Sequence and Structure of Peptoid Oligomers Can Tune the Photoluminescence of an Embedded Ruthenium Dye

Article abstract of DOI:10.1002/chem.201901494

The understanding of structure–function relationships within synthetic biomimetic systems is a fundamental challenge in chemistry. Herein we report the direct correlation between the structure of short peptoid ligands—N-substituted glycine oligomers incorporating 2,2′-bipyridine groups—varied in their monomer sequence, and the photoluminescence of Ru^II centers coordinated by these ligands. Based on circular dichroism and fluorescence spectroscopy we demonstrate that while helical peptoids do not affect the fluorescence of the embedded Ru^II chromophore, unstructured peptoids lead to its significant decay. Transmittance electron microscopy (TEM) revealed significant differences in the arrangements of metal-bound helical versus unstructured peptoids, suggesting that only the latter can have through-space interactions with the ruthenium dye leading to its quenching. High-resolution TEM enabled the remarkable direct imaging of singular ruthenium-bound peptoids and bundles, supporting our explanation for structure-depended quenching. Moreover, this correlation allowed us to fine-tune the luminescence properties of the complexes simply by modifying the sequence of their peptoid ligands. Finally, we also describe the chiral properties of these Ru–peptoids and demonstrate that remote chiral induction from the peptoids backbone to the ruthenium center is only possible when the peptoids are both chiral and helical.

Full text of DOI:10.1002/chem.201901494

A Journal of
Accepted Article
Title: Sequence and Structure of Peptoid Oligomers can Tune the
Photoluminescence of an Embedded Ruthenium Dye
Authors: Galia Maayan, Lieby Zborovsky, and Hagar Tigger-Zaborov
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Sequence and Structure of Peptoid Oligomers can Tune the
Photoluminescence of an Embedded Ruthenium Dye
Lieby Zborovsky, Hagar Tigger-Zaborov and Galia Maayan*
Abstract: The understanding of structure-function relationships
within synthetic biomimetic systems is a fundamental challenge in
chemistry. Herein we report the direct correlation between the
structure of short peptoid ligands – N-substituted glycine oligomers
incorporating 2,2′-bipyridine groups – varied in their monomer
sequence, and the photoluminescence of Ru(II) centers coordinated
by these ligands. Based on circular dichroism and fluorescence
spectroscopy we demonstrate that while helical peptoids do not
affect the fluorescence of the embedded Ru(II) chromophore,
unstructured peptoids lead to its significant decay. Transmittance
electron microscopy (TEM) revealed significant differences in the
arrangements of metal bound helical vs. unstructured peptoids,
suggesting that only the later can have through-space interactions
with the ruthenium dye leading to its quenching. High-resolution
TEM enabled the remarkable direct imaging of singular ruthenium
bound peptoids and bundles, supporting our explanation for
structure-depended quenching. Moreover, this correlation allowed to
fine tune the luminescence properties of the complexes simply by
modifying the sequence of their peptoid ligands. Finally, we also
describe the chiral properties of these Ru-peptoids and demonstrate
that remote chiral induction from the peptoids backbone to the
ruthenium center is only possible when the peptoids are both chiral
and helical.
foldamers,9 peptides10 and peptoids11 containing chromophores
incorporated in their backbone, could be controlled by changing
the chemical environment of the chromophore using solvophobic
12
interactions,9 pH,11b or binding of substrate molecules.
However, the direct relationship between the conformational
order of peptidomimetics and a physical utility such as the
luminescence of an embedded metal complex, that does not
involve changing its chemical environment, was not yet reported.
Linking together two fundamental properties such as high-order
structure and luminescence may provide fundamental insights
on the relationship between structure and function, and open up
a new avenue for possible applications in photocatalysis,
chemical biology, molecular imaging and more.
In an ongoing effort to understand the relationship between
structure and function in synthetic systems, we have designed
and synthesized a set of 13 short peptoid oligomers bearing
2,2’-bipyridine (bipy) ligand at their N-terminus. In the other
positions we placed only 1-6 chiral or achiral substituents, which
are either known helix inducers or which cannot induce helix
formation. We chose to incorporate (bipy)3Ru complexes within
the peptoids due to their unique combination of chemical stability,
excited state lifetimes and reactivity, and luminescence
emission.13 Thus, each peptoid was combined with Ru(II) and its
structural, luminescence and chiral properties before and after
Ru(II) binding were investigated using circular dichroism (CD)
and fluorescence spectroscopies as well as advanced high
resolution electron microscopy (HR TEM) techniques. Our
studies revealed a unique correlation between the secondary
structure of the peptoid ligand(s) and both the luminescence
intensity and the chirality of the embedded ruthenium centers,
which could be tuned simply by modifying the sequence of the
peptoid ligands.
