Investigations of metal-coordinated peptides as supramolecular synthons

Article abstract of DOI:10.1021/jo060395q

This article describes the synthesis and controlled assembly of four model biological-hybrid scaffolds via coordination of a metal complex to four new tripeptides. Each model tripeptide investigated has either a central pyridyl glycyl or a pyridyl alanyl

Full text of DOI:10.1021/jo060395q

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Investigations of Metal-Coordinated Peptides as Supramolecular
Synthons
Warren W. Gerhardt and Marcus Weck*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
marcus.weck@chemistry.gatech.edu
ReceiVed February 24, 2006
This article describes the synthesis and controlled assembly of four model biological-hybrid scaffolds
via coordination of a metal complex to four new tripeptides. Each model tripeptide investigated has
either a central pyridyl glycyl or a pyridyl alanyl residue between two terminally protected glycines. All
tripeptides were coordinated to their complementary recognition unit, a p-methoxy SCS-Pd pincer
1
complex. The assembly events were fully characterized and investigated by H NMR, ES-MS, and
isothermal titration calorimetry (ITC) to elucidate how the substitution and spatial distance of the pyridyl
moiety to the peptide backbone affects the metal coordination. Using these characterization techniques,
we have shown that the metal-coordination events in all cases are fast and quantitative and that the
peptide backbones do not interfere with the self-assembly. The ITC analyses showed that the 4-pyridyl
tripeptides are the tightest binding ligands toward the palladated pincer complexes with the alanyl derivative
being the strongest overall, demonstrating the superiority of the 4-pyridyl peptides over their 3-pyridyl
analogues. The measured association constants are comparable to other pincer-pyridine systems in DMSO
suggesting that the controlled coordination of the metalated pincer/pyridine interaction is an interesting
biological synthon and will allow for the future development of important noncovalent peptide-based
hybrid materials.
Introduction
design well-defined persistent architectures,6-8 thereby creating
self-assembled biologically relevant and unique materials.
Herein, we present as a step toward this goal the coordination
of peptides with metalated pincer complexes, by embedding
complementary recognition units in the peptide side-chain.
In biomacromolecules, metal coordination plays an essential
role in many of life’s processes.9-11 And in the burgeoning field
of supramolecular chemistry, metal coordination already has a
Organic supramolecular structures and assemblies hold great
promise as building blocks for precision nanomaterials and
structures that are otherwise conceptually inaccessible.1-5
Through the use of controlled metal coordination at targeted
positions in natural or synthetic peptides, one might be able to
* To whom correspondence should be addressed. Fax: (+1) 404-894-7452.
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10.1021/jo060395q CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/18/2006
J. Org. Chem. 2006, 71, 6333-6341
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Gerhardt and Weck
vital role in the assembly of purely synthetic systems,12-14
creating geometric and highly controlled architectures12-20 or
functional polymeric materials.21-32 However, its use in the
assembly of biological suprastructures, specifically with pep-
tides, is not nearly as developed. The early use of metal
coordination to control the design and folding of natural peptides
was reported in 1990 by Ghadiri and co-workers33,34 and
Degrado and co-workers.35 Ghadiri coaxed linear peptides to
form helices by cross-linking the peptides with transition-metal
ions via side-chain coordination of two natural residues. Degrado
used coordination of histidine residues to Zn2+ to fold a peptide
into a structural approximation of the enzyme carbonic anhy-
drase.36 Sasaki and co-workers were the first to use unnatural
ligands to self-assemble triple-helix bundles by chelating Fe2+
with bipyridines covalently attached to the peptides’ N-
termini.37,38
FIGURE 1. Pincer metallacycle: E is a neutral two-electron donor;
R is a tethering group.
unnatural residues containing a diiminoacetic acid side-chain
that was able to induce helix nucleation.40
After Hopkin’s seminal work, there were many reports using
peptides with unnatural amino acids with metal coordination
to stabilize or create defined nanostructures such as helixes,41,42
loops,43 cycles,39 and other scaffolds.44-46 Our report is the first
study of peptides containing side-chain recognition units
coordinated to a metalated pincer complex. We envision these
peptides as models. Once fully understood, they can be situated
into a cyclic peptide framework and coordinated with multi-
functional/faceted metalated receptors into larger networks and
structures.
The synthesis of peptides with unnatural residue side-chains
was the next step toward greater complexity and control in
metal-coordinating peptides. Novel synthetic residues preserve
the peptide backbone and can be redefined ad infinitum creating
diversity within supramolecular systems.39 Toward this goal,
Hopkins and co-workers synthesized a peptide with two identical
Metalated pincer complexes (Figure 1) have received con-
siderable attention within the field of supramolecular chemis-
try.47 This chemo-activated, single-site metal-coordination step
has been shown to be fast, quantitative, and chemo reversible
in a variety of solvents.16,41,47-50 Incorporation of one of its
ligands into the side-chain of an unnatural peptide would open
a simple and direct avenue to new peptide-supramolecular
structures, making the metalated pincer complex an ideal
candidate for achieving our goals.
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The pincer ligand is a tridentate ligand that after metalation,
with a Pd or Pt, results in a four-coordinate square planar metal
complex.51-53 The three chelation sites most often have a central
carbon flanked by two neutral donor atoms (E) giving it an ECE
nomenclature.47 In this investigation, we used sulfur as the
donors and Pd as the metal center, i.e., an SCS-Pd pincer
complex. The fourth site is normally occupied by a halide that
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6334 J. Org. Chem., Vol. 71, No. 17, 2006

