Oligo(N-aryl glycines): A new twist on structured peptoids

Article abstract of DOI:10.1021/ja804580n

We explore strategies to enhance conformational ordering of N-substituted glycine peptoid oligomers. Peptoids bearing bulky N-alkyl side chains have previously been studied as important examples of biomimetic "foldamer" compounds, as they exhibit a capacity to populate helical structures featuring repeating cis-amide bonds. Substantial cis/trans amide bond isomerization, however, gives rise to conformational heterogeneity. Here, we report the use of N-aryl side chains as a tool to enforce the presence of trans-amide bonds, thereby engendering structural stability. Aniline derivatives and bromoacetic acid are used in the facile solid-phase synthesis of a diverse family of sequence-specific N-aryl glycine oligomers. Quantum mechanics calculations yield a detailed energy profile of the folding landscape and substantiate the hypothesis that the presence of anilide groups establishes a strong energetic preference for trans-amide bonds. X-ray crystallographic analysis and solution NMR studies verify this preference. Molecular modeling indicates that the linear oligomers can adopt helical structures resembling a polyproline type II helix. High resolution structures of macrocyclic oligomers incorporating both N-alkyl and N-aryl glycine units confirm the ability to direct the presence of trans-amide bonds specifically at N-aryl positions. These results are an important step in developing strategies for the rational de novo design of new structural motifs in biomimetic oligopeptoid systems.

Full text of DOI:10.1021/ja804580n

Published on Web 11/17/2008
Oligo(N-aryl glycines): A New Twist on Structured Peptoids
Neel H. Shah,† Glenn L. Butterfoss,‡,§ Khanh Nguyen,| Barney Yoo,†
Richard Bonneau,‡,§ Dallas L. Rabenstein,| and Kent Kirshenbaum*,†
Department of Chemistry, Center for Genomics & Systems Biology, and Courant Institute of
Mathematical Sciences, Department of Computer Science, New York UniVersity,
New York, New York 10003, and Department of Chemistry, UniVersity of California,
RiVerside, California 92521
Received June 16, 2008; E-mail: kent@nyu.edu
Abstract: We explore strategies to enhance conformational ordering of N-substituted glycine peptoid
oligomers. Peptoids bearing bulky N-alkyl side chains have previously been studied as important examples
of biomimetic “foldamer” compounds, as they exhibit a capacity to populate helical structures featuring
repeating cis-amide bonds. Substantial cis/trans amide bond isomerization, however, gives rise to
conformational heterogeneity. Here, we report the use of N-aryl side chains as a tool to enforce the presence
of trans-amide bonds, thereby engendering structural stability. Aniline derivatives and bromoacetic acid
are used in the facile solid-phase synthesis of a diverse family of sequence-specific N-aryl glycine oligomers.
Quantum mechanics calculations yield a detailed energy profile of the folding landscape and substantiate
the hypothesis that the presence of anilide groups establishes a strong energetic preference for trans-
amide bonds. X-ray crystallographic analysis and solution NMR studies verify this preference. Molecular
modeling indicates that the linear oligomers can adopt helical structures resembling a polyproline type II
helix. High resolution structures of macrocyclic oligomers incorporating both N-alkyl and N-aryl glycine
units confirm the ability to direct the presence of trans-amide bonds specifically at N-aryl positions. These
results are an important step in developing strategies for the rational de novo design of new structural
motifs in biomimetic oligopeptoid systems.
Introduction
mers is limited, but growing. The study of folding principles in
these synthetic systems is now under active investigation.2 A
The de novo design of heteropolymer chains with predictable
folded structures is a formidable challenge for researchers in
the fields of both biopolymer and synthetic polymer chemistry.
Within the context of protein design, each individual amino acid
residue can assume several distinct backbone and side chain
dihedral angles. It is difficult, therefore, to determine a priori
particular amino acid sequences that will favor one set of these
angles over the multitude of alternatives. For proteins, however,
the numerous structures deposited in the Protein Data Bank
facilitate the rational design of sequences that are compatible
with a desired backbone topology.1 In contrast, detailed
structural information for non-natural sequence-specific oligo-
variety of synthetic oligomers have been described that exhibit
a propensity to form stable secondary structures.3-5 These
“foldamers” show promise for applications in biomedicine and
materials science.6,7 Investigating the conformational preferences
of foldamers and their constituent monomer sets is a critical
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Thomas, P. D.; Chan, H. S. Protein Sci. 1995, 4, 561–602. (b) Gellman,
S. H. Acc. Chem. Res. 1998, 31, 173–180. (c) Hill, D. J.; Mio, M. J.;
Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101,
3893–4011. (d) Goodman, C. M.; Choi, S.; Shandler, S.; DeGrado,
W. F. Nat. Chem. Biol. 2007, 3, 252–262. (e) Foldamers: Structure,
Properties, and Applications; Hecht, S.; Huc, I. Eds.; Wiley-VCH:
Weinheim, Germany, 2007.
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† Department of Chemistry, New York University.
‡ Center for Genomics & Systems Biology, New York University.
§ Department of Computer Science, New York University.
| University of California.
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16622 J. AM. CHEM. SOC. 2008, 130, 16622–16632
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2008 American Chemical Society

N-Aryl Peptoids
A R T I C L E S
Scheme 1. Comparison of Peptide and Peptoid Structures
initial step in understanding the sequence–structure relationships
that can guide the predictable design of diverse folded
architectures.
The construction of sequence-specific oligomers with well-
defined conformations typically relies on one of two design
concepts. The first strategy is to employ flexible oligomer
systems that can establish long-range noncovalent interactions
to direct folding. This is evident in biopolymers, where hydrogen
bonding and hydrophobic interactions aid in defining protein
structures.8 These interactions are also seen in non-natural
oligomers, in particular ꢀ-peptides, whose diverse secondary
structures can be defined by various intramolecular hydrogen-
bonding patterns.3 The second design strategy is to use
oligomers incorporating rigid monomer units with conforma-
tionally constrained linkages. This method allows for predictable
organization of the oligomer backbone through local confor-
mational preferences. For example, the semirigid covalent
linkages between monomer units in oligo(m-phenylene-ethy-
nylenes) predispose the oligomers to form helical conforma-
tions.4a-c
We adopt a general approach for designing complex self-
assembled structures in biomimetic macromolecules that entails,
first, the discovery of monomer types predisposed to generate
well-defined conformations that can be propagated through local
interactions upon oligomerization; second, an effort to solve
three-dimensional structures of diverse oligomer sequences in
order to describe these conformations; third, the use of
computational tools employing an energy function to sample
conformational space and evaluate variations in conformational
preferences for different oligomer sequences. Implementation
of this strategy would then rely upon an iterative process to
guide the synthesis of increasingly complex structures. In
practice, analysis of experimental findings can ensure the
suitability of the computational tools, and the energy function
in turn can guide subsequent generations of oligomer sequences.
