Lanthanide Complexes Formed with the Tri- and Tetraacetate Derivatives of Bis(aminomethyl)phosphinic Acid: Equilibrium, Kinetic and NMR Spectroscopic Studies

The lanthanide(III) complexes formed with the tri- and tetraacetate derivatives of bis(aminomethyl)phosphinic acid, L1 and L2, respectively, have been studied by pH potentiometry, spectrophotometry and 1H and 17O NMR spectroscopy. L1 forms [Ln(L1)], [Ln(L1)2]4, protonated [Ln(HL1)] and Ln(H2L1)]+, and [Ln(L1)(OH)]2 hydroxido complexes. Heptadentate L2 forms [Ln(L2)]2 and protonated [Ln(HL2)] and [Ln(H2L2)] complexes in solution and it shows a strong propensity to form [Ln2(L2)]+ dinuclear complexes, which has not been observed previously. The stability constants (log?KLnL) of the complexes increase in the order [Ln(L1)] < [Ln(L2)]2 following the order of increasing number of acetate pendants attached to the bis(aminomethyl)phosphinic acid (BAP) backbone. Within the LnIII series, the log?KLnL values increase from La3+ to Gd3+ and remain practically constant for the heavier lanthanides. Despite the lower basicity, the ligands that contain a phosphinate group generally form similar (L1) or more stable (L2) Ln3+ complexes than the structurally similar N-benzylethylenediamine-N,N',N'-triacetic acid (L3) and propylenediamine-N,N,N',N'-tetraacetic acid (L4), respectively. This indicates that the hard phosphinate group may be coordinated to the Ln3+ ions in the complexes, whereas the larger negative charge of the BAP derivatives may also have an extra stabilizing effect. The kinetic inertness of [Ln(L1)] and [Ln(L2)] is lower than that of similar [Ln(EDTA)] (EDTA = ethylenediamine-N,N,N',N'-tetraacetic acid), but the rate constants that characterize the dissociation of [Ln(L2)]2 are at least two orders of magnitude lower than those obtained for [Ln(L4)]. Variable-temperature 17O transverse and longitudinal relaxation rates and NMR spectroscopic chemical shifts have been measured to assess the water exchange and rotational dynamics of [Gd(L2)]. The chemical shifts evidenced monohydration of the complex. The water exchange rate, kex298 = (2.7 +/- 0.4)+/- 107 s1 is about ten times higher than that of [Ln(DTPA)]2 (DTPA = diethylenetriamine-N,N,N',N?,N?-pentaacetic acid). The rotational correlation time, tRO298 = 270 +/- 30 ps, is long considering the small size of the chelate, which points to aggregation in aqueous solution, in accordance with the high value of the proton relaxivity measured.

Multifrequency EPR studies of zero field splitting of Gd(III) based MRI contrast agents in solids and liquids

Magnetic Resonance Imaging (MRI) offers an amazing number of ways to probe the human body. It is a wonderful symbiosis of engineering, science and medicine. The power of MRI is its ability to probe a variety of physical mechanisms that can then not only reveal delicate anatomical structures but also discover important information about the physiology of tissue function in normal and diseased states. Paramagnetic metal ions, as a result of their unpaired electrons, act as potent MRI contrast agents. They decrease the T1 and T2 relaxation times of nearby water protons. The mechanism of T1 relaxation is generally a through space dipole-dipole interaction. This interaction is between the unpaired electrons of a paramagnetic ion, such as Gd(III) and the water molecules that are in fast exchange within the ion's inner-coordination sphere. The lanthanide ion Gd(III) is by far the most frequently chosen metal centre in MRI contrast agent, because it has a very high magnetic moment (S = 7/2) and a symmetric electronic ground state, 8S7/2. As it is generally highly toxic, acyclic and macrocyclic polyaminocarboxylates ligands have been synthesized forming thermodynamically and kinetically stable complexes with Gd(III). The coupling of the seven unpaired electrons of the Gd(III) ion with the surrounding water proton spins observed in MRI is crucial for the contrast agent effectiveness, expressed as relaxivity. Therefore, understanding the behavior of the electron spin system can provide valuable information for the development of new compounds. Since this ion has S >1, its spin Hamiltonian includes a significant Zero Field Splitting (ZFS) contribution. This work was devoted to the determination by Electron Paramagnetic Resonance (EPR) spectroscopy of parameters governing the strength of ZFS of Gd(III)-based MRI contrast agents in aqueous solution, frozen solution, and powders. The analysis of multiple frequency and temperature EPR data has been conducted on selected commercial and potential MRI contrast agents. The studied complexes have been designed in the aim to optimise other important parameters which influence the relaxivity, like the rotational diffusion of the complexes and the water residence time in the first coordination shell. This work provides a survey of electron spin relaxation through the determination of the ZFS parameters for Gd(III)-based contrast agents (1) in solution where a rigorous derivation of the relaxation rates was applied to the analysis of the full EPR line shape at multiple frequencies, and (2) in glasses and powders where a direct and straightforward determination of ZFS parameters allowed, for the first time, to establish a correlation between the sign of the axial ZFS parameter and the nature of the chelating ligand in Gd(III) complexes.

