Synthesis and conformational study of bioconjugates derived from 1-acetyl-1′-aminoferrocene and α-amino acids

Article abstract of DOI:10.1002/ejic.201402837

1,1′-Disubstituted ferrocene conjugates present useful and efficient bioorganometallic constraint design to reduce the conformational flexibility of small peptides. In this study we present the first systematic conformational analysis of nonsymmetric ferrocene peptidomimetics (Boc-AA-NH-Fn-COMe; Boc = tert-butoxycarbonyl; AA = Gly, L-Ala, L-Val; Fn = 1,1′-ferrocenylene) and their monosubstituted analogues (Boc-AA-NH-Fc; Fc = ferrocenyl; AA = Gly, L-Ala, L-Val). The spectroscopic data (IR, NMR and CD) were corroborated by DFT calculations and indicated the engagement of the NH group closest to the ferrocene unit in intrachain hydrogen bonds. This medium-strength bond is not disrupted by the introduction of a hydrogen-bonding acceptor on the other ferrocene ring, but rather is accompanied by an additional interchain hydrogen bond, which causes the restricted rotation of ferrocene rings and gives rise to a chiral arrangement of the ferrocene core in a P helical manner.

Full text of DOI:10.1002/ejic.201402837

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DOI:10.1002/ejic.201402837
1
Synthesis and Conformational Study of Bioconjugates
Derived from 1-Acetyl-1Ј-aminoferrocene and α-Amino
Acids
[a]
[a]
[b]
ˇ
´
ˇ ´
ˇ
Mojca Cakic Semencic,* Veronika Kovac, Ivan Kodrin,*
[a]
[a]
ˇ ´
´
Lidija Barisic, and Vladimir Rapic
6
Keywords: Conformation analysis / Density functional calculations / Bioinorganic chemistry / Peptidomimetics / Hydrogen
bonds / Circular dichroism
1,1Ј-Disubstituted ferrocene conjugates present useful and
efficient bioorganometallic constraint design to reduce the
conformational flexibility of small peptides. In this study we
present the first systematic conformational analysis of non-
symmetric ferrocene peptidomimetics (Boc-AA-NH-Fn-
DFT calculations and indicated the engagement of the NH
group closest to the ferrocene unit in intrachain hydrogen
bonds. This medium-strength bond is not disrupted by the
introduction of a hydrogen-bonding acceptor on the other
ferrocene ring, but rather is accompanied by an additional
interchain hydrogen bond, which causes the restricted rota-
tion of ferrocene rings and gives rise to a chiral arrangement
of the ferrocene core in a P helical manner.
11
16
21
COMe; Boc = tert-butoxycarbonyl; AA = Gly,
= 1,1Ј-ferrocenylene) and their monosubstituted analogues
(Boc-AA-NH-Fc; Fc = ferrocenyl; AA = Gly, -Ala, -Val). The
L-Ala, L-Val; Fn
L
L
spectroscopic data (IR, NMR and CD) were corroborated by
and in the solid state. We focused our research on type II
derivatives, because in comparison to types I and III, only
conjugates derived from Fca maintain an antiparallel orien-
tation of peptide chains and thus can be truly considered
as peptide turn mimetics.
Introduction
46
51
56
61
66
71
26
31
36
41
The design of secondary structure mimetics of short
peptides is of vital importance not only for understanding
the fundamentals of protein folding, but also for the devel-
opment of functional peptidic compounds. 1,1Ј-Disubsti-
tuted ferrocenes are recognized as molecular scaffolds for
peptide mimetic models because the almost free-rotating
cyclopentadiene (Cp) rings are separated by about 3.3 Å,
which is ideal for interstrand hydrogen-bonding interaction
in the conjugates with natural amino acids or peptides.[1,2]
If the peptide groups directly attached to ferrocene are
taken into account, then three major classes of bioconju-
gates need to be considered: peptidomimetics derived from
ferrocene-1,1Ј-dicarboxylic acid (Fcd, I),[3–6] 1Ј-aminoferro-
cene-1-carboxylic acid (Fca, II),[7–11] and 1,1Ј-diaminofer-
rocene (Fcda, III).[12] Extensive spectroscopic studies of
conjugates I–III revealed the presence of two strong intra-
molecular hydrogen bonds supporting the helically chiral
arrangement of the ferrocene subunit in symmetrically sub-
stituted derivatives (Scheme 1, n = m = 1) both in solution
In contrast to conjugates bearing the identical podand
peptide chains (n = m = 1, 2, ...), nonsymmetric bioorgano-
metallics II (n ϶ m) do not show a tendency to adopt a
single conformer in solution.[13–16] Detailed structural
analysis of peptides derived by C-terminal elongation of
Fca [II, n = 0, m = 1, R = H, CH3, CH(CH3)2][8,10] revealed
the presence of multiple conformational isomers, stabilized
by single intramolecular hydrogen bond, in the solution.
Some conformers are destabilized when the steric demand
of the amino acid side chain increases, and even changing
the solvent can promote some conformers over others. In
the solid state, dipeptide Boc-Fca-l-Ala-OMe is stabilized
by an eight-membered interchain NHAla···OCBoc
hydrogen-bonded ring (numeration of rings is shown in Fig-
ure 12), whereas achiral Ac-Fca-Gly-OMe (Ac = acetyl)
features a nine-membered hydrogen-bonded ring spanning
NHFc and the carbonyl group of glycine. N-Terminal cou-
pling of Fca was used to obtain the second class of type II
analogues (Y-AA-Fca-OMe; Y = Boc, Ac; AA = l-Ala, d-
Ala), which are formally constitutional isomers of peptides
derived by C-elongation of Fca. Combined spectroscopic
and theoretical approach indicated the presence of several
conformations in solution stabilized by intramolecular
hydrogen bonds. As a consequence of imposed interchain
[a] Department of Organic Chemistry, University of Zagreb,
Faculty of Food Technology and Biotechnology,
Pierottijeva 6, 10000 Zagreb, Croatia
E-mail: mcakic@pbf.hr
http://www.pbf.unizg.hr
[b] Department of Chemistry, University of Zagreb, Faculty of
Science,
Horvatovac 102a, 10000 Zagreb, Croatia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402837.