This study embarks upon such an approach.
ylene group which can confer flexibility, and there is no intrinsic
capacity within the backbone to establish long-range interactions,
as there are no hydrogen-bond donors (Scheme 1). Nevertheless,
the incorporation of bulky, branched N-alkyl substituents has
been shown to generate local steric interactions that can direct
conformational preferences.10a A variety of peptoids bearing
these side chains exhibit a polyproline type I helical secondary
structure with repeating cis-amide bonds.5,10 In solution,
however, this structure is the dominant member of a multicon-
formational ensemble.5,10c
A critical impediment to the design of peptoids with stable
secondary structures is that there is substantial conformational
heterogeneity associated with the ability to populate both cis-
and trans-amide bond geometries.10c Peptoids bearing bulky,
branched N-alkyl side chains exhibit only a modest energetic
preference for cis-amide bonds, with a cis-trans-free energy
difference typically under 1 kcal/mol.10a,11 Efforts have been
made to modulate this conformational preference by tuning
stereoelectronic interactions between proximal backbone amide
groups and side chains.11b In short oligomers, however, this
approach generally does not yield energetic preferences sub-
stantially above 1 kcal/mol. It is not yet evident if such energy
differences between cis- and trans-amide bonds will be sufficient
to establish conformational homogeneity in longer oligomers.
Alternatively, strategies have been explored to establish long-
range constraints that can better define peptoid backbone
conformation. One strategy that has proven effective in reducing
Our investigations are focused on a class of peptidomimetics
known as peptoids. These oligomers are composed of a wide
variety of N-substituted glycine monomer units that can be
efficiently linked in a sequence-specific manner.9 Cursory
inspection would suggest that peptoids are poor candidates for
folding. The peptoid backbone repeating unit includes a meth-
(5) Armand, P.; Kirshenbaum, K.; Goldsmith, R. A.; Farr-Jones, S.;
Barron, A. E.; Truong, K. T. V.; Dill, K. A.; Mierke, D. F.; Cohen,
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Werder, M.; Hauser, H.; Abele, S.; Seebach, D. HelV. Chim. Acta
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375. (b) Kirshenbaum, K.; Barron, A. E.; Goldsmith, R. A.; Armand,
P.; Bradley, E. K.; Truong, K. T. V.; Dill, K. A.; Cohen, F. E.;
Zuckermann, R. N. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4303–
4308. (c) Wu, C. W.; Kirshenbaum, K.; Sanborn, T. J.; Patch, J. A.;
Huang, K.; Dill, K. A.; Zuckermann, R. N.; Barron, A. E. J. Am. Chem.
Soc. 2003, 125, 13525–13530.
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Schmutz, M. Chem.sEur. J. 2002, 8, 3448–3457. (c) Hou, J.-L; Yi,
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2008, 47, 1808–1818.
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Blackwell, H. E. J. Am. Chem. Soc. 2007, 129, 8928–8929.
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Soc. 2007, 129, 3218–3225. (b) Holub, J. M.; Jang, H.; Kirshenbaum,
K. Org. Lett. 2007, 9, 3275–3278. (c) Maulucci, N.; Izzo, I.; Bifulco,
G.; Aliberti, A.; De Cola, C.; Comegna, D.; Gaeta, C.; Napolitano,
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2008, 3927–3929.
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U.S.A. 1951, 37, 205–211. (b) Dill, K. A. Biochemistry 1990, 29, 7133–
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1991, 20, 447–490.
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Chem. Soc. 1992, 114, 10545–10647.
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A R T I C L E S
Shah et al.
Scheme 2. Amide Bond Geometry in Peptides and Peptoidsa
Scheme 3. Dihedral Angles in N-Aryl Peptoidsa
a (A) Atom labels within an N-aryl glycine monomer. (B) A representa-
tive N-aryl glycine peptoid oligomer with an acetylated N-terminus and
amidated C-terminus. ω refers to the ac-C-R(i-1), ac-CO(i-1), N(i), ac-C-R(i)
dihedral angle. ꢂ refers to the ac-CO(i-1), N(i), ac-C-R(i), ac-CO(i) dihedral
angle. ψ refers to the N(i), ac-C-R(i), ac-CO(i), N(i+1) dihedral angle. ꢁ1 refers
to the ac-CO(i-1), N(i), N-C- R(i), N-C- ꢀ(i) dihedral angle. For ortho- or
meta-substituted N-phenyl side chains, N-C-ꢀ refers to the carbon atom
closest to the substituent. The subscript (i) refers to a particular residue
number.
a The cis/trans notation for peptoid amide groups is chosen for
consistency with the notation for polypeptides and proteins, although E/Z
notation could also be applied.
peptoid conformational heterogeneity is the inclusion of covalent
macrocyclic constraints.12 Head-to-tail cyclized peptoid oligo-
mers can be synthesized with high efficiency and result in
products that exhibit sufficient conformational ordering to yield
X-ray crystal structures.12a,c Although this strategy has proven
effective in defining a new series of stable peptoid conforma-
tions, it is limited to cyclic products.
structural unit representative of the amide bond found in N-aryl
peptoids. Pedersen et al. showed in the 1960s that the stable
conformation of this small organic molecule, both in solution
and in its crystalline state, displayed the amide oxygen trans to
the phenyl ring (ω ≈ 180°), with the phenyl ring perpendicular
to the plane of the amide bond (ꢁ1 ≈ 90°) (Scheme 3).14 We
sought to determine the energetic difference between the cis-
and trans-amide isomers and thus verify the utility of ab initio
quantum mechanics calculations in evaluating the conforma-
tional preferences of small peptoid-resembling structures. N-
methylacetanilide was allowed to freely minimize at the HF/
G-31G* level of theory from both cis- and trans-amide starting
structures. The geometry of the optimized trans-amide structure
displayed the same dihedral angles as previously determined
by experiment (Figure 1). Energy calculations using HF,
B3LYP, and MP2 methods consistently establish that a trans-
amide bond (ω ≈ 180°) is favored by greater than 2.5 kcal/mol
and ꢁ1 has a single broad minimum centered around 90° (Figure
2).
