How useful is ferrocene as a scaffold for the design of β-sheet foldamers?

Article abstract of DOI:10.1002/anie.200801460

(Chemical Equation Presented) Crease crossing peptide embossing: The first tripeptide ferrocene foldamers to adopt an extended β-sheet-like structure in solution and solid state are presented. The flexibility of peptide substituents in 1,n′-ferrocene-bispeptide conjugates was thought to prevent the formation of extended β-sheet foldamers. The results suggest that ferrocene is a valuable scaffold for extended β-sheets.

Full text of DOI:10.1002/anie.200801460

Communications
DOI: 10.1002/anie.200801460
Peptide Chemistry
How Useful Is Ferrocene as a Scaffold for the Design of b-Sheet
Foldamers?**
Somenath Chowdhury, Gabriele Schatte, and Heinz-Bernhard Kraatz*
The design of structurally well-defined synthetic peptide
foldamers is motivated to some degree by the aim to study
mechanisms of protein folding[1] or biochemical processes,[2]
to generate molecules with potential biological applications,[3]
or to generate novel materials.[4] Foldamers adopting b-sheet-
like conformations have held a particular fascination and
have tremendous potential as model systems in furthering our
understanding of diseases, including Alzheimerꢀs and Hun-
tingtonꢀs diseases. Non-natural scaffolds have been particu-
larly important for obtaining foldamers with a defined, stable
b-sheet-like structure.[5,6] Ferrocene (Fc) has been investi-
gated as a molecular scaffold that supports b-sheet-like
interactions between two podant peptide chains that are
linked to the two parallel cyclopentadienyl rings.[7] 1,n’-
disubstituted Fc-conjugates of amino acids exhibit a distinct
non-proteinic “cross-strand” H-bonding pattern, in which a
10-membered H-bonded ring is formed (compound 1 in
Scheme 1). This result suggests that simple extension might
Scheme 1. H-bonding and conformations of disubstituted ferrocene-
result in a foldamer that possesses an extended b-sheet-like
structure. However, extending the two substituents by one
amino acid each to form a dipeptide, as illustrated by
compound 2 in Scheme 1, shows the two additional amino
acids pointing in opposite directions, while the H-bonding
pattern of the proximal amino acids is maintained. Modifi-
cation of the two C termini of the peptide conjugate by a N-
heterocycle yields a g turn motif, in which the peptide strands
are engaged in intrastrand H-bonding,[8] thus raising serious
concerns of the true value of the Fc scaffold to support an
extended b-sheet-like structure.
The problem appears to be with the colinear alignment of
the peptide chains and potential steric congestion about the
Fc core. Forcing peptides into a specific conformation by
cyclization has led to a H-bonding pattern that, although not
identical, bears some resemblance to what is observed in
peptide conjugates with the different lengths of peptide strands.
proteinic b-sheets, in terms of the formation of H-bonded
rings and peptide dihedral angles. On an intermolecular level,
H-bonding interactions form associated conjugates into
unprecedented supramolecular assemblies.[9] The key to this
success was the use of glycine (Gly) as a flexible amino acid
that can accommodate a wide range of dihedral angles
proximal to the Fc core. Based on these results, we
hypothesized that if Gly is chosen as proximal amino acid
followed by an amino acid with high b-sheet propensity, such
as valine (Val) or isoleucine (Ile),[10] it might be possible to
form acyclic Fc-peptide foldamers that support extended H-
bonding interactions between the two pendant peptide
substitutents (3 in Scheme 1). Based on these considerations,
we selected the sequences Gly-Val-Cys (Cys = cysteine) and
Gly-Ile-Cys and coupled them to 1,1’-Fc-dicarboxylic acid
through the N-terminal Gly residues. Herein, the results of
our investigations are presented, which demonstrate that
these foldamers exhibit a stable H-bonding interaction with
an alternating b-sheet-like conformation of the peptide
backbone.
Suitably protected dipeptides, Boc-Val-Cys(Bn)-OMe (4)
(Boc = tert-butoxycarbonyl, Bn = benzyl) and Boc-Ile-
Cys(Bn)-OMe (6) and tripeptides Boc-Gly-Val-Cys(Bn)-
OMe (5) and Boc-Gly-Ile-Cys(Bn)-OMe (7) were synthe-
sized by the standard solution peptide-coupling method using
HBTU as a coupling reagent. The target compounds Fc[CO-
Gly-Val-Cys(Bn)-OMe]2 (8) and Fc[CO-Gly-Ile-Cys(Bn)-
OMe)]2 (9) were synthesized by coupling the corresponding
tripeptides to ferrocenedicarboxylic acid and characterized
[*] Prof. H.-B. Kraatz
Department of Chemistry, The University of Western Ontario
1151 Richmond Street, London, ON, N6A 5B7 (Canada)
Fax: (+1)519-661-3022
E-mail: hkraatz@uwo.ca
S. Chowdhury
Department of Chemistry, University of Saskatchewan
110 Science Place, Saskatoon, SK, S7N 5C9 (Canada)
Dr. G. Schatte
Saskatchewan Structural Science Centre
University of Saskatchewan
110 Science Place, Saskatoon, SK, S7N 5C9 (Canada)
[**] We acknowledge support from NSERC in the form of an operating
grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801460.
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Chemie
spectroscopically (see Supporting Information). The 1H NMR
spectra of compounds 8 and 9 in CDCl3 show the Gly and Cys
amide NH proton resonance signals downfield from that of
the Val residue in 8 (Ile in compound 9). Importantly, the
chemical shifts of the amide NH proton signals of Gly and Cys
in both compounds are concentration-independent, whereas
those of Val and Ile are concentration-dependent, suggesting
the involvement of these amide protons in intermolecular H-
