Full Paper
terized using 1D and 2D NMR (gCOSY, gHMBCAD) spectrosco-
tons in intramolecular H bonding and F2 in intermolecular
H bonding (Figure 1, Figure S2 in the Supporting Information,
Table 1).
py in a non-gelling solvent CDCl as well as in a gelling solvent
3
[
D ]toluene (see the Supporting Information). The NMR spec-
8
troscopy studies were performed at room temperature (228C)
for CDCl3 and at higher temperature (508C) in the case of
For disubstituted Fc–peptide conjugate 3, F1 and F2 amides
appear around d=7 ppm (in both CDCl3 and [D ]toluene),
8
[
D ]toluene as some of the Fc–peptide conjugates remain in
8
most likely suggesting the involvement of intramolecular hy-
gel state at room temperature. It was noted that the Fc–pep-
tide conjugates show a similar pattern of proton resonance in
drogen bonds. The proton NMR spectra in CDCl at RT show
3
that both F1 and F2 NHs appear more downfield (almost equal
magnitude) compared to monosubstituted Fc conjugate 1 as
well as disubstituted Fc conjugate 4. Both VC-NMR and VS-
NMR spectra of Fc conjugate 3 show an almost equal magni-
tude shift in amide protons of F1 and F2, while the shift for A3
NH is higher than the other two amide protons. It is anticipat-
ed that both F1 and F2 amide protons are simultaneously hy-
drogen bonded with ferrocenyl C=O of the other strand, which
both solvents (Figure 1 for CDCl and Figure S1 in the Support-
3
ing Information for [D ]toluene), suggesting that they most
8
likely have similar hydrogen-bonding patterns and conforma-
tion in both solvents.
[
73]
is known as bifurcating intramolecular H bond. Moreover, all
aliphatic protons of C10 alkyl chain were more shifted up-field
(
0.4–0.5) than the ester CH (0.02) with an increase in tempera-
3
ture, suggesting the presence of van der Waals interactions be-
tween alkyl chains (Figure S3 in the Supporting Information).
Interestingly, in disubstituted Fc–peptide conjugate 5, two
peptide chains are attached to the ferrocene moiety with
a three-carbon alkyl chain, all three amide protons resonate
below d=7 ppm at room temperature. This indicates the ab-
sence of intramolecular H bonds and hence the absence of
Herrick conformation. Moreover, the VC- and VS-NMR spectra
confirm the presence of three intermolecular H bonds.
1
Figure 1. H NMR stack plots of different Fc–peptide conjugates (CDCl
5
3
,
mm, 228C) displaying the amide region: a) FcCO–lFlFlA-OMe (1a),
Next, it was crucial to investigate how differences in the hy-
drogen-bonding pattern affect the conformation around the
ferrocene core. Therefore, circular dichroism (CD) was used for
the elucidation of conformation and metallocene chirality of
b) Fc[CO-lFlFlA-OMe]
2
(2a), c) Fc[CO-C10][CO-lFlFlA-OMe] (3), d) Fc[CO-
-CO-lFlFlA-OMe] (5). The spectra
show the relative positions of amide resonances of the same tripeptide
lFlFlA) in different compounds, presumably giving a clue of hydrogen
OMe][CO-lFlFlA-OMe] (4), and e) Fc[C
3
2
(
bonding and differences in conformations. Indicated above the peaks are
the corresponding amino acids for each compound.
[62,67]
ferrocene peptide conjugates 1a, 2a, 3, 4 and 5 (Figure 2).
CD of Fc–peptide conjugate 1a (in chloroform) shows a very
weak positive Cotton effect at 460 nm, which is expected for
monosubstituted Fc–peptide. While, Fc conjugates 2a and 3
exhibit a strong Cotton effect in the range of 480–490 nm,
suggesting the presence of intramolecular interstrand hydro-
gen bonds, which results in axial chirality around the ferrocene
Variable concentration NMR (VC-NMR) and variable-solvent
NMR (VS-NMR) spectroscopies were performed to assess the
nature of hydrogen bonding in different Fc–peptide conju-
gates. These NMR spectroscopy data are summarized in
Table 1 and the corresponding spectra can be seen in Fig-
ure S2 in the Supporting Information.
[
62,67]
core.
By combining the hydrogen-bonding pattern from
the NMR spectra and CD results, we believe P-helical “Herrick”
and “van Staveren”-like conformations are formed in Fc conju-
gate 2a and 3, respectively (Figure 2B). These results are well
aligned with the NMR spectroscopy data which confirm the
presence of intramolecular hydrogen bonds for both 2a and 3.
Although, Fc conjugate 5 is a disubstituted system, interesting-
ly it exhibits a very week CD signal around the 450–500
region, suggesting the formation of an open conformation
(“Xu” or anti) that does not adopt a typical structure by intra-
molecular hydrogen bonds between two peptide chains (Fig-
For monosubstituted Fc–peptide conjugate 1a, all three
amide protons appear at d below 7 ppm in CDCl as well as in
3
[
D ]toluene, which indicates the absence of intramolecular
8
[
67,72]
H bonds.
The chemical shifts of all the amide NH proton
signals are both concentration- and solvent-dependent, sug-
gesting the involvement of all amide protons in intermolecular
H bonding (Figure 1, Table 1). Similarly, this establishes the
presence of three intermolecular H bonds in Fc–peptide conju-
gate 4. In case of disubstituted Fc–peptide conjugate 2a, F1
NH appears much downfield, d=8.20 ppm, at room tempera-
ture, suggesting strong hydrogen bonding. The chemical shifts
of the amide NH proton signals of F1 and A3 are concentration
independent, whereas that of F2 is concentration-dependent.
However, VS-NMR indicates that the amide proton signal of F1
is solvent-independent, whereas those of F2 and A3 are sol-
vent-dependent, suggesting the involvement of F1 amide pro-
[
67,71]
ure 2B).
The open conformation supports three intermo-
lecular hydrogen bonds per peptide chain (by a sheet-like con-
formation) and this again supports the NMR spectroscopy
studies. However, Fc conjugates 1a and 4 show much en-
hanced CD signals in toluene presumably due to the formation
of the self-assembled gel state that induces supramolecular
chirality in the ferrocene region (vide infra and Figure S4 in the
Supporting Information).
Chem. Eur. J. 2015, 21, 11560 – 11572
11562
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