Introduction
The relationship between structure and function is well
established in biological systems, leading to the unique
properties of natural polymers. Mimicking such structure-function
relationships requires the design of versatile sequences that (i)
include functional groups such as metal-binding ligands, 1
catalysts and chromophores, and (ii) enable control over
secondary structures, such as helices. Helical metal-binding
scaffolds can further form enantiopure metal complexes, having
a wide potential in cooperative and asymmetric catalysis, 2
material science and chemical biology.3
Results and Discussion
Design and synthesis of peptoid oligomers and their Ru(II)
complexes. The set of peptoids depicted in Fig. 1 was
synthesized on solid support from (S)- or (R)-1-
phenylethylamine (Nspe or Nrpe, respectively), (S)-1-
Peptoids,
oligomers
of
N-substituted
glycine,
are
peptidomimetics that can adopt helical secondary structures with
a helical pitch of three residues per turn if bulky-chiral side
chains are incorporated within their sequence.4 The helicity of
peptoids can be easily modified by utilizing different side-groups,
forming secondary structures with different degrees of helicity,
thus making them excellent candidates for studying the interplay
between structure and function.5 It was previously reported, for
example, that single-handed peptoid helices could induce
chirality at an embedded achiral catalyst for applications in
asymmetric catalysis. 6 Thorough sequence-structure-function
studies revealed that the enantioselectivity of the catalytic
peptoids strongly depends on the degree of conformational
order of the scaffold.6 It was also demonstrated that the
cyclohexylethylamine
(Nr1tbe),4l benzylamine (Npm), (S)-1-methoxypropylamine
(Nsmp) and [(4”-methyl-2’,2”-bipyridine4’-yl)-
(Nsch),4d
(R)-1-tert-butylethylamine
methyl]oxy]ethylamine (Nbpm) submonomer synthons in an
efficient iterated two step protocol.14 It was previously shown
that oligomers containing at least 66% of chiral-bulky side chains,
specifically Nspe, Nrpe, Nsch or Ns1tbe, out of all the
substituents within the sequence, could fold into helical peptoid
structures.4 It was also demonstrated that the degree of helical
character is increased with increasing number of monomeric
units.4b In contrast, peptoids incorporating a majority of Nsmp
groups are unstructured,15 and the introduction of achiral benzyl
substituents within homo-oligomers of Nspe decreases the
7
, 8
luminescence intensity of synthetic helicates,
aromatic
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degree of the peptoids helicity.16 Thus, the set of peptoids in Fig.
1 includes helical peptoids, chiral non-helical peptoid, an achiral
peptoid, Nspe peptoids in different lengths, and Nspe pentamers
incorporating Npm groups varied in their number and position,
which are anticipated to be varied in their degree of helicity.
The homoleptic Ru2+ complexes were prepared using
a
19
previously described procedure.
The complexes were
precipitated with NH4PF6 centrifuged and separated. The Ru(II)-
peptoids complexes were analyzed and purified using HPLC
(>95%) and their identity was verified by ESI-MS.
UV-Vis and emission spectroscopy. The metal-free peptoids
exhibited absorption bands near λ = 280 nm, in acetonitrile,
corresponding to the π–π* transition of the bipyridyl group (see
SI). The corresponding homoleptic Ru2+ complexes reveal a 5-
10 nm shifts in this absorption band with a significantly
increased intensity. In all cases, with the exception of Ru(2SP)3
and Ru(5SM)3, an additional band near λ = 450 nm, which
corresponds to the metal-to-ligand charge transfer (MLCT) band
of the ruthenium tri-bipyridine complex is observed17 (Table 1).
This band appears near λ = 454 and 445 nm for (2SP)3Ru and
(5SM)3Ru, respectively. Upon irradiation at 450 nm, in air, in
acetonitrile, all the complexes show an emission band in the
wavelength range of 545-800 nm indicating they are
photoluminescent in solution. The luminescence energy of the
Ru-peptoid complexes is red shifted by 13-19 nm compared with
that of Ru(bipy)3(PF6)2, which is 606 nm (Table 1, entry 1), and
this is consistent with the red shift observed for the MLCT
absorption band of the peptoid complexes.
Table 1. Table 1: Experimental values of absorbance and emission maximal
wavelength (λmax), quantum luminescence efficiencies (Φ) and excited
state lifetimes (τ) of peptoid complexes at rt., in acetonitrile, in air.
a]
[
Ru(L)3
L=
Absorbance
(MLCT)
λmax
Emission
λmax
Entry
Φ
τ(µs)
Figure 1. Peptoid oligomers for constructing Ru complexes. (a) Monomeric
units used in the design of the peptoids. (b) Chemical sequences of peptoids
studied in this work. Modular peptoid synthesis permits systematic control of
the chain length, monomer type, and the position and number of chiral
(colored, color choice arbitrary) and achiral (black) substituents.
