Scrutinising the role of intramolecular hydrogen bonding in water exchange dynamics of Gd(iii) complexes

Article abstract of DOI:10.1039/d1dt00204j

We report a series of structurally related Gd(iii) complexes designed to modulate the exchange of the coordinated water molecule, which is an important parameter to be controlled to achieve optimal performance of contrast agents for application in magneti

Full text of DOI:10.1039/d1dt00204j

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Scrutinising the role of intramolecular hydrogen
bonding in water exchange dynamics of Gd(III)
complexes†
Cite this: Dalton Trans., 2021, 50,
5506
Loredana Leone, a Sara Camorali,a Antía Freire-García,b
a
Carlos Platas-Iglesias, *b David Esteban Gomez b and Lorenzo Tei
*
We report a series of structurally related Gd(III) complexes designed to modulate the exchange of the co-
ordinated water molecule, which is an important parameter to be controlled to achieve optimal perform-
ance of contrast agents for application in magnetic resonance imaging (MRI). The ligands contain a DO3A
scafold functionalised with 2’-methoxyphenacyl or 4’-methoxyphenacyl groups (DO3A-oMAP and
DO3A-pMAP),
a 2’-aminophenacyl group (DO3A-oAnAP) or a 2’,4’-dihydroxyphenacyl moiety
(DO3A-DiHAP). The results are compared with those obtained previously for the analogues containing
2’- or 4’-hydroxyphenacyl groups (DO3A-oHAP and DO3A-pHAP, respectively) and the parent system
with an unsubstituted acetophenone pendant arm (DO3A-AP). 1H NMR studies performed on the Eu(III)
complexes show that ligand functionalisation causes a very minor effect on the relative populations of the
SAP and TSAP isomers present in solution, with the SAP isomer representing 70–80% of the overall
population. The emission spectra of the Eu(III) complexes confirm the presence of a water molecule co-
ordinated to the metal center and point to similar coordination environments around the metal ion. The
analysis of the 1H NMRD profiles and 17O NMR data recorded for the Gd(III) complexes evidences that
water exchange is modulated by the ability of peripherical substituents to establish hydrogen bonds with
the coordinated and/or second sphere water molecules. DFT calculations were used to model the
transition states responsible for the dissociative water exchange mechanism, providing support to the
crucial role of hydrogen-bonds in accelerating water exchange.
Received 20th January 2021,
Accepted 29th March 2021
DOI: 10.1039/d1dt00204j
rsc.li/dalton
the human tissues.1,2 The increase of the longitudinal relax-
ation rate, induced by one millimolar concentration of the
Introduction
Magnetic resonance imaging (MRI) is a powerful diagnostic
technique widely used to obtain detailed anatomical func-
tional images of the human body. As a further tool to highlight
certain pathologies, MRI contrast agents (CAs) are often
required in order to increase the contrast between the healthy
tissue and the lesion. Nowadays, clinically employed CAs are
low molecular weight Gd(III) complexes with linear or macro-
cyclic polyaminocarboxylate ligands, which are capable to
enhance the longitudinal relaxation rate of water protons in
paramagnetic ion, is called relaxivity (r1) and it depends on
several structural and dynamic features of the Gd(III) complex.3
Some of these key parameters are the rotational correlation
time (τR), which relates to the molecular size and stereochemi-
cal rigidity of the complex, and the residence lifetime of the
coordinated water molecule(s) (τM) in exchange with the bulk
water.1,2 The latter parameter, can be also expressed as kex
=
1/τM where kex is the exchange rate of the water molecule co-
ordinated to the metal centre. Water exchange can be modu-
lated by modifying the structural and electronic features of the
complex and it can be determined by variable-temperature
17O-NMR studies.4
A useful strategy for the design of an efficient MRI CA relies
on the structural modification on the pendant arms of one of
the most investigated macrocyclic ligands, DOTA (DOTA =
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate). The Gd(III)
complex of DOTA is used as a contrast agent in clinical
practice under the name DOTAREM®.3 For example, the pres-
aDipartimento di Scienze e Innovazione Tecnologica (DiSIT). Università degli Studi
del Piemonte Orientale “Amedeo Avogadro”, Viale T. Michel 11, I-15121 Alessandria,
Italy. E-mail: lorenzo.tei@uniupo.it
bCentro de Investigacións Científicas Avanzadas (CICA) and Departamento de
Química, Facultade de Ciencias, Universidade da Coruña, 15071 A Coruña, Galicia,
Spain. E-mail: carlos.platas.iglesias@udc.es
†Electronic supplementary information (ESI) available: 1H NMR spectra, absorp-
tion and emission spectra, additional r1 and 17O NMR data and geometries
obtained with DFT calculations. See DOI: 10.1039/d1dt00204j
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ence of functional groups forming hydrogen-bonds with the
coordinated water molecule,5 or with second sphere water
molecules in proton exchange with a coordinated hydroxyl
Results and discussion
Synthesis
group,6 have been recently shown to lead to an increase in the Ligands DO3A-oMAP, DO3A-pMAP, DO3A-oAnAP and
relaxivity of small molecular weight Gd(III) complexes. DO3A-DiHAP were synthesised following the procedure reported
However, in these examples the H-bond formation did not in Scheme 2. In particular, the synthesis started from the reac-
involve modulation of kex.
tion of DO3A(tBu)3 with o- or p-methoxy-2-bromoacetophenone
In a recent work,7 we have explored two novel chelators con- to obtain the ortho and para-methoxy derivatives DO3A(tBu)3-o,
sisting of DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic pMAP. Trifluoroacetic acid deprotection of the tert-butyl esters
acid), bearing 2′-hydroxyphenacyl and 4′-hydroxyphenacyl yielded the DO3A-oMAP and DO3A-pMAP ligands. On the other
groups respectively as the fourth pendant arm (DO3A-oHAP hand, to obtain the DO3A-oAnAP derivative, DO3A(tBu)3 was
and DO3A-pHAP, Scheme 1). Our results showed that GdDO3A- reacted first with o-nitro-2-bromoacetophenone, followed by
oHAP is endowed with a higher kex due to its capability to form reduction of the nitro group with H2 on Pd/C to obtain the
hydrogen bonds between the ortho-phenol(ate) groups and the amino derivative. Subsequent trifluoroacetic acid deprotection
water molecules involved in the dissociative exchange mecha- gives the third ligand DO3A-oAnAP. Regarding the last chelator,
nism, thus stabilizing the eight-coordinate transition state. On DO3A-DiHAP, reaction of DO3A(tBu)3 with commercially avail-
the other hand, GdDO3A-pHAP, with the phenol(ate) group able 2′,4′-dihydroxy-2-bromoacetophenone yielded DO3A(tBu)3-
pointing outwards from the complex, had kex values similar to DiHAP, and the final ligand was obtained by deprotection of the
the model system GdDO3A-AP, which lacks H-bond acceptor t-butyl esters by TFA/DCM (1 : 1). This reaction mixture was then
groups. In the present work, we expand this family of macro- concentrated in vacuum and purified by preparative HPLC-MS.
