Positively charged GdIIIcryptates: slow, associative water exchange

Electron paramagnetic resonance of lanthanides

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd³⁺. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides. Magnetic resonance imaging contrast agents use the long relaxation time of the S-state Gd³⁺ ion, and NMR shift reagents use the fast relaxation of selected other lanthanides. Other than Gd³⁺ and the isoelectronic Eu²⁺ ion, spin relaxation of the lanthanides is so fast that their EPR spectra can be observed only in the liquid helium temperature range. In this chapter the EPR properties of each of the lanthanides is briefly summarized, with an emphasis on electron spin relaxation.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

Isoquinoline-based lanthanide complexes: bright NIR optical probes and efficient MRI agents

In the objective of developing ligands that simultaneously satisfy the requirements for MRI contrast agents and near-infrared emitting optical probes that are suitable for imaging, three isoquinoline-based polyaminocarboxylate ligands, L1, L2 and L3, have been synthesized and the corresponding Gd(3+), Nd(3+) and Yb(3+) complexes investigated. The specific challenge of the present work was to create NIR emitting agents which (i) have excitation wavelengths compatible with biological applications and (ii) are able to emit a sufficient number of photons to ensure sensitive NIR detection for microscopic imaging. Here we report the first observation of a NIR signal arising from a Ln(3+) complex in aqueous solution in a microscopy setup. The lanthanide complexes have high thermodynamic stability (log K(LnL) =17.7-18.7) and good selectivity for lanthanide ions versus the endogenous cations Zn(2+), Cu(2+), and Ca(2+) thus preventing transmetalation. A variable temperature and pressure (17)O NMR study combined with nuclear magnetic relaxation dispersion measurements yielded the microscopic parameters characterizing water exchange and rotation. Bishydration of the lanthanide cation in the complexes, an important advantage to obtain high relaxivity for the Gd(3+) chelates, has been demonstrated by (17)O chemical shifts for the Gd(3+) complexes and by luminescence lifetime measurements for the Yb(3+) analogues. The water exchange on the three Gd(3+) complexes is considerably faster (k(ex)(298) = (13.9-15.4) × 10(6) s(-1)) than on commercial Gd(3+)-based contrast agents and proceeds via a dissociative mechanism, as evidenced by the large positive activation volumes for GdL1 and GdL2 (+10.3 ± 0.9 and +10.6 ± 0.9 cm(3) mol(-1), respectively). The relaxivity of GdL1 is doubled at 40 MHz and 298 K in fetal bovine serum (r(1) = 16.1 vs 8.5 mM(-1) s(-1) in HEPES buffer), due to hydrophobic interactions between the chelate and serum proteins. The isoquinoline core allows for the optimization of the optical properties of the luminescent lanthanide complexes in comparison to the pyridinic analogues and provides significant shifts of the excitation energies toward lower values which therefore become more adapted for biological applications. L2 and L3 bear two methoxy substituents on the aromatic core in ortho and para positions, respectively, that further modulate their electronic structure. The Nd(3+) and Yb(3+) complexes of the ligand L3, which incorporates the p-dimethoxyisoquinoline moiety, can be excited up to 420 nm. This wavelength is shifted over 100 nm toward lower energy in comparison to the pyridine-based analogue. The luminescence quantum yields of the Nd(3+) (0.013-0.016%) and Yb(3+) chelates (0.028-0.040%) are in the range of the best nonhydrated complexes, despite the presence of two inner sphere water molecules. More importantly, the 980 nm NIR emission band of YbL3 was detected with a good sensitivity in a proof of concept microscopy experiment at a concentration of 10 μM in fetal bovine serum. Our results demonstrate that even bishydrated NIR lanthanide complexes can emit a sufficient number of photons to ensure sensitive detection in practical applications. In particular, these ligands containing an aromatic core with coordinating pyridine nitrogen can be easily modified to tune the optical properties of the NIR luminescent lanthanide complexes while retaining good complex stability and MRI characteristics for the Gd(3+) analogues. They constitute a highly versatile platform for the development of bimodal MR and optical imaging probes based on a simple mixture of Gd(3+) and Yb(3+)/Nd(3+) complexes using an identical chelator. Given the presence of two inner sphere water molecules, important for MRI applications of the corresponding Gd(3+) analogues, this result is particularly exciting and opens wide perspectives not only for NIR imaging based on Ln(3+) ions but also for the design of combined NIR optical and MRI probes.