1
bipy
451
454
459
460
461
460
445
462
460
458
459
459
460
461
606
625
619
620
625
622
625
625
621
620
623
624
623
622
0.018[b]
0.005
0.015
0.017
0.016
0.019
0.002
0.006
0.022
0.017
0.017
0.016
0.011
0.016
0.156
0.141
0.164
0.187
0.187
0.189
0.128
0.150
0.187
0.165
0.171
0.159
0.167
0.165
2
2SP
3
3SP
4
5SP
It is known that substitution on the bipyridine ring strongly affects
the photoluminescence properties of the obtained ruthenium
complex. 17 Thus, Nbpm was chosen because the peptoid
binding linker is attached to the 6’ position and because the
quantum luminescence efficiency of tri(4,4’-dimethyl-2,2’-
bipyridyl) ruthenium complex is much higher than that of
analogous complexes with (6,6’-dimethyl-2,2’-bipyridyl) or (5,5’-
dimethyl-2,2’-bipyridyl) as ligands.17 Nbpm was synthesized by a
two-step procedure starting from the commercially available 4,4’-
dimethyl-2,2’-bipyridine as a precursor (See SI). The Nbpm
group was placed at the N-terminus of each peptoid aiming to
explore the effect of different sequences and structures on the
luminescence properties of an embedded Ru(bipy)3(PF6)2
complex. The modularity of the “sub-monomer” protocol permits
the attachment of different substituents at defined positions
along the peptoid backbone, enabling systematic structure-
function investigations.4 All peptoids were synthesized by the
“sub-monomer” approach, 18 analyzed and purified by high-
pressure liquid chromatography (HPLC, >95%). The molecular
weights measured by electrospray ionization mass spectrometry
(ESI-MS) were consistent with the expected masses (see SI).
5
5RP
6
7SP
7
5SM
8
5PM
9
5SC
10
11
12
13
14
5RTB
5SP1
5SP1_2
5SP4
5SP3_4
[a] Measured in ACN at r.t in air. [b] Reference 20.
Circular dichroism (CD) spectroscopy of the peptoid
pentamers. In contrast to polyproline peptides that are
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composed of either cis or trans amide bonds, peptoid monomers
can favor both cis and trans orientation of the amide bond. This
can be easily evidenced from CD spectroscopy, which is a key
tool for describing the secondary structure of peptoids. It has
been previously demonstrated that Nspe, Nrpe, Nsch and
Ns1tbe substituted peptoids adopt right-handed helical
structures akin to polyproline type-I (PP-I) helices.4 The CD
spectra of peptoids having only phenylethyl side chains
(phenylethyl peptoids) exhibit a characteristic double minima
(Nspe) or a double maxima (Nrpe) with bands near 200 nm and
220 nm, that are associated with trans-amide bond and cis-
amide bond conformations respectively.4c Peptoids containing
only tertiary butyl groups or cyclohexyl groups, on the other
hand, were shown to adopt all-cis PP-I helices and their CD
spectra resemble these of PP-I peptide helices, with two minima
near 190 and 225 nm and a maximum near 210 nm.4l The CD
spectra of 5SP and 5RP having Nspe or Nrpe monomers
respectively, show the expected characteristic double minima or
maxima with bands at 203 and 220 nm. The CD spectrum of
5SC shows characteristic two minima at 196 and 222 nm and a
maximum at 208 nm, and the spectra of 5RTB exhibits two
maxima at 198 and 223 nm and one minimum at 211 nm as was
expected for oligomers that contain Nsch and Nr1tbe groups,
correspondingly.4l The CD spectrum of 5SM exhibits a minimum
near 190 nm, characteristic of a chiral yet unstructured peptoid
efficiency (Φ) of the Ru(pentamer)3 complexes, based on their
absorbance data at 450 nm and the integrated fluorescence
intensity in different concentrations (see SI). We found that when
measuring in the same conditions, the values for all the helical
complexes are close to that of Ru(bipy)3(PF6)2 (Φ=0.018),20
while the values are 3 or 9 times lower in the cases of Ru(5SM)3
and Ru(5PM)3, respectively (Table 1). As a control and another
example of an unstructured flexible ligand, we have synthesized
the di-methylated version of the amine Nbpm, [(4”-methyl-2’,2”-
bipyridine-4’-yl)-methyloxy]ethyl (dimethyl)amine (bp-4DM)21
and its corresponding ruthenium complex Ru(bp-4DM)3 (see SI).
The complex Ru(bp-4DM)3 showed a luminescence intensity 10
times lower than that of (bipy)3Ru as measured in acetonitrile in
air at r.t (see SI), with Φ=0.002, an order of magnitude lower
than that of Ru(bipy)3 (Φ=0.018). Overall, these observations are
remarkable because they suggest that only in some cases there
is a specific interaction between the substituent on the ligand
and the ruthenium center that leads to its quenching, and that
these interactions are only possible when the ligand substituent
is flexible and unstructured. Specifically, we identify a direct
correlation between structure and function: the peptoids helicity
and the luminescence of embadded ruthenium complexes.