cyclic ligands by changing the nature of the H-bond donor Complexation of the free ligands was accomplished by using
group and/or increasing their number on the aromatic moiety the lanthanide trichloride salts (Ln(III) = Gd, Eu) in water at pH
of the ligand. Therefore, we have synthesised and character- 7. Free lanthanide ion excess was eliminated by precipitation
ised four new DO3A-AP-like ligands with a fourth pendant arm and filtration of the hydroxide at basic pH.
consisting of 2′-methoxyphenacyl or 4′-methoxyphenacyl
groups (DO3A-oMAP and DO3A-pMAP), a 2′-aminophenacyl
Spectroscopic study on Eu(III) complexes
group (DO3A-oAnAP) or
a 2′,4′-dihydroxyphenacyl moiety To get more insight into the structural and dynamic properties
(DO3A-DiHAP, Scheme 1). The main aim of this study is to of Ln-DO3A-oAnAP, LnDO3A-DiHAP and LnDO3A-MAP com-
evaluate the impact on kex and relaxivity of changing the plexes, the Eu(III) complexes were investigated by using 1H
nature of the H-bond donor group (–NH2 vs. –OH), and NMR spectroscopy at three temperatures (283, 298 and 310K,
1
whether further substitution of the aromatic ring can also ESI†). In case of EuDO3A-DiHAP, the H NMR spectra at pH 4
influence these properties (DO3A-DiHAP). The derivatives con- and 9 showed that the (de)protonation of the phenol does not
taining –OMe groups were designed as controls, as they lack change substantially the ratio of the isomers present in solu-
H-bond donor groups. Spectroscopic studies using 1H NMR tion (Fig. S1†). Thus, in this work all other spectroscopic
and absorption and emission electronic spectroscopy were measurements were recorded only at neutral pH. The solution
carried out to gain information on the structures of the com- structure of these DO3A-acetophenone-like complexes is
plexes in solution. A DFT study was also conducted to rational- expected to resemble that of the corresponding LnDOTA com-
ise the trends observed in exchange rates of the coordinated plexes.8 The latter complexes are characterised by the presence
water molecule.
of two different coordination isomers defined by the same con-
formation of the macrocyclic ring but with different orien-
tation of the side arms (i.e., capped square-antiprismatic geo-
metry, SAP, and capped twisted square antiprismatic geometry,
TSAP). These isomers exhibit two different sets of signals in
the paramagnetic 1H NMR spectra with a relative population
that is affected by the size of the Ln(III) ion.8 The substitution
of one acetic arm with substituted phenacyl moieties reduces
the symmetry removing the proton equivalence, although the
carbonyl oxygen is expected to coordinate the metal ion and
therefore the rigidity of the system should be maintained. The
1H NMR spectra of EuDO3A-DiHAP and EuDO3A-oAnAP at pH
7 and at 283 K (Fig. S2 and S3†) show one predominant set of
signals (70–80%) and another set present in ca. 20–30%
amount. The chemical shifts of the four predominant axial
ring protons are in the range 25–40 ppm, suggesting the pres-
ence of SAP isomers.9 By comparing the integral of the two
sets of signals of axial ring-protons, the ratio between the SAP
Scheme 1 DO3A-acetophenone (DO3A-AP) and their derivatives dis-
cussed in the present work.
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Scheme 2 Synthetic route followed for the preparation of the ligands.
and TSAP isomers for EuDO3A-oAnAP and EuDO3A-DiHAP at expressions developed by Horrocks and Beeby.15,16 This ana-
neutral pH can be estimated as about 76 : 24 and 75 : 25, lysis afforded hydration numbers close to 1 for all complexes
respectively. For the other Eu-complexes (Fig. S4 and S5†) the with substituents at position 4 of the phenyl ring. The
SAP/TSAP ratio were 76 : 24 for EuDO3A-oMAP and 82/18 for EuDO3A-oAnAP and EuDO3A-oHAP complexes present some-
EuDO3A-pMAP.
what higher calculated hydration numbers, likely due to the
The emission spectra of the Eu(III) complexes with DO3A- deactivation effect caused by the –OH and –NH2 groups
oHAP, DO3A-pHAP and DO3A-oAnAP are characterised by very involved in hydrogen bonds with the oxygen atom of the aceto-
weak emission intensities associated to the 5D0
→
7FJ tran- phenone moiety.
sitions (J = 0–4) typical of the complexes with this metal ion
The low quantum yields determined for the complexes with
(Fig. S10–S14, ESI†).10 The lifetimes of the excited 5D0 state DO3A-o,pHAP and DO3A-oAnAP are likely related to a poor
measured in water (τH O) fall within the range 0.39–0.51 ms efficiency of the energy transfer, associated to a low energy of
2
(Table 1). The emission quantum yields determined for these the ligand-centred triplet state. Indeed, the absorption spectra
complexes with hydroxyl or amine substituents on the aceto- of these complexes show maxima at rather low energy (λmax
phenone group are very low (<1.6%, Table 1).
>335 nm, Table 1). Application of the methodology developed
The complexes with the ligands containing methoxyl substi- by Werts to this family of structurally related complexes leads to
tuents show however rather strong luminescence, with emis- similar radiative lifetimes of Eu(III), τrad, which fall in the range
sion quantum yields of ∼6–7%. The quantum yield deter- 9.3–10.1 ms.17 The comparable values of τrad obtained suggests
mined for EuDO3A-pMAP (6.9%) compares reasonably well similar coordination environments around the metal ion. The
with the value reported in the literature (9.8%),11 considering analysis of the photophysical data indicates that the low
the uncertainty of these measurements. The EuDO3A-o,pMAP quantum yields of the DO3A-o,pHAP and DO3A-oAnAP com-
complexes are also characterised by longer lifetimes of the plexes is related to low sensitisation efficiencies, ηsens, which are
Eu(III) 5D0 excited state (0.63 ms, Table 1). The latter values are however virtually quantitative in the case of the EuDO3A-o,
in the upper range of the lifetimes reported for monohydrated pMAP complexes, in agreement with previous data.11
DO3A Eu(III) derivatives (ca. 0.33–0.68 ms).12–14 The lifetimes
of the D0 excited state were also measured in D2O solution to a rather intense D0 → F0 transition, which can be attributed
The emission spectra of all complexes are characterised by
5
5
7
estimate the number of coordinated water molecules using the to the low symmetry of the crystal field originated by the
Table 1 Absorption and emission properties of the Eu(III) complexesa
Ligand
λmax/nm
τH O/ms
τD O/ms
ΦEu/%
qc
ΔJ = 2: ΔJ = 1
τrad/ms
ηsens/%
2
2
DO3A-oMAP
DO3A-pMAP
DO3A-oAnAP
DO3A-oHAP
DO3A-pHAP
DO3A-APb
338/267
306
396/271
337/266
352/306
265
0.630(1)
0.628(1)
0.389(2)
0.510(2)
0.456(3)
0.62
2.285(1)
2.243(1)
0.91(1)
2.104(7)
0.814(5)
2.26
6.0
1.1
1.1
1.5
1.5
0.9
1.1
0.95
1.10
1.12
0.96
1.13
—
10.1
9.5/6.65b
9.4
9.8
9.3
0.96
6.9/9.8b
<0.2
0.4
1.0/0.99b
∼0.04
0.08
1.6
0.6
0.33
0.71
7.37
a Conditions provided in Fig. 1. b Data from ref. 11. c Calculated according to ref. 16.
