Metalloporphyrin receptors for histidine-containing peptides

Article abstract of DOI:10.1016/j.cclet.2014.03.034

Two new ditopic metalloporphyrin receptors constructed by combining metalloporphyrin with crown ethers have been prepared and characterized. ¹H NMR and MS spectra confirmed the complexation of receptor with peptide driven by coordination interaction and hydrogen bonding. UV/vis experiments revealed that the receptors exhibited high binding affinity to histidine-containing peptides. These receptors could differentiate short peptides of C-terminal histidine and N-terminal histidine and formed the most stable complexes with tripeptide.

Full text of DOI:10.1016/j.cclet.2014.03.034

Chinese Chemical Letters 25 (2014) 659–662
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Chinese Chemical Letters
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Original article
Metalloporphyrin receptors for histidine-containing peptides
b
Hui Liu a, , Zhan-Ting Li
*
a Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemical Engineering & Pharmacy, Wuhan Institute of Technology,
Wuhan 430073, China
b State Key Laboratory of Bio-Organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Two new ditopic metalloporphyrin receptors constructed by combining metalloporphyrin with crown
ethers have been prepared and characterized. 1H NMR and MS spectra confirmed the complexation of
receptor with peptide driven by coordination interaction and hydrogen bonding. UV/vis experiments
revealed that the receptors exhibited high binding affinity to histidine-containing peptides. These
receptors could differentiate short peptides of C-terminal histidine and N-terminal histidine and formed
the most stable complexes with tripeptide.
Received 6 January 2014
Received in revised form 6 March 2014
Accepted 13 March 2014
Available online 27 March 2014
Keywords:
ß 2014 Hui Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Metalloporphyrin
Crown ether
Synthetic receptor
Peptide
1. Introduction
terminus [22,23]. The metal cation Zn(II) in the center of porphyrin
could provide additional coordination site to bind nitrogen atom of
Selective recognition of short peptides by synthetic receptors
has attracted a great deal of interest in past decade because of their
important roles in nature [1–5]. The intrinsic properties of short
peptides originated from flexible conformation and irregular
topology placed a number of challenges in the design of receptors
[6]. In the recently reported examples, one strategy has been
proved to be effective for selective recognition of peptides, this is,
the binding of peptide N terminus and side chain of peptide in
ditopic fashion [7–10].
histidine. Aza-crown ethers of varied size were incorporated to
porphyrin via amide linkage as recognition site of ammonium ion,
which allowed the receptor preorganized as the distance between
N terminus and side chain of peptides (Scheme 1).
2. Experimental
NMR spectra were recorded on a Varian spectrometer operating
at 300 and 400 MHz for 1H and 13C respectively in the indicated
Crown ethers are ideal binding units for ammonium ions in
amino acids [11–14]. On the other hand, metalloporphyrins have
been documented to coordinate the nitrogen atom of imidazole
[15–18]. Porphyrin platform can further provide a large molecular
surface for dispersive interaction with peptide backbone [19–21].
In this context, the aim of this letter is to develop new receptors by
combining crown ethers with metalloporphyrin and to investigate
their recognition behavior toward histidine-containing short
peptides.
solvents. Chemical shifts were expressed in parts per million (d)
using residual solvent protons as internal standards. MALDI-TOF
mass spectra were recorded on a Voyager-DE STR mass spectrom-
eter (AB SCIEX, USA). UV/vis absorption spectra were measured
with
a Cary 100 UV/vis spectrophotometer (Varian, USA).
Elemental analysis was carried out at the SIOC analytical center.
Unless otherwise indicated, all starting materials were obtained
from commercial suppliers and were used without further
purification. All solvents were dried before use following standard
procedures. All reactions were performed under an atmosphere of
dry nitrogen. Compound 5 was prepared following our previous
method [24,25]. Column chromatography was carried out using
silica gel (300–400 mesh). All of the modified peptides were
prepared following standard procedures in the solution (see
Supporting information).
The strapped porphyrin developed by our group was served as a
scaffold to align peptide backbone in one direction from the N to C
*
Corresponding author.
E-mail address: witty_lau@hotmail.com (H. Liu).
1001-8417/$ – see front matter ß 2014 Hui Liu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2014.03.034