Correlation of relaxivity with coordination number in six-, seven-, and eight-coordinate Mn(II) complexes of pendant-arm cyclen derivatives

The syntheses and characterization of several complexes of Mn(II) with cyclen derivatives having variable numbers of pendant N-acetic acid or N-acetamide arms are reported. X-ray crystallographic results are presented for Mn(DOTAM)Cl(2) x 2 H(2)O (monoclinic C2/c, a = 18.5798(15), b = 13.6006(11), c = 10.5800(8) A, beta = 110.490(1) degrees, Z = 4), [Mn(DO3AM)][MnCl(4)] x EtOH (monoclinic P2(1)/n, a = 8.366(8), b = 19.483(2), c = 16.3627(16) A, beta = 99.254(2) degrees, Z = 4), and Mn(H(2)DOTA) (monoclinic C2/c, a = 16.374(3), b = 6.6559(13), c = 16.750(3) A, beta = 98.381(3) degrees, Z = 4), which exhibit 8-, 7-, and 6-coordinate Mn(II), respectively. (1)H relaxivity data in water at 20 MHz and 37 degrees C is presented and interpreted in terms of a mechanism involving transient binding of water in an associative intermediate. Relaxivity studies in mixed water/methanol solvents are consistent with this interpretation.

Gadolinium(III) complexes as MRI contrast agents: ligand design and properties of the complexes

Magnetic resonance imaging is a commonly used diagnostic method in medicinal practice as well as in biological and preclinical research. Contrast agents (CAs), which are often applied are mostly based on Gd(III) complexes. In this paper, the ligand types and structures of their complexes on one side and a set of the physico-chemical parameters governing properties of the CAs on the other side are discussed. The solid-state structures of lanthanide(III) complexes of open-chain and macrocyclic ligands and their structural features are compared. Examples of tuning of ligand structures to alter the relaxometric properties of gadolinium(III) complexes as a number of coordinated water molecules, their residence time (exchange rate) or reorientation time of the complexes are given. Influence of the structural changes of the ligands on thermodynamic stability and kinetic inertness/lability of their lanthanide(III) complexes is discussed.

Luminescent Eu(III) and Gd(III) Trisbipyridine Cryptates: Experimental and Theoretical Study of the Substituent Effects

Synthesis, absorption spectra and luminescebce properties of a series of lanthanide trisbipyridine cryptates Ln within R-Bpy x R-Bpy x R-Bpy, where Ln = Eu, Gd and R = H, COOH, COOCH3, CONH(CH2)2NH2 are described. Comparison of the unsubstituted parent compound with the substituted compounds shows that bipyridine substitution doesn't alter significantly the photophysical properties of the lanthanide cryptate. The absorption maximum is slightly red-shifted when three bipyridines are substituted, whereas substituting one bipyridines has a negligible effect on the absorption spectra. The experimental triplet state energy is between 21600 and 22 100 cm(-1) for the series of compounds and the luminescence lifetimes at 77 K are between 0.5 and 0.8 ms in HO2 and equal to 1.7 ms in D2O. The experimental characterizations are completed by DFT and TD-DFT calculations to assess the ability of these approaches to predict absorption maxima, triplet state energies and structural parameters of lanthanide cryptates and to characterize the electronic structure of the excited states. The calculations on the unsubstituted parent and substituted compounds show that absorption maxima and lowest 3pipi* triplet state energies can be accurately determined from density functional theory (DFT) and time-dependent (TD) DFT calculations.

Physicochemical and MRI characterization of Gd3+-loaded polyamidoamine and hyperbranched dendrimers

Generation 4 polyamidoamine (PAMAM) and, for the first time, hyperbranched poly(ethylene imine) or polyglycerol dendrimers have been loaded with Gd3+ chelates, and the macromolecular adducts have been studied in vitro and in vivo with regard to MRI contrast agent applications. The Gd3+ chelator was either a tetraazatetracarboxylate DOTA-pBn4- or a tetraazatricarboxylate monoamide DO3A-MA3- unit. The water exchange rate was determined from a 17O NMR and 1H Nuclear Magnetic Relaxation Dispersion study for the corresponding monomer analogues [Gd(DO3A-AEM)(H2O)] and [Gd(DOTA-pBn-NH2)(H2O)]- (kex298=3.4 and 6.6x10(6) s-1, respectively), where H3DO3A-AEM is {4-[(2-acetylaminoethylcarbamoyl)methyl]-7,10-bis(carboxymethyl-1,4,7,10-tetraazacyclododec-1-yl)}-acetic acid and H4DOTA-pBn-NH2 is 2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. For the macromolecular complexes, variable-field proton relaxivities have been measured and analyzed in terms of local and global motional dynamics by using the Lipari-Szabo approach. At frequencies below 100 MHz, the proton relaxivities are twice as high for the dendrimers loaded with the negatively charged Gd(DOTA-pBn)- in comparison with the analogous molecule bearing the neutral Gd(DO3A-MA). We explained this difference by the different rotational dynamics: the much slower motion of Gd(DOTA-pBn)--loaded dendrimers is likely related to the negative charge of the chelate which creates more rigidity and increases the overall size of the macromolecule compared with dendrimers loaded with the neutral Gd(DO3A-MA). Attachment of poly(ethylene glycol) chains to the dendrimers does not influence relaxivity. Both hyperbranched structures were found to be as good scaffolds as regular PAMAM dendrimers in terms of the proton relaxivity of the Gd3+ complexes. The in vivo MRI studies on tumor-bearing mice at 4.7 T proved that all dendrimeric complexes are suitable for angiography and for the study of vasculature parameters like blood volume and permeability of tumor vessels.