Structure-function studies. At this point we aimed to
understand the correlation between the structure of the Ru-
peptoid complexes and their luminescence properties, as well as
and 5PM, being achiral, does not exhibit a CD spectrum (see SI). the factors that enable quenching only by unstructured flexible
In all cases, the CD spectra of (pentamer)3Ru complexes show
bands in the 190-230 nm reign that are similar to their
corresponding unbound peptoid pentamers (Fig. 2a).
peptoids. A key question in this context is whether a single
unstructured peptoid can cause emission quenching or is it a
different structural arrangement created by the assembly of
three peptoid oligomers via one ruthenium ion that enables the
quenching. In other words, we wanted to know if the
luminescence is actually dependent on the helicity (secondary
structure) of the peptoid scaffold or on another, higher ordered,
structure. To understand the influence of a single peptoid ligand
on the emission properties of the corresponding ruthenium
complex, we have prepared the two heteroleptic ruthenium
complexes, Ru(bipy)2(5SP) and Ru(bipy)2(5PM). The complexes
were synthesized by reacting one equivalent of peptoid with one
equivalent of Ru(bipy)2Cl2 in refluxing ethanol under nitrogen
atmosphere (see SI). The complexes were precipitated with
NH4PF6 and purified by HPLC. Interestingly, both Ru(bipy)2(5SP)
and Ru(bipy)2(5PM) have comparable emission intensities (see
SI), very similar to that of Ru(bipy)3. The quantum efficiencies of
luminescence of Ru(bipy)2(5SP) and Ru(bipy)2(5PM) are
Φ=0.018 and Φ=0.016, respectively. This is in contrast to the
analogous homoleptic complexes Ru(5SP)3 and Ru(5PM)3 that
show very different emission properties. From this experiment it
is evident that one peptoid ligand is not sufficient to cause the
luminescence quenching, thus the emission properties of the
heteroleptic complex are not influenced by the secondary
structure of the substituting peptoid ligand. These properties
should be therefore a consequence of some higher-order
arrangement of the three peptoid oligomers attached to each
Ru(bipy)3 center.
Figure 2. Structure-function relationships within Ru(peptoid pentamer)3
complexes. (a) CD spectra measured at rt. in acetonitrile: per residue molar
ellipticity of Ru(5SP)3, Ru(5RP)3, Ru(5SC)3, Ru(5RTB)3, Ru(5SM)3 and
Ru(5PM)3 (30µM each). (b) Normalized emission spectra of Ru(bipy)3,
Ru(5SP)3, Ru(5RP)3, Ru(5SC)3, Ru(5RTB)3, Ru(5SM)3 and Ru(5PM)3, (5µM
each in acetonitrile, in air, at rt.).
Luminescence of the peptoid-Ru complexes. Although all the
(pentamer)3Ru complexes show fluorescence emission (see
Table 1), we noticed an interesting and unique trend within the
intensity of the emission; while the emission spectra of the
ruthenium complexes of helical peptoids 5SP, 5RP, 5SC and
5RTB exhibit intensity very similar to this of Ru(bipy)3(PF6)2
complex, the intensities of the emission spectra of the chiral
unstructured Ru(5SM)3 and the achiral Ru(5PM)3 complexes are
significantly lower (Fig. 2b). We have also calculated the
experimentally derived value of the luminescence quantum
In the lack of crystals suitable for X-ray analysis, we have
decided to employ two different transmission electron
microscopy (TEM) techniques, a conventional TEM and a high-
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resolution (HR) TEM, the later is known to enable direct
observation of three-dimensional peptoid structures.22 We
initially performed conventional TEM measurements to samples
containing 10 µM Ru(5SP)3, Ru(5PM)3, Ru(5SM)3 or Ru(5RTB)3
in acetonitrile (the complexes are completely soluble) that were
deposited on polymer-coated copper grids negatively stained
with phosphotungstic acid. The grids were dried in air prior to
analysis, leaving the Ru-bundle complexes adsorbed on the
polymer. Fig. 3a shows representative TEM micrographs of
Ru(5RTB)3 and Ru(5SP)3 (inset), in which worm-like assemblies
with an average length of 77.8 ± 29.7 nm and 228.3 ± 78.5 nm,
respectively and an average width of 10.5 ± 3.6 nm and
25.2 ± 6.33 nm, respectively, are inspected. Fig. 3b presents the
TEM micrograph of Ru(5SM)3, which shows sphere-like
assemblies with an average diameter of 43.5 ± 10.32 nm.
Interestingly, TEM analysis of Ru(5PM)3 revealed mixed
morphology containing both worm-like assemblies and spheres
(Fig. S74). These micrographs provide evidence that ruthenium
complexes from both helical and unstructured peptoids form 3D
arrangements with structure-dependent morphology.
bundle. Energy-dispersive X-ray spectroscopy (EDS) analysis
performed on a sample containing Ru(5RTB)3 confirmed the
presence of ruthenium in this sample (Fig. S75). The ATR-FT IR
spectra of Ru(5RTB)3 and Ru(5SP)3 measured from the TEM
solutions before and after these were dried, were similar for
each peptoid (Fig. S76), suggesting that the structures seen in
the TEM micrographs are also present in solution.