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previous examples in which positive CD-signals were ob-
served when l-amino acids were employed and vice
versa.[8,10] To investigate how the omission of one hydrogen-
bond-donating/accepting amino acid unit influences con-
formational preferences of these amides, we prepared a
complete set of Boc-AA-NH-Fn-COMe derivatives (AA =
Gly, l-Ala, l-Val) and performed a detailed conformational
analysis with CD, IR, and NMR spectroscopic techniques
in combination with DFT calculations. The synthesis and
properties of the previously reported monosubstituted de-
rivatives Boc-AA-NH-Fc,[8,18] which can be considered as
simple model compounds, are included here for compari-
son. The experimental results have been accompanied by
DFT calculations to gain better insight on the hydrogen
bond patterns involved in the most stable conformers of
each compound.
86
91
96
Results and Discussion
101
Synthesis of Ferrocene Derivatives
N-Boc-protected aminoferrocene (1) and 1-acetyl-1Ј-
aminoferrocene (2) were prepared according to previously
described procedures.[8,19] Removal of the tert-butyloxy-
carbonyl group from 1 and 2 was performed by the action 106
of gaseous HCl and the resulting aminoferrocenes were
subsequently coupled with activated Boc-AA-OH (AA =
Gly, l-Ala or l-Val) to give 3a (80%), 3b (82), 3c (95%), 4a
(89%), 4b (83%), and 4c (84%), respectively (Scheme 2).
Scheme 1. Peptidomimetics derived from ferrocene-1,1Ј-dicarbox-
ylic acid (Fcd, I), 1Ј-aminoferrocene-1-carboxylic acid (Fca, II),
1,1Ј-diaminoferrocene (Fcda, III), and various l- and d-amino ac-
ids (m, n = 1, 2, etc.) are stabilized by two strong intramolecular
hydrogen bonds both in the solid state and in solution.
IR, NMR, and CD Studies
111
The aim of this research was to shed light on hydrogen-
constraints, the relative position of two cyclopentadienyl bond patterns in conformers of short ferrocene-containing
rings with specific pseudo-torsion angle is unaffected by the bioconjugates 4 by using IR, NMR, and CD spectroscopic
absolute configuration of the alanine chiral centers or by techniques in combination with theoretical calculations. A
76
81
the steric demand of the protective group.[16] We have re- comparison of these data with those of the model com- 116
cently shown that conjugates of type II, bearing homochiral pounds 3 was also performed. Amido groups of monosub-
dipeptide sequence Boc-(AA)2-NH-Fn-COMe (AA = Gly, stituted conjugates 3 are unable to engage in interchain
l-Ala, l-Val), are stabilized by interchain hydrogen bonds hydrogen-bonding, which may force cyclopentadienyl rings
accompanied by less stable intrachain contacts.[17] Although of the ferrocene unit into a fixed conformation due to ab-
derived from l-amino acids, these derivatives are charac- sence of hydrogen-bonding sites on the other cyclopenta- 121
terized by negative bands in the CD spectra and by M- dienyl ring. Infrared absorption spectra of amides 4 in
helicity of the ferrocene subunit. This finding contrasts with dichloromethane solutions (c = 10–2 m) display stretching
Scheme 2. Syntheses of Fca conjugates 3 and 4: (a) HCl(g)/EtOAc; (b) 1. HOBt/EDC, CH2Cl2; 2. Boc-AA-OH, CH2Cl2.
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bands below 3400 cm–1, belonging to hydrogen-bonded NH
groups together with signals above 3400 cm–1 due to the
126 free NH groups (Table 1). The relative intensities of free
and hydrogen-bonded NH bands allow the population of
free and bonded NH groups to be estimated.
Table 1. IR data [ν, cm–1] for 3a–c and 4a–c in CH2Cl2 (c = 10–2 m)
and in KBr.
NHfree
CH2Cl2
NHbonded
NHfree
KBr
NHbonded
3a
3b
3c
4a
4b
4c
3420 (m)
3425 (m)
3427 (m)
3412 (m)
3423 (m)
3423 (m)
3322 (vw)
3322 (vw)
3329 (vw)
3322 (w)
3314 (w)
3312 (w)
–
–
–
3337 (s)
3296 (m)
3305 (m)
3315 (s)
3300 (s)
3299 (s)
3409 (m)
3419 (w)
3431 (w)
Figure 2. FTIR absorption spectra of NH region of 3b in CH2Cl2
(c = 1ϫ10–2 m).
Conjugates 4 display weak, broad absorption bands cen-
tered at 3322, 3314, and 3312 cm–1, respectively, ac-
131 companied by medium intensity signals assigned to the free
NH groups, indicating a greater content of non-hydrogen-
bonded conformers. The ratio of the free and bonded sig-
nals remains unchanged upon dilution from c = 10–2 to
10–3 m, indicating that hydrogen bonding is intramolecular
136 in nature, as expected (Figure 1). The IR absorption spectra
of conjugates 3 display very weak hydrogen-bonded NH ab-
sorptions, suggesting less significant (if any) participation
of hydrogen bonds (Figure 2). In the solid state (KBr disk),
conjugates 3 exhibit absorptions of NH stretching vi-
141 brations in the 3247–3337 cm–1 range, suggesting that all
NH groups are hydrogen-bonded under these conditions.
Additional bands at approximately 3420 cm–1 are registered
in the spectra of derivatives 4, indicating the presence of
nonbonded NH groups too.
ferrocene moiety (δ NHFc ca. 7.7 ppm), observed in the
spectra of all conjugates, indicates its participation in a
hydrogen bond. Although the appearance of NHAA reso- 156
nances at high field suggests that these groups are not in-
volved in hydrogen bonds, we cannot exclude the possibility
of H-bonding interaction due to the well-documented ten-
dency of urethane NH groups to resonate at high field in
CDCl3.[20,21] NHFc amide protons of monosubstituted 161
derivatives 3 have nearly the same chemical shifts as those
observed for 4, implying almost equal population of
hydrogen-bonded NH-groups in CDCl3 solution.
Table 2. Chemical shifts [δ, ppm] of the amide protons for conju-
gates 3a–c and 4a–c in CDCl3 (c = 10–1 m).
NHFc
NHAA
3a
3b
3c
4a
4b
4c
7.74
7.67
7.85
7.76
7.62
7.79
5.47
5.09
5.44
5.55
5.17
5.27
Similarly, as described for IR absorption measurements,
the small variation of NHFc resonances (Δδ Ͻ 0.2 ppm) 166
during NMR dilution experiments in the concentration
range of 1.5–100 mm indicates that initial downfield shift of
these protons is mainly attributed to intramolecular
hydrogen bonding (Figure 3 and Figure 4).
