Local and tunable n→π* interactions regulate amide isomerism in the peptoid backbone

Article abstract of DOI:10.1021/ja071310l

We report that n→π* interactions are operative in peptoids and play a major role in controlling amide isomerism. These interactions can be tuned using α-chiral amide side chains known to promote peptoid folding. To our knowledge, this is the first report of n→π* interactions between amides in non-prolyl systems. Furthermore, we have characterized an n→π* interaction between backbone carbonyls and side chain aromatic rings that can dramatically stabilize the cis-amides required for peptoid helix formation. The tunability of both types of n→π* interactions in peptoids has significant implications for peptoid folding and could be exploited for the design of new peptoid architectures. Copyright

Full text of DOI:10.1021/ja071310l

Published on Web 07/03/2007
Local and Tunable nfπ* Interactions Regulate Amide Isomerism in the
Peptoid Backbone
Benjamin C. Gorske, Brent L. Bastian, Grant D. Geske, and Helen E. Blackwell*
Department of Chemistry, UniVersity of WisconsinsMadison, 1101 UniVersity AVenue,
Madison, Wisconsin 53706-1322
Received February 23, 2007; E-mail: blackwell@chem.wisc.edu
Peptoids, or N-substituted glycine oligomers, are a prominent
peptidomimetic class with wide-ranging applications that reflect
the diversity of monomers from which they are constructed.1
Analogous to polyprolines, the peptoid backbone is composed
entirely of tertiary amides (Figure 1A), and peptoids with N-R-
chiral side chains can form polyproline I-like (PPI) helices.2
Conformational heterogeneity arising from backbone amide isomer-
ism, however, has complicated the de novo design of additional
peptoid structural motifs.2 We hypothesized that carbonyl-carbonyl
and carbonyl-aromatic nfπ* interactions could be harnessed to
regulate this isomerization. Previous studies have shown that nfπ*
interactions between tertiary prolyl amides (nfπ*Am) can stabilize
the trans-amides of PPII helices.3,4 However, reports of nfπ*
interactions between amides and electron-deficient aromatic rings
(nfπ*Ar) and their impacts on oligomer folding are scarce.5 Here,
we demonstrate not only that competing nfπ*Am and nfπ*Ar
interactions operate in peptoids but also that R-chiral amide side
chains can be used to tune the strengths of these interactions in
both the backbone and the side chains. Such interactions have
significant implications for peptoid folding and could be exploited
for the design of new peptoid architectures.
Figure 1. (A) Generic peptoid structure. (B) Three-dimensional representa-
tions of nfπ*Am and (C) nfπ*Ar interactions (indicated by yellow arrows)
in the peptoid model systems examined in this study.
Table 1. Structure of the Peptoid Model System and the
Structures of the C-Termini and Amide Side Chains Examined in
This Studya
The nfπ*Am interaction is characterized by donation of electron
density from a carbonyl oxygen lone pair into the π* orbital of an
adjacent amide carbonyl.3 Thus far, studies of these effects have
mainly focused on proline derivatives, in which the stereoconstraints
imposed by the proline ring play a pivotal role.3,6 In peptoids, only
trans-amide donors can be stabilized by this interaction, as they
satisfy the geometrical requirements for orbital overlap (Oi-1‚‚‚CidOi
distance e3.2 Å, angle ) 109 ( 10°; Figure 1B). Conversely, an
nfπ*Ar interaction between a backbone carbonyl and proximal side
chain aromatic group would exclusively stabilize the cis-amide,
again due to the geometrical considerations (carbonyl oxygen to
ring centroid distance 2.8-3.8 Å, dihedral angle between aryl and
carbonyl planes e90°; Figure 1C).5d We hypothesized that compet-
ing nfπ*Am and nfπ*Ar interactions could be tuned to regulate
K
cis/trans in peptoids. We therefore designed a peptoid model system
a Single stereocenters do not influence Kcis/trans in these systems and are
not shown. The cis-rotamer was defined as that which orients the side chain
and the oxygen atom on the same side of the amide bond.
that allowed us to probe for these nfπ* interactions while
mimicking the local environment in a typical N-R-chiral polypep-
toid. Both new and previously reported peptoid side chains and
C-termini were incorporated (Tables 1 and 2), and their impacts
for these effects; nfπ*Am interactions between carbonyls in the
piperidinyl series were also excluded because removal of the
C-terminal carbonyl (methyl amide series 5-9) did not affect the
rotameric ratios.
Having investigated the impact of nfπ*Ar interactions on Kcis/trans
in these peptoid model systems, we next examined the C-terminal
carbonyl in order to probe for nfπ*Am interactions. We anticipated
that dimethyl amides would be less effective than piperidinyl amides
in stabilizing positive charge at the nitrogen. Therefore, donation
of electron density by the dimethyl amide nitrogen to the carbonyl
should be reduced, increasing its acceptance of nfπ*Am interactions
on Kcis/trans were assessed by H NMR.3,6
1
Examination of a series of piperidine-capped, benzyl peptoids
(1-4, Table 2) revealed that, relative to pe (3), electron-withdrawing
groups on the aromatic ring (fe and np; 1, 2) stabilize the cis-amide,
suggesting a role for nfπ*Ar interactions in these systems. Indeed,
pyridinium 4mpy (8) afforded an exceptional increase in Kcis/trans
(270%) relative to pe. In contrast, the bulkier cyclohexyl (ch, 4/7)