EPFL2006

A Starburst-Shaped Heterometallic Compound Incorporating Six Densely Packed Gd3+ Ions

Alain Borel, Eva Jakab Toth, Joao Bruno Livramento Fig., André Merbach

The heterotritopic ligand [bpy(DTTA)2]8- has two diethylenediamine-tetraacetate units for selective lanthanide(III) coordination and one bipyridine function for selective FeII coordination. In aqueous solution and in the presence of these metals, the ligand is capable of self-assembly to form a rigid supramolecular metallostar structure, {Fe[Gd2bpy(DTTA)2(H2O)4]3}4-. We report here the physicochemical characterization of the dinuclear complex [Gd2bpy(DTTA)2(H2O)4]2- and the metallostar {Fe[Gd2bpy(DTTA)2(H2O)4]3}4- with regard to potential MRI contrast agent applications. A combination of pH potentiometry and 1H NMR spectroscopy has been used to determine protonation constants for the ligand [bpy(DTTA)2]8- and for the complexes [Fe{bpy(DTTA)2}3]2- and [Y2bpy(DTTA)2]2-. In addition, stability constants have been measured for the dinuclear chelates [M2bpy(DTTA)2]n- formed with M = Gd3+ and Zn2+ (log KGdL = 18.2; log KZnL = 18.0; log KZnHL = 3.4). A multiple field, variable-temperature 17O NMR and proton relaxivity study on [Gd2bpy(DTTA)2(H2O)4]2- and {Fe[Gd2bpy(DTTA)2(H2O)4]3}4- yielded the parameters for water exchange and the rotational dynamics. The 17O chemical shifts are indicative of bishydration of the lanthanide ion. The exchange rates of the two inner-sphere water molecules are very similar in the dinuclear [Gd2bpy(DTTA)2(H2O)4]2- and in the metallostar (kex298 = 8.1+/-0.3 x 10^6 and 7.4+/-0.2 x 10^6 s-1, respectively), and are comparable to kex298 for similar GdIII poly(amino carboxylates). The rotational dynamics of the metallostar has been described by means of the Lipari-Szabo approach, which involves separating global and local motions. The difference between the local and global rotational correlation times, lO298 = 190�15 ps and gO298 = 930+/-50 ps, respectively, shows that the metallostar is not completely rigid. However, the relatively high value of S2 = 0.60+/-0.04, describing the restriction of the local motions with regard to the global one, points to a limited flexibility compared with previously reported macromolecules such as dendrimers. As a result of the two inner-sphere water molecules, with their near-optimal exchange rate, and the limited flexibility, the metallostar has a remarkable molar proton relaxivity, particularly at high magnetic fields (r1 = 33.2 and 16.4 mM-1 s-1 at 60 and 200 MHz, respectively, at 25 �C). It packs six efficiently relaxing GdIII ions into a small molecular space, which leads, to the best of our knowledge, to the highest relaxivity per molecular mass ever reported for a GdIII complex. The [bpy(DTTA)2]8- ligand is also a prime candidate as a terminal ligand for constructing larger sized, FeII (or RuII)-based metallostars or metallodendrimers loaded with GdIII on the surface.