We are developing new tools to control peptoid conforma-
tions, thereby expanding the scope of accessible secondary,
tertiary, and quaternary structures. We seek readily available,
chemically diverse monomer types that establish intramolecular
interactions and lead to predictable sequence-structure relation-
ships. Previous structural investigations of peptoids have focused
on the use of relatively flexible N-alkyl glycine monomer units
that may confound the design of well-defined peptoid folds. This
study evaluates the conformational properties of N-aryl glycine
peptoids (Scheme 1). Bradley and co-workers initially suggested
that the incorporation of N-aryl peptoid side chains may reduce
conformational heterogeneity; however, this phenomenon was
not analyzed further.13 Here, we present an experimental and
computational evaluation of N-aryl side chains, establishing that
the inclusion of these groups within peptoid oligomers provides
a strong energetic preference for trans-amide bond geometries
(Scheme 2). We demonstrate that N-aryl glycine building blocks
can be used to generate new peptoid secondary structures that
are conformationally defined in solution. This new family of
peptoid structures may facilitate the de novo design of chemi-
cally diverse biomimetic architectures.
Pedersen, Suschitzky, and Itai have proposed that the origin
of the trans-amide bond preference in N-methylacetanilide and
other N-methylarylamides is a result of electronic repulsion
between the electron-dense center of the phenyl ring and the
Results and Discussion
(14) (a) Pedersen, B. F.; Pedersen, B. Tetrahedron Lett. 1965, 34, 2995–
3001. (b) Pedersen, B. F. Acta Chem. Scand. 1967, 6, 1415–1424.
(15) (a) Garner, G. V.; Meth-Cohn, O.; Suschitzky, H. J. Chem. Soc., C
1971, 1235–1236. (b) Itai, A.; Toriumi, Y.; Saito, S.; Kagechika Shudo,
H. J. Am. Chem. Soc. 1992, 114, 10649–10650. (c) Saito, S.; Toriumi,
Y.; Tomioka, N.; Itai, A. J. Org. Chem. 1994, 60, 4715–4720. (d)
Yamasaki, R.; Tanatani, A.; Azumaya, I.; Saito, S.; Yamaguchi, K.;
Kagechika, H. Org. Lett. 2003, 5, 1265–1267.
The Conformation of N-Methylacetanilides. We conducted
a computational study of N-methylacetanilide, the smallest
(13) Bradley, E. K.; Kerr, J. M.; Richter, L. S.; Figliozzi, G. M.; Goff,
D. A.; Zuckermann, R. N.; Spellmeyer, D. C.; Blaney, J. M. Mol.
DiVersity 1997, 3, 1–15.
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N-Aryl Peptoids
A R T I C L E S
Table 1. Energy Difference between Cis- and Trans-Amide Bonds
in Substituted N-Methylacetanilidesa
Figure 1. Model structure of N-methylacetanilide, optimized at the HF/
6-31G* level of theory.
Figure 2. Quantum mechanics calculations of N-methylacetanilide: (A)
Relative energy as a function of ω. Note that the region between 25° and
160° was not sampled because the amide nitrogen begins to assume a
tetrahedral geometry in this region. (B) Relative energy as a function of ꢁ1.
Note that ω was held fixed at 180°. For all plots, HF is HF/6-31G*//HF/
6-31G*, B3LYP is B3LYP/6-311+G(2d,p)//HF/6-31G*, and MP2 is MP2/
6-311+G(d,p)//HF/6-31G*.
a Calculated using various ab initio quantum mechanics methods. HF
is HF/6-31G*//HF/6-31G*, B3LYP is B3LYP/6-311+G(2d,p)//HF/
6-31G*, and MP2 is MP2/6-311+G(d,p)//HF/6-31G*.
amide oxygen, which would be in close proximity in the
cis-isomer.14,15 These studies suggest that various substituted
N-methylacetanilides should retain trans-amide bond preference
to varying degrees, depending on the electronic characteristics
of the substituents. To test this hypothesis, the energy differences
between cis and trans isomers of a set of substituted N-
methylacetanilides (1–10) were calculated (Table 1). Each
compound showed a substantial energetic preference for the
trans-amide bond, ranging from 1.05 to 3.55 kcal/mol at the
HF/G-31G* level of theory. Electron-withdrawing groups tended
to decrease (and electron-donating groups increased) this
preference.
To verify this trend experimentally, a subset of the molecules
found in Table 1 were synthesized, and their cis/trans-amide
conformer ratios were analyzed in solution by nuclear magnetic
resonance (NMR) spectroscopy (see Supporting Information).
These compounds uniformly displayed a strong preference for
populating the trans-amide bond geometry. For example, in
deuterated methanol at 25 °C, N-methylacetanilide 1 existed as
92.9% trans-amide conformer. Compound 6, bearing an electron-
donating methoxy substituent, existed as 95.7% trans-amide
conformer. Compound 8, bearing an electron-withdrawing fluoro
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A R T I C L E S
Shah et al.
Scheme 4. Structures of N-Aryl Peptoid Oligomers
mance liquid chromatography (see Figure 3 for representative
RP-HPLC chromatogram, characteristic of N-aryl peptoid
syntheses). Products were purified by RP-HPLC and their
identities were confirmed by electrospray mass spectrometry.
Structural Characterization of N-Aryl Peptoid Oligomers.
Peptoid dimer 11 was readily crystallized by slow evaporation
in methanol. X-ray crystallographic studies of the plate-like
crystals were conducted to determine a high-resolution crystal
structure (Figure 4). The X-ray structure of peptoid dimer 11
confirms that the N-aryl-substituted backbone amide bonds are
in the trans-amide bond geometry. Additionally, the values for
the ꢁ1 side chain dihedral angles are also consistent with the
computationally predicted rotamer (Table 2). The unit cell of
the crystal contains four molecules including two pairs of
identical structures in which the conformations from one pair
are enantiomers of the corresponding pair. The molecules within
the lattice pack to form a hydrogen-bonding network. In
particular, each dimer and its adjacent enantiomer form a pair
of intermolecular hydrogen bonds between the C-terminal
amides and N-terminal carbonyl oxygens. In addition, each
dimer and another enantiomer form a pair of hydrogen bonds
between the C-terminal carbonyl oxygens and side chain phenol
protons (see Supporting Information).
