 25 °C
For compounds 23, 25, 27, and 29, we observed three main
sets of peaks, which we assigned to the stretching frequencies
of the non-hydrogen-bonded amide NHb (3428, 3430, 3429, and
3420 cm-1), non-hydrogen-bonded N-oxy amide NHa (3388,
3392, 3391, and 3386 cm-1), and hydrogen-bonded amide NHb
(3324, 3320, 3322, and 3316 cm-1) bonds, respectively. The
presence of major hydrogen-bonded amide NHb and minor non-
hydrogen-bonded amide NHb bands in 23 and 29 indicates that
they adopt extensive 10-membered ring intramolecular hydrogen
bond conformations. The relatively smaller hydrogen-bonded
and the relatively larger non-hydrogen-bonded amide NHb bands
in 25 and 27, compared with those in 23 and 29, suggest that
the tendency of 25 and 27 to experience 10-membered-ring
intramolecular hydrogen bond formation is weaker than that of
23 and 29. In addition, the relatively larger hydrogen-bonded
and relatively smaller non-hydrogen-bonded amide NHb bands
in 25 and 27, compared with those for 21, indicate that the
tendency of 25 and 27 to experience 10-membered-ring in-
tramolecular hydrogen bond formation is stronger than that of
21.
NHa (ppm)
NHb (ppm)
NHc (ppm)
∆δb
(dilu.) (DMSO)
∆δc
∆δb
(dilu.) (DMSO)
∆δc
∆δb
(dilu.) (DMSO)
∆δc
δa
δa
δa
21 8.62 1.06
23 7.94 0.82
25 8.45 0.81
27 8.48 0.70
29 8.37 0.83
31 7.81 1.07
33 8.33 1.37
1.66
2.15
1.87
1.75
1.96
2.30
2.10
6.20 0.56
7.05 0.2
0.91
d
6.45 0.39
6.39 0.38
7.02 0.39
10.57 0.2
10.43 0.33
0.82
0.86
0.73
0.34
0.35
6.90 0.25
0.32
a δ: Amide NH proton’s chemical shift obtained from the 1H NMR
spectrum of the indicated compound at a concentration of 0.78 mM in
CDCl3. b The value of ∆δ in the dilution studies was calculated using the
equation ∆δ ) δNH (100 mM) - δNH (0.78 mM). c The value of ∆δ in the
DMSO-d6 addition studies was calculated using the equation ∆δ ) δNH
(9% DMSO-d6 in CDCl3) - δNH (CDCl3). d The signal overlapped with
the aromatic protons after the addition of DMSO.
IR Spectroscopic Studies of γ-Aminoxy Peptides. Data
from the N-H stretching region in IR spectra can provide
insight into the degree of hydrogen bond formation in nonpolar
solvents because the time scale of IR spectroscopic measure-
ments is sufficiently short to distinguish clearly between the
N-H stretchings of hydrogen-bonded and non-hydrogen-bonded
states. According to our previous studies, the IR spectral
absorption bands of the non-hydrogen-bonded amide N-H and
N-oxy amide N-H bonds appear in the regions 3450-3400
and 3400-3340 cm-1, respectively, while the absorption bands
corresponding to the hydrogen-bonded amide N-H and N-oxy
amide N-H bonds appear at wavenumbers less than 3370 and
3250 cm-1, respectively.17
It is interesting to note that there were small bands in the
region 3200-3300 cm-1 in the IR spectra of 21, 25, and 27.
We assign these bands to the stretching frequencies of the
hydrogen-bonded NHa groups, which suggests the formation
of eight-membered-ring intramolecular hydrogen bonds between
the N-oxy amide NHi and CdOi groups in 21, 25, and 27 (Figure
3).