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point to similar structures of the complexes in solution. For
instance, significant changes in the ΔJ = 2/ΔJ = 1 intensity
ratios (from 0.6 to 5.2) were observed upon changing the axial
donor in Eu(III) DOTA-tetraamide complexes.19 The complexes
also show very similar splitting of the 5D0
→
7F1 transition,
with the exception of the EuDO3A-oHAP complex. The energies
of the three components observed for the EuDO3A-oAnAP,
EuDO3A-o,pMAP and EuDO3A-pHAP complexes with respect to
the 5D0 → 7F0 transition are 270 6, 403 2 and 468 2 cm−1
.
The corresponding values measured for EuDO3A-oHAP are
296, 371 and 473 cm−1 (Table S1, ESI†). Since all these com-
plexes present similar populations of the SAP and TSAP
isomers in solution, the different splitting of the 5D0
→
7F1
manifold in EuDO3A-oHAP must be related to subtle structural
changes of the metal coordination environment.
Fig. 1 Partial emission spectra of the Eu(III) complexes normalised to
pH dependence
the intensity of the 5D0
→
7F2 transition. Conditions: EuDO3A-pHAP
The pH dependence of the relaxivity (r1) of Gd(III) complexes of
this class of DO3A-like ligands was measured to assess the
possible change in r1 with (de)protonation of the functional
group(s) on the aromatic ring (Fig. 2A). For GdDO3A-o,pMAP
the r1 vs. pH graphs show no evident change in r1 in the range
of pH 2–12 and thus a retention of the coordination sphere of
the metal ion in this pH range. As for GdDO3A-oAnAP and
GdDO3A-DiHAP, the relaxivity varies slightly in the range of
pH 2–12. It drops from pH 2 to pH 7 and then slightly rises
towards basic pH for GdDO3A-oAnAP. In the case of
GdDO3A-DiHAP, the relaxivity rises slightly from pH 5 to pH
11. These results show that: (i) for GdDO3A-oAnAP, the proto-
nation of the aromatic amine can cause an increase in relaxiv-
ity due to a small contribution of second-sphere water mole-
cules hydrogen bonded to the protonated amine or to an acid-
catalysed proton exchange contribution;6 (ii) the deprotonation
(10−4 M, pH 5.1, λexc = 314 nm, bandpass = 1 nm); EuDO3A-oHAP (pH
4.9, λexc = 350 nm, bandpass = 2 nm); EuDO3A-oAnAP (pH 7.4, λexc
=
400 nm, bandpass = 2 nm); EuDO3A-pMAP (pH 7.4, λexc = 312 nm,
bandpass = 1 nm); EuDO3A-oMAP (pH 7.4, λexc = 350 nm, bandpass =
1 nm).
coordination of the ligand. The presence of three components
for the 5D0
→
7F1 transition is also in line with a low sym-
metry.18 The emission profile is also characterised by similar
ratios of the ΔJ = 2 and ΔJ = 1 transitions (0.95–1.13, Table 1).
The intensity of the magnetic dipole 5D0 → 7F1 transition is vir-
tually independent of the metal coordination environment,
5
7
while the electric dipole character of the D0 → F2 transition
makes it very sensitive to variations in the metal coordination
sphere.12,19 Thus, the similar ΔJ = 2/ΔJ = 1 intensity ratios
Fig. 2 Left: Plots of 1H relaxivity for GdDO3A-AP-derivatives as a function of pH (20 MHz and 298 K): right: UV spectra (λ = 200–450 nm) of
GdDO3A-DiHAP (4 × 10−4 M) in different aqueous buffer solutions ranging from pH 4.5 to 9.9. The absorbances are normalized to zero for λ =
600 nm. Insert: Plot of the total absorbance difference vs. pH to determine the pKa. The total absorbance difference is the sum of the absolute
absorbance difference values at the chosen wavelengths (i.e. 300 and 350 nm). The pKa value was worked out by nonlinear regression as reported in
ref. 20.
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of the dihydroxyphenyl moiety leads to a small r1 increase, and delocalization of the negative charge to form an enolate
probably due to the formation of a negatively charged, more anion that coordinates the metal ion. However, the variation of
hydrophilic complex.
r1 from basic to acidic pH observed was very small. This means
In order to get further insight on the pH dependence, we that either there is no change in the structural and dynamic
carried out variable pH UV-vis measurements on the free parameters related to the Gd-complex, or that counteracting
ligands and on the correspondent Gd(III)-complexes. In case of variations in different parameters do not alter r1 significantly.
the DO3A-DiHAP ligand, the –OH deprotonation results in a In our previous study, the detailed 17O NMR analysis at acidic
red shift of the absorption band from 280 to 340 nm, associ- and basic pH of GdDO3A-oHAP showed that water exchange
ated to a pKa of 6.93 0.09 (0.1 M NaCl, 298 K, Fig. S15†), dynamics are not substantially affected by pH changes, there-
whereas the GdDO3A-DiHAP complex shows a shift from 300 fore, in the present work we focused on determining the
to 350 nm and a pKa of 6.05 0.04 (Fig. 2B). The ΔpKa of relaxometric properties of the complexes at physiological pH.
≈0.88 units can be attributed to an electron withdrawing effect Thus, the r1 values for GdDO3A-oAnAP, GdDO3A-DiHAP and
of the metal ion that allows the delocalization of the negative GdDO3A-o,pMAP at 20 MHz, 298 and 310 K and pH 7.4 are
charge also on the carbonyl oxygen, in part forming an enolate listed in Table 2. The r1 values are consistent with the presence
anion. In the case of DO3A-oAnAP, the absorption spectra did of one coordinated water molecule (q = 1), although the r1
not experience changes with pH, and thus it was not possible values are slightly higher than the values measured in analo-
to determine the pKa in the chosen working range.