NMR spectroscopy was used to analyze the solution-phase
structures of peptoids 11 and 13. The conformational hetero-
geneity of each peptoid was assessed by 1-D 1H NMR in
deuterated acetonitrile and methanol. These experiments indicate
that both oligomers exist as one substantially dominant con-
formation in solution (see Supporting Information). 2-D nuclear
Overhauser effect spectroscopy (NOESY) was used to determine
the geometry of the amide bonds in the major solution-phase
conformer of each peptoid. Cis- and trans-amide bond geom-
etries of oligo(N-aryl glycines) can be distinguished on the basis
of distinct patterns of nuclear Overhauser effect interactions
(NOEs) (see Supporting Information, Scheme S3). Specifically,
the distance between backbone methylene protons of adjacent
residues is typically between 3.0 and 3.5 Å in the cis geometry
and between 4.5 and 5.0 Å in the trans geometry, the
approximate threshold distance for NOE observations. Thus,
the presence of a cis-amide bond should result in a substantial
NOE between methylene protons of adjacent residues, whereas
a trans-amide bond would preclude this cross-peak. Additionally,
only the presence of a trans-amide bond would establish NOEs
between the backbone methylene protons of one residue to the
aromatic side chain ortho-protons of the C-terminal adjacent
monomer. The NOESY spectra of peptoids 11 and 13 demon-
strate a pattern of NOEs dictated by repeating trans-amide bonds
throughout the backbone (see Supporting Information). These
dominant conformers had greater than 90% abundance in
solution relative to all observable conformers. Thus, NMR
spectroscopy demonstrates that N-aryl peptoid oligomers exhibit
conformational ordering distinguished by the presence of
repeating trans-amide bonds.
substitutent, existed as 91.9% trans-amide conformer. These data
support the calculated structural trend reported in Table 1 and
correspond with previously observed values in deuterated
dichloromethane at -60 °C.15d The computational and experi-
mental results collectively indicate that chemically diverse
aromatic substituents can be incorporated into N-methylaceta-
nilides while retaining the trans-amide bond conformer preference.
Synthesis of N-Aryl Peptoid Oligomers. Peptoid dimer 11 and
trimers 12 and 13 (Scheme 4), bearing three unique N-phenyl
side chains, were synthesized on Rink amide resin using a
variation of the solid-phase submonomer protocol described by
Zuckermann et al. (Scheme 5). 9 Typically, N-alkyl-substituted
glycine units are generated by the iteration of successive short
bromoacetylation and amine displacement steps (∼20 min).
Given the deactivated character of aryl amines, however, longer
reaction times are required.9 After the first 20 min bromoacety-
lation, each subsequent amine displacement with an aryl amine
was carried out for 16 h. All bromoacetylation reactions
following substitution by an aryl amine were carried out for 90
min. When desired, the N-termini of peptoid oligomers were
acetylated by a 90 min bromoacetylation reaction, followed by
a 2 h hydride displacement with sodium borohydride in
dimethylsulfoxide. Products were cleaved from the resin with
aqueous trifluoroacetic acid. To confirm the applicability of this
protocol to longer oligomer lengths, an acetylated N-(phenyl)-
glycine homotetramer 14 and homohexamer 15 were also readily
synthesized (Scheme 4). All peptoids were synthesized in good
yield as determined by analytical reversed-phase high-perfor-
Computational Studies of N-Aryl Peptoid Oligomers. Ad-
ditional insights into the conformational behavior of N-aryl
peptoids were obtained by a computational analysis of the
energy landscape for the oligomer backbone conformations.
Given the evidence for restricted side chain rotation (ꢁ1 ≈ 90°)
and the strong tendency to form trans-amide bonds (ω ≈ 180°),
we focused on evaluating the conformations available for the
other rotatable backbone bonds defined by the ꢂ and ψ dihedral
angles. Thus, the structure of representative molecule 16 was
studied, considering only ꢂ and ψ as unknown rotational degrees
9
16626 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

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A R T I C L E S
Scheme 5. Synthesis of N-Aryl Peptoid Oligomer 12
of freedom (Scheme 6). Compound 16 represents the smallest
component of an N-aryl peptoid oligomer that includes interac-
tions between neighboring side chains. We calculated a Ram-
achandran-type analysis of the conformational energy landscape
for 16 by sampling both ꢂ and ψ dihedrals at 15° increments
through the entire range of possible values at the B3LYP/6-
311+G(2d,p)//HF/6-31G* level of theory (Figure 5).16 The
resulting data show a center-symmetric landscape with two
global minima around (ꢂ,ψ) ) A(60°,-150°) and B(-60°,150°).
These two mirror image conformations, A and B, are at least 2
kcal/mol more stable than any other local minimum on the
landscape.
overall conformation, as observed previously for polyproline
type II structures.11b,17
In an N-aryl peptoid oligomer, either of these two local
conformations could exist at each monomer position, leading
to a potential mixture of secondary structures. However,
molecular modeling studies of N-(phenyl)glycine hexamer 15
show that steric clashes make many combinations less likely to
be populated. On the other hand, the repeating conformations
(A)6 and (B)6 are energetically favorable and form extended
right and left-handed helices, respectively (Figure 6). These
secondary structures contain repeating trans-amide bonds and
resemble the polyproline type II helix that is commonly observed
in proline-rich protein sequences. The modeled helix has roughly
3.1 residues per turn and a helical pitch of approximately 9 Å
per turn. The spacing between repeating aromatic side chains
is sufficiently large to preclude significant aromatic/aromatic
interactions. However, stabilizing nfπ* interactions between
adjacent trans-amide bonds may play a role in defining the
Figure 4. Crystal structure of peptoid dimer 11: (A) single molecule; (B)
unit cell (hydrogen atoms removed for clarity).
Figure 3. RP-HPLC chromatogram of crude peptoid 12.
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A R T I C L E S
Shah et al.