For the triamides 31 and 33, we observed four bands (3441,
3387, 3325, and 3211 cm-1 for 31; 3428, 3394, 3300, and 3214
cm-1 for 33), which we assign to the stretching frequencies of
the non-hydrogen-bonded amide NHc (3441 and 3428 cm-1),
the non-hydrogen-bonded N-oxy amide NHa and NHb (3387
and 3394 cm-1), the hydrogen-bonded amide NHc (3325 and
3300 cm-1), and the hydrogen-bonded N-oxy amide NHa and
NHb (3211 and 3214 cm-1) bonds, respectively. The large
hydrogen-bonded amide NHc bands and the small non-hydrogen-
bonded amide NHc bands indicate that the NHc groups of 23
and 29 exist mainly in 10-membered-ring intramolecular
hydrogen bond conformations. Considering that the NHa groups
did not form intramolecular hydrogen bonds and that the NHb
groups formed intramolecular hydrogen bonds (according to the
results of our 1H NMR spectroscopy studies), the bands at 3387
and 3394 cm-1 can be assigned primarily to the signals of the
Figure 2 presents the N-H stretching regions of the FT-IR
spectra of 21, 23, 25, 27, 29, 31, and 33. The spectra were
recorded for these samples in dichloromethane at a concentration
(2 mM) at which intermolecular hydrogen bonding is unlikely
to occur. For 21, we observed two major bands (3446 and 3392
cm-1, assigned to the stretching frequencies of the non-
hydrogen-bonded amide NHb and NHa bonds) and two minor
bands (3338 and 3280 cm-1, assigned to the stretchings of the
hydrogen-bonded NHb and NHa bonds). The presence of a minor
intramolecular hydrogen-bonded amide band for the NHb bond
of 21 indicates that the 10-membered-ring intramolecular
hydrogen-bonded conformation did not predominate.
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A R T I C L E S
Chen et al.
Figure 1. (Left-hand column) Amide proton chemical shifts plotted as a function of the logarithm of the concentration of peptides 21, 23, 25, 27, 29, 31,
and 33 in CDCl3 at room temperature; (right-hand column) amide proton chemical shifts plotted as a function of the amount of DMSO-d6 added to 5 mM
solutions of peptides 21, 23, 25, 27, 29, 31, and 33 in CDCl3 (0.5 mL) at room temperature.
non-hydrogen-bonded N-oxy amide NHa groups and those at
3325 and 3300 cm-1 can be assigned primarily to the signals
of hydrogen-bonded NHb groups; thus, the existence of two
consecutive 10-membered-ring intramolecular hydrogen bonds
9
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γ
-Aminoxy Peptides
A R T I C L E S
Figure 2. N-H Stretching regions of the FT-IR spectra of 21, 23, 25, 27, 29, 31, and 33 (2 mM in CH2Cl2) at room temperature (after subtraction of the
spectrum of pure CH2Cl2).
predominates for the conformations of 31 and 33 in CH2Cl2
solution.
2D-NOESY Studies of γ-Aminoxy Peptides. We performed
2D-NOESY studies of 21, 23, 25, and 29 in CDCl3 to probe
their conformations in solution (Figure 4). Because of their
overlapping signals, no 2D-NOESY spectra were recorded for
27, 31, and 33. For 23, 25, and 29, we detected obvious NOE
signals between Hb and the γ proton, but no such signal for 21.
This finding suggests that the backbones of 23, 25, and 29 were
bent, while that of 21 was not.
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A R T I C L E S
Chen et al.
stability to that of 34a. Conformer 34e, with its extended
backbone, is less stable than those conformers that feature
intramolecular hydrogen bonds. Overall, the calculations predict
that compound 34 can adopt both eight- and 10-membered-ring
hydrogen bond conformations, which would exist in equilibrium.
This finding is corroborated by the experimental results. For
example, in the IR spectrum of the unsubstituted γ-aminoxy
peptide 21, the peaks appearing at 3392 and 3338 cm-1
correspond to the free aminoxy amide NHa and the 10-
membered-ring hydrogen-bonded amide NHb units, respectively,
whereas the signals at 3346 and 3280 cm-1 correspond to the
free amide NHb and the eight-membered-ring hydrogen-bonded
amide NHa units, respectively.
Figure 3. Two possible intramolecular hydrogen bonds within a γ-aminoxy
peptide.
CD Studies of γ-Aminoxy Peptides. Circular dichroism
(CD) spectroscopy has been used successfully to characterize
the secondary structures of natural peptides31-33 and unnatural
peptides such as â-peptides4,34-36 and aminoxy peptides.16,17,22,23,37
Figure 5 presents the CD spectra of 23, 25, 29, 31, and 33
recorded at room temperature in 2,2,2-trifluoroethanol (TFE);
these CD signals have been normalized with respect to the
concentration and number of backbone N-O turns of each
compound. In the CD spectrum of 29, a strong absorption peak
appeared at 203 nm; its intensity was similar to that of the broad
peaks at ca. 210 nm in the spectra of 23 and 31.22 In contrast,
for 25 and 33 we observed only weak broad peaks between
205 and 230 nm. The lower intensity of these two peaks is
probably due to the conformational flexibility of 25 and 33, in
which the γ4-Me group does not exert a strong influence on
the secondary structures.