Figure 4. HR TEM micrographs of samples containing 10 µM peptoids in
acetonitrile that were deposited on holey carbon copper grids, providing direct
imaging (black-atom contrast) of (a) Ru(5RTB)3 (Talos 200C operated at 200
kV) and (b) Ru(5SP)3 (Titan Themis G2 300 operated at 80 kV). Insets:
magnified images of the individual metallopeptoids and metallopeptoid
bundles (also marked in red in (a)).
These results, together with the luminescence data, provide a
strong and remarkable confirmation for the direct relationship
between structure and function within these Ru-peptoid
complexes. Previous studies on peptides bearing amino acid
chromophores demonstrated that the peptide bond itself is an
intramolecular quencher and that the fluorescence decay
depends on local backbone conformations about the
chromophore moiety.20 It was also suggested that the backbone
carbonyl group can act as a fluorescent quencher of indole
chromophores.21 This quenching occurs via a through-space
electron transfer mechanism, which depends on the number,
distance, and possibly the orientation of peptide bonds about the
indole ring.10,22 It is also known that amines can quench the
emission of Ru(bipy)3 complexes depending on their
concentration, structure and on the solvent. 23 The peptoid
backbone contains amide, carbonyl and amine groups, thus, the
quenching of the luminescence that is observed when
unstructured peptoid ligands coordinate to the ruthenium ion
may be attributed to the through-space interactions of the
peptoid backbone with the ruthenium center. Our hypothesis is,
that in the case of helical peptoid ligands, a more rigid structure
is formed keeping the peptoid backbone far apart from the metal
center, thus minimizing the interaction of the peptoid scaffold
with the ruthenium center (Figure 5a). In contrast, when
unstructured peptoids are coordinated, the metal center is
surrounded and possibly covered by the peptoid ligands, which
can interact with the peptoid backbone, resulting in through-
space luminescence quenching (Fig. 5b). This hypothesis is
supported by the TEM images presented in Figures 3 and 4,
showing elongated structures in the case of helical ppetoids and
spherical structures in the case of unstructured peptoids, as well
Figure 3. TEM micrographs of samples containing 10 µM peptoids in
acetonitrile that were deposited on polymer-coated copper grids negatively
stained with phosphotungstic acid (yielding white-atom contrast). (a)
Ru(5RTB)3 and Ru(5SP)3 (inset), showing the formation of worm-like
assemblies and (b) Ru(5SM)3 oligomers assembled to spheres.
To gain more information about the structural composition of the
worm-like assemblies we have performed HR TEM
measurements to the same solutions of Ru(5RTB)3 or Ru(5SP)3
that were used in the conventional TEM measurements.
Samples from these solutions were deposited on holey carbon
coated copper grids. The grids were dried in air prior to analysis.
The obtained high-resolution images revealed great detail about
the structural organization of the worm-like assemblies, showing
individual chains that can represent either separate peptoid
helices or bundles of three helices held via coordination to the
embedded Ru(bipy)3 complex (Fig. 4a-b).23
The average chain length of 2.2-2.4 nm is in agreement with the
length of an Nr1tbe containing peptoid homopentamer, the
crystal structure of which was recently published.4l Specific
arrangements of helical bundles can be also observed (marked
in red circles in Fig. 4a) with an average space width between
peptoids of 2.5Å (marked by two white pointing arrows in the
inset of Fig. 4b), which can be assigned to hydrophobic
interactions and/or π-π stacking between helices from the same
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as by the results obtained in case of the complex Ru(bp-4DM)3
(see above) in which the luminescence of the ruthenium center
is also quenched by its interactions with the flexible ligand that
contains amine groups.
From the chain-length dependence plots (Fig 6c-d) we can
conclude that the luminescence is a chain length dependent
property with the critical chain length of three monomers. Below
this length, the luminescence is quenched and above this length,
the peptoid scaffold adopts a helical secondary structure and the
luminescence is maintained and only slightly enhanced with the
increase in the peptoid chain length.
Figure 5. A cartoon representing our hypothesis regarding the formation of
either Ru2+ complexes from (a) three helical peptoids, which do not interact
with the ruthenium center, or from (b) three unstructured flexible ligands that
can interact with the ruthenium center leading to through-space quenching.