To determine the extent of solvent accessibility of the 171
amide NH protons in 3 and 4, that is, the strength of intra-
molecular hydrogen bonds, titration experiments with a
strong hydrogen-bond acceptor were performed.[22] The
chemical shift of solvent-exposed amide proton is strongly
affected by addition of dimethyl sulfoxide (DMSO); in con- 176
Figure 1. Concentration-dependent FTIR absorption spectra of the
NH region of 4b in CH2Cl2 (measurement range from 1 to 10 mm).
146
To address the question to what extent are NHFc and trast, amide NH protons involved in a strong intramolec-
NHAA groups of 4 involved in hydrogen bonding, we under- ular hydrogen bond tend to experience little or no effect
took an NMR study. Groups of interest were easily as- from the addition of the coordinating solvent. By increasing
signed based on NOE spectroscopy and coupling patterns the percentage of DMSO in CDCl3 from 0 to 30%, the net
(Table 2). In general, the chemical shifts of the amide pro- changes in the chemical shift values for NHFc group of 3b 181
151 tons should be higher (δ Ͼ 7 ppm) in hydrogen-bonded and 4b are 1.22 and 1.41 ppm, respectively (Figures 5 and
structures than in the non-hydrogen-bonded states (δ Ͻ 6). This dramatic downfield shift clearly indicates that
7 ppm). The downfield resonance of NH attached to the hydrogen bonds involving NHFc protons of 3 and 4 are eas-
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Figure 3. Concentration-dependent 1H NMR chemical shifts of the
amide protons of 4b in CDCl3.
Figure 6. Chemical shift dependence of the NHFc and NHAla reso-
nances of 3b as a function of [D6]DMSO concentration in CDCl3
solution.
changes of the NHAA group (Δδ/ΔT = –3, –2.33, and
–2 ppb/K for 4a, 4b, and 4c, respectively) suggest that these
groups are not hydrogen bonded under these conditions. 201
Larger Δδ/ΔT values found for the NHFca group (–4.67,
–4.67, and –6 ppb/K for 4a, 4b, and 4c, respectively) indi-
cate that the initially shielded environment of these protons
becomes exposed with temperature increase. Given that the
NHFca chemical shift values proved to be concentration- 206
independent below 6 mm at 298 K, we can exclude the pos-
sibility of self-assembly and attribute this temperature de-
pendence to an unfolding process of individual molecules.
Figure 4. Concentration-dependent 1H NMR chemical shifts of the
amide protons of 3b in CDCl3.
ily disrupted by addition of DMSO. A comparable increase
186 in the slope of the titration curves of NHAla resonances in-
dicates similar solvent accessibility of both amide protons
(NHFc and NHAA).
Figure 7. Temperature-dependent 1H NMR spectra of 4b in CDCl3
(c = 5ϫ10–3 m) from 298–328 K (* denotes residual CHCl3).
Figure 5. Chemical shift dependence of the NHFc and NHAla reso-
nances of 4b as a function of [D6]DMSO concentration in CDCl3
solution.
To determine whether derivatives 4a–c adopt any ordered
conformations in solution, NOESY experiments were per- 211
formed. The distance between the two cyclopentadienyl
To ascertain the dependence of amide proton chemical rings of 3.3 Å means that nuclear Overhauser effects (NOE)
shifts on temperature, we carried out temperature-depend- contacts provide useful information regarding interchain in-
191 ent NMR experiments in CDCl3 solution due to the insta- teractions between podand peptide chains.[16] NOE signals
bility of derivatives 4a–c in DMSO at elevated temperature between methyl protons of the acetyl group and methyl pro- 216
(Figure 7 and Supporting Information). In general, a low tons of the Boc group and valine side chain of 4c indicate
temperature coefficient (Δδ/ΔT = 2.4Ϯ0.5 ppb K–1) is ex- the same “superimposable” orientation of substituents on
pected for an amide NH proton exposed to solvent and cyclopentadienyl rings supported by interchain hydrogen
196 larger temperature dependencies are an indication of an ini- bonds (Figure 8). Further support for interchain interaction
tially shielded NH that becomes exposed with increasing is provided by the observation of NOESY cross-peaks be- 221
temperature.[23] Temperature-dependent chemical shift tween methyl protons of the tert-butoxycarbonyl group and
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the ferrocene protons of the ring bearing an acetyl group addition, we recorded the CD spectra in the presence of 251
(7,10Fc and 8,9Fc). Similar NOE contacts were observed for different concentrations (2, 10, and 20%) of DMSO. The
4a and 4b (see the Supporting Information).
intensity of the CD signal of 4b was reduced to 60% upon
addition of as little as 2% of hydrogen-bond disrupting sol-
vent. As expected, further addition of DMSO causes a sub-
stantial decrease in the intensity of the CD curve, indicating 256
destabilization of intramolecular hydrogen bonds and for-
mation of solvates. In contrast, addition of 2% DMSO
caused only partial disruption (20%) of the hydrogen-bond
network of valine derivative 4c, the CD pattern of which
remained conserved even in the presence of 20% DMSO, 261
with 57% reduction in the CD intensity.
Figure 9. CD spectra of 3b and 4b in CH2Cl2 (1 mm in CH2Cl2).
Figure 8. Important NOE interactions of 4c.
226
The CD spectra in the visible region of chiral derivatives
3b,c and 4b,c are shown in Figure 9 and Figure 10. The data
are gathered in Table 3. In the UV/Vis spectra of all conju-
gates in CH2Cl2 or CH2Cl2/DMSO, typical ferrocene ab-
sorption is observed at approximately 445 nm. Compounds
231 3b and 3c, which are unable to form interchain hydrogen
bonds, display a weak signal, with Mθ = 683 and
–133 degm–1 cm–1, as expected for monosubstituted ferro-
cene peptides.[18] Derivatives 4b and 4c display positive Cot-
ton effect (Mθ = 2751 and 1953 deg m–1 cm–1) around the
236 UV/Vis absorption maxima, indicating P-helicity of the fer-
rocene moiety. This finding is in accordance with previously
described examples in which right-handed helicity is ob-
served when l-amino acids are used as a source of chiral-
ity.[8] Although the intensities of CD signals are approxi-
241 mately four times weaker than those observed for symmetri-
cally substituted derivatives (Scheme 1, n = m = 1, Mθ ca.
10000 degm–1 cm–1), these results indicate an ordered chiral
environment around the ferrocene moiety. Given that
monosubstituted derivatives 3b and 3c exhibit very weak
246 Cotton effect, medium-intensity CD curves observed in the
spectra of 4b and 4c can only arise from intramolecular
hydrogen bonding between the two podand chains.[6] The
Figure 10. CD spectra of 3c and 4c in CH2Cl2 (1 mm in CH2Cl2).