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Metal-Coordinated Peptides as Supramolecular Synthons
FIGURE 2. Pincer amino acids synthesized by van Koten.
FIGURE 4. Activation and coordination of p-methoxy SCS-Pd pincer
complex 5 to tripeptide 1 in DMSO.
coordination capabilities of tripeptides 1-4 with p-methoxy
SCS-Pd pincer complex 5 (Figure 4) in DMSO because a self-
assembled biomaterial should have a strong noncovalent interac-
tion that is fast and quantitative in a polar environment. The
behavior of each unique coordination event was characterized
by 1H NMR spectroscopy, isothermal titration calorimetry (ITC),
and ES-mass spectroscopy to determine which substitution
pattern and spacing distance is ideal for these model synthons,
to establish association constants (Ka) and to ascertain any
influences of the peptide backbone on this coordination.
Ultimately this detailed analysis will allow us to develop novel
cyclic peptide supramolecular synthons coordinated with meta-
lated pincer complexes giving architecturally complex bioma-
terials and scaffolds.
FIGURE 3. Four model tripeptides containing an unnatural pyridyl
glycine or pyridyl alanine as the central residue.
can be quantitatively complexed by a donor ligand (vida supra)
forming a strong directional metal-ligand bond.48,55
Currently, the only reported use of a metalated pincer complex
with a peptide or an amino acid was described by van Koten
and co-workers.56,57 In this case, NCN-Pt pincer and Pd pincer
complexes were covalently attached to the N-terminus of an
amino acid (Figure 2) and then a peptide chain. However, in
their articles, the authors did not describe this system as an
assembly unit. Nevertheless, we evaluated their elegant approach
as a possible strategy toward the controlled noncovalent
synthesis of peptide-based materials. Unfortunately, the func-
tionalization at the N-terminus of the peptide clearly limits the
use of this design as a general synthon in metal-coordination-
based peptidal architectures.
For our approach, we chose a pyridyl side-chain residue as a
selective recognition unit for the metal coordination to the SCS-
Pd pincer complex. A pyridyl moiety was chosen as the
functional side-chain because (a) there are already a handful of
reports using nitrogenous heteroaromatic side-chains as receptors
for self-assembly in this area of supramolecular chem-
istry41-46 and (b) the use of pyridine as a ligand for metalated
pincer complexes has been established over the past de-
cade.15,25,26,32,47,48,58,59
Results and Discussion
Metalated pincer complexes are known to coordinate to other
nitrogen-containing heteroaromatic units such as pyrazine.32
Therefore, if histidine with its imidazole side-chain would
coordinate quantitatively to metalated pincer complexes, histidyl
peptides or related derivatives would be the simplest and most
direct route toward our goal. Therefore, we first examined the
coordination of imidazole and methyl imidazole with a SCS-
1
Pd pincer complex as a preliminary study. However, the H
NMR spectra of the metal coordination of both imidazole and
methyl imidazole to metalated pincer complexes showed no
clear or strong coordination pattern indicating that no clean or
controlled single metal coordination event took place.
Peptide Synthesis. On the basis of the unsatisfactory imi-
dazole/methyl imidazole-Pd pincer study, we used pyridine,
which is the most common ligand for metalated pincer
complexes,47 in our design. The four distinct tripeptides (1-4)
were made with a pyridyl glycine or alanine amino acid via
solution-phase peptide synthesis using two different strategies
(Scheme 1 shows the synthesis of the pyridyl alanines and
Scheme 3 shows the synthesis of the pyridyl glycines). The
C-termini of all tripeptides were protected as methyl esters, and
the N-termini were protected with a phthaloyl group. After
purification by RP-HPLC, these tripeptides crystallized easily
from a CH3CN/water mixture (1 was the exception). The crystal
structures of 2-4 can be found in the Supporting Information.
Pyridyl Alanine Tripeptide Synthesis. Protected versions
of the 3- and 4-pyridyl alanine amino acids are commercially
available, and tripeptides 2 and 4 were synthesized using
conventional peptide coupling techniques. A nonfunctional
isosteric phenylalanine tripeptide 6 was also synthesized using
this route with comparable yields (Scheme 2).
In this initial contribution, we synthesized and investigated
four tripeptides as supramolecular synthons 1-4 (Figure 3), each
with the unnatural pyridyl side-chain residue in the center being
flanked by two terminally protected glycine residues.
Each of these four peptides has a different substitution pattern
and spacing of the central pyridine side-chain from the peptide
backbone. These variations are expected to lead to different
coordination geometries and strengths. We studied the metal-
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The syntheses of the Boc-protected dipeptides 7-9 were
carried out using dicyclohexyl carbodiimide (DCC) as the
J. Org. Chem, Vol. 71, No. 17, 2006 6335