gous conditions for other q = 1 Gd-complexes of comparable
molecular weight. As r1 depends on the magnetic field
strength, temperature, and several important molecular para-
Relaxometric properties
To the best of our knowledge, except for our previous com- meters of the paramagnetic metal complex, a complete 1H and
munication,7 relaxometric studies on Gd-complexes having a 17O NMR relaxometric study was carried out to obtain detailed
ketone donor group have not been reported so far. We could information of the physicochemical properties of the com-
only find the relaxivity values at 20 MHz, 310 K and pH 7.0 for plexes. Thus, the variation of r1 as a function of the magnetic
a di- and a tri-nuclear Gd-complex with phenyl-di- (or tri-) acyl field strength, the so-called nuclear magnetic resonance dis-
groups connecting two or three DO3A moieties. The r1 values persion profile (1H NMRD), was measured for all complexes at
were 6.1 mM−1 s−1 for the trinuclear system and 5.4 mM−1 s−1 298, and 310 K in the proton Larmor frequency range
for the dinuclear analogue.21 On the other hand, the other 0.01–120 MHz, corresponding to magnetic field strengths
reported LnDOTA-like chelates bearing a pendant arm with a varying between 2.34 × 10−4 and 3 T (Fig. 3 and S16, ESI†). The
ketone donor have been used for luminescence studies.11,22–24 profiles show the characteristic shape of a low molecular
Sherry and co-workers reported another interesting study on weight Gd-complex with a plateau at low fields, a dispersion
Eu(III)-ketone coordination, demonstrating slow water around 4–8 MHz, and another plateau with lower relaxivity in
exchange induced by the coordination of the carbonyl oxygen the high-frequency region (>20 MHz). This behaviour is quite
due to its lower electron density with respect to amide or car- typical for Gd chelates whose relaxivity is largely dominated by
boxylate oxygen donors.25
rotational dynamics.26
As discussed above, in case of the dihydroxyphenacyl
The temperature dependence of the solvent 17O NMR trans-
derivatives the coordination of the Gd(III) ion changes in the verse relaxation rates, R2, and shifts, Δω, allowed obtaining
physiological pH range with deprotonation of the –OH group more accurate and quantitative information on the kinetics of
Table 2 Best-fit parameters obtained from the analysis of the 1/T1 1H NMRD profiles (298 and 310 K) and 17O NMR data for GdDO3A-AP, GdDO3A-
oAnAP, GdDO3A-oHAP (at pH 4), GdDO3A-pHAP, GdDO3A-oMAP and GdDO3A-pMAPa
Complex
Isomer
298r1b (mM−1 s−1
)
310r1b (mM−1 s−1
)
298τR (ps)
298τM (ns)
Δ2 (1019 s−2
)
298τv (ps)
EM (kJ mol−1
)
GdDO3A-oMAP
SAP
TSAP
SAP
TSAP
SAP
TSAP
SAP
TSAP
SAP
TSAP
SAP
TSAP
SAP
TSAP
5.7 0.1
5.0 0.1
93
90
2
3
1050 10
6.5 1.0
1100 20
11.0 1.2
690 10
15.5 1.1
4.5 0.1
4.3 0.3
5.4 0.3
4.7 0.2
5.0 0.4
4.6 0.3
4.6 0.2
10 0.5
9.8
5.0
8.5
3.0
7.0
5.1 0.1
3.2 0.3
5.8 0.2
3.8 0.4
5.2 0.3
6.8 0.4
5.1 0.2
5.0 0.1
5.1
14.8
5.7
24.8
5.8
51
71
52
80
51
57
52
57
58.9
50
34
40
54
32
1
4
1
3
2
5
1
3
GdDO3A-pMAP
GdDO3A-oAnAP
GdDO3A-DiHAP
GdDO3A-APc
5.7 0.1
5.3 0.1
5.4 0.2
5.1
5.1 0.2
4.6 0.2
4.7 0.2
4.6
91.6 2.3
81.8 0.7
100
695
5
8.5 0.7
1200
25
210
2.2
950
7.1
GdDO3A-oHAPc
GdDO3A-pHAPc
6.4
5.4
105
5.8
4.9
95
3.3
3.4
a The parameters fixed in the fitting procedure are: q = 1, rGdO = 2.5 Å, rGdH = 3.0 Å, aGdH = 4.0 Å, 298DGdH = 2.25 × 10−5 cm2 s−1, ER = 16 kJ mol−1
Ev = 1 kJ mol−1, A/ħ = −3.0 × 106 rad s−1 b Relaxivities at 20 MHz. c From ref. 7.
,
.
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Fig. 3 1H NMRD profiles recorded at 298 (black) and 310 K (red) for (A) GdDO3A-oAnAP at pH 7.4 and (B) GdDO3A-DiHAP at pH 7.4.
water exchange (measurements at 11.75 T on 10–20 mM solu- almost steady from 275 to 290 K, increase with temperature
tions of the complexes at physiological pH). For all chelates from 290 to 320–330 K, where a maximum is observed, and
the resulting profiles of 17O R2 vs. T (Fig. 4 and S17, ESI†) then they decrease at higher temperatures. These profiles
suggest the presence of two species with very different water contain contributions from a small amount of fast exchanging
exchange dynamics. In fact, the shapes of the curves are TSAP isomer that is hidden behind the larger portion of slow
distant from the simple pseudo-exponential trend expected for exchanging SAP isomer, which gives the peak at 320–330 K
systems containing one coordinated water molecule in rather typical of Gd-chelates characterized by a long water
exchange with the bulk solvent. As already shown for GdDO3A- exchange lifetime (τM ∼1 μs), i.e. GdDOTA-monoamides.30 The
o,pHAP and GdDO3A-AP,7 the two species present in solution 17O R2 profiles recorded for GdDO3A-oMAP and GdDO3A-pMAP
can be considered the SAP and TSAP isomers observed in the present a maximum at about the same temperature, anticipat-
1H NMR spectra of the corresponding Eu-complexes. Similar ing very similar kex values (Fig. S17†). This maximum is slightly
observations were recently reported for GdHPDO3A and shifted to lower temperatures for GdDO3A-oAnAP and
derivatives27,28 and previously for two GdDOTA-bisamide com- GdDO3A-DiHAP, which indicates that water exchange is slightly
plexes.29 These studies demonstrated that the TSAP isomer, faster for the dominant SAP isomer. A more pronounced effect
present in lower concentration, has a water exchange lifetime was observed previously for the GdDO3A-oHAP complex, where
significantly shorter than that observed for the SAP isomer. the phenol(ate) group assisted the kex increase by H-bonding
Notably, in all chelates discussed in the present work, the con- with water molecules involved in the exchange process.7
tributions of the TSAP isomers become important for T <
The variable-temperature 17O R2 profiles were fitted accord-
290 K and predominant at lower temperatures. In particular, ing to the well-established set of Swift–Connick equations31
in the profiles of the reported Gd-complexes, the R2 values are using a model that considers the presence in solution of two
Fig. 4 Transverse 17O relaxation rates measured at 11.74 T and pH 7.4 for: (A) GdDO3A-oAnAP (9.6 mM) and (B) GdDO3A-DiHAP (15.9 mM). The red
and blue lines represent the calculated contributions of the isomeric species SAP and TSAP, respectively.
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isomeric species whose relative population is extrapolated environments very similar to the parent (unsubstituted)
1
from the H NMR spectra of the Eu-complexes (the TSAP/SAP GdDO3A-AP complex (see Computational details below, and
ratio is considered constant over the range of temperatures Table S2, ESI†). Noteworthy, the methoxy substituent in
under examination, Fig. S1–S5, ESI†). Moreover, the NMRD GdDO3A-oMAP points outside the coordination sphere of the
data were analysed using the standard Solomon Bloembergen complex, as a result of steric hindrance (Fig. S36, ESI†). Thus,
Morgan model for the inner-sphere relaxation mechanism32 it is not surprising that the three complexes present similar
and Freed’s model for the outer-sphere components.33 The exchange rates of the coordinated water molecule.
water exchange parameters that affect the inner-sphere contri-
Geometry optimizations performed for the Gd(III) com-
bution were fixed at weighted averages of the values obtained plexes of DO3A-oHAP, DO3A-oAnAP and DO3A-DiHAP yield
for the two isomers by fitting the 17O NMR data (Table 2). very similar distances between the metal ion and the oxygen
Given the large number of parameters involved in the fitting, atom of the coordinated water molecule (rGdO, Table 3).