Table 3. CD spectroscopic data of 3a–c and 4a–c in CH2Cl2 and
CH2Cl2/DMSO (c = 1ϫ10–3 m).
λmax [nm] (Mθ, degM–1 cm–1)
CH2Cl2
CH2Cl2 +
CH2Cl2 +
CH2Cl2 +
2% DMSO 10% DMSO 20% DMSO
3a
3b
3c
4a
4b
4c
476 (37)
466 (683)
471 (–133)
468 (–47)
490 (2751)
486 (1953)
–
–
–
458 (255)
471 (–414)
–
492 (1661)
491 (1567)
463 (–194)
472 (–568)
–
491 (645)
493 (1072)
467 (–337)
472 (–569)
–
490 (266)
492 (849)
Computational Studies
In our previous studies, DFT calculations proved to be
magnitude of the CD signal of glycine derivatives 3a and valuable for predicting the possible geometries of investi-
4a is negligible, as expected for achiral conjugates. In gated compounds.[13–17,24] The same approach was per- 266
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formed in this study and the calculated data were compared
with the experimental results to obtain a complete picture
regarding hydrogen-bond patterns in derivatives 3a–c and
4a–c. To address these questions, initial molecular mechan-
271 ics calculations were performed; only the most stable con-
formers were subjected to further quantum mechanics cal-
culations, with B3LYP-D3 and 6-311+G(d,p) basis set used
for all atoms except for iron, for which the LanL2DZ set
was used. The results are summarized in Tables 4 and 5,
276 and the geometries of the most stable conformers, displayed
in Figures 13 and 14, were calculated with solvent chloro-
form modeled as the polarizable continuum (IEF-PCM).
The structures are named using the recommended stereo-
chemical descriptors displayed in Figure 11.[2] The 1,nЈ-di-
281 substituted ferrocene compounds with chiral substituents
are usually defined by a value of ω (pseudo-torsion angle)
together with a P- or M-stereochemical descriptor to de-
scribe the helical chirality of the ferrocene moiety. The ori-
entation of the Cp-amide bonds is usually designated by
286 E/Z stereochemical descriptors considering the relative
positions of two highest priority groups. Additionally,
calculated conformers were classified according to the
hydrogen-bond patterns displayed in Figure 12.
Figure 11. Nomenclature of 1,nЈ-disubstituted ferrocenes with chi-
ral substituents. (a) M/P isomers defined by a pseudo-torsion angle
ω between two substituents; (b) E/Z isomers describing the orienta-
tion of Cp-amide bond.
Table 4. Relative energies (E, kJmol–1) of the most stable conform-
ers 3a–c in chloroform at 298 K (populated Ͼ 5% for the selected
ensemble of conformers), IHB patterns according to Figure 13,
X–Y distances (in Å) of the selected X–H···Y hydrogen bonds.
most stable conformers a seven-membered ring (also known
as γ-turn) is connected by a NH···O hydrogen bond (pattern
A, Figure 12). Only in the case of valine amino acid (3c)
was this pattern also found in the less stable conformer (3c- 296
5), because the iPr group may adopt different relative posi-
tions depending on the torsion angle describing rotation
around the Cα–CiPr single bond. It is also interesting that
conformer 3c-2, with the same pattern A, occurred by con-
necting a seven-membered ring in the opposite direction 301
when compared with 3c-1; that is, the iPr group is oriented
toward the hydrogen bond in 3c-2. Another pattern (B) is
characterized by a five-membered ring connected by a
NH···N hydrogen bond. In the case of valine amino acid,
splitting of levels was also observed due to rotation of the 306
iPr group, which is the most noticeable difference between
Conformer Erel IHB pattern NHFc···O=CAA NHFc···NAA
3a-1
3a-2
0.00
2.69
A
B
2.92
–
–
2.76
3b-1
3b-2
0.00
3.93
A
B
2.94
–
–
2.74
3c-1
3c-2
3c-3
3c-4
3c-5
0.00
2.17
2.32
4.41
5.56
A
A
B
A
B
3.02
2.84
–
2.95
–
–
–
2.73
–
2.73
In monosubstituted derivatives 3a–c, two sets of con- conformers 3c-3 and 3c-5.
291 formers were observed. In both, the NHFc group is engaged
Interestingly, although less sterically crowded, the con-
in the formation of an intrachain hydrogen bond. In the formers with substituent twisted away from the ferrocene
Table 5. Relative energies (E, kJmol–1) of the most stable conformers 4a–c in chloroform at 298 K (populated Ͼ 5% for the selected
ensemble of conformers), stereochemical descriptors, pseudo-torsion angles (ω, deg), IHB patterns according to Figure 14, X–Y distances
(in Å) of the selected X–H···Y hydrogen bonds.
Conformer
Stereochemical
descriptor
ω
Erel
IHB pattern
NHFc···O=CAA
NHFc···NAA
NHAA···O=CAc
[a]
4a-1
4a-2
4a-3
M-1,5Ј; Z,Z
–42.1
–8.8
21.3
0.00
3.09
3.55
C
D
D
3.01
–
–
–
2.74
2.77
2.90
2.95
2.89
M-1,1Ј; Z,Z[b]
P-1,1Ј; E,Z[b]
4b-1
4b-2
4b-3
M-1,5Ј; Z,Z
P-1,1Ј; E,Z
M-1,1Ј; Z,Z
–41.0
21.3
–14.5
0.00
6.34
6.77
C
D
D
2.99
–
–
–
2.72
2.71
2.90
2.88
2.96
4c-1
4c-2
M-1,5Ј; Z,Z
P-1,1Ј; E,Z
–40.9
22.2
0.00
1.85
C
D
3.03
–
–
2.71
2.90
2.91
[a] Together with enantiomeric P-1,2Ј for 4a-1. [b] Together with enantiomeric P-1,1Ј for 4a-2 and 4a-3.
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Figure 12. The hydrogen-bond patterns (A–D) observed in the ensemble of the most stable conformers based on a computational study
of monosubstituted (3a–c) and disubstituted (4a–c) derivatives [R = H, CH3, CH(CH3)2]. Numerations of five-, seven-, and nine-mem-
bered rings are displayed.