Sequence-structure-function relationships. It is known from
previous research that the degree of peptoids helicity depends
on their length and monomer sequence. In order to further
explore our intriguing observations we wanted to see if we can
actually tune the function (luminescence) of phenylethyl Ru-
peptoid complexes by systematically altering the helicity of
Ru(5SP)3. To do so, we first examined the CD spectra and
luminescence properties of ruthenium complexes based on
Nspe peptoids different in their chain length, namely Ru(2SP)3,
Ru(3SP)3, and Ru(7SP)3. The CD spectra of the free peptoids
(see SI) and their ruthenium complexes (Figure 6a) were
recorded in acetonitrile at room temperature. As expected, the
CD spectrum of Ru(2SP)3 displayed only weak CD signals,
indicating no helical secondary structure. This is in line with the
previous observation that at least three monomers are required
in order to form helical secondary structure with a helical pitch of
three residues per turn.4b The CD signal of Ru(3SP)3 has a lower
intensity than the signal measured for Ru(5SP)3, with double
minima of equal intensity at 200 nm and 220 nm, suggesting that
this has a helical structure with an equal population of cis and
trans amide bonds.4c The CD spectra of Ru(7SP)3 shows the
strongest signal with an intensity by 66% higher than that of
Ru(5SP)3. The major band was observed at 220 nm, indicating a
higher population of cis amide bonds.
The emission spectrum at the range of 550-800 nm was
measured for these three complexes upon irradiation at 450 nm
(Figure 6b) and the quantum efficiency of luminescence was
experimentally derived. The complex Ru(7SP)3 demonstrates
the highest emission intensity, comparable to that of Ru(bipy)3.
The emission intensity is slightly decreased when decreasing
the length of the helical peptoid scaffold from seven monomers
in Ru(7SP)3 to five monomers in Ru(5SP)3 and to three
monomers in Ru(3SP)3 (Figure 6b). For the unstructured dimer
2SP, the emission intensity of the corresponding ruthenium
complex Ru(2SP)3 is lower by an order of magnitude compared
to that of Ru(7SP)3, Ru(5SP)3 and Ru(3SP)3 (Figure 6b). The
same trend is observed for the value of Φ, which is 0.005 for
Ru(2SP)3 – three to four times lower than the values measured
for Ru(7SP)3 (0.019), Ru(5SP)3 (0.017) and Ru(3SP)3 (0.015).
Fig. 6. Peptoid chain length dependent luminescence in Nspe-peptoid-Ru
complexes. (a) CD spectra of Ru(2SP)3, Ru(3SP)3, Ru(5SP)3, Ru(7SP)3 (30µM
each in acetonitrile). (b) Normalized luminescence spectra of Ru(bipy)3,
Ru(2SP)3, Ru(3SP)3, Ru(5SP)3 and Ru(7SP)3 (5µM in acetonitrile). (c) Norm-
alized emission vs. number of monomers Ru(nSP)3. (d) Luminescence
quantum efficiency vs. number of monomers Ru(nSP)3.
Following these observations, we decided to modify the
sequence of 5SP by using a combination of chiral Nspe groups
and non-chiral Npm groups incorporated at different positions
along the peptoid sequence (see Fig. 1b). Based on previous
experience, we expect that the position and number of the Nspe
groups should affect the helicity of the peptoid16,28 and its metal
complex,28 thus we aimed to probe the influence of the change
in helicity of the peptoid scaffold on the luminescence of the
corresponding ruthenium complex. We synthesized four peptoid
oligomers all bearing an Nbpm group at the terminal 5th position
but have different combination of Nspe and Npm groups.
Oligomer 5SP1 has one Nspe group at the 1st position, oligomer
5SP1_2 has two Nspe groups at the 1st and 2nd positions and
oligomers 5SP4 and 5SP3_4 have one Nspe groups at the 4th
and two Nspe groups at the 3rd and 4th positions respectively.
The
corresponding
ruthenium
complexes
Ru(5SP1)3,
Ru(5SP1_2)3, Ru(5SP4)3 and Ru(5SP3_4)3 were synthesized
according to the usual procedure and their CD spectra and
luminescence properties were measured (Fig. 7a and b,
respectively).
The CD spectrum of complex Ru(5SP1)3 shows a signal with an
intensity three times lower than that of Ru(5SP)3 with double
minima of nearly equal intensity at about 200 nm and 220 nm
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consistent with the initiation of a helical structure. The CD
spectrum of Ru(5SP1_2)3 has a similar shape but with twice as
µs) as measured in acetonitrile in air at room temperature and
are at the typical range of Ru(bipy)3 complexes in air.17
high intensity, consistent with
a
more pronounced helical
Remote induced chirality from the chiral peptoid monomers
to the achiral Ru(bipy)3 center. It was previously shown that
the chirality of (bipy)3Ru complexes could be resolved by
attaching a helical scaffold to the bipy ligand, which allows the
predetermination of the complex chirality.19,29 As discussed
above, all the Ru-peptoid complexes produce absorption bands
near 285 and 460 nm; an example of UV-Vis spectra of 5SP and
its Ru complex is depicted in Fig. 8a. Ruthenium binding to 5SP
also produces a new band in the CD spectra, of very low
intensity at about 295 nm corresponding to the π–π* transition of
the bipy ligand. This band is very weak and does not show the
expected Cotton effect. Repeating the measurement at higher
concentrations (300 µM vs. 30 µM) resulted in a stronger CD
signal that showed clear Cotton effect with a minimum at 293 nm
(Fig. 8b, red line). CD measurements at even higher
concentrations (1.5 mM) at the wavelength range of 400-600 nm
show a 75 nm wide exciton couplet with a crossing point at 450
nm, which is related to the wavelength of the MLCT band (Fig.