660
H. Liu, Z.-T. Li / Chinese Chemical Letters 25 (2014) 659–662
Scheme 1. Metalloporphyrin receptors and modified peptides.
The UV/vis titration experiments were performed according to
(br, 2H), À0.85 (br, 2H), À0.69 (br, 4H), 0.61 (br, 4H), 0.90 (t,
the following procedure.
A
solution of receptor (2.0 mL,
6H), 1.25–1.55 (m, 20H), 1.88 (p, 4H), 2.29–3.06 (m, 20H), 3.67 (m,
4H), 4.42 (t, 4H, J = 6.75 Hz), 7.09 (d, 1H, J = 9.0 Hz), 7.29 (s, 1H),
7.46 (t, 1H, J = 7.5 Hz), 7.56 (d, 1H, J = 5.4 Hz), 7.76 (t, 1H, J = 5.1 Hz),
7.99 (s, 1H), 8.22–8.45 (m, 9H), 8.75–8.88 (m, 8H). 13C-NMR
1.1 Â 10À6 mol/L) in chloroform was titrated with 10
mL of
histidine-containing peptide (2.0 Â 10À5 mol/L to 5.0 Â 10À5 mol/
L in chloroform, adjusted with 10 fold HCl). After each addition the
solution was allowed to equilibrate for 5 min and the absorption
intensity was recorded in the wavelength region of 350–600 nm at
25 8C.
(100 MHz, CDCl3):
d 14.1, 22.7, 25.1, 26.2, 29.1, 29.3, 29.4, 29.7,
31.9, 65.4, 70.4, 112.8, 113.8, 116.3, 117.7, 119.4, 119.9, 127.6,
128.9, 129.5, 129.8, 131.4, 131.6, 131.8, 132.1, 134.3, 134.7, 147.8,
149.5, 150.5, 150.9, 159.8, 160.5, 167.0, 171.8, 172.0. MS (MALDI-
TOF) (m/z): 1451 (M+H+). Anal. Calcd. for C85H101N5O12Zn: C,
70.40; H, 7.02; N, 4.83. Found: C, 70.61; H, 7.02; N, 4.64.
2.1. General procedure for the synthesis of metalloporphyrin receptors
To a solution of porphyrin acid 5 in dichloromethane oxalyl
chloride and several drops of DMF were added. The mixture was
stirred at room temperature for 5 h. After evaporating the solvents
under vacuum, the residue was dissolved in dichloromethane and
then aza-crown ether and triethyl amine were added and stirred
overnight at room temperature. After removal of solvent, the
resulting residue was purified by column chromatography to afford
aza-crownetherporphyrin. The freebaseporphyrinwasdissolvedin
dichloromethane/methanol (3:1) and zinc acetate was added with
stirring. The mixture was stirred under reflux overnight. The solvent
was removed in vacuo and the resulting residue was subjected to
column chromatography to afford metalloporphyrin receptor as a
purple solid in high yield (Scheme 2).
3. Results and discussion
To investigate the binding modes of the metalloporphyrin
receptor and the histidine-containing peptides, 1H NMR experi-
ments were firstly carried out. When 5 equiv. of HisN8 was added
to a solution of metalloporphyrin 1 or 2 in CD3OD/CDCl3 (1/1), the
signals of the protons of the imidazole group shifted upfield (ca.
0.17 ppm) as a result of the coordination interaction between the
imidazole nitrogen atom and the zinc cation (Fig. 1). The strong
Compound1:Purplesolid(68%).Mp > 250 8C.1HNMR (300 MHz,
CDCl3):
d
À1.39 (m, 2H), À1.21 (m, 2H), À1.00(m, 2H), À0.80 (m, 2H),
À0.60 (br, 4H), 0.65 (br, 4H), 0.90 (t, 6H), 1.25–1.56 (m, 20H), 1.89 (p,
4H), 3.35–3.86 (m, 20H), 4.49 (t, 4H, J = 6.75 Hz), 7.28 (m, 2H), 7.46
(d-t, 1H, J = 0.9, 7.5 Hz), 7.76 (d-t, 1H, J = 1.8, 5.1 Hz), 7.86 (d-d, 1H,
J = 2.25, 8.55 Hz), 8.19 (d-d, 2H, J = 1.05, 7.95 Hz), 8.32 (d-d, 1H,
J = 7.5, 1.8 Hz), 8.37–8.48 (m, 7H), 8.81 (m, 8H). 13C NMR (100 MHz,
CDCl3): d 14.1, 22.7, 25.1, 26.1, 27.7, 28.3, 28.9, 29.1, 29.3, 29.4, 31.9,
65.5, 70.1, 112.9, 113.8, 116.4, 117.8, 119.5, 120.0, 127.7, 128.0,
129.1, 129.6, 129.8, 131.6, 131.7, 131.9, 132.2, 132.8, 134.4, 134.7,
147.7, 149.5, 150.4, 150.7, 159.8, 160.6, 167.0, 172.0, 172.3. MS
(MALDI-TOF) (m/z): 1407 (M+H+). Anal. Calcd. for C83H97N5O11Zn: C,
70.90; H, 6.95; N, 4.98. Found: C, 70.39; H, 7.04; N, 4.91.
Fig. 1. Partial 1H NMR spectra (300 MHz) in CD3OD/CDCl3 (1/1) at 25 8C: (a)
Receptor 1; (b) Receptor 1 + 5 equiv. HisN8; (c) HisN8.
Compound 2: Purple solid (71%). Mp > 250 8C. 1H NMR
(300 MHz, CDCl3):
d
À1.39 (br, 2H), À1.26 (br, 2H), À1.00
Scheme 2. Synthetic route of metalloporphyrin receptors 1 and 2.

H. Liu, Z.-T. Li / Chinese Chemical Letters 25 (2014) 659–662
661
Table 1
Binding constants (L/mol) and the associated free energy change (kcal/mol) of
peptides with receptors 1 and 2.
Peptide
1
2
Binding constant
D
G
Binding constant
DG
HisN8
1.1 Â 105
1.9 Â 105
4.4 Â 105
5.2 Â 105
À6.9
À7.2
À7.7
À7.8
1.2 Â 105
2.5 Â 105
5.5 Â 105
7.8 Â 105
À7.0
À7.4
À7.9
À8.0
HisGlyN8
GlyHisN8
GlyGlyHisN8
a
b
Values are the average of two separate measurements and with error of Æ15%.
Obtained in chloroform.
Following the NMR study, mass spectrometry was used to
probe the non-covalent interactions between the metallopor-
phyrin receptors and the histidine-containing peptides. The
formation of a stable complex from receptor 1 and GlyHisN8
driven by coordination interaction and hydrogen bonding was
confirmed by MALDI-TOF analysis (Fig. 2). It gave a peak at m/z
1785.1, which corresponded to 1:1 complex of 1 and GlyHisN8,
although the intensity was weak.
Fig. 2. MALDI-TOF mass spectra of 1ÁGlyHisN8 complex.
The binding constants were determined by UV/vis titration
experiments in chloroform. Both receptors 1 and 2 showed the
maximum absorption at the wavelength of 421 nm. Upon the
addition of a histidine-containing peptide to a solution of receptor
1 or 2, a pronounced decrease in the absorption for all those
peptides was observed. No significant shift was observed for
receptors 1 and 2 when a modified histidine (HisN8) was added to
their solutions. Upon addition of dipeptide (HisGlyN8 or GlyHisN8)
two clear isosbestic points appeared at 432 and 559 nm,
respectively. Furthermore, the addition of tripeptide (GlyGly-
HisN8) also resulted in a significant bathochromic shift (ca. 12 nm
and 10 nm) (Fig. 3 and Figs. S4 and S5 in Supporting information).
On the basis of the absorption data obtained at 421 nm, binding
constants were calculated using a 1:1 binding model [29]. The
results are summarized in Table 1, which reveals that those
histidine-containing peptides bind well to the metalloporphyrins.
In contrast to receptor 1, receptor 2 shows slightly higher binding
affinity to all the four peptides. It can be attributed to a higher
binding ability of aza-18-crown-6 toward ammonium ion than
that of aza-15-crown-5 [30].
Fig. 3. Absorption spectral changes of
2
(1.1 Â 10À6 mol/L) upon addition of
HisGlyN8 (1.0 Â 10À7–7.0 Â 10À6 mol/L) in chloroform at 25 8C.
Receptors 1 and 2 could differentiate short peptides with C-
terminal histidine and N-terminal histidine. For example, recep-
tors exhibited higher binding affinity toward GlyHisN8 than
HisGlyN8. It might be attributed to a more matched spatial
distance between ammonium ion and imidazole group in
GlyHisN8. It was also noteworthy that the binding energy
increased with the increasing length of the peptides, which was
attributed to the dispersive interaction of the amide groups with
the porphyrin unit. Receptors 1 and 2 bound best to tripeptide
GlyGlyHisN8, which bears three amide oxygen atoms with
upfield shifts were also observed when other histidine-containing
peptides were used (Figs. S1–S3 in Supporting information),
indicating the placement of the peptide within the ring current of
metalloporphyrin. Due to the overlapping of signals in the region of
3.0–4.0 ppm, the changes of the protons of the crown ether were
hardly estimated. However, the binding strength between crown
ether and ammonium ions should be much weaker than that of
Zn2+ and imidazole according to previous studies [26–28].
Ar
Ar
O
O
O
O
N
N
N
N
N
N
N
N
Zn
Ar
Zn
O
O
N
N
N
Ar
NH
O
H
N
NH
N
O
H
N
O
O
H
H
O
O
H
N
H
H
O
H
N
O
O
O
N
N
H
H
O
O
O
2-GlyHisN8
1-HisGlyN8
Fig. 4. Proposed structures of the complexes formed from receptors and peptides.

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H. Liu, Z.-T. Li / Chinese Chemical Letters 25 (2014) 659–662
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In summary, the combination of a strapped metalloporphyrin
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Acknowledgment
This research was financially supported by Basic Science
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Appendix A. Supplementary data
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