311 unit are higher in energy in comparison with those with the
All of the displayed hydrogen bonds in Figures 13 and 14
same substituent twisted toward it. The relative energies of were confirmed by Bader’s AIM theory. Koch and Popelier
these conformers in Gly-, Ala-, and Val-containing peptides criteria were used to characterize hydrogen bonds according
are 7.86, 9.29, and 9.39 kJmol–1, respectively. This could to values of topological parameters of the bond critical 346
be partially explained by the favorable CH···O and CH···N point between hydrogen-bond acceptor and hydrogen atom
316 interactions between the Cp ring and the corresponding [electron density ρ(r), Laplacian of the electron density
hydrogen-bond accepting atoms. Bader’s AIM theory con- ٌ2ρ(r) and the energy density H(r)].
firmed the existence of the bond critical points between
these atoms.
These results should be tested against the experimental
data. In the IR absorption spectra of compounds 3 there is 351
In disubstituted derivatives 4a–c, an additional acetyl evidence for the existence of nonbonded NH groups. The
321 group on the other cyclopentadienyl ring is a potential computational data confirm the previously stated observa-
hydrogen-bond acceptor. In comparison to 3a–c, which are tions because the amino acid NHAA group is not involved
characterized only by intrachain hydrogen bonds due to the in any type of hydrogen bond, in contrast to the NHFc
absence of the other substituent, herein the interchain group, which forms a NH···O or NH···N hydrogen bond. 356
hydrogen bond always prevails in the selected ensemble of According to the IR absorption spectra of conjugates 4, the
326 the most stable conformers. A nine-membered ring is intensities of their stretching bands corresponding to
formed by a hydrogen bond between the amino acid NHAA bonded NH groups is stronger in comparison to those in
group and the acetyl carbonyl oxygen. In this way the conjugates 3. This might imply greater engagement of the
longer substituent is usually bent in a way to adopt a favor- NH groups in formation of the intramolecular hydrogen 361
able orientation to form an additional seven-membered ring bonds and is in relatively good agreement with calculated
331 through a NH···O hydrogen bond (pattern C). This can be data whereby both NH groups of conjugates 4 are involved
directly compared to the previously described hydrogen in some kind of hydrogen bond.
bond pattern A in 3a–c. The same observations are also
The engagement of the NHFc group in the formation of
valid for less stable conformers of 4a–c. Again, the nine- hydrogen bonds, confirmed by NMR experiments, is in ac- 366
membered ring is additionally accompanied by an intra- cordance with the computational data. It should be stated
336 chain NH···N hydrogen bond. This pattern can also be that the DFT study also showed a preference for the forma-
treated as similar and derived from previously described tion of an additional hydrogen bond between NHAA and
pattern B. It should be emphasized that conformers with the acetyl group; that is, formation of an interchain
pattern D could adopt two conformations, one with a posi- hydrogen bond. Experimentally it was found that a rela- 371
tive pseudo-torsion angle (e.g., P-1,1Ј in 4b-2 and 4c-2) and tively low amide proton chemical shift in compounds with
341 a second with a negative pseudo-torsion angle (e.g., M-1,1Ј a Boc protecting group (between 5.0 and 5.5 ppm) is usually
in 4b-3 and in higher energy states of 4c).
attributed to non-hydrogen bonded states of NHAA groups.
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Figure 13. The most stable conformers of 3a–c with selected hydrogen bonds.
In contrast, the NOE contacts between the methyl protons ides containing the Boc protecting group, tert-butyloxy 381
376 of the Boc group and the ferrocene unit, together with the carbonyl NH signals occur at higher field than other NH
1
CD spectra of 4b and 4c and observed Cotton effect, corre- signals in the peptide H NMR spectrum.[20,21] This is ex-
spond to the existence of the interchain hydrogen bond. In plained by an additional oxygen lone-pair resonance par-
an attempt to explain these three observations, we per- ticipation of tert-butyloxycarbonyl group and cannot be ob-
formed additional calculations. It was known that in pept- served with an acetyl group.[25] In our simplified model 386
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Figure 14. The most stable conformers of 4a–c with selected hydrogen bonds.
amide compounds, we have tried to confirm the effect of Table 6. Calculated parameters of modeled amides. Chemical shifts
of NHs (δ, ppm), the relative weights of resonance structures I, II,
and III (Figure 15), and selected NBO charges.
acetyl, methyloxy carbonyl, and tert-butyloxycarbonyl
groups on the chemical shift of the NH amide proton.
Acetyl
Methyloxy- tert-Butyloxy-
Calculations have been performed in chloroform mod-
391 eled with IEF-PCM using the same computational model
[B3LYP-D3/6-311+G(d,p)] used for the ferrocene com-
pounds; the results are summarized in Table 6. The NMR
shielding tensors were computed by using the Gauge-Inde-
pendent Atomic Orbital (GIAO) method included in
396 Gaussian09. The alkyloxy group directly linked to the carb-
onyl leads to lower chemical shift of NHs of about one ppm
in comparison to alkyl group. These chemical shifts are still
lower than experimental values, mostly because they are not
engaged in any kind of interactions in the modeled com-
401 pounds (e.g., expected hydrogen bonds). The contribution
of the resonance form II (the double C=N bond order) de-
creases going from an acetyl to a tert-butyloxy group,
carbonyl
carbonyl
δ [ppm]
5.24
5.29
4.19
NRT analysis
NBO charges
I [%]
II [%]
III [%]
55.4
33.4
–
49.5
29.2
7.4
43.9
25.6
8.3
N
C
–0.629
0.675
–0.694
–
–0.651
0.921
–0.653
0.928
Ocarbonyl
Oalkyloxy
–0.694
–0.582
–0.699
–0.613
Furthermore, NBO (Natural Bond Orbital) charges on
whereas the resonance form III [the double C=O(alkyloxy) the nitrogen atom become more negative when the alkyloxy
bond order] becomes more significant according to the group is linked to the carbonyl group, which increases the
406 NRT (Natural Resonance Theory) analysis (Figure 15).
local electron density. This may explain the lower chemical
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Figure 15. The most important resonance structures of modeled amides according to the NRT analysis.