Theoretical Calculations. To further understand the con-
formational features of the unsubstituted γ-aminoxy and γ4-
Me-, γ4-Et-, γ4-Ph-, and γ3,4-cyclohexyl-substituted aminoxy
peptides, we performed theoretical calculations on the model
compounds 34-39 (Figure 6) using the Gaussian 98 software
package.38 All of the geometries were fully optimized using the
B3LYP39,40/6-31G (d, p) method followed by harmonic vibration
frequency calculations to ensure that each structure was a
minimum. Energies were evaluated through MP241-43/6-31G
(d, p) calculations on the B3LYP/6-31G (d,p) geometries. The
effect of solvent was evaluated by the PCM model using the
B3LYP/6-31G (d, p) method. A dielectric constant of 8.93 was
set to model CH2Cl2 as solvent. The relative free energies of
the conformers were calculated with the MP2/6-31G (d, p)
energies plus the enthalpy and entropy corrections along with
the solvent energy corrections, as shown in eq 1.
With a methyl group introduced to the γ carbon atom in an
(S)-configuration, model compound 35 can adopt several
conformations; Figure 8 displays the six most stable of them.
Conformer 35a, which is derived from 34a, but with a shorter
hydrogen bond length and a better hydrogen bond angle, is the
most stable. Conformers 35b and 35c, which are derived from
34b and 34c, respectively, are less stable than conformer 35a.
It is notable that while 34b is less stable than 34c, 35b is more
stable than 35c, presumably because the methyl group is gauche
to the N-O bond in 35b (with a distance of 2.74 Å between
one hydrogen atom on the methyl group and the aminoxy amide
proton), whereas it is gauche to the CR-Câ bond in 35c (with
a distance of 2.47 Å between one hydrogen atom on the methyl
group and the CR hydrogen atom). Obviously, the degree of
steric repulsion is stronger in 35c than it is in 35b. Conformers
35d and 35e, with their eight-membered-ring hydrogen bonds,
and the extended conformer 35f are also less stable than
conformer 35a. Conformer 35d is more stable than 35e because
the methyl group is aligned anti to the N-O bond in 35d, while
the Cγ-H bond is eclipsed with the N-O bond in 35e.
Interestingly, the energy difference between 35a and 35d is
larger than that between 34a and 34d, presumably because the
methyl group is gauche to the N-O bond in 35a (with a distance
of 2.73 Å between one hydrogen atom on the methyl group
and the amide hydrogen atom), whereas it is gauche to the CR-
Câ bond in 35d (with a distance of 2.29 Å between one hydrogen
atom on the methyl group and the CR hydrogen atom), resulting
in stronger steric repulsion in 35d than in 35a. As a result, the
energy increase from 34d to 35d is higher than that from 34a
to 35a. These results suggest that the γ4-Me aminoxy model
compound 35 having the (S)-configuration favors the left-handed
10-membered-ring hydrogen bond conformation, but because
the free energy difference between the dominant conformer 35a
and conformer 35d is not large (0.7 kcal/mol), their intercon-
version is facile. Our experimental results for compound 25
indicate that the 10-membered-ring hydrogen bond is favored
∆G ) ∆E(MP2) + [∆E(B3LYP, solvent) -
∆E(B3LYP)] + enthalpy correction - Τ∆S (1)
Figure 7 displays the five most stable conformations of model
compound 34. Ten-membered-ring hydrogen bonds are found
in conformers 34a-c, of which 34a is the most stable (hydrogen
bond length: 2.00 Å; hydrogen bond angle: 163.0°); conformers
34b and 34c are less stable because of their unfavorable torsional
angles æ and φ, respectively (see Table 2). Conformer 34d,
having an eight-membered-ring hydrogen bond, has a similar
Figure 4.
9
750 J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008

γ
-Aminoxy Peptides
A R T I C L E S
Figure 7. Calculated conformers of model compound 34. The calculated
relative free energies (kcal/mol) in the gas phase and in CH2Cl2 (in
parentheses) and the N-H‚‚‚O hydrogen bond lengths and angles are
provided.
over the 8-membered-ring hydrogen bond (stronger signals at
3392 and 3320 cm-1 than that at 3430 cm-1).