Table 2. Dihedral Angles Observed for Enantiomeric Chains in
Dimer 11 Crystal Structure
chain
monomer
ω
ꢂ
ψ
ꢁ1
A
N-(4-hydroxy-3-nitrophenyl)-
glycine
177.45
80.27 -173.33 -100.59
N-(phenyl)glycine
-175.68
95.98
162.81
173.34
111.82
100.59
B
N-(4-hydroxy-3-nitrophenyl)- -177.45 -80.22
glycine
N-(phenyl)glycine
175.64 -95.98 -162.81 -111.82
Scheme 6. Structure of Compound 16a
a Note that ω and ꢁ1 populate values near 180° and 90°, respectively
(see Figure 2 and Scheme 3).
Figure 6. Model of N-(phenyl)glycine peptoid hexamer 15 (A, parallel to
helix axis; B, perpendicular to helix axis) in a low energy conformation
resembling a polyproline type II helix (see methods for details).
of a cyclic peptoid hexamer, incorporating exclusively N-alkyl
side chains, displays a backbone amide bond motif that follows
the pattern (-cis-cis-trans-cis-cis-trans-)cyclic. Oligomer
17 was designed to mimic this motif, since the N-aryl side chains
are three residues apart and should populate trans-amide bonds
at these positions. By this rationale, oligomer 18 was designed
to generate a different backbone motif with adjacent trans-amide
bonds.
Figure 5. Ramachandran energy landscape of compound 16. Relative
energies (kcal/mol) are reported at the B3LYP/6-311+G(2d,p)//HF/6-31G*
level of theory for structures every 15° in φ and ꢂ. Regions in white are
>10 kcal/mol above the lowest energy structure.
Peptoid macrocycle 17 was readily crystallized by slow
evaporation from ethanol, and its high-resolution structure was
determined by X-ray crystallography. The crystal structure of
molecule 17 shows two trans-amide bonds at the sites with
N-aryl substituents, and four cis-amide bonds at the other four
locations. This pattern verified the initial prediction that N-aryl
side chains preferentially direct trans-amide bond formation
relative to N-alkyl side chains (Figure 7). The ꢂ and ψ dihedral
angles in both (N-phenyl)glycine monomers fall near the minima
predicted in the landscape above (Table 3). In addition, the ꢁ1
dihedral angles for the N-aryl residues were 111.3° and 71.41°,
within the range of the broad minimum predicted by computa-
tion of N-methylacetanilide (Figure 2B).
Cyclic Peptoids Bearing N-Aryl Glycine Units. Head-to-tail
macrocyclization has proven to be an effective approach to
generate novel, stable peptoid structures.12a In particular, cyclic
N-alkyl peptoid hexamers and octamers form tight hairpin turns
containing a mixture of cis- and trans-amide bonds. We explored
the incorporation of N-aryl glycines into peptoid macrocycles
and its effect on macrocycle structure. We anticipated that cyclic
oligomers incorporating both N-alkyl and N-aryl glycine mono-
mers would populate a combination of both cis- and trans-amide
bonds throughout the backbone, and the placement of N-aryl
glycines would site-specifically direct the formation of trans-
amide bonds. Thus, cyclic peptoid hexamers 17 and 18 were
synthesized from their linear precursors as previously described
(see Supporting Information).12a The compounds were designed
with two N-aryl side chains at positions (i and i + 3) or (i and
i + 1), respectively (Scheme 7). A previously reported structure
(16) Ramachandran, G. N.; Ramakrishnan, C.; Sasisekharan, V. J. Mol.
Biol. 1963, 7, 95–99.
(17) Horng, J.-C.; Raines, R. T. Protein Sci. 2006, 15, 74–83.
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16628 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

N-Aryl Peptoids
A R T I C L E S
Scheme 7. Structures of Cyclic Peptoids Containing N-Aryl and
N-Alkyl Glycine Monomers
Figure 7. X-ray crystal structure of cyclic peptoid hexamer 17. Bottom
image includes only backbone atoms.
Table 3. Dihedral Angles Observed for Cyclic Hexamer 17 Crystal
Structure
residue
monomer
ω
ꢂ
ψ
ꢁ1
1
2
3
4
5
6
(S)-N-(1-phenylethyl)glycine
N-(phenyl)glycine
(S)-N-(1-phenylethyl)glycine
(S)-N-(1-phenylethyl)glycine
N-(phenyl)glycine
19.75
172.29
53.91 -162.24 -93.78
67.49 -139.44
111.3
16.31 -81.24 -179.8
-98.88
Cyclic peptoid 18 did not form crystals with dimensions
necessary for X-ray diffraction. However, the macrocycle
displayed conformational homogeneity in deuterated acetonitrile
when analyzed by 1-D 1H NMR (see Supporting Information).
With additional HSQC, COSY, and ROESY NMR data, all of
the backbone and non-aromatic side chain proton resonances
for peptoid 18 were assigned. The HSQC data for compound
18 showed a series of interesting trends that provide insight
into the structure of this molecule in solution (Figure 8). In
particular, the 13C shifts for the backbone methylene groups of
the N-aryl glycine monomers (residues 4 and 5) are further
downfield than those of the N-alkyl glycine monomers, making
them readily distinguishable for spectral analysis. Additionally,
the methine protons on the (S)-1-phenylethyl side chains are
clustered into two distinct groups. ROESY data indicate that
the three methine protons that are furthest downfield (methine
protons at residues 1, 3, and 6) correspond to sites of cis-amide
bonds, and the further upfield proton (methine proton at residue
2) corresponds to a trans-amide bond position (see Supporting
Information). This observation is consistent with previously
determined NMR structures of peptoids bearing (S)-1-phenyl-
ethyl side chains.10c,18 The ROESY data establish that both
N-phenyl-substituted amide bonds (18, residues 4 and 5) adopt
a trans-conformation, as expected. The NMR study of cyclic
-5.51 -62.83
153.08 -125.37
-168.19 -72.49
139.03
71.41
(S)-N-(1-phenylethyl)glycine -12.36
79.33
176.36 -138.13
peptoid 18 provided distance constraints that were used to
generate a model backbone structure (see Supporting Informa-
tion) containing three cis- and three trans-amide bonds (Scheme
7). This structure shows a novel cyclic scaffold in which the
presence of adjacent trans-amide bonds was predicted a priori.