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Gerhardt and Weck
SCHEME 1. Synthesis of Tripeptides 2 and 4
SCHEME 2. Synthesis of Nonfunctional Isosteric Tripeptide 6
condensing agent and the nucleophilic additive 1-hydroxyben-
zotriazole (HOBt) in THF with yields of 88%, 95%, and 90%
for 7, 8, and 9, respectively. The Boc group of dipeptide 9 was
removed using a 1:2 TFA/CH2Cl2 solution. However, Boc
deprotection of dipeptides 7 and 8 with TFA led to trifluoramide
terminated dipeptides under a variety of coupling conditions.
Therefore, these Boc groups were removed with a saturated
methanolic solution of HCl. The best results for the tripeptidation
coupling step were realized with benzotriazole-1-yl-oxy-trispyr-
rolidino-phosphonium hexafluorophosphate (PyBOP) as the
condensing agent and the nucleophilic additive 1-hydroxy-7-
azabenzotriazole (HOAt), giving overall yields of 78%, 64%,
and 87% for 2, 4, and 6, respectively.
Pyridyl Glycine Tripeptide Synthesis. The synthetic route
toward glycyl derivative 1 was not as straightforward as that
for the alanyl derivatives. Not commercially available, the
4-pyridyl glycine residue was first synthesized but never isolated
as the carboxylic acid because of an inherent instability, which
led to spontaneous decarboxylation upon acidification of the
lithium carboxylate salt to isolate the pyridinium amino acid
residue. This exacerbated the synthesis of the pyridyl glycyl
amino acid of 1; therefore, a different synthesis in comparison
to that for 2 and 4 was developed that went directly to the
dipeptide. This was done using a modified Strecker synthesis
developed by Ugi and further expanded by Kunz (Scheme
3).60-66
The synthetic protocol used a protected amino sugar as an
auxiliary, forming an imine with the appropriate pyridine
carboxaldehyde precursor, that is attacked by the isonitrile to
give the protected dipeptides 10 and 11 with yields greater than
90%. The N-terminus protections were removed with a two-
part, one-pot methanolic/aqueous HCl deprotection. After re-
esterification, the HCl salts of 10 and 11 were then converted
to tripeptides 1 and 3 using the same tripeptidation step as the
alanyl derivatives, with overall yields of 86% and 70% for 1
and 3, respectively.
p-Methoxy SCS-Pd Pincer Synthesis. The synthesis of the
palladated pincer complex 5 started with the methylation of 12
to 13 using iodomethane, which was then palladated to yield 5
following standard literature procedures (Scheme 4) with an
overall yield of 92%.58,67,68 Palladated pincer complex 5 could
be further recrystallized from CHCl3 as large yellow crystals.
Coordination. Peptides 1-4 were synthesized to fully
characterize and understand the coordination pattern and as-
sociation strength of each new peptide ligand. The limited
solubility of all four peptides and their coordinated complexes
in less-polar organic solvents (CHCl3 and CH2Cl2) that are
traditionally used to assemble metalated pincer-ligand com-
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