411 shift of the NH protons in ferrocene derivatives with the intrachain hydrogen bonds. This medium-strength hydrogen
Boc protecting group. bond is not disrupted by the introduction of a hydrogen-
Following the results of these computational studies, one bond-accepting acetyl group on another cyclopentadiene 451
might conclude that the distribution of conformers is not ring. The formation of an additional interchain hydrogen
seriously affected by the connection of an acetyl substituent bond between NHAA and the acetyl carbonyl of disubsti-
416 on other cyclopentadienyl ring. The intrachain hydrogen tuted derivatives 4 is proposed based on a comparison of
bonds in monosubstituted conjugates 3 are accompanied by their CD properties with that of monosubstituted conju-
one additional interchain hydrogen bond in disubstituted gates 3. The observation of NOE contacts between podand 456
derivatives 4 due to the presence of an acetyl group. Conse- chains of 4 has provided further evidence for interchain
quently, the helicity of the disubstituted derivatives 4b and interaction, which constrains Cp rings into a fixed confor-
421 4c is also affected and M-helical are more populated in mation. The corresponding computational DFT study
comparison to P-helical conformers, at least according to [B3LYP-D3/6-311+G(d,p)&LanL2DZ using IEF-PCM to
DFT studies. A weak Cotton effect observed in the CD model chloroform] confirmed the existence of very similar 461
spectra of 3b,c could be attributed to the intrachain conformer distribution, irrespective of the nature of the
hydrogen bond forming a right- or a left-handed turn. amino acid. The NHFc group is engaged in formation of a
426 Furthermore, the very weak signal in the CD spectra with seven-membered ring (γ-turn) through an intrachain
a negative sign (–133 degm–1 cm–1) of the Cotton effect is NH···O hydrogen bond, or a five-membered ring through
possibly a consequence of the conformer distribution of 3c an intrachain NH···N hydrogen bond. The acetyl group in 466
in comparison to 3b (683 degm–1 cm–1). Although the most disubstituted derivatives affects the formation of one ad-
stable conformer has right-handed helicity, conformers with ditional interchain intramolecular NHAA···OAc hydrogen
431 a left-handed helicity are more abundant in total. This is bond.
especially evident in valine-containing compounds in which
Furthermore, the omission of one amino acid from con-
the energy difference between both left-handed 3c-2 and 3c- jugates Boc-(AA)2-NH-Fn-COMe[17] causes a sign reversal 471
3 relative to the most stable conformer is smaller than in of the CD signal of the resulting spectrum recorded for 4.
alanine derivatives.
This significant change in CD pattern can be explained by
the higher abundance of P-helices compared with those of
M-helices in conjugates 4.
436 Conclusions
This study confirms that Fca bioconjugates of type 4 are 476
able to engage in intramolecular hydrogen-bonding interac-
tions and the results extend our understanding of the con-
formational properties of nonsymmetric ferrocene peptides.
We have presented the first systematic conformational
analysis of disubstituted ferrrocene bioconjugates (Boc-AA-
NH-Fn-COMe; AA = Gly, l-Ala, l-Val; 4a, 4b and 4c,
respectively). The synthesis, spectroscopic characterization
441 (IR, NMR and CD), and solution-phase DFT calculations
were performed for disubstituted derivatives 4 and analo-
gous, yet simplified, monosubstituted derivatives 3 (Boc-
AA-NH-Fc; AA = Gly, l-Ala, l-Val; 3a, 3b and 3c, respec-
tively).
Experimental Section
General: Commercial reagents, EDC [EDC = N-(3-dimethylamino-
propyl)-N-ethylcarbodiimide hydrochloride, Aldrich] and HOBt
(HOBt = 1-hydroxybenzotriazole, Aldrich), were used as received.
Most synthetic procedures were carried out under argon. CH2Cl2
used for synthesis and spectroscopy was dried (P2O5), distilled from
481
446
The IR absorption and NMR spectroscopic analysis of
these compounds in solution in nonpolar CDCl3, and also
in DMSO, indicates the engagement of the NHFc group in CaH2, and stored over molecular sieves (4 Å). Monosubstituted 486
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ferrocene derivatives 3a–c and 1-acetyl-1Ј-aminoferrocene (2) were
prepared according previously described procedures.[18,19] Products
were purified by preparative thin-layer chromatography on silica
gel (Merck, Kieselgel 60 HF254) using CH2Cl2/ethyl acetate mix-
tures. Melting points were determined with a Reichert Thermovar
HT1 BT 11 melting-point apparatus. IR absorption spectra were
recorded as CH2Cl2 solutions and KBr pellets with a Bomem MB
100 mid FTIR spectrophotometer. UV/Vis and CD spectra were
(C-5, Fn), 66.6 (C-8, Fn), 66.4 (C-9, Fn), 62.9 (C-7, Fn), 62.7 (C-
10, Fn), 60.4 [CH(Val)], 30.6 [CHβ(Val)], 28.5 [CH3(Boc)], 27.6
(OMe), 19.5, 17.9 [CH3(Val)] ppm. HRMS (MALDI): m/z calcd. 551
for C22H30N2O4Fe 442.1550; found 442.1554. C22H30FeN2O4
(442.34): calcd. C 59.74, H 6.84, N 6.33; found C 59.85, H 6.79, N
6.40.
491
496
Computational Details: Starting geometries of all selected ferrocene
compounds (3a–c and 4a–c) were generated with MacroModel
v10.3[26–28] molecular modeling program using several different
search methods and force fields, mostly OPLS_2005.[29] In specific
cases, the ferrocene unit has been frozen with constant pseudo-
torsion angle during the conformational search, artificially simulat-
ing rotation of two Cp rings. All of the conformers were then opti-
mized with Gaussian09 (Revision D.01)[30] at the B3LYP-D3/
LanL2DZ level of theory using Grimme’s dispersion with the origi-
nal D3 damping function.[31–33] The most stable conformers were
reoptimized in chloroform at the B3LYP-D3/6-311+G(d,p) level of
theory, using IEF-PCM to describe implicit solvent effects.[34,35]
Iron was modeled by using the ECP set LanL2DZ. Molecules were
visualized by using Chem3D 2012[36] and GaussView 5[37] pro-
grams. The topological analysis of the selected compounds was per-
formed with AIM2000 program.[38,39]
556
561
566
recorded with
a Jasco-810 spectropolarimeter in CH2Cl2 or
CH2Cl2/DMSO mixtures. 1H and 13C NMR spectra were recorded
with a Bruker Avance 600 MHz spectrometer in CDCl3 and CDCl3/
DMSO solutions with Me4Si as internal standard. HRMS spectra
were recorded with a MALDI-TOF mass spectrometer.
Synthesis of 4a, 4b, and 4c. General Procedure: The following pro-
501 cedure is similar to those described previously.[10] A solution of
Boc-NH-Fn-COMe (2, 343 mg, 1 mmol) in CH2Cl2 (10 mL) was
cooled and treated with gaseous HCl. After stirring for 2 h at 0 °C,
the solvent was removed under reduced pressure and the residue
was treated with Et3N (0.3 mL, 2 mmol) in CH2Cl2 (ca. pH 8). The
506
free amine was coupled with Boc-AA-OH (2 mmol) previously acti-
vated with 2.2 equiv. of EDC/HOBt reagents. After stirring for 3 h
at room temperature, the mixture was washed with a saturated
aqueous solution of NaHCO3 (30 mL), 10% aqueous solution of
citric acid (30 mL), and brine (30 mL), dried, filtered, and concen-
trated. Purification of crude products using preparative thin-layer
chromatography on silica gel with CH2Cl2/EtOAc (10:1) gave red
crystals of the respective products.