Figure 5. CD spectra of peptides 25 (0.1 mM in TFE) and 23, 29, 31, and
33 (0.4 mM in TFE) at 25 °C.
The conformations of compound 36 can be derived from those
of compound 35 by replacing one of the hydrogen atoms on
the Cγ methyl group with a methyl group. Figure 9 indicates
that the relative stabilities of the most stable conformers of
compound 36 are similar to those of compound 35. One notable
change is that the calculated free energy difference between 36a
and 36d is ca. 0.1 kcal/mol in CH2Cl2, while the corresponding
energy difference between 35a and 35d is ca. 0.7 kcal/mol. In
both 36a and 36d, the additional methyl group is aligned gauche
to the Câ-Cγ bond, but the distances between the closest
hydrogen atoms on the additional methyl group and on the Câ
atom are 2.29 Å in 36a and 2.36 Å in 36d. Because of the
stronger repulsion in 36a than in 36d, the free energy increase
from 35a to 36a is higher than that from 35d to 36d. These
Figure 6. Model γ-aminoxy peptides 34-39.
Table 2. Torsional Angles of γ-Aminoxy Peptides Optimized Using B3LYP/6-31G (d, p)
structure
φ
θ
æ
ψ
structure
φ
θ
æ
ψ
structure
φ
θ
æ
ψ
34a
34b
34c
34d
34e
35a
35b
35c
35d
35e
35f
-169.3
-163.7
62.2
61.6
-75.5 -61.4
68.5 -102.1
36b
36c
36d
36e
36f
37a
37b
37c
37d
37e
38a
38b
-159.0
57.3
-73.7 -62.7
-88.4
124.7
-88.0
38c
38d
38e
39a
-79.9 -52.3
-74.9 -50.1
163.0
-93.6
-86.3
167.1
118.3
167.2
71.4 -145.7
57.3 -57.3 167.6
-86.5
73.0 -147.2
-69.5 -56.9
71.9 -144.7
-70.1
-159.3
-161.9
-165.9
-161.5
-159.5
-161.4
-70.3 -55.9
-179.9 -61.3 -169.3 -147.7
-116.4
-155.8
-166.6
-162.1
59.5
-71.4
62.3
61.2
62.1
69.7 -112.4
69.5 -104.2
69.6 -110.9
67.4 -171.7 -178.7
59.9
57.1
-164.8
-159.8
-74.5 -62.1
-70.6 -55.5
-117.7
-156.8
-161.3
62.6
58.0
69.8 -106.3
67.1 -105.6
-88.2
167.1
39b
39c
39d
-88.5
166.7
72.5 -146.3
-69.0
123.5
-87.2
122.5
-89.3
56.6 -89.3
62.6 70.6 -112.9
165.1
121.1
-74.4 -61.9
-66.0 -59.6
-158.5
-166.0
-166.9
70.9 -142.6
-73.3 -61.5
-64.4 -58.3
-62.3 -58.1
-87.6
60.4
66.6 -170.4 -175.8
63.4 70.7 -108.1
-71.9
65.8 -166.8 -171.2
59.5
62.7
68.1 -134.2
75.0 -150.0
69.2 -109.4
-83.8 110.0
36a
9
J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008 751

A R T I C L E S
Chen et al.
Figure 8. Calculated conformers of model compound 35. The calculated relative free energies (kcal/mol) in the gas phase and in CH2Cl2 (in parentheses)
and the N-H‚‚‚O hydrogen bond lengths and angles are provided.
Figure 9. Calculated conformers of model compound 36. The calculated relative free energies (kcal/mol) in the gas phase and in CH2Cl2 (in parentheses)
and the N-H‚‚‚O hydrogen bond lengths and angles are provided.
results nicely explain our experimental observation that com-
pound 27 having the (S)-configuration and an ethyl side chain
adopts the left-handed 10-membered-ring hydrogen bond con-
formation (major) and the 8-membered-ring hydrogen bond
conformation (minor), and the 8-membered-ring hydrogen bond
is more favorable in 27 than it is in 25 (stronger absorption at
3241 cm-1 for 27 than that for 25).
Figure 10 displays the five most stable conformations of
model compound 37, which has an (R)-configuration and a
phenyl group at the γ4 position. Conformer 37a is the most
(31) Lyu, P. C.; Liff, M. I.; Marky, L. A.; Kallenbach, N. R. Science 1990,
250, 669-673.
(32) Oneil, K. T.; Degrado, W. F. Science 1990, 250, 646-651.
(33) Marqusee, S.; Robbins, V. H.; Baldwin, R. L. Proc. Natl. Acad. Sci. U.S.A.
1989, 86, 5286-5290.
(35) Raguse, T. L.; Lai, J. R.; Gellman, S. H. J. Am. Chem. Soc. 2003, 125,
5592-5593.
(34) Rueping, M.; Schreiber, J. V.; Lelais, G.; Jaun, B.; Seebach, D. HelV. Chim.
Acta 2002, 85, 2577-2593.
(36) Wang, X. F.; Espinosa, J. F.; Gellman, S. H. J. Am. Chem. Soc. 2000, 122,
4821-4822.
9
752 J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008

γ
-Aminoxy Peptides
A R T I C L E S
Figure 10. Calculated conformers of model compound 37. The calculated relative free energies (kcal/mol) in the gas phase and in CH2Cl2 (in parentheses)
and the N-H‚‚‚O hydrogen bond lengths and angles are provided.
stable with its 10-membered-ring hydrogen bond (hydrogen
bond length: 1.99 Å; hydrogen bond angle: 163.5°). Conform-
ers 37b and 37c are relatively less stable. Conformer 37b suffers
from severe distortion because of an unfavorable torsional angle
æ (-88.2°). Conformer 37c is much less stable than 37a because
of a combination of a bad torsional angle φ (-74.4°) and a
smaller angle between the phenyl ring and the C-O bond (25.6°
in 37c; 39.9° in 37a), which leads to stronger steric repulsion
in 37c than in 37a. Conformer 37d with its eight-membered-
ring hydrogen bond is also much less stable than 37a because
of the smaller angle between its phenyl ring and the C-O bond
(19.5° in 37d; 39.9° in 37a). The energy difference between
the dominant conformer 37a and the other conformers is at least
1.7 kcal/mol, significantly larger than those for 35 and 36
because the phenyl ring is bulkier than either methyl or ethyl
groups. These results reveal that the γ4-aminoxy model com-
pound 37 should strongly favor the 10-membered-ring hydrogen
bond conformer 37a and that the formation of the 10-membered-
ring hydrogen bond in model compound 37 is more favorable
than that in model compounds 35 and 36.
presence of the rigid cyclohexyl ring. Conformers 38c and 38d
with their eight-membered-ring hydrogen bonds are less stable
than 38a because of their unfavorable torsional angles φ and θ.
The energy difference between the dominant conformer 38a and
the other conformers is greater than 1.6 kcal/mol because the
cyclohexyl side chain is bulky and rigid; in addition, the other
conformers of 38 suffer from 4more severe distortion than does
38a. Therefore, the γ-aminoxy amide having a cis-disubstituted
cyclohexyl side chain at its â and γ positions is predicted to
strongly favor the 10-membered-ring intramolecular hydrogen
bond conformer 38a.
From our theoretical calculations of the model compounds
34-38, we conclude that the γ4-aminoxy peptides can form 10-
membered-ring intramolecular hydrogen bonds (γ N-O turns)
and that the extent of the hydrogen bond formation increases
from the unsubstituted γ-aminoxy peptide to the γ4-alkyl-
substituted aminoxy peptides to the γ3,4-cyclohexyl- and γ4-
Ph-substituted aminoxy peptides.
Figure 12 displays the four most stable conformations of the
model compound 39. Conformer 39a is the most stable with its
two contiguous left-handed γ N-O turn units, akin to the most
stable conformer 35a. Overall, 39a corresponds to a helical
structure, referred to as a γ N-O helix. Interestingly, the lengths
of the hydrogen bond of 39a are 1.90 and 1.95 Å, with hydrogen
bond angles of 170.2 and 166.8°, respectively, which are better
than those of 35a. This finding indicates that the formation of
the two 10-membered-ring hydrogen bonds in 39 is a coopera-
tive process, in good agreement with the experimental observa-
tion that triamide 33 forms stronger intramolecular hydrogen
bonds than does diamide 25. Conformers 39b and 39c are less
stable than 39a because of the presence of less-stable γ N-O
turn units (akin to that of 35b in 39b and 35c in 39c) in addition
to one γ N-O turn unit akin to that in 35a. Conformer 39d has
two contiguous eight-membered-ring hydrogen bond units (akin
Figure 11 displays the five most stable conformations of
model compound 38, in which the (S)-â and (S)-γ carbon atoms
are part of a cis-disubstituted cyclohexyl ring. Conformer 38a
is the most stable with its 10-membered-ring hydrogen bond.