2006

Luminescent lanthanides bioprobes emitting in the visible and/or near-infrared ranges

Steve Comby

This Ph.D. work deals with the development of lanthanide-based bioprobes for biomedical analyses and imaging. For this purpose, two different approaches have been followed, one focusing on visible-emitting complexes, while the second addresses the sensitization of NIR-emitting LnIII ions. The homoditopic ligands H2LC2 and H2LC3 have been tailored to self-assemble with lanthanide ions (LnIII) into neutral triple-stranded bimetallic helicates of overall composition [Ln2(L)3] and also to provide a versatile platform for further derivatization. The grafting of polyoxyethylene groups onto the pyridine (H2LC2) or the benzimidazole (H2LC3) units ensures water solubility, while maintaining large thermodynamic stability and adequate antenna effect for the excitation of LnIII ions. The conditional stability constants are large at physiological pH, with logβ23 reaching values in the range 22-25 for helicates with H2LC2 and H2LC3. The ligands triplet states feature adequate energy to efficiently sensitize the EuIII luminescence (QEu L = 21 and 11 % for L = H2LC2 and H2LC3, respectively) in aerated water at pH 7.4. Conversely, energy back transfer occurring in [Tb2(LC3)3] precludes a good sensitization of the TbIII luminescence, and only the TbIII helicate with H2LC2 displays sizeable quantum yield (QTbL = 11 %). Furthermore, it has been demonstrated that ligand H2LC2 sensitizes the emission of other visible- and NIR-emitting LnIII ions, such as SmIII or YbIII, for which in cellulo luminescence is evidenced, a rare observation. The EuIII emission spectra of both helicates arise from a main species with pseudo-D3 symmetry and without coordinated water. Cytotoxicity experiments reveal no significant effect of the helicates on the viability of several cancerous cell lines and cell imaging properties of the EuIII helicates are demonstrated for the HeLa cell line by luminescence microscopy. Bright EuIII emission is seen after relatively short time (15-30 min) and low helicate incubation concentrations (10 µM). Similar images were obtained for [Sm2(LC2)3] and [Tb2(LC2)3] using slightly higher loading concentration. The helicates stain the cytoplasm and the permeation mechanism is likely to be endocytosis. The described luminescent helical stains are robust chemical species which remain mostly undissociated in the cell medium and which resist to the presence of other complexing agents, as demonstrated in the case of H2LC2. All these properties make these helicates potential bioprobes for interacting with biological material and for cell imaging applications. As near-infrared (NIR) light, in addition to be less energetic than visible light, displays better penetrability into the human body, the development of bioprobes emitting in the near-infrared range is of prime importance for deeper tissue imaging, especially for early diagnosis of tumors. We therefore investigate the ability of new polydentate podands to sensitize the NIR luminescence of NdIII and YbIII ions. These ligands have been designed in a way to take advantage of the chelating effect of bidentate (Tsox, TsoxMe) or tridentate 8-hydroxyquinolinate (8-HQ) subunits (T2soxMe). The interaction between the ligands and LnIII ions in aqueous solution leads to the formation of stable 1:1 chelates (pLn in the range of 12-16), all the chromophoric 8-HQ units being coordinated to the metal center, exploiting the entropic effect generated by the anchor. At physiological pH, one major species is present for all the ligands regardless of the LnIII ion used. The low energy triplet states of the ligands are particularly well-suited for an efficient sensitization of the NdIII- or YbIII-centered luminescence (QNdL = 0.02-0.04 % and QYbL = 0.13-0.37 %) in buffered aqueous solution. ErIII NIR luminescence is also detected in water for the chelates with Tsox and TsoxMe. Lifetime determinations point in all cases to the absence of coordinated water molecules in the inner coordination sphere of the LnIII ion. It has also been demonstrated that methylation of the amide functions of Tsox removes the quenching mechanism induced by the proximate N-H groups and increases both the lifetimes and quantum yields of the TsoxMe chelates, with the YbIII complex being one of the most NIR luminescent lanthanide-based edifices described so far. Ligand TsoxMe thus appears to be the best candidate of the series regarding to the development of NIR probes for bioanalyses. Furthermore, other applications, for instance in telecommunications, are still conceivable since the incorporation of the Yb-TsoxMe chelate into sol-gel silica films has proven to be successful, the chelate withstanding the sol-gel process, while providing a slight enhancement of photophysical properties compared to the same chelate in aqueous solution.

EPFL2008