Design Strategies for Peptoid Oligomers. As the capabilities
to establish synthetic mimics of peptide secondary structures
become more extensive, there is increasing interest in creating
more sophisticated constructs akin to proteins. To be effective
as protein mimics, however, a class of foldamers must exhibit
the ability to support not only structural, but also chemical and
functional diversity. Peptoids were among the first sequence-
specific oligomers generated in combinatorial library format,
as their solid-phase submonomer synthesis protocol facilitates
the inclusion of chemically diverse N-alkyl and N-aryl side
chains.9,19 This synthetic ease has, in turn, led to the design of
peptoids with interesting biological activities and materials
properties.20,21
A modest-sized set of well-defined peptoid conformations has
been described in the past several years.20f These biomimetic
secondary structures include a polyproline type I helix generated
by branched N-alkyl glycine oligomers,5,10 a compact threaded
loop stabilized by hydrogen bonding in a peptoid nonamer,18
(18) Huang, K.; Wu, C. W.; Sanborn, T. J.; Patch, J. A.; Kirshenbaum,
K.; Zuckermann, R. N.; Barron, A. E.; Radhakrishnan, I. J. Am. Chem.
Soc. 2006, 128, 1733–1738.
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A R T I C L E S
Shah et al.
Figure 8. HSQC spectrum of cyclic hexamer 16 obtained from a 5 mM solution in acetonitrile-d3 at room temperature.
and macrocyclic peptoid hairpins that mimic a protein ꢀ-turn
motif.12a In this study, we demonstrate the ability to restrict
peptoid backbone conformational space to the same regions
found in protein polyproline type II helices. The ability to design
peptoids with complex functions will ultimately rely on the
combination of secondary structural elements into more elabo-
rate tertiary and quaternary structures. The growing collection
of secondary structures now described for peptoids should
provide the building blocks to generate these higher-order
architectures.
oligomers (ω, ꢁ1, ꢂ, and ψ) have energetic preferences which
lead to well-defined conformations, notably a structure resem-
bling that of a polyproline type II helix.
The de novo design of functional foldamers will depend on
the ability to predict structure from sequence. Here, we have
shown that N-aryl glycine monomer units provide a means to
achieve this goal in peptoids. The local conformations of N-aryl
glycine oligomers are well-defined and promote backbone
conformational stability. The ability to direct the presence of
trans-amide bonds at any location within a peptoid sequence
will enhance the capacity to predict overall backbone structure,
as evidenced in the successful design of N-alkyl/N-aryl hybrid
cyclic hexamers. Peptoids provide an attractive platform for
access to chemically diverse molecular architectures. The use
of N-aryl glycine monomer units in conjunction with other
strategies may ultimately allow for the development of predict-
able structure–function relationships in this important class of
peptidomimetic foldamers.
Conclusion
In this report, we describe the conformational preferences of
N-aryl glycine peptoids. Oligomers from dimer to hexamer
length were efficiently synthesized on solid-phase. The relative
energies of cis- and trans-amide bonds in these molecules were
evaluated by computational methods, and the energetic prefer-
ence for trans-amide bond conformers was shown to tolerate a
wide array of aryl substituents. The structures of N-aryl peptoid
oligomers were studied by solution NMR spectroscopy and
X-ray crystallography. These studies confirm that the compounds
prefer trans-amide bonds along the backbone and exhibit the
predicted side chain rotamers. Additional computational studies
indicate that the critical rotatable bonds in N-aryl peptoid
Experimental Section
Materials. Bromoacetic acid, aniline, 4-amino-2-nitrophenol,
4-methoxyaniline, 4-nitroaniline, 4-fluoroaniline, piperidine, citric
acid, iodomethane, and sodium hydride (60% w/w dispersion in
mineral oil) were purchased from Sigma-Aldrich (St. Louis, MO).
S-(-)-1-phenylethylamine was purchased from TCI (Portland, OR).
N,N′-Diisopropylcarbodiimide, N,N-diisopropylethylamine (DIEA),
and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) were purchased from
Chem-Impex International (Wood Dale, IL). Trifluoroacetic acid
was purchased from Acros (Belgium). PyBOP, Fmoc-protected Rink
amide resin (0.69 mmol ·g-1 loading level) and 2-chlorotrityl resin
(1.3 mmol ·g-1 loading level) were purchased from Nova Biochem
(San Diego, CA).
(19) (a) Horn, T.; Lee, B.-C. ; Dill, K. A.; Zuckermann, R. N. Bioconjugate
Chem. 2004, 15, 428–435. (b) Lee, B.-C.; Zuckermann, R. N.; Dill,
K. A. J. Am. Chem. Soc. 2005, 127, 10999–11009.
(20) (a) Patch, J. A.; Kirshenbaum, K.; Seurynck, S. L.; Zuckermann, R. N.;
Barron, A. E. Pseudo-peptides in Drug DiscoVery; Nielsen, P.E. Ed.;
Wiley-VCH: Weinheim, Germany, 2004; pp 1-35. (b) Utku, Y.;
Dehan, E.; Ouerfelli, O.; Piano, F.; Zuckermann, R. N.; Pagano, M.;
Kirshenbaum, K. Mol. BioSyst. 2006, 2, 312–317. (c) Holub, J. M.;
Garabedian, M. J.; Kirshenbaum, K. QSAR Comb. Sci. 2007, 26, 1175–
1180. (d) Lim, H.-S.; Archer, C. T.; Kodadek, T. J. Am. Chem. Soc.
2007, 129, 7750–7751. (e) Chongsiriwatana, N. P.; Patch, J. A.;
Czyzewski, A. M.; Dohm, M. T.; Ivankin, A.; Gidalevitz, D.;
Zuckermann, R. N.; Barron, A. E. Proc. Natl. Acad. Sci. U.S.A. 2008,
105, 2794–2799. (f) Yoo, B.; Kirshenbaum, K Curr. Opin. Chem. Biol.
2008, xx. doi:10.1016/j.cbpa.2008.08.015.
Synthesis of N-Methylacetanilides (1, 2, 4, 6, and 8). Aniline
or an aniline derivative (1 mmol) was dissolved in 2 mL of dry
(21) (a) Statz, A. R.; Meagher, R. J.; Barron, A. E.; Messersmith, P. B.
J. Am. Chem. Soc. 2005, 127, 7972–7973. (b) Shah, N. H.; Kirshen-
baum, K. Macromol. Rapid Commun. 2008, 29, 1134–1139.