Design of Gd(III)-Based Magnetic Resonance Imaging Contrast Agents: Static and Transient Zero-Field Splitting Contributions to the Electronic Relaxation and Their Impact on Relaxivity

A multiple-frequency (9.4-325 GHz) and variable-temperature (276-320 K) electron paramagnetic resonance (EPR) study on low molecular weight gadolinium(III) complexes for potential use as magnetic resonance imaging (MRI) contrast agents has been performed. Peak-to-peak linewidths Delta Hpp and central magnetic fields have been analyzed within the Redfield approximation taking into account the static zero-field splitting (ZFS) up to the sixth order and the transient ZFS up to the second order. Longitudinal electronic relaxation is dominated by the static ZFS contribution at low magnetic fields (B < 0.3 T) and by the transient ZFS at high magnetic fields (B > 1.5 T). Whereas the static ZFS clearly depends on the nature of the chelating ligand, the transient ZFS does not. For the relatively fast rotating molecules studied water proton relaxivity is mainly limited by the fast rotation and electronic relaxation has only a marked influence at frequencies below 30 MHz. From our EPR results we can conclude that electronic relaxation will have no influence on the efficiency of Gd(III)-based MRI contrast agents designed for studies at very high magnetic fields (B > 3T).

Unexpected Aggregation of Neutral, Xylene-Cored Dinuclear GdIII Chelates in Aqueous Solution

We have synthesized ditopic ligands L(1), L(2), and L(3) that contain two DO3A(3-) metal-chelating units with a xylene core as a noncoordinating linker (DO3A(3-) = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate; L(1) = 1,4-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(2) = 1,3-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L(3) = 3,5-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzoic acid). Aqueous solutions of the dinuclear Gd(III) complexes formed with the three ligands have been investigated in a variable-temperature, multiple-field (17)O NMR and (1)H relaxivity study. The (17)O longitudinal relaxation rates measured for the [Gd(2)L(1-3)(H2O)(2)] complexes show strong field dependence (2.35-9.4 T), which unambiguously proves the presence of slowly tumbling entities in solution. The proton relaxivities of the complexes, which are unexpectedly high for their molecular weight, and in particular the relaxivity peaks observed at 40-50 MHz also constitute experimental evidences of slow rotational motion. This was explained in terms of self-aggregation related to hydrophobic interactions, pi stacking between the aromatic linkers, or possible hydrogen bonding between the chelates. The longitudinal (17)O relaxation rates of the [Gd(2)L(1-3)(H2O)(2)] complexes have been analysed with the Lipari-Szabo approach, leading to local rotational correlation times tau(1)(298) of 150-250 ps and global rotational correlation times tau(g)(298) of 1.6-3.4 ns (c(Gd): 20-50 mM), where tau(1)(298) is attributed to local motions of the Gd segments, while tau(g)(298) describes the overall motion of the aggregates. The aggregates can be partially disrupted by phosphate addition; however, at high concentrations phosphate interferes in the first coordination sphere by replacing the coordinated water. In contrast to the parent [Gd(DO3A)(H2O)(1.9)], which presents a hydration equilibrium between mono- and dihydrated species, a hydration number of q = 1 was established for the [Ln(2)L(1-3)(H2O)(2)] chelates by (17)O chemical shift measurements on Ln = Gd and UV/Vis spectrophotometry for Ln = Eu. The exchange rate of the coordinated water is higher for [Gd(2)L(1-3)(H2O)(2)] complexes k(ex)(298) = 7.5-12.0 x 10(6) s(-1)) than for [Gd(DOTA)(H2O)](-). The proton relaxivity of the [Gd(2)L(1-3)(H2O)(2)] complexes strongly decreases with increasing pH. This is related to the deprotonation of the inner-sphere water, which has also been characterized by pH potentiometry. The protonation constants determined for this process are logK(OH) = 9.50 and 10.37 for [Gd(2)L(1)(H2O)(2)] and [Gd(2)L(3)(H2O)(2)], respectively.