8c, red line).
structure. The CD spectrum of Ru(5SP4)3 shows a very low
intensity pick near 220 nm while this of Ru(5SP3_4)3 exhibits a
signal with similar intensity to that of Ru(5SP)3 but with the larger
pick at around 220 nm, suggesting an overall helical structure of
the peptoid with a higher population of cis amide bonds. The
complex Ru(5SP4)3 shows the lowest luminescence intensity
with a value located between that of the completely disordered
Ru(5PM)3 and the helical Ru(5SP)3 (Figure 4b). Complexes
Ru(5SP1)3, Ru(5SP1_2)3 and Ru(5SP3_4)3, show luminescence
intensity slightly lower than that of (5SP)3Ru, and very close in
value to one another.
Fig. 7. Sequence and Structure dependent luminescence in phenyl peptoid-Ru
complexes. (a) CD spectra of Ru(5SP)3, Ru(5SP1)3, Ru(5SP1_2)3, Ru(5SP4)3,
Ru(5SP3_4)3 and (5PM)3Ru (30µM each in acetonitrile). (b) Normalized
luminescence spectra of Ru(bipy)3, Ru(5SP)3, Ru(5SP1)3, Ru(5SP1_2)3,
Ru(5SP4)3, Ru(5SP3_4)3 and Ru(5PM)3 (5µM in acetonitrile). (c) Normalized
emission vs. CD intensity at 220 nm of all 6 peptoid-Ru complexes. (d)
Luminescence quantum efficiency vs. CD intensity at 220 nm of all 6 peptoid-
Ru complexes.
Overall, we could demonstrate
a clear trend pointing at
Fig. 8. Chiral induction from helical peptoid pentamers. (a) UV-Vis spectra of
5SP (30µM in acetonitrile) and Ru(5SP)3 (10µM in acetonitrile). (b) CD spectra
measured at rt. of Ru(5SP)3, Ru(5RP)3, Ru(5SM)3 and Ru(5PM)3 (300 µM in
acetonitrile). (c) CD spectra at rt. of the four complexes above (1.5mM in
acetonitrile). (d) CPL spectrum and emission dissymmetry values of Ru(5SP)3
(500 µM in acetonitrile in air).
increased luminescence intensity (or quenching) as a function of
the peptoid scaffold helicity; as the peptoid is less structured, it
can enable more efficient quenching of the (bipy)3Ru center and
as it is more helical, showing increased intensity of the CD band
near 220 nm, the luminescence of the (bipy)3Ru center
increases as well (Fig. 7c). The experimental values of quantum
efficiency for complexes Ru(5SP1)3, Ru(5SP1_2)3, Ru(5SP4)3
and Ru(5SP3_4)3 support this trend with the values of Φ= 0.017,
Φ= 0.016, Φ= 0.011 and Φ= 0.016 respectively (Table 1, and Fig.
7d). Moreover, tuning of luminescence could also be achieved,
as we observed significant differences in luminescence with
correspondence the different degrees of helicity of the various
peptoid ligands. The emission lifetimes (see Table 1) of all
peptoid complexes are close to that of Ru(bipy)3(PF6)2 (0.156
The directionality of the band corresponds to Δ absolute
configuration at the metal center.30 These signals reflect the
induction of chirality from the peptoid scaffold to the metal center
resulting in the stereoselective formation of the Δ stereoisomer.
As expected, the CD spectra of the corresponding complex
Ru(5RP)3 in the high concentrations show inversed signals to
those of Ru(5SP)3 at the near UV and the far UV regions (Fig.
8b-c, blue lines). The exciton couplet of Ru(5RP)3 at 450 nm
corresponds to that of a Λ ruthenium complex.31 These results
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support the observation that the stereochemistry of the formed
ruthenium complex is dictated by the chirality of the peptoid
scaffold and the handedness of the peptoid helix. Both Ru(5SC)3
and Ru(5RTB)3 also produced bands in the CD spectra at the
near UV and the far UV regions (see SI). The CD spectrum of
Ru(5SC)3 exhibits two intense exciton couplets at the
wavelength range of 270-310 nm and 400-600 nm, similar to
those of Ru(5RP)3 suggesting the formation of a Λ ruthenium
complex. The spectrum of Ru(5RTB)3 is very similar to this of
Ru(5RP)3, with a band near 293 nm and an exciton couplet near
450 nm, also suggesting the formation of a Λ ruthenium complex.