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dichloromethane. Acetic anhydride (189.05 µL, 2 mmol) and DIEA
(522.53 µL, 3 mmol) were added to the solution and the mixture
was stirred at room temperature until the aniline was no longer
evident by thin layer chromatography in 10% ethyl acetate in
dichloromethane (reaction times varied from 2 to 18 h depending
on the aromatic substituent). Next, 2 mL of dichloromethane were
added to the reaction mixture, and the solution was washed twice
with 5 mL of 10% w/w aqueous citric acid.
After washing, the organic layer was dried under reduced
pressure, and the residue was redissolved in 4 mL of dry N,N-
dimethylformamide (DMF). Sodium hydride dispersed in mineral
oil (100 mg, 2.5 mmol) and iodomethane (155.64 µL, 2.5 mmol)
were added to the solution, and the mixture was stirred overnight.
The DMF was then removed under reduced pressure, and the
remaining residue was dissolved in 4 mL of dichloromethane. This
solution was then washed twice with 5 mL of 10% w/w citric acid,
and the organic layer was dried under reduced pressure. The residue
was then redissolved in 4 mL of methanol and extracted twice with
5 mL of hexane. The methanol was removed under reduced
pressure, and the remaining residue was lyophilized to a powder.
Products less than 95% purity (based on NMR) were purified on a
silica gel column with a gradient of 5-50% ethyl acetate in
dichloromethane. Products were analyzed and confirmed by elec-
trospray mass spectrometry and NMR (see Supporting Information).
The desired compounds were isolated in the following yields: 1,
20.1%; 2, 8.5%; 4, 41.7%; 6, 24.6%; and 8, 37.7%.
N-Aryl Peptoid Synthesis. Solid-phase synthesis of peptoid
oligomers was carried out in fritted syringes on Rink amide resin
using a variation of the peptoid submonomer synthesis reported
by Zuckermann et al.9 N-Substituted glycine monomers were
generated from their corresponding amine submonomers. N-
(phenyl)glycine was generated by incorporation of aniline. N-(3,4-
Dimethylphenyl)glycine was generated by incorporation of 3,4-
dimethylaniline. N-(4-Hydroxy-3-nitrophenyl)glycine was generated
by incorporation of 4-amino-2-nitrophenol. (S)-N-(1-Phenyleth-
yl)glycine was generated by incorporation of S-(-)-1-phenylethyl-
amine. All equivalencies are given with respect to resin loading
level.
In a typical oligomer synthesis, 100 mg of resin with a loading
level of 0.69 mmol ·g-1 was swollen in 2 mL of N,N′-dimethyl-
formamide (DMF) for 30 min. Following swelling, the Fmoc
protecting group was removed by treatment with 1.5 mL of 20%
piperidine in DMF twice for 20 min. After deprotection and after
each subsequent synthetic step, the resin was washed twice with 2
mL of DMF, twice with 2 mL of dichloromethane, and twice again
with 2 mL of DMF, one minute per wash.
Peptoid synthesis was carried out with alternating bromoacylation
and amine displacement steps. For each bromoacylation step, 20
equiv bromoacetic acid (1.2 M in DMF) and 24 equiv N,N′-
diisopropylcarbodiimide (neat) were added to the resin, and the
mixture was agitated for 90 min, except for the first bromoacetyl-
ation, which was carried out for only 20 min. After washing, 20
eq. of the required amine (1.0 M in DMF) were added to the resin
and agitated for 16 h. When required, peptoid N-termini were
acetylated on solid-phase by the following method. First, the
N-terminus was bromoacetylated for 90 min as described above.
After washing, the bromide was displaced by adding 5 eq. of sodium
borohydride (0.25 M in dimethylsulfoxide) to the resin and agitating
the mixture for 2 h.
Peptoid Macrocycle Synthesis. Linear peptoid precursors for
macrocyclization were synthesized on 2-chlorotrityl resin similar
to the procedure described above (See Supporting Information for
linear structures). For the first bromoacetylation step, 100 mg of
resin were suspended in 5 equiv bromoacetic acid (1.0 M in dry
dichloromethane) and 20 equiv DIEA (neat) and agitated for 30
min. For each successive bromoacetylation, 20 equiv bromoacetic
acid (1.2 M in DMF) and 24 equiv N,N′-diisopropylcarbodiimide
(neat) were added to the resin, and the mixture was agitated for 20
min following displacements with S-(-)-1-phenylethylamine or 90
min following displacements with aniline. Amine submonomers
were allowed to react with the resin after each bromoacetylation
step. The resin was mixed with 20 equiv S-(-)-1-phenylethylamine
(1.0 M in DMF) and agitated for 20 min or mixed with 20 equiv
aniline (1.0 M in DMF) and agitated for 16 h. When the desired
sequence was achieved, linear peptoid products were cleaved from
the resin by treatment with 20% HFIP in dichloromethane (40
mL·g-1 resin) for 30 min. After filtration, the cleavage cocktail
was concentrated by rotary evaporation under reduced pressure for
large volumes or under a stream of nitrogen gas for volumes less
than 1 mL. Cleaved samples were then resuspended in 50%
acetonitrile in water and lyophilized to powders.
A typical cyclization was conducted with the purified linear
products in dry, deoxygenated DMF. The linear oligomer (24 µmol)
was suspended in 10.50 mL of DMF in a 50 mL conical tube.
PyBOP solution (750 µL of 96 mM PyBOP freshly prepared in
DMF) and DIEA solution (750 µL of 192 mM DIEA freshly
prepared in DMF) were added to the peptoid. The reaction vessel
was flushed with nitrogen and sealed to exclude air. The reaction
proceeded overnight at room temperature, and 10 µL of reaction
mixture were diluted with 140 µL of 50% ACN in H2O to quench
the reaction. The diluted sample was analyzed by HPLC to confirm
completion. After the allotted reaction time, DMF was removed
by lyophilization, and the samples were resuspended in 50%
aqueous acetonitrile for purification.
Peptoid Characterization and Purification. Peptoid products
were cleaved from the Rink amide resin by treatment with 95%
aqueous trifluoroacetic acid for 10 min (40 mL ·g-1 resin). After
filtration, the cleavage cocktail was concentrated by rotary evapora-
tion under reduced pressure for large volumes or under a stream
of nitrogen gas for volumes less than 1 mL. Cleaved samples were
then resuspended in 50% acetonitrile in water to the desired
concentration.