Supporting Information (see footnote on the first page of this arti-
571
1
cle): H, 13C NMR and MS spectra of all newly synthesized com-
pounds; temperature-dependent 1H NMR spectra of 4a and 4c;
NOESY spectra of conjugates 4a and 4b.
511
Boc-Gly-NH-Fn-COMe (4a): Yield 358 mg (89%); m.p. 120–
122 °C. 1H NMR (600 MHz, CDCl3): δ = 7.76 (s, 1 H, NHFc),
5.55 (br. s, 1 H, NHGly), 4.79 (s, 2 H, 7-H, 10-H, Fn), 4.70 (s, 2
H, 2-H, 5-H, Fn), 4.51 [s, 2 H, CH2(Gly)], 4.02 (s, 2 H, 8-H, 9-H,
Fn), 3.90 (s, 2 H, 3-H, 4-H, Fn), 2.36 (s, 3 H, COMe), 1.50 [s, 9
H, CH3(Boc)] ppm. 13C NMR (75 MHz, CDCl3): δ = 202.1
(COMe), 167.8 [CO(Gly)], 154.6 [CO(Boc)], 95.4 (C-6), 79.8
[Cq(Boc)], 79.6 (C-1, Fn), 72.9 (C-3, C-4, Fn), 70.2 (C-2, C-5, Fn),
65.8 (C-8, C-9, Fn), 62.2 (C-7, C-10, Fn), 44.4 [CH2(Gly)], 28.0
[CH3(Boc)], 27.9 (OMe) ppm. HRMS (MALDI): m/z calcd. for
C19H24N2O4Fe 400.1080; found 400.1095. C19H24FeN2O4 (400.26):
calcd. C 57.02, H 6.04, N 7; found C 57.11, H 6.13, N 6.88.
Acknowledgments
516
521
This research was supported by the Ministry of Science, Education
and Sports of the Republic of Croatia (grant numbers 058-1191344-
3122 and 119-1191342-1339) and by the Croatian Science Founda-
tion under the project 7444. Computational resources were
provided by the Croatian National Grid Infrastructure (www.
cro-ngi.hr) at Zagreb University Computing Centre (SRCE).
576
581
[1] T. Moriuchi, T. Hirao, Chem. Soc. Rev. 2004, 33, 294–301.
[2] S. I. Kirin, H.-B. Kraatz, N. Metzler-Nolte, Chem. Soc. Rev.
2006, 35, 348–354.
[3] A. Nomoto, T. Moriuchi, S. Yamazaki, A. Ogawa, T. Hirao,
Chem. Commun. 1998, 1963–1964.
[4] T. Moriuchi, A. Nomoto, K. Yoshida, A. Ogawa, T. Hirao, J.
Am. Chem. Soc. 2001, 123, 68–75.
[5] T. Moriuchi, A. Nomoto, K. Yoshida, T. Hirao, J. Organomet.
Chem. 1999, 589, 50–58.
526
531
Boc-L-Ala-NH-Fn-COMe (4b): Yield 343 mg (83%); m.p. 51–53 °C.
1H NMR (600 MHz, CDCl3): δ = 7.62 (s, 1 H, NHFc), 5.17 (d, J
= 6.1 Hz, 1 H, NHAla), 4.75–4.71 (m, 3 H, 2-H, 7-H, 10-H, Fn),
4.57 (s, 1 H, 5-H, Fn), 4.47 (s, 2 H, 8-H, 9-H, Fn), 4.19 [s, 1 H,
CH(Ala)], 4.04 (s, 1 H, 3-H, Fn), 4.01 (s, 1 H, 4-H, Fn), 2.34 (s, 3
H, COMe), 1.48 [s, 9 H, CH3(Boc)], 1.45 [s, 3 H, CH3(Ala)] ppm.
13C NMR (75 MHz, CDCl3): δ = 201.8 (COMe), 170.8 [CO(Ala)],
156.7 [CO(Boc)], 95.2 (C-6), 79.8 [Cq(Boc)], 79.6 (C-1, Fn), 73.0
(C-3, C-4, Fn), 70.2 (C-2, Fn), 70.1 (C-5, Fn), 66.0 (C-8, Fn), 65.6
(C-9, Fn), 62.3 (C-7, Fn), 61.9 (C-10, Fn), 50.1 [CH(Ala)], 27.9
586
591
596
601
606
[6] T. Hirao, J. Organomet. Chem. 2009, 694, 806–811.
[7] L. Barisˇic´, M. Dropucˇic´, V. Rapic´, H. Pritzkow, S. I. Kirin, N.
Metzler-Nolte, Chem. Commun. 2004, 2004–2005.
ˇ
[8] L. Barisˇic´, M. Cakic´, K. A. Mahmoud, Y. Liu, H.-B. Kraatz,
H. Pritzkow, S. I. Kirin, N. Metzler-Nolte, V. Rapic´, Chem. Eur.
J. 2006, 12, 4965–4980.
536 [CH3(Boc)], 27.1 (OMe), 17.5 [CH3(Ala)] ppm. HRMS (MALDI):
m/z calcd. for C20H26N2O4Fe 414.1236; found 414.1245.
C20H26FeN2O4 (414.29): calcd. C 57.98, H 6.33, N 6.76; found C
58.09, H 6.50, N 6.69.
[9] L. Barisˇic´, V. Rapic´, N. Metzler-Nolte, Eur. J. Inorg. Chem.
2006, 4019–4021.
ˇ
[10] M. Cakic´ Semencˇic´, D. Siebler, K. Heinze, V. Rapic´, Organome-
tallics 2009, 28, 2028–2037.
ˇ
[11] M. Cakic´ Semencˇic´, K. Heinze, C. Forster, V. Rapic´, Eur. J.
Boc-L-Val-NH-Fn-COMe (4c): Yield 371 mg (84%); m.p. 57–58 °C.