some of them were fixed to known or reasonable values as Furthermore, the complex with DO3A-AP shows a similar cal-
shown in Table 2. The rotational correlation time τR, consider- culated rGdO value compared with the functionalised deriva-
ing that two isomers have similar rotational dynamics, was tives, in spite of the lower water exchange rate of the former.
determined for all complexes by fitting the NMRD profiles. The rGdO values were found previously to correlate reasonably
Values in the range 80–105 ps were found, in agreement with well with the water exchange rate in nine-coordinate Gd(III)
τR values reported for Gd-complexes of analogous molecular complexes, with a longer distance, and thus a weaker coordi-
volume.34 The parameters associated with the electronic relax- nation, generally corresponding to a faster exchange.38 Thus,
ation times T1,2e (Δ2 and τV) are also in line with the values we conclude that additional factors not related to the strength
obtained previously for similar complexes.27,30 Noteworthy, in of the Gd–Owater bond are responsible for the different water
the case of GdDO3A-DiHAP at pH 7.4, a mixture of species are exchange rates. The 17O hyperfine coupling constants (A/ħ) of
present, neutral and anionic, so the parameters obtained rep- the coordinated water molecules obtained from scalar relativis-
resent an average between those of the two species.
tic calculations are also very similar (∼3 × 106 rad s−1, Table 3),
The τM values obtained for the SAP isomers (0.69–1.1 μs) and equal to the value assumed for the analysis of the 17O
are longer than those determined for the TSAP isomers (τM in NMR shifts and relaxation data (A/ħ = −3.0 × 106 rad s−1, see
the 6.5–15.5 ns range), in line with previous results.35 This has above).
been attributed to the steric compression around the water
The main difference among the different calculated struc-
binding site in the TSAP isomers, which facilitates the depar- tures is related to the Gd–O bond distance involving the carbo-
ture of the water molecule following a dissociatively activated nyl oxygen atom of the acetophenone group (Table S2, ESI†),
water exchange mechanism.4 The τM values determined for the which takes values of 2.429 (GdDO3A-oAnAP), 2.443
(dominant) SAP isomers of the structurally-related Gd(III) com- (GdDO3A-DiHAP), 2.456 (GdDO3A-oHAP), 2.465 (GdDO3A-
plexes listed in Table 1 fall in three categories: the GdDO3A- oMAP) and 2.481 Å (GdDO3A-AP). These Gd–O distances corre-
oMAP, GdDO3A-pMAP and GdDO3A-pHAP complexes display late with the activating ability of the substituents of the aromatic
298τM values of ∼0.9–1.1 μs, which indicates that the incorpor- ring (NH2 > OH > OMe > H).39 These structural changes are
ation of –OMe substituents, or a –OH group in para position, however not likely responsible for the different splitting pattern
5
7
do not have a significant effect in the exchange rate of the of the D0 → F1 transition observed for GdDO3A-oHAP, as all
298τM
= 1.2 μs for the unsubstituted other complexes, including GdDO3A-oAnAP, show a similar
water molecule
(
GdDO3A-AP derivative). A second group encompasses the splitting of the 7F1 level. Interestingly, the complex showing a
GdDO3A-DiHAP and GdDO3A-oAnAP complexes, with 298τM distinct splitting in the emission spectrum is that showing very
values of ∼0.7 μs. Finally, water exchange is considerably faster fast water exchange. While we do not have a definitive expla-
for GdDO3A-oHAP (298τM ∼0.2 μs).
nation for this effect, the fast dissociative water exchange may
DFT calculations
DFT calculations were conducted to rationalise the different
water exchange rates observed for the Gd(III) complexes listed
in Table 2. Our calculations focused on the more abundant
SAP isomers, for which water exchange rates were determined
to a higher accuracy. Following our previous studies, a few
explicit second-sphere water molecules were introduced in the
models, together with a polarized continuum to account the
effects of bulk water. The inclusion of explicit second-sphere
water molecules was found to be critical to obtain accurate
Gd–Owater distances and 17O hyperfine coupling constants
A/ħ.36,37
Table 3 Water exchange parameters and 17O hyperfine coupling con-
stants of the coordinated water molecules obtained with DFT calcu-
lations for the GdDO3A-AP, GdDO3A-oHAP, GdDO3A-oAnAP and
GdDO3A-DiHAP complexesa
DO3A-AP DO3A-oHAP DO3A-oAnAP DO3A-DiHAP
rGdO/Å
2.516
3.471
28.9
2.516
3.337
22.6
+16.4
17.7
2.7
2.523
3.237
25.9
+10.8
22.68
3.0
2.518
3.247
24.5
+12.7
20.7
2.7
rGdO (TS)/Å
ΔH‡/kJ mol−1
ΔS‡/J K−1 mol−1 +2.7
ΔG‡298/kJ mol−1 28.1
AO/ħ/106 rad s−1
2.6
a Data obtained from calculations on the GdDO3A-AP·3H2O, GdDO3A-
oHAP·4H2O, GdDO3A-DiHAP·4H2O and GdDO3A-oAnAP·3H2O systems.
Geometry optimizations performed for the GdDO3A-oMAP
and GdDO3A-pMAP complexes provided metal coordination
5512 | Dalton Trans., 2021, 50, 5506–5518
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cause the complex to spend a non-negligible time in the de- oAnAP and DO3A-DiHAP show that the water molecule that is
hydrated state.40 Since the crystal field splitting of cyclen-based leaving the metal coordination environment is involved in
complexes containing four pendant arms is very sensitive to hydrogen-bonds with the –OH/NH2 groups of the ligand. On
axial ligation,19,41 a significant population of the dehydrated the contrary, the leaving water molecule in GdDO3A-AP
form is expected to impact the splitting of the 7F1 level.
enlarges the rGdO distance from 2.516 to 3.471 Å on going from
The potential energy surfaces of the complexes were the ground to the transition state but remains in the axial posi-
explored by increasing the rGdO distance in steps of 0.05 Å tion. The energy barriers computed with DFT are characterised
from the equilibrium geometry to ∼4 Å. This eventually led to by positive activation entropies, as expected for a dissociative
a second energy minimum for each complex in which the water exchange mechanism.42 The calculated ΔH‡ and ΔG‡
Gd(III) ion is eight-coordinated. Subsequently, we optimized values follow the trend observed for water exchange, with
the transition states that relate the nine- and eight-coordinated GdDO3A-AP showing the highest energy barrier for water
forms of the complexes, as models for the dissociative water exchange and GdDO3A-oHAP the lowest. The GdDO3A-DiHAP
exchange reaction (Fig. 5). The structures of the transition and GdDO3A-oAnAP complexes present intermediate values
states obtained for the complexes with DO3A-oHAP, DO3A- for both ΔH‡ and ΔG‡. We notice that while the experimental
Fig. 5 Structures of the ground states (left panel) and transition states (right panel) optimized with DFT calculations for the GdDO3A-oHAP·4H2O
(a and b), GdDO3A-oAnAP·3H2O (c and d) and GdDO3A-AP·3H2O (e and f) systems.