Peptoid oligomers were characterized by analytical reversed-
phase high performance liquid chromatography (RP-HPLC) using
an analytical C18 column on a Beckman Coulter System Gold HPLC
system. Products were detected by UV absorbance at 214 nm during
a linear gradient from 5 to 95% solvent B (0.1% trifluoroacetic
acid in HPLC-grade acetonitrile) in solvent A (0.1% trifluoroacetic
acid in HPLC-grade water) in 10 min with a flow rate of 0.7
mL·min-1. The expected molecular mass of each product was
confirmedusingliquidchromatography-massspectrometry(LC-MS)
on an Agilent 1100 series LC/MSD trap XCT with an electrospray
ion source in positive ion mode.
Peptoid products were purified to >95% purity using the same
RP-HPLC apparatus described above with a preparatory C18 column.
Products were detected by UV absorbance at 230 nm during a linear
gradient from 5 to 95% solvent B in solvent A in 50 min with a
flow rate of 2.5 mL ·min-1. Compounds were then lyophilized to
powders.
NMR Spectroscopy. 1-D proton spectra were obtained using
either a Bruker 400 MHz NMR spectrometer, a Bruker 500 MHz
NMR spectrometer, or a Varian 500 MHz NMR spectrometer.
Multidimensional data were obtained using either a Bruker 500
MHz NMR spectrometer or a Varian 500 MHz NMR spectrometer.
NMR experiments were carried out at either 25 or 0 °C. All NMR
samples were prepared as 5-10 mM solutions in CD3CN or
CD3OD. NMR spectra of the CD3OD solutions were measured on
the Varian 500 MHz spectrometer in Shigemi tubes. To increase
resolution in the indirectly detected dimension, some NOESY
spectra were measured with BASHD-NOESY pulse sequences.22
Crystallization of 11. Compound 11 (5 mg in 500 µL of HPLC
grade methanol) was filtered through a 0.5 µm stainless steel syringe
tip filter. The solution was then allowed to evaporate slowly at room
temperature to form crystals. Crystallography data: yellow plate-
(22) (a) Krishnamurthy, V. V. Magn. Reson. Chem. 1997, 35, 9–12. (b)
Kaerner, A.; Rabenstein, D. L. Magn. Reson. Chem. 1998, 36, 601–
607.
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Shah et al.
like crystal, 0.32 × 0.20 × 0.03 mm3, monoclinic, P2(1)/c, a )
10.3748(15) Å, b ) 15.862(3) Å, c ) 10.7396(16) Å, R ) 90°, ꢀ
) 99.820(4)°, γ ) 90°, Z ) 4, V ) 1741.5(5) Å3, Fcalcd ) 1.474
g/cm3. X-ray diffraction data were collected on a Bruker Apex II
X-ray diffractometer.
Crystallization of 17. Compound 17 (5 mg in 500 µL of HPLC
grade ethanol) was filtered through a 0.5 µm stainless steel syringe
tip filter. The solution was then allowed to evaporate slowly at room
temperature to form crystals. Crystallography data: colorless needle-
like crystal, 0.28 × 0.21 × 0.12 mm3, monoclinic, P2(1), a )
9.7832(3) Å, b ) 14.8225(4) Å, c ) 16.4479(5) Å, R ) 90°, ꢀ )
91.127(2)°, γ ) 90°, Z ) 2, V ) 2384.67(12) Å3, Fcalcd ) 1.269
g/cm3.
model with 6 × 20 rounds of restrained minimization followed by
6 × 20 rounds of minimization without external restraints.26
Acknowledgment. We thank Dr. Chin Lin for his helpful
discussions on peptoid NMR characterization. We also acknowledge
Dr. Sam Hawxwell for his helpful conversations regarding crystal-
lization techniques and for aiding in X-ray diffraction studies of
compound 17. We thank Airon Soegiarto for contributing to the
X-ray crystallography of compound 11 and Doug Renfrew for
assisting with QM work. This work was supported by a National
Science Foundation CAREER Award (CHE-0645361). We also
thank the NCRR/NIH for a Research Facilities Improvement Grant
(Grant C06RR-165720) at NYU.
Computational Studies. All QM calculations were done with
the Gaussian 2003 package.23 All calculations used the “SCF)Tight”
option and all MP2 calculations used the “full” option. Basis sets
and constrained degrees of freedom are noted in the text and
legends. The model of the N-(phenyl)glycine hexamer was gener-
ated using the SIgMA package.24 A force field was generated by
adapting parameters from the cedar all-atom energy function.25
Backbone torsions where held at predicted minima (ꢂ at -60° and
ψ at 150°) by 20 kcal/mol harmonic restraints, and structure was
subject to several thousand cycles of energy minimization. The
structures were then further optimized using the semiempirical self-
consistent-charge density functional tight-binding (SCCDFTB)
Supporting Information Available: Complete ref 23; designa-
tor abbreviations for N-substituted glycine peptoid monomers;
NMR and mass spectrometry characterization of synthesized
N-methylacetanilides; mass spectrometry and RP-HPLC char-
acterization data for compounds 11-15, 17, and 18; additional
representations of the crystal structures of dimer 11 and cyclic
hexamer 17; 1-D and 2-D NMR spectra of compounds 11, 13,
17, and 18; NMR-based model structure of cyclic hexamer 18;
additional computational data; and additional visualizations of
the Ramachandran-like conformational analysis of 16. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(23) Frisch, M. J.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT,
2004.
(24) Mann, G.; Yun, R. H.; Nyland, L.; Prins, J.; Board, J.; Hermans, J.
Computational Methods for Macromolecules: Challenges and Ap-
plications; Schlick T., Gan, H. H., Eds.; Springer-Verlag: Berlin,
Germany, 2002; pp 129-145.
(25) (a) Hermans, J.; Berendsen, H. J. C.; van Gunsteren, W. F.; Postma,
J. P. M. Biopolymers 1984, 23, 1513–1518. (b) Ferro, D. R.; McQueen,
J. E.; McCown, J. T.; Hermans, J. J. Mol. Biol. 1980, 136, 1–18.
JA804580N
(26) (a) Elstner, M.; Frauenheim, T.; Kaxiras, E.; Seifert, G.; Suhai, S.
Phys. Status Solidi B 2000, 217, 357–376. (b) Hu, H.; Elstner, M.;
Hermans, J. Proteins 2003, 50, 451–463.
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16632 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008