and more electron-rich phenolate (mph, 9) side chains demoted the
cis-amide rotamer by 40% relative to pe. The para positioning of
the aryl substituents in 2 and 9 excluded a purely steric rationale
9
8928
J. AM. CHEM. SOC. 2007, 129, 8928-8929
10.1021/ja071310l CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Peptoid Model System Structures, Amide cis/trans
Ratios in CD3CN, and Corresponding Free Energy Differences
consistent with the solid-state conformation exhibiting the nfπ*Am
interaction. We also obtained NOESY NMR data for cis-6 and cis-8
that provide additional evidence of nfπ*Ar interactions in these
model systems. These data indicate that the conformation in which
the aromatic ring eclipses the carbonyl oxygen (Figure 2B) is
significantly populated by 8 but not by 6, suggesting that nfπ*Ar
interactions stabilize the cis-amide. Calculations of partial charges
for the atoms participating in the nfπ*Ar interactions suggest that
electrostatics do not contribute appreciably to this stabilization.5a
Additional calculations of LUMO energies for cis-(5-8) and
interaction distances and angles for trans-(13-15) also corroborate
the existence and tunability of nfπ* interactions in these peptoid
systems (see Supporting Information).
The data presented herein strongly suggest that nfπ* interactions
play a significant role in controlling peptoid amide isomerism, both
in solution and in the solid state. To our knowledge, this work
represents the first report of nfπ*Am interactions outside of prolyl
systems and expands their relevance beyond the scope of peptides.
Furthermore, we have characterized an nfπ*Ar interaction that may
promote PPI helices in peptoids by stabilizing cis-amides and
disrupting hydrogen bonds between the peptoid termini and
backbone carbonyls.8 The strengths of the nfπ* interactions can
be effectively tuned by careful selection of R-chiral side chains,
which can be installed using standard peptoid synthesis methods.1,8a
The energetic significance of the interactions (up to 1.4 kcal or
greater) is remarkable, considering the flexibility of the peptoid
glycine unit relative to proline. Ongoing work in our laboratory is
directed toward elucidating the role and significance of nfπ*
interactions in related polypeptoids in order to facilitate the design
of new peptoid structures.
∆
Gcis
/trans
a
peptoid
R1
R2
Kcis
(kcal/mol)b
/
trans
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Pip
Pip
Pip
Pip
Me
Me
Me
Me
fe
np
pe
ch
np
pe
ch
4mpy
mph
np
pe
ch
np
pe
ch
fpnan
3.84 ( 0.19
3.43 ( 0.19
2.04 ( 0.27
1.22 ( 0.01
3.27 ( 0.18
2.12 ( 0.07
1.30 ( 0.05
7.82 ( 0.52
1.25 ( 0.02
3.06 ( 0.01
1.69 ( 0.14
0.58 ( 0.06
1.05 ( 0.01
0.67 ( 0.04
0.29 ( 0.04
>10
-0.79
-0.73
-0.42
-0.12
-0.70
-0.44
-0.15
-1.22
-0.13
-0.66
-0.31
+0.33
-0.03
+0.24
+0.73
<-1.4
Me
dma
dma
dma
MeO
MeO
MeO
Pip
a Determined by integrating 1H NMR spectra of 15 mM solutions at 24
°C. b ∆G ) -RTln(Kcis/trans).
Acknowledgment. We thank the NSF (CHE-0449959), Re-
search Corporation, and the Shaw Scientist Program of the Greater
Milwaukee Foundation for financial support of this work, Dr. Ilia
Guzei for X-ray crystallographic analyses, and Prof. Ronald Raines
for numerous helpful discussions. H.E.B. is an Alfred P. Sloan
Foundation Fellow.
Figure 2. (A) X-ray crystal structure of 16. The nfπ* interaction is shown
as a yellow line with associated angles and distances. The hydrogen bond
is shown as a green line. (B) Newman projections depicting the most
populated conformations for cis-6 and -8, as indicated by the NOEs
shown: red ) stronger for 8 vs 6, black ) identical for 8 and 6.
Supporting Information Available: Peptoid synthesis and char-
acterization, X-ray and NMR data, and calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
and decreasing Kcis/trans.
3a Examination of a series of such peptoids
(10-12, Tables 1 and 2) indeed revealed 10, 20, and 50% reductions
in Kcis/trans for np, pe, and ch, respectively, compared to the
analogous piperidinyl series (2-4). The large decrease in Kcis/trans
observed for ch suggests that the nfπ*Am interaction is significantly
enhanced by ch relative to pe and np. We speculate that ch sterically
enforces a conformational bias that facilitates nfπ*Am interactions
and stabilizes the trans-amide rotamer. In order to further demon-
strate the potential for nfπ*Am interactions in these systems, we
also synthesized a series of C-terminal methyl esters (13-15), which
we expected to function as excellent nfπ*Am interaction acceptors.3
As predicted, the Kcis/trans values for 13-15 were further reduced
(50-65%) relative to the dimethyl amide series (10-12), indicating
an even stronger nfπ*Am interaction.
In addition to amides in the peptoid backbone, we discovered
that amides in peptoid side chains can also participate in nfπ*Am
interactions (e.g., in peptoid anilide 16, Tables 1 and 2). The trans-
rotamer of 16 appears to be completely suppressed in solution, and
its solid-state structure reveals a strong nfπ*Am interaction from
the N-terminal acetamide to the anilide carbonyl (O‚‚‚CdO distance
and angle of 2.6 Å and 106°, respectively; Figure 2A).7 In order to
confirm the conformational plausibility of this side chain to
backbone nfπ*Am interaction in solution, we obtained NOESY
NMR spectra for 16. The NOE contact patterns are qualitatively
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