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Table 4 Hydrogen bonding data obtained with DFT calculations for the minimum energy geometries and transition states of the GdDO3A-AP,
GdDO3A-oHAP, GdDO3A-oAnAP and GdDO3A-DiHAP complexesa
DO3A-oHAP
DO3A-oAnAP
DO3A-DiHAP
Minimum
Minimum
TS
1.777
2.688
152.3
Minimum
TS
2.067
2.972
147.6
TS
2.099
2.883
135.9
D–H⋯A/Å
D⋯A/Å
D–H⋯A/°
1.688
2.545
144.0
1.896
2.618
126.0
1.672
2.539
145.2
a Data obtained from calculations on the GdDO3A-AP·3H2O, GdDO3A-oHAP·4H2O, GdDO3A-DiHAP·4H2O and GdDO3A-oAnAP·3H2O systems.
trend is qualitatively well reproduced by our calculations, the purification. The 1H and 13C NMR spectra were recorded using
a Bruker Avance III 500 MHz (11.4 T) spectrometer equipped
with 5 mm PABBO probes and BVT-3000 temperature control
unit. Chemical shifts are reported relative to TMS and were
referenced using the residual proton solvent resonances. HPLC
analyses and mass spectra were performed on a Waters
HPLC-MS system equipped with a Waters 1525 binary pump.
calculated activation energies are lower than those obtained
from the analysis of the 17O NMR data. We tested different
density functionals and found that activation energies are very
sensitive to the choice of the functional, while the use of the
large-core (4f in core) or small-core pseudopotentials provided
very similar results.
Analytical measurements were carried out on
a Waters
The higher barrier calculated for GdDO3A-oAnAP compared
with GdDO3A-oHAP can be reasonably attributed to the weaker
hydrogen bonds established in the aniline derivative (Table 4).
The –NH2 group establishes relatively weak intramolecular
hydrogen bond with the carbonyl oxygen atom of the acetophe-
none group in the ground state geometry, as indicated by the
N–H⋯O angle (126°), which is far from the ideal linear value.
The hydrogen bond involving the leaving water molecule and
the –NH2 group in the transition state is also weak when com-
pared to that established in the GdDO3A-oHAP complex.
Unexpectedly, the hydrogen bond established by the –OH
group with the leaving water molecule is weaker in
GdDO3A-DiHAP compared with GdDO3A-oHAP. This is
explained by the electron donating effect of the hydroxyl group
at position 4, which reinforces the intramolecular hydrogen
bond with the carbonyl group in the ground state.
XBridge-Phenyl (5 μm 4.6 × 150 mm) column, while a Waters
XBridge-Phenyl Prep OBD (5 μm, 19 × 100 mm) column was
used for preparative purposes.
HPLC analytical method (Method 1) = A: TFA 0.1% in H2O;
B: MeOH; flow 1 mL min−1; 0–3 min: 1% B, 3–18 min: from 1
to 100% B, 18–19 min 100% B, 19–20 min 1% B. HPLC pre-
parative method (Method 2) = TFA 0.1% in H2O; B: MeOH;
flow 20 mL min−1; 0–3 min: 30% B, 3–13 min: from 30 to 77%
B, 13–14 min from 77 to 100% B, 14–16 min 100%
B. Electrospray ionization mass spectra (ESI MS) were recorded
using a SQD 3100 Mass Detector (Waters), operating in positive
or negative ion mode, with 1% v/v formic acid in MeOH as the
carrier solvent. DO3A(tBu)3-oMAP and DO3A(tBu)3-pMAP were
synthesized as reported previously.7
Synthesis
1-(2-(2-Nitrophenyl)-2-oxoethyl)-4,7,10-tris-(t-butoxycarbonyl
methyl)-1,4,7,10-tetraazacyclododecane (DO3A(tBu)3-oNO2AP).
Conclusions
A
solution in CH3CN (5 mL) of DO3A(tBu)3 (200 mg,
0.39 mmol) K2CO3 (100 mg, 0.72 mmol) and 2-bromo-2′-nitro-
acetophenone (208 mg, 0.80 mmol) was left stirring under N2
atmosphere for 5 h at room temperature and then filtered,
evaporated in vacuo and then purified by silica gel chromato-
graphy (95 : 5 to 90 : 10 CH2Cl2–MeOH) to afford (DO3A(tBu)3-
o-NO2AP) (180 mg, 0.26 mmol, yield 67%). 1H NMR (CDCl3,
500 MHz): δ (ppm) = 8.07 (d, J = 8 Hz, –m-Ph–, 1H), 7.73 (d, J =
8 Hz, –o-Ph–, 1H), 7.68–7.66 (m, –m′,p-Ph–, 2H), 3.67–2.09 (m,
This work has demonstrated that water exchange in Gd(III)
complexes can be conveniently modulated by introducing peri-
pheral hydrogen bond donor groups. The water exchange rates
are increased by stabilization of the eight-coordinate transition
state thanks to the formation of a hydrogen bond between the
peripheral substituent and the leaving water molecule. The
results reported in this paper provide an additional strategy for
fine tuning the physicochemical parameters of Gd(III)-based
MRI contrast agents.
macrocycle, (–NCH2
(–NCH2COOC(CH3 3
) , 27H). 13C NMR (CDCl3, 125 MHz): δ
(ppm) = 202.4 (NCH2C Ph), 173.1 (–NCH2C OC(CH3)3), 146.2
(–C2-Ph), 135.3 (–C1-Ph–), 134.2 (–C5-Ph–), 131.5 (–C4-Ph–),
128.0 (–C6-Ph–), 124.5 (–C3-Ph–), 82.0 (–NCH2COOC(CH3)3),
63.3 (–NCH2COPh), 55.6 (–NCH2COOC(CH3)3), 54.7–48.6
H3)3). ESI-MS (m/z): found
̲ COOC(CH3)3, –NC̲H̲2COPh, 24H), 1.37 (s,
̲
̲
O̲
̲O̲
Experimental and computational
section
̲
̲
̲
(macrocycle), 27.80 (–NCH2COOC(C
̲
678.5 [M + H]+ (calc for C34H56N5O9: 678.40).
General methods
All chemicals were purchased from Sigma-Aldrich or Alfa
1-(2-(2-Aminophenyl)-2-oxoethyl)-4,7,10-tris-(t-butoxycarbonyl
Aesar unless otherwise stated and were used without further methyl)-1,4,7,10-tetraazacyclododecane
(DO3A(tBu)3-oAnAP).