bonding (Figure 1), which is confirmed by single crystal X-ray
structural studies of the two compounds (see below).
These results were supported by two single-crystal X-ray
diffraction studies of compounds 8 and 9. Suitable crystals
were obtained by slow solvent evaporation from chloroform
at 08C. Both compounds crystallize in the space group
P212121. The molecular structures are shown in Figure 3.
Figure 1. 1H NMR spectra in CDCl3 solution of compound 8 showing
the amide regions. Concentration of solutions (mm) from bottom to
top; 2.00, 1.60, 1.33, 1.14, 1.00, 0.98 and 0.80. Peak A corresponds to
Gly-NH, B to Cys-NH and C to Val-NH.
Circular dichroism (CD) spectroscopy has been useful in
evaluating the axial stereochemistry of the Fc group in Fc
conjugates. A positive Cotton effect of the Fc centered
transition indicates P-helicity, whereas a negative Cotton
effect indicates M-helicity.[7c] The CD spectra of compounds 8
and 9 recorded in MeCN solution are presented in Figure 2,
clearly showing a positive Cotton effect for the Fc-based
transition at around 480 nm, which is an indication of a rigid
P-helical structure about the Fc core. The negative signal
around 225 nm indicates the presence of a sheet-like con-
formation in both compounds.
Figure 3. Molecular structures of a) compound 8 and b) compound 9
as obtained from single crystal X-ray diffraction studies, showing the
extended sheet-like conformation. Hydrogen atoms are omitted for
clarity. c) Stereoviewof compound 8, highlighting the two peptide
backbones.
The crystal structures of compound 8 and 9 exhibit a
Herrick-type H-bonding pattern, which is typical for 1,n’-Fc-
peptide conjugates.[7a,c] The two peptide chains do not diverge
but are aligned parallel with respect to each other, allowing
for additional H-bonding interactions between the amide NH
=
of Cys on one strand with the ester C O of the other strand
Figure 2. CD spectra of compounds 8 and 9 in CH3CN solution at
258C, showing the presence of P-helical arrangement of the ferrocene
core and the signature of b-sheet conformations of the two peptide
substitutents.
(see Figure 3 and Supporting Information). The H-bond
lengths involving the Gly-NHs are shorter, which is compat-
ible with the downfield shift of the NH resonances in the
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1H NMR spectrum. The longer H-bonding interactions
The FT-IR spectrum of compound 8 in the solid state exhibits
strong amide I bands at 1634 and 1680 cmÀ1 and an amide II
band at 1531 cmÀ1, which is the characteristic peak pattern for
a sheet conformation. The absorption at 1680 cmÀ1 is
particularly indicative of sheet characteristics.[11,12] The IR
absorptions of compound 9 (1638 cmÀ1, 1679 cmÀ1, and
1533 cmÀ1) are similar to those of compound 8 (see Figure S1
in Supporting Information), again providing a consistent
picture for a sheet-like structure of this Fc foldamer in the
solid state. In the amide A region, strong absorptions are
observed at 3294 cmÀ1 for 8 and 3282 cmÀ1 for 9, which is
compatible with H-bonding of the NH group.
In conclusion, we have presented the first example of an
acyclic Fc-peptide foldamer that clearly exhibits an extended
H-bonded structure. In contrast to previous work, it is
possible to prevent the two podant peptide chains from
diverging using a very flexible Gly proximal to the Fc core.
The Ramachandran plot of the foldamer clearly shows that
Gly supports a large range of dihedral angles and provides
sufficient flexibility for the system to adopt a peptide
structure that is dictated by subsequent addition of “high b-
sheet-propensity” amino acids.
involving the Cys-NH may be due to the steric repulsion
between the bulky protecting groups on Cys. It is important to
note that ROESY studies clearly show that the solid state
structures are maintained in solution (CDCl3). Thus for
compound 8, ROE correlations are observed between
protons of the ester OMe and the Val-CH3, the ester OMe
and the phenyl Hs, and between the b-H of Cys and the
phenyl Hs (see Figure S11 and Figure S12 in Supporting
Information). These ROE signals correspond well with the
solid state structure. The distance between two S atoms in a
single molecule are 6.059 Š and 5.255 Š for 8 and 9,
respectively, which is outside the sum of the van der Waals
radii of sulfur (1.85 Š).
A second set of H-bonding contacts exist that link
molecules on an intermolecular level, which again matches
the findings in the solution 1H NMR spectroscopy. Each
molecule is linked to two other molecules through intermo-
lecular H-bonding (O11···N22* = 2.869 Š, and N12···O21** =
3.064(4) Š in
8
and O11···N22* = 2.926(4) Š and
N12···O21** = 2.894(4) Š in 9) to form the sheet like
continuous assembly (Figure 4 and Supporting Information).
The intermolecular H-bonding interactions between head-to-
tail-connected ferrocene peptides results in the formation of a
14-membered H-bonded ring as are found in antiparallel b-
sheet-like structures. Again, it is important to stress that the
observed H-bonding network in both compounds is signifi-
cantly different to proteinic structural motifs.
FT-IR spectroscopy is another powerful technique in
examining the secondary structure of peptides.[11] Absorptions
in the amide I region offer a characteristic signature that
allows us to distinguish an a helix, a b sheet and a random coil.
Received: March 27, 2008
Published online: July 30, 2008
Keywords: ferrocene · foldamers · molecular scaffolds ·
.
peptides · self-assembly
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1
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