Investigating the
relaxation of water
1
H nuclei induced
by paramagnetic Mn(II) complexes is important to understand the mechanisms
that control the efficiency of contrast agents used in diagnostic
magnetic resonance imaging (MRI). Herein, a series of potentially
hexadentate triazacyclononane (TACN) derivatives containing different
pendant arms were designed to explore the relaxation of the electron
spin in the corresponding Mn(II) complexes by using a combination
of
1
H NMR relaxometry and theoretical calculations. These
ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid
(H
3
NOTA) and three derivatives in which an acetate group
is replaced by sulfonamide (H
3
NO2ASAm), amide (H
2
NO2AM), or pyridyl (H
2
NO2APy) pendants. The analogue of
H
3
NOTA containing three propionate pendant arms (H
3
NOTPrA) was also investigated. The X-ray structure of the
derivative containing two acetate groups and a sulfonamide pendant
arm [Mn(NO2ASAm)]
−
evidenced six-coordination of
the ligand to the metal ion, with the coordination polyhedron being
close to a trigonal prism. The relaxivities of all complexes at 20
MHz and 25 °C (1.1–1.3 mM
–1
s
–1
) are typical of systems that lack water molecules coordinated to
the metal ion. The nuclear magnetic relaxation profiles evidence significant
differences in the relaxivities of the complexes at low fields (<1
MHz), which are associated with different spin relaxation rates. The
zero field splitting (ZFS) parameters calculated by using DFT and
CASSCF methods show that electronic relaxation is relatively insensitive
to the nature of the donor atoms. However, the twist angle of the
two tripodal faces that delineate the coordination polyhedron, defined
by the N atoms of the TACN unit (lower face) and the donor atoms of
the pendant arms (upper face), has an important effect in the ZFS
parameters. A twist angle close to the ideal value for an octahedral
coordination (60°), such as that in [Mn(NOTPrA)]
−
, leads to a small ZFS energy, whereas this value increases as the
coordination polyhedron approaches to a trigonal prism.
We present two ligands
containing a N-ethyl-4-(trifluoromethyl)benzenesulfonamide
group attached to either a 6,6′-(azanediylbis(methylene))dipicolinic
acid unit (H3DPASAm) or a 2,2′-(1,4,7-triazonane-1,4-diyl)diacetic
acid macrocyclic platform (H3NO2ASAm). These ligands were
designed to provide a pH-dependent relaxivity response upon complexation
with Mn2+ in aqueous solution. The protonation constants
of the ligands and the stability constants of the Mn2+ complexes
were determined using potentiometric titrations complemented by spectrophotometric
experiments. The deprotonations of the sulfonamide groups of the ligands
are characterized by protonation constants of log K
i
H = 10.36
and 10.59 for DPASAm3– and HNO2ASAm2–, respectively. These values decrease dramatically to log K
i
H = 6.43 and 5.42 in the presence of Mn2+, because of the
coordination of the negatively charged sulfonamide groups to the metal
ion. The higher log K
i
H value in [Mn(DPASAm)]− is related to the formation of a seven-coordinate complex, while
the metal ion in [Mn(NO2ASAm)]− is six-coordinated.
The X-ray crystal structure of Na[Mn(DPASAm)(H2O)]·2H2O confirms the formation of a seven-coordinate
complex, where the coordination environment is fulfilled by the donor
atoms of the two picolinate groups, the amine N atom, the N atom of
the sulfonamide group, and a coordinated water molecule. The lower
conditional stability of the [Mn(NO2ASAm)]− complex
and the lower protonation constant of the sulfonamide group results
in complex dissociation at relatively high pH (<7.0). However,
protonation of the sulfonamide group in [Mn(DPASAm)]− falls into the physiologically relevant pH window and causes a significant
increase in relaxivity from r
1p = 3.8
mM–1 s–1 at pH 9.0 to r
1p = 8.9 mM–1 s–1 at pH 4.0 (10 MHz, 25 °C).
We report a detailed investigation of the potential as magnetic resonance imaging (MRI) contrast agents of Fe(III) complexes with H4EDTA derivatives containing different spacers: trans-cyclohexane-1,2-diamine (t H4CDTA), cis-cyclohexane-1,2-diamine (c H4CDTA),...
We present a quantitative analysis of the thermodynamic
stabilities
of Mn(II) complexes, defined by the equilibrium constants (log
K
MnL
values) and the values of pMn obtained as
−log[Mn]
free
for total metal and ligand concentrations
of 1 and 10 μM, respectively. We used structural descriptors
to analyze the contributions to complex stability of different structural
motifs in a quantitative way. The experimental log
K
MnL
and pMn values can be predicted to a good accuracy
by adding the contributions of the different motifs present in the
ligand structure. This allowed for the identification of features
that provide larger contributions to complex stability, which will
be very helpful for the design of efficient chelators for Mn(II) complexation.
This issue is particularly important to develop Mn(II) complexes for
medical applications, for instance, as magnetic resonance imaging
(MRI) contrast agents. The analysis performed here also indicates
that coordination number eight is more common for Mn(II) than is generally
assumed, with the highest log
K
MnL
values generally observed for hepta- and octadentate ligands. The
X-ray crystal structure of [Mn
2
(DOTA)(H
2
O)
2
], in which eight-coordinate [Mn(DOTA)]
2–
units are bridged by six-coordinate exocyclic Mn(II) ions, is also
reported.
Rigid derivatives of the acyclic ligand PDTA4 (H4PDTA = propylenediamine-N,N,N′,N′-tetraacetic acid) were prepared by functionalization of a cis-1,3-cyclobutanediamine spacer. The new ligands contain either four acetate groups attached to the...
Triazines and their degradation products are transported to the aquatic environment, and once there, the probability to reach the marine environment is very high. In this paper, solid phase extraction (SPE) and extraction by matrix solid phase dispersion (MSPD) to analyse nine triazines (ametryn, atrazine, cyanazine, prometryn, propazine, simazine, simetryn, terbuthylazine and terbutryn) and eight degradation products (desethylatrazine, desethyldesisopropylatrazine, desethyl-2-hydroxyatrazine, desethylterbuthylazine, desisopropylatrazine, desisopropyl-2-hydroxyatrazine, 2-hydroxyatrazine and 2-hidroxyterbuthylazine) in seawater and marine sediments samples were used. The analysis was carried out using liquid chromatography with tandem mass spectrometry (LC-ESI-MS/MS). The methods were optimized and validated to achieve a selective and sensitive determination of the analytes from different sample, regardless of its complexity. Under the optimum conditions, the proposed methods provided adequate limits of quantification (0.05-0.45 μg L and 0.23-4.26 μg kg in seawater and marine sediments, respectively). Intra- and inter-day relative standard deviation were below 1.41% for all compounds. Recoveries were evaluated, and acceptable values that ranged from 87.5-99.4 and 60.9-99.7% for the seawater and sediment samples, respectively, were obtained. The proposed methods were applied to the analysis of the target compounds in seawater samples and marine sediments from a coastal area of Galicia (NW of Spain).
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