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After two vacuum/H2 cycles to replace air inside the reaction cedure as for the synthesis of DO3A-oAnAP, starting from
vessel with hydrogen, a mixture of (DO3A(tBu)3-o-NO2AP) 90 mg (0.14 mmol) of DO3A(tBu)3-p-MeOAP. 1H NMR (D2O,
(180 mg, 0.26 mmol) and Pd/C (15 wt% of the substrate, 500 MHz): δ = 7.98 (d, –C2-6-Ph, J = 8.5 Hz, 2H), 7.12 (d,
27 mg), in MeOH (10 mL) was vigorously stirred at room temp- C3-5-Ph, J = 8.5 Hz, 2H), 3.93 (s, –OCH3
erature under 1 atm of H2 for 4 h. The reaction mixture was fil- (m, macrocycle, 16H; m, –NCH2COOH, 6H, –NCH2
tered through a celite pad and the filtrate was concentrated in 13C NMR (D2O, 125 Hz): δ = 190.7 (NCH2C
Ph), 174.0
vacuum to obtain (DO3A(tBu)3-o-NH2AP) (151 mg, 0.23 mmol, (–NCH2C OH), 164.6 (–C4-Ph), 130.8 (–C2-6-Ph–), 126.4 (–C1-
yield 90%). H NMR (CDCl3, 500 MHz): δ (ppm) = 7.58 (d, J = Ph–), 114.4 (–C3-5-Ph–), 59.3 (–NCH2COPh), 55.7 (–OCH3), 54.7
8 Hz, –m-Ph–, 1H), 7.23 (t, J = 7.5 Hz, –p-Ph–, 1H), 6.74 (d, J = (–NCH2COOC(CH3)3), 53.4–48.2 (macrocycle). HPLC analysis
8.3 Hz, –o-Ph–, 1H), 6.57 (t, J = 7.5 Hz, –m′-Ph–, 1H), 3.46–2.24 (Method 1): tr = 10.8 min. ESI-MS (m/z): found 495.45 [M + H]+,
(m, macrocycle, (–NCH2COOC(CH3)3, –NCH2
COPh, 24H), 1.44 248.42 [M + 2H]+/2 (calc for C23H35N4O8: 495.55).
(s, (–NCH2COOC(CH3 3 1-(2-(2,4-Dihydroxyphenyl)-2-oxoethyl)-4,7,10-tris-(t-butoxy
) , 27H). 13C NMR (CDCl3, 125 MHz):
δ (ppm) = 200.7 (NCH2C Ph), 172.7 (–NCH2C OC(CH3)3), carbonylmethyl)-1,4,7,10-tetraazacyclododecane (DO3A(tBu)3-
150.56 (–C2-Ph), 134.5 (–C4-Ph–), 129.1(–C6-Ph–), 117.6 DiHAP). A solution in CH3CN (5 mL) of DO3A(tBu)3 (200 mg,
(–C5-Ph–), 116.5 (–C3-Ph–), 115.8 (–C1-Ph–), 82.0 0.39 mmol), K2CO3 (100 mg, 0.72 mmol) and 2-bromo-2′,4′-
(–NCH2COOC(CH3)3), 60.1 (–NCH2COPh), 56.0 (–NCH2COOC dihydroxyacetophenone (180 mg, 0.78 mmol) was stirred
(CH3)3), 55.5–48.7 (macrocycle), 27.80 (–NCH2COOC(CH3)3). under N2 atmosphere for 5 h at room temperature and then fil-
ESI-MS (m/z): found 648.4 [M + H]+ (calc for C34H5N5O7: tered. The reaction mixture was concentrated in vacuum and
647.43). then purified by preparative HPLC-MS (Method 2) to obtain
̲
, 3H), 4.14–3.24
̲
̲
CO–, 2H).
̲O̲
̲O̲
1
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲O̲
̲
̲
̲
̲
1
1-(2-(2-Aminophenyl)-2-oxoethyl)-1,4,7,10-tetraaza cyclodo- (DO3A-Di-HyAP) (120 mg, 0.18 mmol, 47%). H NMR (CD3CN,
decane-1,4,7-triacetic acid (DO3A-oAnAP). DO3A(tBu)3-o- 500 MHz): δ (ppm) = 7.59 (d, J = 8.5 Hz, (–C6-Ph–), 1H), 6.51 (d,
NH2AP (151 mg, 0.23 mmol) was dissolved in DCM : TFA J = 8.5 Hz, (–C5-Ph–), 1H), 6.40 (s, (–C3-Ph–), 1H), 3.71–3.11 (m,
(1 : 1/v : v) and the mixture was stirred at rt overnight. After macrocycle, (–NCH2
evaporation in vacuo, the residue was dissolved in HCl 1 M (–NCH2COOC(CH3 3
) , 27H). 13C NMR (CD3CN, 125 MHz): δ
(1 ml) and evaporated in vacuo. The last operation was (ppm) = 190.10 (NCH2C Ph), 173.8 (–NCH2C OC(CH3)3),
̲ COOC(CH3)3, –NC̲H̲2COPh, 24H), 1.34 (s,
̲
̲
O̲
̲O̲
repeated twice and finally the aqueous solution was freeze 168.7 (–C4-Ph–), 168.3 (–C2Ph–), 131.7 (–C6-Ph–), 115.00 (–C1-
dried to obtain the ligand (DO3A-AnAP) as the HCl salt in Ph–), 108.90 (–C5-Ph–), 102.70 (–C3-Ph–), 82.0 (–(–NCH2COOC
quantitative yield, without further purification (173 mg, (C
̲
̲H2COPh), 54.3 (–NC̲
1
̲
̲
̲
̲
̲
̲
̲O̲
̲
̲
1-(2-(2-Methoxyphenyl)-2-oxoethyl)-1,4,7,10-tetraaza cyclodo- 125 MHz): δ (ppm) = 189.9 (NCH2COPh), 173.8 (–NCH2COOH),
decane-1,4,7-triacetic acid (DO3A-oMeOAP). The HCl salt of 166.70–166.72 ((–C2-Ph–), and (–C4-Ph–), 132.9 (–C6-Ph–),
DO3A-o-MeOAP (77 mg) was prepared following the same pro- 116.1 (–C1-Ph–), 107.2 (–C3-Ph–), 102.9 (–C5-Ph–), 63.41
cedure as for the synthesis of DO3A-oAnAP, starting from (–NCH2COPh), 53.2 (–NCH2COOH), 51.5–48.0 (macrocycle).
90 mg (0.14 mmol) of DO3A(tBu)3-o-MeOAP. 1H NMR (D2O, HPLC analysis (Method 1): tr = 11.6 min. ESI-MS (m/z): found
500 MHz): δ = 7.94 (d, –C6-Ph, J = 5.6 Hz, 1H), 7.73 (t, –C4-Ph–, 497.42 [M + H]+, 249.2 [M + 2H]+/2 (calc for C22H33N4O9:
J = 7.5 Hz, 1H), 7.24 (d, C3-Ph, J = 8.5 Hz, 1H), 7.15 (t, –C5-Ph, 497.22).
J = 7.5 Hz, 1H), 3.99 (s, –OCH3
16H; m, –NCH2COOH, 6H, –NCH2
125 Hz): δ = 192.4 (NCH2C Ph), 173.9 (–NCH2C
(–C2-Ph), 137.1 (–C4-Ph–), 130.7 (–C6-Ph–), 122.7 (–C1-Ph–),
121.0 (–C5-Ph–), 112.8 (–C3-Ph–), 63.6 (–NCH2COOH), 55.7 The ligands (50 mg) were dissolved in H2O (1 mL) and LnCl3
(–OCH3), 55.09 (–NCH2COPh), 53.4–48.2 (macrocycle). HPLC (1.1 eq.) was added maintaining the pH around 6.5–7 and the
analysis (Method 1): tr = 11.4 min. ESI-MS (m/z): found 495.37 solution was stirred overnight at room temperature. The pH
[M + H]+, 248.22 [M + 2H]+/2 (calc for C23H35N4O8:495.55).
was then raised to 9.5 to allow precipitation of excess Ln(III) as
̲
, 3H), 4.19–3.12 (m, macrocycle,
CO–, 2H). 13C NMR (D2O,
OH), 160.3
̲
̲
̲
̲
̲O̲
General procedure for the preparation of Ln(III) complexes
̲
̲
̲
1-(2-(4-Methoxyphenyl)-2-oxoethyl)-1,4,7,10-tetraaza cyclodo- Ln(OH)3, which after 2h was centrifuged and then filtered
decane-1,4,7-triacetic acid (DO3A-pMeOAP). The HCl salt of through a 0.2 μm filter. Finally, the pH was brought back to 7
DO3A-p-MeOAP (75 mg) was prepared following the same pro- and the solution was lyophilized to obtain the pure complex.