Conformer 38b is relatively less stable because of severe
distortion of its torsional angle æ (-83.9°) caused by the
(37) Yang, D.; Li, W.; Qu, J.; Luo, S. W.; Wu, Y. D. J. Am. Chem. Soc. 2003,
125, 13018-13019.
(38) Frisch, M. J.; et al. Gaussian 98, revision A.11.3, Gaussian, Inc.: Pittsburgh,
PA, 2002.
(39) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(40) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(41) Moller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618-622.
(42) Frisch, M. J.; Headgordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166,
275-280.
(43) Frisch, M. J.; Headgordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166,
281-289.
9
J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008 753

A R T I C L E S
Chen et al.
Figure 11. Calculated conformers of model compound 38. The calculated relative free energies (kcal/mol) in the gas phase and in CH2Cl2 (in parentheses)
and the N-H‚‚‚O hydrogen bond lengths and angles are provided.
Figure 12. Calculated conformers of model compound 39. The calculated relative free energies (kcal/mol) in the gas phase and in CH2Cl2 (in parentheses)
and the N-H‚‚‚O hydrogen bond lengths and angles are provided.
Conclusion
to that in 35d) of the same handedness. This structure is
calculated to be much less stable than structure 39a because
the torsional angles φ (-64.4 and -62.3°, respectively) in 39d
are deviated greatly from that in 35d (-70.6°). These results
indicate that the (S,S)-γ4-aminoxy model peptide 39 should favor
a helical structure formed from two left-handed γ N-O turn
units akin to that in conformer 35a.
On the basis of experimental and theoretical findings for a
series of γ-aminoxy peptides, namely those containing unsub-
stituted and γ4-Ph-, γ4-alkyl-, and γ3,4-cyclohexyl-substituted
aminoxy acid residues, we conclude that γ-aminoxy peptides
can form 10-membered-ring intramolecular hydrogen bonds
9
754 J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008

γ-Aminoxy Peptides
A R T I C L E S
between adjacent residues, that is, γ N-O turns. The extent of
formation of the 10-membered-ring intramolecular hydrogen
bonds increases from the unsubstituted γ-aminoxy peptide to
the γ4-alkyl-substituted aminoxy peptides to the γ4-phenyl- and
Acknowledgment. This study was supported by the Univer-
sity of Hong Kong and the Hong Kong Research Grants Council
(grants HKU 7367/03M, HKU 2/06C, and HKUST 6083/02M).
D.Y. acknowledges the Bristol-Myers Squibb Foundation for
an unrestricted grant in Synthetic Organic Chemistry.
γ
3,4-cyclohexyl-substituted aminoxy peptides. The presence
of two consecutive homochiral 10-membered-ring intramole-
cular hydrogen bonds results in the formation of a novel
helical structure in which the formation of the consecutive
γ N-O turns is a cooperative process. The experimental
studies and theoretical calculations on γ N-O turns and
γ N-O helices in γ-aminoxy peptides have provided many
insights into the nature of folding of γ-aminoxy peptides. The
understanding of the effect of the side chains on local confor-
mational features of γ-aminoxy peptides should stimulate
the design of new foldamers and their applications in drug
development.
Supporting Information Available: Experimental details for
the preparation and characterization of compounds 1-33; HPLC
traces for the determination of the values of ee of compounds
6, 11, and 19; 2D-NOESY spectra of peptides 23, 25, and 29;
calculated relative energies of all γ-aminoxy peptides; calculated
energies and corrections of all γ-aminoxy peptides; geometries
(Cartesian coordinates) of all γ-aminoxy peptides optimized
using B3LYP/6-31 (d, p); complete ref 38. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0772750
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J. AM. CHEM. SOC. VOL. 130, NO. 2, 2008 755