541
1H NMR (600 MHz, CDCl3): δ = 7.79 (s, 1 H, NHFc), 5.27 (br. s,
1 H, NHVal), 4.75 (s, 2 H, 7-H, 10-H, Fn), 4.70 (s, 1 H, 2-H, Fn),
4.57 (s, 1 H, 5-H, Fn), 4.49 (s, 2 H, 8-H, 9-H, Fn), 4.02 (s, 2 H, 3-
H, 4-H, Fn), 3.98–3.94 [m, 1 H, CH (Val)], 2.38 (s, 3 H, COMe),
2.21–2.18 [m, 1 H, CHβ(Val)], 1.48 [s, 9 H, CH3(Boc)], 1.03–0.97
[m, 6 H, CH3(Val)] ppm. 13C NMR (75 MHz, CDCl3): δ = 202.5
(COMe), 170.6 [CO(Val)], 156.1 [CO(Boc)], 95.5 (C-6), 79.9
[Cq(Boc)], 79.6 (C-1, Fn), 73.7 (C-3, C-4, Fn), 70.7 (C-2, Fn), 70.6
Inorg. Chem. 2010, 1089–1097.
[12] S. Chowdhury, K. A. Mahmoud, G. Schatte, H.-B. Kraatz,
Org. Biomol. Chem. 2005, 3, 3018–3023.
[13] J. Lapic´, D. Siebler, K. Heinze, V. Rapic´, Eur. J. Inorg. Chem.
2007, 2014–2024.
[14] J. Lapic´, V. Djakovic´, I. Kodrin, Z. Mihalic´, M. Cetina, V.
Rapic´, Eur. J. Org. Chem. 2010, 13, 2512–2524.
546
ˇ
[15] S. Djakovic´, D. Siebler, M. Cakic´ Semencˇic´, K. Heinze, V.
Rapic´, Organometallics 2008, 27, 1447–1453.
Eur. J. Inorg. Chem. 0000, 0–0
11
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Job/Unit: I42837
/KAP1
Date: 26-11-14 13:41:52
Pages: 13
www.eurjic.org
FULL PAPER
611
616
621
626
631
636
641
[16] L. Barisˇic´, M. Kovacˇevic´, M. Mamic´, I. Kodrin, Z. Mihalic´, V.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Starov-
erov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Ad-
amo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Mar-
tin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09,
revision D.01, Gaussian, Inc., Wallingford CT, 2009.
D. Becke, J. Chem. Phys. 1993, 98, 5648–5652.
C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1998, 37, 785–789.
S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys.
2010, 132, 154104.
E. Cances, B. Mennucci, J. Tomasi, J. Chem. Phys. 1997, 107,
3032–3041.
E. Cances, B. Mennucci, J. Math. Chem. 1998, 23, 309–326.
ChemBio3D Ultra 2012 v13, PerkinElmer Inc.
GaussView 5.0, R. Dennington, T. Keith, J. Millam, Semichem
Inc., Shawnee Mission, KS, 2009.
Rapic´, Eur. J. Inorg. Chem. 2012, 1810–1822.
ˇ
[17] V. Kovacˇ, M. Cakic´ Semencˇic´, I. Kodrin, S. Roca, V. Rapic´,
Tetrahedron 2013, 69, 10497–10506.
646
651
656
[18] J. L. Jios, S. I. Kirin, N. N. Buceta, T. Weyhmuller, C. O.
Della Vedova, N. Metzler-Nolte, J. Organomet. Chem. 2007,
692, 4209–4214.
[19] J. Lapic´, A. Pezerovic´, M. Cetina, S. Djakovic´, V. Rapic´, J. Mol.
Struct. 2011, 990, 209–216.
[20] M. Iqbal, P. Balaram, J. Am. Chem. Soc. 1981, 103, 5548–5552.
[21] Y. V. Venkatachalapathi, B. V. V. Prasad, P. Balaram, Biochem-
istry 1982, 2, 5502–5509.
[22] T. P. Curran, A. B. Lesser, R. S. H. Yoon, J. Organomet. Chem.
2007, 692, 1243–1254.
[23] E. S. Stevens, N. Sugawara, G. M. Bonora, C. Toniolo, J. Am.
Chem. Soc. 1980, 102, 7048–7050.
[31]
[32]
[33]
[24] S. Djakovic´, I. Kodrin, V. Smrecˇki, P. Novak, Z. Mihalic´, D.
ˇ
Ziher, J. Lapic´, V. Rapic´, Tetrahedron 2014, 70, 2330–2342.
[25] S. Ganesh, R. Jayakumar, J. Pept. Res. 2002, 59, 249–256.
[26] Maestro, version 9.7, Schrödinger, LLC, New York, NY, 2014.
[27] MacroModel, version 10.3, Schrödinger, LLC, New York, NY,
2014.
661
[34]
[35]
[36]
[37]
[28] F. Mohamadi, N. G. J. Richards, W. C. Guida, R. Liskamp, M.
Lipton, C. Caufield, G. Chang, T. Henrickson, W. C. Still, J.
Comput. Chem. 1990, 11, 440–467.
[29] J. L. Banks, H. S. Beard, Y. Cao, A. E. Cho, W. Damm, R.
Farid, A. K. Felts, T. A. Halgren, D. T. Mainz, J. R. Maple, R.
Murphy, D. M. Philipp, M. P. Repasky, L. Y. Zhang, B. J.
Berne, R. A. Friesner, E. Gallicchio, R. M. Levy, J. Comput.
Chem. 2005, 26, 1752–1780.
666
671
[38]
[39]
F. Biegler-König, J. Schömbohm, D. J. Bayles, J. Comput.
Chem. 2001, 22, 545–559.
F. Biegler-König, J. Schönbohm, J. Comput. Chem. 2002, 23,
1489–1494.
[30] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Received: September 4, 2014
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Job/Unit: I42837
/KAP1
Date: 26-11-14 13:41:52
Pages: 13
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FULL PAPER
Ferrocene Bioconjugates
ˇ
ˇ ´
ˇ
M. C. Semencic,* V. Kovac, I. Kodrin,*
ˇ ´
´
L. Barisic, V. Rapic ......................... 1–13
Synthesis and Conformational Study of
Bioconjugates Derived from 1-Acetyl-1Ј-
aminoferrocene and α-Amino Acids
676
681
Keywords: Conformation analysis / Density
functional calculations
/
Bioinorganic
Conformational analysis (IR, NMR,CD
and DFT) revealed that nonsymmetric bio-
organometallics [Boc-AA-NH-Fn-COMe;
Boc = tert-butoxycarbonyl; AA = Gly,
l-Ala, l-Val; Fn = 1,1Ј-ferrocenylene] ad-
opt helically ordered conformations sup-
ported by intramolecular hydrogen bonds.
chemistry / Peptidomimetics / Hydrogen
bonds / Circular dichroism
Eur. J. Inorg. Chem. 0000, 0–0
13
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim