Synthesis and Unprecedented Complexation Properties of β-Cyclodextrin-Based Ligand for Lanthanide Ions

Article abstract of DOI:10.1021/acs.inorgchem.8b00937

Here, we report the synthesis and detailed studies on the coordination chemistry of a novel chemically modified polyaminocarboxylate (5) based on β-cyclodextrin (CD) scaffold for lanthanides. The target ligand is prepared in a highly efficient manner (seven total steps) from β-CD using the readily available iminodiacetic acid as a starting material. A propargyl group is attached to the iminodiacetate via N-alkylation, and the obtained derivative is efficiently conjugated to the β-CD scaffold via the copper(I)-mediated 1,3-dipolar cycloaddition. The generated 1,2,3-triazolmethyl residues advantageously provide a competent chelating group while displacing the metal coordination center away from the primary rim of β-CD, to afford the required conformational flexibility. The functional groups from each of the two adjacent glucopyranosyl units of β-CD complete a uniquely created octavalent coordination sphere for lanthanides while still sparing one site for dynamic water coordination. To help study the coordination chemistry of CD ligand 5, we also design a relevant maltoside ligand 6, which faithfully represents one submetal-binding section of ligand 5. Thanks to HRMS and NMR studies, we successfully elucidate the coordination chemistries of synthesized ligands. The octavalent coordination sphere of ligand 5 shows strong binding affinity to lanthanides. By potentiometric titration experiments, ligand 5 is found to bind gadolinium(III), forming 1:1, 1:2, and 1:3 multinuclear complexes with lanthanides, thus possessing great capacity for catalyzing the dynamic water-exchange. Further NMR studies also reveal that the formed ligand 5/Gd(III) complexes show significantly better abilities to alter T1 relaxivities of coordinated water than DOTA-Gd(III) and also some of the synthetic CD probes reported in the literature.

Full text of DOI:10.1021/acs.inorgchem.8b00937

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis and Unprecedented Complexation Properties of
β‑Cyclodextrin-Based Ligand for Lanthanide Ions
‡,∇
Pier-Luc Champagne,†,∇ Cecile Barbot, †Ping Zhang,† Xuekun Han,† Issam Gaamoussi,‡
́
‡
,‡
,†
́
Marie Hubert-Roux, Gabriel E. Bertolesi, Geraldine Gouhier,* and Chang-Chun Ling*
†Alberta Glycomics Centre, Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N
1N4, Canada
‡
́
̀
Normandie Universite, COBRA, UMR 6014, FR 3038, INSA Rouen, CNRS, IRIB, IRCOF 1 rue Tesniere 76821
Mont-Saint-Aignan, France
S
* Supporting Information
ABSTRACT: Here, we report the synthesis and detailed
studies on the coordination chemistry of a novel chemically
modified polyaminocarboxylate (5) based on β-cyclodextrin
(CD) scaffold for lanthanides. The target ligand is prepared in a
highly efficient manner (seven total steps) from β-CD using the
readily available iminodiacetic acid as a starting material. A
propargyl group is attached to the iminodiacetate via N-
alkylation, and the obtained derivative is efficiently conjugated
to the β-CD scaffold via the copper(I)-mediated 1,3-dipolar
cycloaddition. The generated 1,2,3-triazolmethyl residues
advantageously provide a competent chelating group while
displacing the metal coordination center away from the primary rim of β-CD, to afford the required conformational flexibility.
The functional groups from each of the two adjacent glucopyranosyl units of β-CD complete a uniquely created octavalent
coordination sphere for lanthanides while still sparing one site for dynamic water coordination. To help study the coordination
chemistry of CD ligand 5, we also design a relevant maltoside ligand 6, which faithfully represents one submetal-binding section
of ligand 5. Thanks to HRMS and NMR studies, we successfully elucidate the coordination chemistries of synthesized ligands.
The octavalent coordination sphere of ligand 5 shows strong binding affinity to lanthanides. By potentiometric titration
experiments, ligand 5 is found to bind gadolinium(III), forming 1:1, 1:2, and 1:3 multinuclear complexes with lanthanides, thus
possessing great capacity for catalyzing the dynamic water-exchange. Further NMR studies also reveal that the formed ligand 5/
Gd(III) complexes show significantly better abilities to alter T1 relaxivities of coordinated water than DOTA-Gd(III) and also
some of the synthetic CD probes reported in the literature.
cyclic ligand (log KGd‑DOTA ≈ 25.2), which is ∼3 orders of
magnitude higher than the linear analogue (log KGd‑DTPA
INTRODUCTION
■
≈
The use of contrast agents (CAs) has transformed magnetic
resonance imaging (MRI) and allowed to become one of the
most powerful and versatile diagnostic tools in modern clinical
medicine.1−3 CAs are commonly based on complexes of
gadolinium(III), which is a paramagnetic lanthanide that
possesses optimal coordination chemistry. For example, the
highly toxic metal provides a 9-coordination sphere available
for potential dynamic water coordination, resulting in catalytic
alternation of relaxivities of proton nuclei of water. This creates
significant contrasts to relaxivities of noncoordinated water
molecules located in other tissues, leading to improved
resolution of acquired images. Currently, ∼35% of MRI
scans require the use of a CA. To circumvent the toxicity of
Gd(III), polyaminocarboxylate-based ligands4 such as the
linear diethylenetriaminepentaacetic acid (DTPA) and cyclic
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid
(DOTA), as well as their derivatives, are used in clinics to
form highly stable complexes (1 and 2; see Figure 1) with
Gd(III); the determined affinities are remarkably high with the
22.5). However, because of the adverse effects5 associated with
gadolinium complexes, such as the gadolinium-associated
nephrogenic systemic fibrosis (NSF),6,7 since 2006, there has
been a trend to disfavor the use of linear DTPA-based CAs,
because of their increased risk of undergoing in vivo
transmetalation with endogenous ions, which leads to the
release of free Gd(III) ions.8 Thus, the continuous develop-
ment of novel CAs with improved stability and relaxivities is
needed.
Cyclodextrins (CDs) represent a class of promising scaffolds
for the design of a new generation of MRI probes, because of
their biocompatibility, large molecular weights, and multi-
valency, as well as their hydrophobic cavities. During the last
two decades, there has been considerable interest to develop
CD-based MRI probes.9−27 However, most of these reports
Received: April 7, 2018
© XXXX American Chemical Society
A
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Figure 1. Structures of some commercial CAs: Gd-DTPA(H2O) (1), Gd-DOTA(H2O) (2), and some reported CD-based ligands 3 and 4 for
gadolinium(III), as well as new synthetic targets 5 and 6.
were based on a common strategy consisting of either
covalently grafting one or multiple units of known chelating
motifs such as DTPA or DOTA onto a monomeric9−13 or
oligo/polymeric CD14−21 backbone or forming an inclusion
complex.11,16,17,21−27 The new derivatives produced generally
possess improved relaxivities, because of their reduced
tumbling rate, influenced by the markedly increase in
molecular weights in the newly generated systems, compared
to the commercial CAs with low molecular weights. Logically,
all Gd(III) complexes generated using such strategy should
possess similar stability as the commercial CAs (1 and 2).
On the other hand, another strategy to develop CD-based
MRI probes consists of taking advantage of the native
macrocyclic geometry to create an innovative coordinating
sphere for Gd(III). Such an approach has the potential to
generate new MRI probes with improved properties. However,
this approach has only reached limited success, because, to
date, there have been very few attempts reported in the
literature, and no CD derivatives with binding affinity to
Gd(III) comparable to DTPA or DOTA have been
reported.28−31 The reported literature examples can be
classified into two families, based on functionalization. The
first was based on per-6-amino-α/β/γ-CDs by carrying out a
per-N,N-dialkylation with acetates to afford EDTA-type ligands
such as the β-CD analogue 3 (see Figure 1).28 Based on
molecular modeling, it was proposed that every two units of
iminodiacetates attached to C6 positions of adjacent
glucopyranosyl units in CDs were used to complex one
metal ion, generating a coordination sphere of six valencies for
each Gd(III) center (two amino groups and four acetates).
Unfortunately, the binding affinities of these Gd complexes
were not reported. We estimate that they might likely be much
lower than those of Gd-DOTA or Gd-DTPA complexes,
because of the lower number of coordination per Gd(III)
center, as well as potential tension introduced by the CD
scaffold, because of the proximity of the metal coordination
center to the primary rim of CDs, preventing each chelating
arm from adopting optimal geometry for coordination. The
second family consists of per-O-alkylations of CD derivatives
with acetates. Starting from 3,6-anhydro-α-CD,29 Delangle et
al. generated a O2-polyacetate that was reported to bind to one
Gd(III) metal with a stability constant (log KGd) of 7.5 and
exhibit low toxicity, as well as no nephrotoxicity or hemolysis
on a rat model. Gouhier and co-workers have also developed
novel MRI probes such as the per-O6-acetate 4 from β-CD via
O6-alkylations;30,31 these ligands were found to sequester one
Gd(III) metal, and the formed complexes showed favorable
alterations of water relaxivity, revealing their ability to form
secondary hydration spheres. However, the coordination
geometry of these per-O6-acetates with Gd(III) remains
poorly understood, and their binding affinities with Gd(III)
were also similar to that of the O2-polyacetate.32 Thus, despite
the many advantages of using CDs as a convenient scaffold for
the design of MRI probes, numerous challenges remain to be
overcome before a clinically viable CA based on CDs can be
obtained.
Here, we report a novel β-CD-based MRI probe 5,
decorated with seven copies of iminodiacetates that are
connected to the C6 positions of β-CD via a N-(1H-1,2,3-
triazol-4-yl)methyl linker. To assist our detailed binding
studies, we also designed a ligand 6 based on the methyl α-
maltoside disaccharide that essentially represents a local copy
of the coordination sphere found in compound 5 (see Figure
1). The unique feature of our system is the afforded one extra
coordination valency to Gd(III) by the 1H-1,2,3-triazole unit
from each arm, to potentially complete a 8-coordination sphere
for lanthanide(III), resulting possibly high affinity and high
stability of the complex, while still leaving room for the
coordination of one water molecule. Moreover, we conducted
the first detailed binding studies with Gd(III) and determined
the stability constants of formed complexes. Our results
revealed great potential of using this novel coordination
chemistry to generate high-affinity ligands based on CD
scaffolds to sequester lanthanide ions.
B
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a
Scheme 1. Efficient Synthetic Route To Target Compound 5
a
Reagents and conditions: (a) MeOH/AcCl, 60 °C, overnight (∼100%); (b) propargyl bromide/DIPEA/CH2Cl2, 40 °C, overnight (65%); (c)
CuI/DIPEA/acetone, 55 °C, overnight (77%); and (d) NaOMe/MeOH/CH2Cl2, then NaOH/H2O-MeOH, 70 °C, overnight (84%).
presence of DIPEA; the reaction afforded the desired per-C6-
substituted conjugate 11, isolated in 77% yield by column
chromatography using a mixture of dichloromethane−meth-
anol−triethylamine (98.5:0.5:1) as eluent.
SYNTHESIS
■
The desired target ligand 5 was effectively synthesized through
a highly convergent route illustrated in Scheme 1. The
commercially available iminodiacetic acid 7 was first O-
methylated in anhydrous methanol at 60 °C overnight under
strong acidic condition to provide the bis(methyl ester) 8,
isolated in the hydrochloric acid salt form (∼100% yield). The
salt 8 was then N-alkylated with propargyl bromide in refluxing
dichloromethane in the presence of excess N,N-diisopropyle-
thylamine (DIPEA); this afforded the corresponding tertiary
amine 9, isolated by column chromatography on silica gel in
good yield (65%). The next step is the key conjugation
between compound 9 and the fully acetylated per-6-azide 10,
synthesized according to the literature from per-6-bromo-β-
CD in two steps.33 The 1,3-dipolar cycloaddition was carried
out using 1.5 equiv of alkyne per azide in refluxing acetone
using a catalytic amount of copper(I) iodide as a catalyst in the
The purity and identity of compound 11 was confirmed by
1H and 13C nuclear magnetic resonance (NMR) spectroscopy
and electrospray high-resolution mass spectrometry (ESI-
HRMS). For example, a single 1,2,3-triazole peak was observed
at 7.73 ppm, combined with the anomeric protons of all
glucopyranoses being observed at 5.53 ppm as a doublet (J =
3.3 Hz); this was further confirmed with the observed
downfield shifts of all H-6a and H-6b protons from below
3.8 ppm (compound 10) to 4.93 and 4.83 ppm, respectively,
due to the significant deshielding effect of the newly formed
aromatic 1,2,3-triazole rings. Furthermore, the ESI-HRMS
spectrum in positive ion mode showed the expected doubly
charged ion at m/z 1646.5788, corresponding to the expected
formula: C133H184N28O70 (M+2H)2+ (calculated m/z:
C
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1646.5844). The fully protected intermediate 11 was then
formamide (DMF). Followed by a per-O-acetylation with
acetic anhydride in pyridine, the crude compound 14 was then
directly substituted by sodium azide in anhydrous DMF at 80
°C to afford the corresponding 6,6′-diazide 15, which was
isolated by column chromatography on silica gel as an
anomeric mixture (α/β ≈ 1:1, ∼48% yield over three steps).
With the help of HPLC on normal phase silica gel column, and
using a gradient of 0 → 5% ethyl acetate−dichloromethane as
the eluent, the desired α-anomer 15 was obtained in pure form.
We subsequently conjugated the pure α-6,6′-diazide 15 to the
previously synthesized alkyne 9 using similar conditions as
above; this afforded the desired conjugate 16 in 71% yield.
Following sequential deprotection steps as above, the pure
ligand 6 was obtained by gel filtration on Sephadex LH-20
(92% yield).
́
deprotected by first performing a Zemplen transesterification
to remove the 14 O-acetates on the secondary face, followed by
a saponification reaction of all of the remaining methyl esters
using NaOH in a mixture of deionized water and methanol.
The obtained crude product was finally purified by gel filtration
on Sephadex G-15 to provide the desired target ligand 5 in
high yield (84% yield).
To quickly prepare the disaccharide ligand 6, the maltose 12
was first subjected to a Fisher glycosylation in refluxing
anhydrous methanol in the presence of Amberlite IR-120 (H+)
(see Scheme 2). The crude mixture was then purified by
a
Scheme 2. Synthesis of Methyl α-Maltoside 6
The structures of both compounds 5 and 6 were confirmed
by ESI-HRMS in negative ion mode. For instance, for CD
conjugate 5, we observed a triply charged ion at m/z 834.5876,
which corresponded to the expected formula: C91H123N28O56
(M-3H)3− (calculated m/z: 834.5885). The persubstitution of
the compound 5 was confirmed by the observed axial
symmetry in the 1H NMR spectrum. For instance, we
observed a singlet at 8.21 ppm and a doublet at 5.18 (J =
2.8 Hz), which were assigned to the one set of 1,2,3-triazole
protons and anomeric protons, respectively (Figure 2), found
in compound 5. Similarly, for the maltoside 6, we observed the
related ion corresponding to the expected formula
C27H39N8O17 (M−H)− at m/z 747.2422 (calculated m/z:
1
747.2439). In the H NMR spectrum of the compound 6 in
D2O (see Figure 2), we observed two singlets related to the
1,2,3-triazole peaks at 8.08 and 8.03 ppm, and two anomeric
protons at 5.31 and 4.56 ppm (doublets), which respectively
correspond to the two sets of chelating functionalities attached
to the two α-glucopyranosyl units in the molecule. For the
other types of protons, each of them also appeared as two sets
of signals.
a
Reagents and conditions: (a) MeOH/Amberlite 120, reflux,
overnight (30%); (b) PPh3/NBS/DMF, 70 °C, overnight, then
Ac2O/Pyridine, overnight; (c) NaN3/DMF, 80 °C, 18 h (48%, 3
steps); (d) CuI/DIPEA/acetone, overnight (71%); and (e) NaOMe/
MeOH 4 h, then NaOH/H2O, reflux, 6 h (92%).
STUDIES OF PROTONATION OF LIGAND 5
■
The structure of ligand 5 contains numerous protonation sites,
including 14 carboxylates, 7 tertiary amines, and 7 1,2,3-
triazole rings (see Scheme 3). Based on pKa values of related
functional groups such as N-methyl-1,2,3-triazolium (1.25),34
acetic acid (∼4.76), and triethylammonium (∼10.75), we can
ignore the protonation of the 1,2,3-triazole rings in the
molecule, since it only becomes relevant at very acidic pH
column chromatography on silica gel to afford the desired
methyl glycoside 13 as a 1:1 anomeric mixture (∼30% yield).
The two primary hydroxyl groups were then converted to the
6,6′-dibromide at 70 °C using N-bromosuccinimide (NBS)−
triphenylphosphine as the reagent in anhydrous dimethyl
1
Figure 2. H NMR spectra of synthesized β-CD ligand 5 and maltoside 6 (600 MHz, D2O, 25 °C).
D
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a
Scheme 3. Ligand 5 Can Undergo Stepwise Protonations
a
A few examples of various protonated states of ligand 5 are shown.
Figure 3. (Left) Potentiometric titrations of ligand 5 (L), T = 298 K, [L] = 0.0625−1 μmol, [extra-HCl] = 0−37.3 μmol in NMe4Cl (0.1 M); total
initial volume: 4 mL; buret: [NMe4OH] = 0.05 M. (Right) Calculated distributions of different protonated species at different pH values.
values. Thus, in less-acidic solutions, ligand 5 contains
essentially 21 protonation sites, designated as [LH21]7+; a
stepwise deprotonation process would lead to other
intermediate stages, such as the neutral species LH14, the
fully deprotonated intermediate from carboxylates [LH7]7−,
and finally the completely deprotonated ligand L14− from both
carboxylates and ammoniums.
In order to gain insight on the involvement of the
carboxylates in the coordination with the metallic center,
attenuated total reflection (ATR) Fourier transform infrared
(FTIR) spectroscopy studies were performed on ligand 5. It is
well-known that deprotonation of the carboxylic acid would
lead to the absence of any strong bands around 1700 cm−1 in
the IR spectrum; the resulting carboxylate typically presents
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two vibrational modes around 1600 and 1400 cm−1, because of
the symmetric (νs_COO) and antisymmetric (νas_COO) stretch-
ing modes. The ATR-FTIR spectrum of ligand 5 (see Figure
S46 in the Supporting Information) showed two stretching
bands at 1619.7 and 1396.1 cm−1, indicating that the
carboxylic functionalities of the isolated ligand 5 was indeed
fully deprotonated.
values of ammonium and carboxylic acid functional groups,
this pKa could be ascribed to the [LH7]7− species. pKa values
lower than 6.20 could be assigned to carboxylic acid moieties,
while pKa values of >6.20 could be attributed to ammonium
functionalities. Thus, the major species in the solution of
neutral pH should correspond to one with complete
deprotonation of carboxylic acids, while the first ammonium
group was yet quite totally deprotonated [LH6]8− (pKa ∼7.59),
which exists in equilibrium with [LH7]7− (pKa ∼6.20) and
[LH5]9− (pKa ∼7.93). More alkaline solutions revealed pKa
values of 8.07, 8.56, 9.08, and 8.80, which were assigned to
[LH4]10−, [LH3]11−, [LH2]12−, and [LH]13− respectively.
Potentiometric titrations were performed to study the
deprotonation of ligand 5 (see Figure 3). The initial solution
of ligand 5 alone gave pH ∼8.4; this value correlates well with
extensive deprotonation observed in the ATR-FTIR spectrum.
Therefore, we gradually added a solution of HCl in
trimethylammonium chloride (0.1 M) to the solution to
obtain a series of solutions with varied HCl/5 ratios (0 →
597), while maintaining the total volume constant. To each
prepared solution, a titrant solution of tetramethylammonium
hydroxide (0.05 M) was added while the pH of the solution
recorded; this allowed us to obtain a series of titration curves
(Figure 3). As can be seen, with increasing HCl/5 ratios, the
form of the obtained curves becomes increasingly similar to
that of pure HCl, while with decreasing HCl/5 ratios, the form
of the curves becomes more representative to ligand 5.
The protonation constant βh for the following equilibrium is
defined by eq 1, where L represents the fully deprotonated
form (L14−) of ligand 5 (LH14). H is the proton (charges are
omitted).
̈
Normally, the successive pKa values of the Bronsted pairs
increase according to the successive dissociation into a
polyprotic acid. Nevertheless, the delta pKa values of the
pairs [LH2]12−/[LH]13−, [LH3]11−/[LH2]12−, [LH4]10−
/
[LH3]11−, and [LH5]9−/[LH4]10− differ from h “statistical
factor” pKn+1 − pKn = 0.6, suggesting that the involved
dissociable groups interact with each other. On the other hand,
in more acidic solutions, three pKa values were determined
4.99, 3.63, and 2.57which were assigned to be the sequential
protonated species from [LH7]7−, namely, [LH8]6−, [LH9]5−,
and [LH10]4−, respectively. The determined pKa value
experiences a rapid decrease from 6.20 to 2.57, as each
successive protonation produces a less negatively charged
carboxylate, reducing charge−charge repulsion between
adjacent carboxylates. pKa values corresponding to highly
protonated species ranging from [LH11]3− to [LH21]7+ were
found to be lower than < 2, which is a pH value that is too low
to allow their determination.
[LHh]
L + hH F LHh βh =
[H]h[L]
(1)
Since dissociation constants Ka are commonly defined by the
expression
Based on the determined pKa values, a diagram of
percentage distributions of different protonated species
according to pH, is shown in Figure 3.
[H][LHh−1
]
Ka =
[LHh]
(2)
(3)
MTT ASSAYS OF β-CD LIGAND 5
■
it is evident that
We analyzed the in vitro cytotoxic effect of β-CD ligand 5 in
two available mammalian tumoral cell lines in our group, HeLa
cervix and HepG2 hepatic-carcinomas, using the MTT
assays.35 When grown in Dulbecco’s modified eagle’s medium
(DMEM) supplemented with 10% calf serum without
antibiotics at 37 °C (see the Supporting Information), both
cell lines only showed a slight cytotoxic effect (20−40%
inhibition) at concentrations of β-CD ligand 5 higher than 1
mM after 24 h of treatment, suggesting that β-CD ligand 5 is
fairly nontoxic (see Figure 4).
pKa = log βh − log βh−1
Table 1 reports all the protonation constants determined by
titration curve refinement and the corresponding acidity
Table 1. Some Determined Stepwise Protonation Constants
a
of Ligand 5
species
LH
LH2
LH3
LH4
LH5
LH6
LH7
LH8
log βLHh
pKa
8.80
17.88
26.44
34.51
42.44
50.03
56.23
61.22
64.85
67.42
8.80
9.08
8.56
8.07
7.93
7.59
6.20
4.99
3.63
2.57
<2.5
STUDIES OF COMPLEXATION OF Gd(III) WITH
COMPOUNDS 5 AND 6
■
Addition of GdCl3·6H2O to a solution of either synthesized
ligand 5 or 6 led to the formation of Gd(III) complexes. In the
case of maltoside 6, when 1 equiv of GdCl3·6H2O was added,
only 1:1 maltoside 6/Gd(III) complex was detected by ESI-
MS in negative ion mode (see Figure 5a, as well as Figure S9 in
the Supporting Information), since the main ion is observed at
m/z 902.14 and corresponds to singly charged ion [M−4H
+Gd(III)]−(C27H36N8O17Gd)− (m/z found: 902.1442, calcu-
lated: 902.1446). Similarly, when 1 equiv of GdCl3·6H2O was
added to the multidented ligand 5, the 1:1 ligand 5/Gd(III)
complex was observed as the main species in the solution, as
evidenced by ESI-MS (see Figure 5b, as well as Figure S6 and
Table S1 in the Supporting Information). The MS spectrum
was rather complex due to the presence of many ions with
LH9
LH10
LH11−LH21
a
Charges were omitted.
constants. All pKa values were ascribed to successive
deprotonation of the carboxylic acids, followed by ammonium
functions.
As can be seen from Table 1, a pKa value of 6.20 was
determined to be near the neutral pH. Based on the known pKa
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For all Gd(III) complexes with ligand 5 and 6, we noticed
that they were all extensively deprotonated. For example, for
the Gd(III) complex of maltoside 6, the ligand was found to be
fully deprotonated, suggesting that both the amines and all the
four carboxylates are available to complex Gd(III). For the
Gd(III) complexes correlated to ligand 5, the high-resolution
mass spectroscopy (HRMS) spectra revealed a loss of 9
protons, whereas for dinuclear and trinuclear complexes, the
HRMS spectra revealed further loss of 11 and 13 protons,
respectively. In all these cases, we can conclude that there were
at least two free amines and four carboxylates available for each
Gd(III), as in the case of complex of maltoside 6/Gd(III).
To gain more insight on the coordination sphere of formed
1
Gd(III) complexes, we used H NMR titration to study the
Figure 4. In vitro cytotoxicity assays (MTT) of β-CD ligand 5 in
HeLa and HepG2 cell lines. HeLa and HepG2 cells were maintained
in DMEM supplemented with 10% fetal calf serum without antibiotics
at 37 °C in a 5% CO2 humidified incubator. To determine the toxicity
to β-CD ligand 5, Hela and HepG2 cells were grown in a 96-well plate
(150 μL/well) in the presence of different concentrations of the β-CD
ligand 5 for 24 h. Each well was then incubated with 15 μL of 5 mg/
mL of thiazolyl blue tetrazolium bromide for 2 h at 37 °C. After three
washes with PBS, the blue formazan crystals were solubilized in
dimethyl sulfoxide (DMSO; 150 μL) and the absorbance was
determined at 595 nm on a multimode microplate reader.
complexation of both ligands 5 and 6 with the diamagnetic
yttrium(III) chloride. It is known that both Gd(III) and Y(III)
have similar coordination spheres.36 Thus, using the simple
maltoside 6 as a model, we performed a series of NMR
titration experiments in deuterium oxide at 25 °C, by adding a
solution a solution of yttrium(III) chloride in the same solvent
(see Figure 6).
As can be seen, with the addition of 0.25−0.75 equiv of the
1
yttrium(III) chloride, the H NMR spectra of the sample
experiences a dramatic change within the regions of the 1,2,3-
triazole protons, the two pairs of methylene protons (3.98 and
4.26 ppm) attached to the 1,2,3-triazole, as well as the
methylene protons of the N-acetates (∼3.5 ppm). This
contrasts with the two anomeric protons (∼5.3 ppm, 4.55
ppm) and anomeric O-methyl group (2.94 ppm), which only
showed little shifts during the titrations. When the amount of
yttrium(III) was increased to ∼1 equiv and above, no further
1
significant changes in the H NMR spectra were observed,
confirming the 1:1 stoichiometry observed from mass
spectrometry. The significant changes in chemical shifts of all
methylene groups mentioned above are expected, since they
are almost certainly involved in the coordination with the
metal ion. The very significant changes in chemical shifts of the
two types of 1,2,3-triazole rings strongly suggested their direct
involvement in metal chelation. Indeed, the participation of
two 1,2,3-triazole rings in coordination combined with the two
adjacent iminodiacetates provides an octavalent coordination
sphere to the metal center (see Figure 7).
In addition, we noticed that the two types of 1,2,3-triazole
protons signals split into two sets of signals (8.16/8.03 ppm vs
7.84/7.73 ppm), suggesting that the 1:1 maltoside 6/Y(III)
complex might in fact exist in two stereoisomers in solution.
To confirm this hypothesis, we performed a variable-temper-
ature experiment by gradually heating an NMR sample
containing 1:1 ratio of YCl3/α-maltoside 6 in D2O from 30
°C to 70 °C (see Figure 6, top). As can be seen, when the
temperature of the experiment was raised to 40 °C, the four
broad peaks of 1,2,3-triazole signals gradually shifted into two
broad singlets that continued to be better resolved at higher
temperatures. At 70 °C, the two singlets became completely
resolved and the spectrum became much more simplified,
suggesting faster interexchange between the two stereoisomers
at higher temperatures. One way to explain the two
stereoisomers is illustrated in Figure 7. As can be seen, for
each iminodiacetate group, a pseudoplane is observed between
the nitrogen, two oxygens of the N-acetates and lanthanide ion,
with the N-(1,2,3-triazol-4-yl)methyl group being perpendic-
ular to the peudo-plane; when the chelating groups from the
two adjacent α-D-glucopyranosyl units coordinate to the
Figure 5. ESI-QTOF-MS (negative ion mode) spectra of (a)
maltoside 6/Gd(III) (1:1), (b) CD 5/Gd(III) (1:1), and (c) CD
5/Gd(III) (1:3) complexes.
different charged states and adducts. Ions at m/z 442.6122,
535.7288, and 675.4021 correspond to [M−9H+Gd]6−
( C 9 1 H 1 1 7 N 2 8 O 5 6 G d ) 6 −
,
[ M − 9 H + N a + G d ] 5 −
(C91H117N28O56NaGd)5−, and [M−9H+2Na+Gd]4−
(C91H117N28O56Na2Gd)4− species, respectively. On the other
hand, when more than 1 equiv of GdCl3·6H2O was mixed with
ligand 5, the other multinuclear complexes were gradually
detected. As an example, when 3 equiv of GdCl3·6H2O were
added, the ESI-MS spectrum revealed the presence of mainly
dinuclear ([M−11H+2Gd]5−; [M−11H+K+2Gd]4−) and
trinuclear ([M−13H+3Gd]4−; [M−13H+Na+3Gd]3−) species
(see Figure 5c, as well as Figure S8 and Table S1 in the
Supporting Information).
G
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Figure 6. (a) Variable-temperature 1H NMR experiments (30−70 °C, D2O, 600 MHz) of a sample containing 1:1 Y(III)/maltoside 6 (14 mM) in
D2O. (b) NMR titration (D2O, 25 °C, 600 MHz) of a solution of maltoside 6 with Y(III) chloride.
lanthanide ion, the two pseudoplanes could intersect each
other either in a right-hand fashion or left-hand fashion,
resulting in the formation of two putative stereoisomers.
similar manner as maltoside 6 (see Figure 7), where each metal
center likely involves two 1,2,3-triazoles and two iminodiace-
tates attached to adjacent glucopyranosyl units in a β-CD. The
observed three metal coordination centers would totally
engage the chelating groups of six glucopyranosyl units;
leaving the chelating groups in the seventh unit not engaged in
binding, or available for additional functionalization. However,
in theory, the chelating groups attached to two diagonally
positioned glucopyranosyl units (such as 6A, 6D) of the β-CD
could team up to form a complex with one Gd(III). This does
not appear to be viable as molecular modeling showed that
such a mode of complexation would introduce considerable
torsional strains that leads to significant distortion of the CD
macrocycle.
1
Analogously, we have also conducted H NMR titration
experiments with CD ligand 5 by varying the amounts of
yttrium(III) chloride from 0.25 equiv to 4 equiv. As expected,
the obtained NMR spectra were far more complex than that of
the maltoside 6 complex. The signals generally appeared very
broad and overlapped; obviously, this was due to the formation
of different multinuclear complexes, as well as stereoisomers at
the metal centers. This presented considerable difficulties for
us to perform full analysis of the spectra. However, analogous
to the complex of ligand 6, we have indeed observed significant
changes in chemical shifts on protons related to the 1,2,3-
triazole rings, as well as the three types of methylene protons
found in ligand 5 (see Figure S10 in the Supporting
Information). Since HRMS experiments confirmed that CD
ligand 5 is capable of complexing up to three transition metals,
we concluded that ligand 5 most likely chelates the metal in a
DETERMINATION OF FORMATION CONSTANTS
OF LIGAND 5 WITH Gd(III)
The complexation of ligand 5 with Gd(III) was studied by
■
potentiometric titrations. While maintaining the amount of
H
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Figure 7. Proposed model of coordination in the complexes of maltoside 6 with lanthanides such as Y(III) and Gd(III). The two pairs of adjacent
iminodiacetates and N-(1,2,3-triazol-4-yl)methyl units provide an octavalent coordination sphere for the metal center. Two interconverting
stereoisomers could form, depending on how the chelating groups from each α-D-glucopyranosyl unit coordinate to the lanthanide ion.
ligand 5 (0.5 μmol) and HCl (9.95 μmol) constant within the
cell, a solution of GdCl3·6H2O was then added to reach certain
ratios. The obtained solution was then titrated with the
(∼15%) in the case of a 2:1 Gd:L ratio. Gd3L (∼67%) is in
equilibrium with Gd3LH (∼18%) and Gd2LH (∼5%) in the
case of a 3:1 Gd:L ratio.
addition of tetramethylammonium hydroxide (NMe4 OH−).
The formation constant (logKGd.5) for the monometallic
complex was on the order of 25. These complexes are
remarkably more stable than those described for the native CD
(log βGd = 2.5),37 even several orders of magnitude higher
than the values of some strong aminocarboxylate ligands38
such as EDTA (log βGd = 17.7) and DTPA (log βGd = 22.5).
The ligand 5 allowed the complexation of a secondary metallic
cation with a formation constant equal to 36.28 in the case of
Gd2L species. The introduction of this second lanthanide
cation was more difficult (log βGd(210) − log βGd(110) =
11.19), certainly because of electrostatic repulsions. A similar
conclusion can be reached by comparing log βGd(310) with
log βGd(210). Effectively, log βGd(310) − log βGd (210) =
7.63.
+
We noticed that with the solution containing the greatest Gd:L
ratios, the addition of same amount of base caused slower
raising of pH values (see Figure 8). This could be explained by
the displacement of protons from the protonated groups
during Gd(III) complexation, thus acidifying the medium.
The stability constants of the Gd(III) complexes are
expressed by the general formula
[MmLlHh]
mM + lL + hH F MmLlHh βh =
[M]m[L]l[H]h
(4)
with M being the metal ion, L the ligand, and H the proton
(the charges are omitted), and dissociation constant Kmlh can
be defined in a manner similar to the case of free ligand as
The variation of pM with pH for a particular metal−ligand
complex can give useful information about the stability of the
complex under different pH conditions. The pM value reveals
the concentration of free metal ion left uncomplexed by a
ligand; thus, it provides a more complete picture of the
effectiveness of a ligand in chelating the metal center. The pM
is usually calculated as a function of pH for a standard set of
conditions39 (initial concentration: [M] = 1 μM, [L] = 10 μM)
where pM = −log [M]free. Thus, the pGd value must be as high
as possible to ensure negligible dissociation, because Gd(III) is
toxic. Under such conditions, the calculated pGd value for 1:1
Gd-DOTA complex was 12.1 (from protonation constants of
DOTA and complexation constants of Gd-DOTA40). Since
our titration experiments were performed in mM concen-
trations which are 3 orders of magnitude higher than the
concentrations used to calculate pGd under standard
conditions, a direct calculation of pGd using our data would
result in overestimation. We thus plotted the pGd as a function
of pH using a 1 μM concentration in Gd(III), by varying the
concentrations of ligand 5 to obtain the respective 1:1, 2:1, and
3:1 stoichiometries of Gd(III)-ligand 5. As shown in Figure 9,
the maximum pGd value was obtained at pH ∼12. At pH 7.4,
the determined pGd values were 15.60, 8.04, and 6.87 for the
1:1, 2:1, and 3:1 stoichiometries of Gd(III)-ligand 5
complexes, respectively. Since the 1:1 Gd(III)-ligand 5
complex gives many free donor atoms available for
[H][MmLlHh−1
]
Kmlh
=
[MmLlHh]
(5)
Analogously,
pKmlh = log β − log β
(6)
mlh
mlh−1
The stability constants of the Gd(III) complexes are
reported in Table 2, and distribution curves given in Figure
8 for Gd(III):ligand 5 ratios of 1:1, 2:1, and 3:1.
Since the water coordination number was ∼1 (see next
section), speciation forms, which take into account one water
molecule in the coordination sphere, were considered with
their associated hydroxylated forms GdLH−1, Gd2LH−1,
Gd2LH−2, Gd3LH−1, Gd3LH−2, Gd3LH−3.
The GdL complex was formed at pH 9, when all carboxylate
sites were deprotonated. The other successive protonations
could be attributed to other ammonium sites protonated while
not coordinated to gadolinium. Example: GdLH4 refers to a
Gd complex with four protonated and three deprotonated
ammonium groups.
The distribution curves clearly show that the lanthanide ion
was totally complexed at pH 2.5. They underline that, at
neutral pH, GdLH3 (∼63%) is in equilibrium with GdLH4 and
GdLH2 (∼18.5%) in the case of a 1:1 Gd:L ratio. Gd2LH
(∼40%) is in equilibrium with Gd2LH2 (∼30%) and Gd2L
I
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Figure 8. Upper portion shows potentiometric titrations in the presence of Gd(III) (top) with [L] = 0.5 μmol; [extra-HCl] = 9.9456 μmol in
NMe4Cl (0.1 M); total initial volume = 4 mL; and buret, [NMe4OH] = 0.05 M. Lower portion shows distribution curves: (middle left) 1:1
complex with [Gd] = [L] = 0.5 μmol, (middle right) 2:1 complex with [Gd] = 2 × [L] = 0.5 μmol, and (bottom) 3:1 complex with [Gd] = 3 × [L]
= 0.5 μmol. Considering the entire set of results obtained for the Gd(III) ligand 5, the best chemical model that fits the data corresponds to (1)
mononuclear GdL species for Gd:L ratio of <1, (2) binuclear species Gd2L for Gd:L ratios of >1, or (3) trinuclear species Gd3L for Gd:L ratios of
>2.
coordination, we use the pGd value of the 3:1 Gd(III)-ligand 5
complex (6.87) to compare with the pGd value of Gd(III)-
DOTA (12.1), because it has the minimal numbers of free
donor atoms available for coordination. As can be seen, the
pGd value of 3:1 Gd(III)-ligand 5 is significantly lower than
the pGd value of 1:1 Gd(III)-DOTA, suggesting that the
ligand 5 forms less-stable complexes with Gd(III) than DOTA.
It is important to note that the thermodynamic stability of
gadolinium complexes is not the only parameter to predict the
in vivo toxicity of gadolinium complexes of ligand 5. The
presence of other endogenous metal ions, such as Zn(II),
Fe(II), Cu(II), and Ca(II) could compete with the complex-
ation of Gd(III), resulting in transmetalation.41,42 Further
studies on kinetic stability of the Gd(III)-ligand 5 complex
must be performed to probe the suitability of ligand 5 for use
as an MRI probe.
WATER COORDINATION NUMBER
■
The hydration number was determined using the well-
established luminescence method on the corresponding Eu3+
J
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Table 2. Some Determined Stepwise Protonation Constants
a
of Ligand 5
species
mlh
log βmlh
pKa
Mononuclear
GdLH−1
GdL
GdLH
GdLH2
GdLH3
GdLH4
GdLH5
GdLH6
following
Dinuclear
Gd2LH−2
Gd2LH−1
Gd2L
11−1
110
111
112
113
114
115
116
16.08
25.09
33.74
41.95
50.04
56.97
60.51
64.00
9.01
8.66
8.21
8.09
6.93
3.54
3.49
<2.5
22−2
22−1
210
211
212
213
214
215
216
17.71
27.15
36.28
44.13
51.38
56.33
60.27
63.54
66.45
9.44
9.13
7.85
7.25
4.95
3.94
3.27
2.95
<2.5
Gd2LH
Figure 10. Proposed model of octavalent coordination and
monohydration in the complexes of β-CD ligand 5 with three
lanthanide ion centers.
Gd2LH2
Gd2LH3
Gd2LH4
Gd2LH5
Gd2LH6
following
Trinuclear
Gd3LH−3
Gd3LH−2
Gd3LH−1
Gd3L
Supporting Information), we calculated the water coordination
number (q) from the following equation, based on the well-
established isotope effect of the Eu3+ complexes:
i
j
y
z
1
1
31−3
31−2
31−1
310
311
312
13.80
23.91
33.83
43.91
50.70
55.36
60.09
64.99
j
j
z
z
q = t ×
−
− kcorr
j
j
j
z
z
z
10.10
9.92
10.09
6.79
4.66
4.73
4.90
<2.5
τ
τ
H2O
D2O
(7)
k
{
where t and kcorr represent a constant in each case, which is
dependent on the lanthanide used; in the case of Eu3+, where t
and the correction factor kcorr are constants, depending on the
types of relation used; in the case of Parker’s relation, t = 1.2
ms−1 and kcorr = −0.25 ms, and in the case of the Supkowski
Gd3LH
Gd3LH2
Gd3LH3
Gd3LH4
following
313
314
and Horrocks relation, t = 1.11 ms−1 and kcorr = −0.31.43 τH O
2
a
Charges were omitted.
and τD O are the measured luminescence lifetimes of ligand 5/
2
Eu3+complexes. The obtained water coordination numbers q1
or q2, based on either the Parker relation or the Horrocks−
Supkowsi relation, are reported in Table 3.
Table 3. Determination of Water Coordination Number (q1
or q2) through Time-Resolved Luminescence Measurements
of Ligand 5/Eu3+ Complexes
Water Coordination Number
τ
τ
D2O
(ms)
H2O
Parker
Horrocks−Supkowsi
relation, q2
complex
(ms)
relation, q1
5:Eu −
0.621
2.582
1.168
1.013
1:1
5:Eu −
0.635
2.329
1.074
0.927
1:2
Figure 9. pGd value of Gd(III)-ligand 5 complexes as a function of
pH and stoichiometry compared to Gd(III)-DOTA. ([Gd]total = 1 μM
and [ligand 5]total, or [DOTA] = 1 μM). pGd calculations (in the case
of ligand 5) were done using protonation and complexation constants
from Tables 1 and 2. In the case of DOTA ligand, protonation
constants logβ LHh were: LH1: 11.74 ; LH2: 21.5 ; LH3: 26.18, LH4:
30.29, LH5: 32.66 and complexation constant log βGd-DOTA equal
to 24.7.40
Consequently, for a pH value between 6 and 7, q is equal to
1. Unfortunately, we were unable to measure the water
coordination number for the 1:3 ligand 5/Eu(III) complex,
because of solubility issues. Figure 10 shows a model of
coordination and monohydration of the multinuclear complex
of CD ligand 5 with lanthanides. The small, yet rigid 1,2,3-
triazole functionalities advantageously displace each metal
coordination center away from the primary rim of β-CD, so
that the iminodiacetate groups could efficiently interact with a
lanthanide center with conformations that minimize undesired
torsional strains.
complexes of ligand 5. Thus, by preparing the complexes in
H2O and D2O, and measuring the time-resolved luminescence
(see Figure 10, as well as Figures S47 and S48 in the
K
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Figure 11. Graphs obtained from relaxivity measurements to confirm the linear relationship between 1/T1 and concentration of: (a) α-maltoside
6/Gd(III), (b) 1:1 β-CD ligand 5/Gd(III), (c) 1:2 β-CD ligand 5/Gd(III), and (d) 1:3 β-CD ligand 5/Gd(III) complexes (pH 6−7).
was synthesized in a highly efficient manner from inexpensive
and readily available iminodiacetic acid. The introduction of a
propargyl group on the nitrogen of the iminodiacetate permits
an efficient incorporation of the chelating groups onto a CD
scaffold via the copper(I)-mediated 1,3-dipolar cycloaddition;
this advantageously provided a 1,2,3-triazole unit that has the
ideal linker length to allow the participation of 1,2,3-triazole in
the coordination of metals. Despite the structural complexity of
β-CD ligand 5, with the help of a related synthetic maltoside
ligand 6, we were able to elucidate the coordination chemistry
of the CD ligand with lanthanides, and we confirmed that the
stereoarrangement of each of the two adjacent malto-
disaccharide subunits of CD creates a unique octavalent
coordination sphere for lanthanide ions, while still being
capable of sparing an additional site for dynamic water
coordination. Although the thermostability of 3:1 Gd(III)-
ligand 5 was found to be lower than the 1:1 Gd(III)-DOTA, it
is much better than other similarly constructed MRI probes
based on CD scaffolds. Future studies on the kinetic inertness
of the complexes in the presence of other metal ions, such as
Zn(II), Fe(II), and others, will be needed to reveal the in vivo
safety of the gadolinium(III) complex with ligand 5. The new
CD ligand was found to form 1:1, 1:2, and 1:3 multinuclear
complexes with lanthanides; thus, its multiple coordination
centers could provide higher catalytic capacity during dynamic
water exchange. Lastly, the formed complexes also showed
better abilities to alter the relaxation time of coordinated water
(T1) than DOTA-Gd(III). Obviously, the increase in proton
relaxivities can be partially explained by the increase of
molecular weights. However, the coordination of the 1,2,3-
triazole nitrogens could influence the exchange rates of water,
which could also impact the proton relaxivities. Future studies
on the water exchange rates in the inner sphere of this system
by NMR studies can provide evidence to better explain the
RELAXOMETRIC STUDIES
■
Relaxivity (T1) measurements were performed with both the
maltoside 6 and CD ligand 5 at 37 °C on a 0.47 T (20 MHz)
instrument using the “Inversion Recovery Pulse Sequence”.
The resulting curves were adjusted to a monoexponential
function to obtain T1. Relaxation values were measured three
times, and the average was calculated (see Figure 11).
For the 1:1 Gd(III) complex of maltoside 6, a relaxivity
value (r1) of 4.61 mM−1 s−1 was obtained, whereas for the 1:1,
2:1, and 3:1 Gd(III)-ligand 5 complexes, relaxivity values of
12.67 mM−1 s−1, 27.69 mM−1 s−1 (13.8 mM−1 s−1/Gd), and
42.14 mM−1 s−1 (14.0 mM−1 s−1/Gd) were observed,
respectively. These represent a respective increase of 2.7, 3.0,
and 3.1 times, compared to the maltoside 6/Gd(III) complex.
An enhancement of relaxivity of 9% and 10% due to the
introduction of one and two new chelated ligands (variation of
mass of 6% and 12%) was observed, because of the decrease in
the rotational time. These values also compare very favorably
to the relaxivity of the commercial Gd-DOTA and Gd-DTPA
(∼3.5−4.3 mM−1 s−1 at 20 MHz).1 Our recently reported per-
6-O-acetates of β-CD/Gd(III) 4 allowed a relaxivity of 6.53
mM−1 s−1 (20 MHz),30 and ligand 3 was reported to have a
relaxivity of 8.8 mM−1 s−1 (12 MHz),28 which is lower than
that of the complexes formed with ligand 5. Thus, the newly
synthesized CD derivative 5 showed some potential as an
interesting MRI probe. However, its in vivo performance will
be ultimately dependent on its kinetic stability in the presence
of other endogenic metal ions.
CONCLUSION
■
Here, we have reported the synthesis and studies on the
coordination chemistry of a rather complex MRI probe 5,
which is based on a modified β-CD host. The target compound
L
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Inorganic Chemistry
Article
́
̌
(9) Kotkova, Z.; Helm, L.; Kotek, J.; Hermann, P.; Lukes, I.
Gadolinium Complexes of Monophosphinic Acid DOTA Derivatives
Conjugated to Cyclodextrin Scaffolds: Efficient MRI Contrast Agents
for Higher Magnetic Fields. Dalton Trans. 2012, 41, 13509−13519.
(10) Skinner, P. J.; Beeby, A.; Dickins, R. S.; Parker, D.; Aime, S.;
Botta, M. Conjugates of Cyclodextrins with Charged and Neutral
Macrocyclic Europium, Terbium and Gadolinium Complexes:
Sensitised Luminescence and Relaxometric Investigations and an
Example of Supramolecular Relaxivity Enhancement. J. Chem. Soc.
Perkin Trans. 2 2000, 1329−1338.
origin of observed increase in proton relaxivities. Apart from
using ligand 5 as an MRI probe, it could be used to sequester
other heavy-metal ions in different applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00937.
(11) Barge, A.; Cravotto, G.; Robaldo, B.; Gianolio, E.; Aime, S.
New CD Derivatives as Self-Assembling Contrast Agents for Magnetic
Resonance Imaging (MRI). J. Inclusion Phenom. Mol. Recognit. Chem.
2007, 57, 489−495.
Experimental details and additional figures and schemes
(including an NMR analysis and computational
sections) (PDF)
́
́
́
(12) Kotkova, Z.; Kotek, J.; Jirak, D.; Jendelova, P.; Herynek, V.;
́
̌
Berkova, Z.; Hermann, P.; Lukes, I. Cyclodextrin-Based Bimodal
Fluorescence/MRI Contrast Agents: An Efficient Approach to
Cellular Imaging. Chem. - Eur. J. 2010, 16, 10094−10102.
(13) Bryson, J. M.; Chu, W.-J.; Lee, J.-H.; Reineke, T. M. A β-
Cyclodextrin “Click Cluster” Decorated with Seven Paramagnetic
Chelates Containing Two Water Exchange Sites. Bioconjugate Chem.
2008, 19, 1505−1509.
AUTHOR INFORMATION
Corresponding Authors
*Fax: +33-235522959. E-mail: geraldine.gouhier@univ-rouen.
fr (G. Gouhier).
■
*Fax: 1-403-289-9488. E-mail: ccling@ucalgary.ca (C.-C.
Ling).
(14) Barge, A.; Caporaso, M.; Cravotto, G.; Martina, K.; Tosco, P.;
Aime, S.; Carrera, C.; Gianolio, E.; Pariani, G.; Corpillo, D. Design
and Synthesis of a γ
ORCID
Chang-Chun Ling: 0000-0003-0802-7729
1
8
β
-Cyclodextrin Oligomer: A New Platform
Author Contributions
with Potential Application as a Dendrimeric Multicarrier. Chem. - Eur.
J. 2013, 19, 12086−12092.
(15) Aime, S.; Botta, M.; Fedeli, F.; Gianolio, E.; Terreno, E.; Anelli,
P. High-Relaxivity Contrast Agents for Magnetic Resonance Imaging
Based on Multisite Interactions between a β-Cyclodextrin Oligomer
and Suitably Functionalized GdIII Chelates. Chem. - Eur. J. 2001, 7,
5261−5269.
∇The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. These authors contributed equally.
Notes
The authors declare no competing financial interest.
(16) Aime, S.; Gianolio, E.; Arena, F.; Barge, A.; Martina, K.;
Heropoulos, G.; Cravotto, G. New Cyclodextrin Dimers and Trimers
Capable of Forming Supramolecular Adducts with Shape-Specific
Ligands. Org. Biomol. Chem. 2009, 7, 370−379.
ACKNOWLEDGMENTS
■
The financial support from Alberta Innovates−Technology
Futures, the Natural Sciences and Engineering Research
Council of Canada, and the University of Calgary is greatly
acknowledged. We thank the European program Battuta and
Labex Synorg for their financial supports. We thank Stephane
Marcotte from INSA Rouen, France, for ICP measurements.
(17) Gianolio, E.; Napolitano, R.; Fedeli, F.; Arena, F.; Aime, S.
Poly-β-Cyclodextrin Based Platform for pH Mapping via a
Ratiometric 19F/1H MRI Method. Chem. Commun. 2009, 6044−6046.
(18) Liu, T.; Li, X.; Qian, Y.; Hu, X.; Liu, S. Multifunctional pH-
Disintegrable Micellar Nanoparticles of Asymmetrically Function-
alized β-Cyclodextrin-Based Star Copolymer Covalently Conjugated
with Doxorubicin and DOTA-Gd Moieties. Biomaterials 2012, 33,
2521−2531.
́
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