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Gd(DO3A-oAnAP). HPLC analysis (Method 1): tr = 15.5 min. NMR Relaxometer (Stelar) equipped with a superconducting
ESI-MS (m/z): found 635.3 [M + H]+, 317.5 [M + 2H]+/2 (calc for magnet HS-110 at 3.0 T. The exact concentration of Gd(III) was
C22H31GdN5O7: 635.14).
determined by measurement of bulk magnetic susceptibility
Gd(DO3A-oMeOAP). HPLC analysis (Method 1): tr
=
shifts of a tBuOH signal. Variable-temperature 17O NMR
10.95 min. ESI-MS (m/z): found 650.2 [M + H]+, 325.6 measurements were recorded on a Bruker Avance III spectro-
[M + 2H]+/2 (calc for C23H33GdN4O8: 650.14).
meter (11.7 T) equipped with a 5 mm probe and standard
temperature control unit. Aqueous solutions of the complexes
Gd(DO3A-pMeOAP). HPLC analysis (Method 1): tr
=
10.92 min. ESI-MS (m/z): found 650.4 [M + H]+, 325.7 containing 2.0% of the 17O isotope (Cambridge Isotope) were
[M + 2H]+/2 (calc for C23H33GdN4O8: 650.14).
used. The observed transverse relaxation rates were calculated
Gd(DO3A-DiHAP). HPLC analysis (Method 1): tr = 10.0 min. from the signal width at half-height.
ESI-MS (m/z): found 652.4 [M + H]+, 326.5 [M + 2H]+/2 (calc for
Computational details
C22H30GdN4O9: 652.14).
DFT calculations were performed using both the pseudopotential
Eu(DO3A-oAnAP). HPLC analysis (Method 1): tr = 11.9 min.
ESI-MS (m/z): found 628.4 [M + H]+, 314.6 [M + 2H]+/2 (calc for
C22H31EuN5O7: 628.14).
approximation44 and scalar relativistic calculations.45 All pseudo-
potential calculations were carried out with the Gaussian 16
program package46 and the wB97XD functional,47 which includes
empirical dispersion. We used the quasirelativistic large-core
pseudopotential of the Stuttgart/Cologne group, which includes
46 + 4f7 electrons in the core for Gd, and the related [5s4p3d]
valence basis set.48 All other atoms were described with the stan-
dard 6-311G(d,p) basis set. Geometry optimizations were per-
formed without restrains and frequency calculations were carried
out to confirm the nature of the optimized geometries as local
energy minima or saddle points. The output of frequency calcu-
lations also provided the zero-point energies and thermal terms
required to calculate the enthalpy and entropy of the systems.
Solvent effects (water) were considered with integral equation
formalism of the polarized continuum model (IEFPCM), using
the default parameters implemented in Gaussian 16.49
Transition states were located using the synchronous transit-
guided quasi-newton method (QST3).50 An ultrafine grid was
used throughout using the integral = ultrafine keyword.
Eu(DO3A-oMeOAP). HPLC analysis (Method 1): tr
=
12.7 min. ESI-MS (m/z): found 643.3 [M + H]+, 322.92
[M + 2H]+/2 (calc for C23H33EuN4O8: 643.14).
Eu(DO3A-pMeOAP). HPLC analysis (Method 1): tr
=
11.8 min. ESI-MS (m/z): found 643.3 [M + H]+, 322.19
[M + 2H]+/2 (calc for C23H33EuN4O8: 643.14).
Eu(DO3A-DiHAP). HPLC analysis (Method 1): tr = 9.8 min.
ESI-MS (m/z): found 645.5 [M + H]+, 310.5 [M + 2H+]/2 (calc for
C22H30EuN4O9: 645.11).
UV-Vis and luminescence measurements
UV analyses were carried out on a Jasco V-550 dual-lamp (deu-
terium) and visible (xenon) spectrophotometer scanning from
700 nm to 200 nm. Solutions at known concentration of the
complexes were prepared and the UV-vis spectra were recorded
at different pHs intervals (pH 6–11 for the free ligand and
4–10 for its Gadolinium complex).
Scalar relativistic calculations were performed using the
second-order Douglas–Kroll–Hess method,51 as implemented
ORCA program package.52 These calculations used the wB97X
functional,53 in combination with the SARC2-DKH-QZVP54
basis set for Gd and the DKH-def2-TZVPP55 basis set for all
other atoms. The resolution of identity and chain of spheres
RIJCOSX approximation56 was used to accelerate the calcu-
lations using the SARC2-DKH-QZVP/JK auxiliary basis set for
Gd and the Autoaux57 procedure to generate auxiliary basis
sets for all other atoms. The grid settings were increased from
the default values using the GridX8 keyword. Solvent effects
(water) were included with the SMD solvation model58
implemented in ORCA. Views of the structures were generated
using the OLEX2 program.59
Steady-state emission spectra of the Eu(III) complexes were
recorded with a Horiba FluoroMax Plus-P spectrofluorometer
using an integration time of 0.1 s. The excitation source was a
150 W ozone-free xenon arc lamp. The instrument was
equipped with a R928P photon counting emission detector and
a photodiode reference detector for monitoring lamp output.
Luminescence lifetimes were measured using the time corre-
lated single photon counting module and a xenon flash lamp as
excitation source. Emission quantum yields were determined
using the Cs3[Eu(pic)3] complex (pic = 2,6-dipicolinate) as refer-
ence (Φ = 24% in TRIS, pH 7.4, 7.5 × 10−5 M).43 UV-vis absorp-
tion spectra for quantum yield determination were obtained
with 0.1 cm quartz cells using a Jasco V-650 spectrometer.
Relaxometric measurements
The water proton longitudinal relaxation rates as a function of
the magnetic field strength were measured in non-deuterated
aqueous solutions on a Fast Field-Cycling Stelar SmarTracer
relaxometer (Stelar s.r.l., Mede (PV), Italy) over a continuum of
magnetic field strengths from 0.00024 to 0.25 T (corres-
ponding to 0.01–10 MHz proton Larmor frequencies). The
relaxometer operates under computer control with an absolute
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
uncertainty in 1/T1 of 1%. Additional longitudinal relaxation The authors thank Centro de Computación de Galicia (CESGA)
data in the range 20–120 MHz were obtained with a High Field for providing the computer facilities. C. P.-I. and D. E.-G.
5516 | Dalton Trans., 2021, 50, 5506–5518
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5518 | Dalton Trans., 2021, 50, 5506–5518
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