Interestingly, the CD spectra of Ru(5SM)3, which is composed of
a chiral peptoid, do not show any signals at 290 nm and 450 nm
even at higher concentrations indicating no induction of chirality
from the chiral Nsmp substituents to the metal center (Fig 8b-c,
green line). As expected, no CD spectra was obtained for
Ru(5PM)3, which is a completely achiral complex.
Ru(5SP1_2)3, Ru(5SP4)3 and Ru(5SP3_4)3 all exhibit
pronounced Cotton effect near 290 nm and exciton couplets at
the 400-600 nm region, implying a preferable Δ stereochemistry
(Fig. 9b). Notably, in the cases of complexes Ru(5SP1)3 and
Ru(5SP1_2)3, there is a remote chiral induction on the metal
center through a chain of three (four amide bonds) or two (three
amide bonds) achiral Npm monomers (through 21 or 18 bonds,
respectively), a phenomena that was previously observed only
for peptide substituted bipyridyl iron complexes.33
The photoluminescence of Ru(5SP)3 allows us to measure its
circularly polarized luminescence (CPL) spectrum. CPL is a
spectroscopic technique for measuring the differential emission
of left and right circularly polarized light. The obtained spectrum
reflects the chirality of the complex in its luminescent (excited)
state. CPL is quantitatively expressed by the emission
anisotropy factor, gem, also known as the dissymmetry factor,
which can be calculated according to eq. 1. The CPL spectrum
of Ru(5SP)3 is depicted in Figure 8d. Ru(5SP)3 has a gem value
of 1.9·10-2 [± 0.1]. Typical values of the dissymmetry factor for
chiral, luminescent organic molecules and chiral, luminescent
transition metal complexes are in the 10-2 - 10-5 order of
magnitude.32
!!
! !!
! !
(1)
g!" λ =
=
!.!"
!.!(! !! )
! !
The complex shows a slight decrease in gem as the wavelength
increases, consistent with previous work conducted on
[Ru(bpy)3]2+ complexes.32 The CPL measurement of Ru(5SM)3
was also recorded and it supports the observation that there is
no chirality induction from the chiral Nsmp substituents to the
ruthenium center as its dissymmetry factor is gem= 0 (see SI).
Thus, the monomers chirality by itself is not sufficient to induce
chirality on the metal center and the helicity of the entire
structure is required.
Figure 9: (a) CD spectra measured at rt. in acetonitrile of Ru(2SP)3, Ru(3SP)3,
Ru(5SP)3 and Ru(7SP)3 (300 µM, full lines and 1.5 mM, dashed lines). (b) CD
spectra measured at rt. in acetonitrile of Ru(5SP1)3, Ru(5SP1_2)3, Ru(5SP4)3
and Ru(5SP3_4)3 (300 µM, full lines and 1.5 mM, dashed lines). (c)
A
schematic representation of the chiral induction on the ruthenium metal center.
Even one remote chiral group is sufficient to obtain chirality at the metal center.
Conclusions
Chiral induction could also be observed from the CD spectra of
Ru(3SP)3 and Ru(7SP)3 measured at high concentrations, with
bands at near 290 nm (in 300 µM solution) and an exciton
couplet at 400-600 nm region (in 1.5 mM). While no bands were
detected in the CD spectrum of Ru(2SP)3 even at high
concentrations (Fig. 9a). Moreover, the intensity of the signal at
280 nm correlates with the intensity of the signal at 200-220 nm.
Thus, the intensity of the signal is increased with the increase in
peptoid length. The absence of a Cotton effect signal for
Ru(2SP)3 at about 280 nm, indicates that the chiral induction is
caused by the helical structure of the peptoid scaffold rather
than by the chirality of the single Nspe monomer that is present
in Ru(2SP)3. Additional CD measurements revealed that the
spectrum of the heteroleptic complex Ru(bipy)2(5SP) is similar to
this of Ru(5SP)3, while the spectrum of Ru(bipy)2(5PM) did not
show any signals (see SI). The complexes Ru(5SP1)3,
Despite the well-established relationships between sequence,
structure and function in biopolymers, knowledge about these
relationships in peptoids and other peptide mimics is still lacking,
hampering their usage in various applications. Thus, the
demonstration of a direct correlation between structure and
function by helical peptoid oligomers is a milestone in the
development of biomimetic foldamers. In this work, we have
explored these relationships and show, for the first time, that the
luminescence intensity of ruthenium centers embedded within
peptidomimetic scaffolds, can be controlled solely by the
structure of the oligomer, specifically its helicity. Based on
circular dichroism and fluorescence spectroscopy we
demonstrate that while helical peptoids do not affect the
fluorescence of the embedded Ru(II) center, unstructured
peptoids lead to its significant decay. TEM measurements
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This research was funded by the Solar Fuels Israel Center of
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The sequence, and thus the 3D structure, of short peptidomimetic (peptoid)
oligomers can dramatically and directly impact the photoluminescence and chiral
properties of an embedded ruthenium complex.
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