The solution structure and dynamics of metal-bound water exchange have been investigated in a series of lanthanide complexes of primary, secondary, and tertiary tetraamide derivatives of 1,4,7,10-tetraazacyclododecane. In the gadolinium complexes at ambient pH, water exchange lifetimes (τm) determined by 17O NMR were sufficiently long (19 μs for [Gd·2]3+, 298 K, 17 μs for [Gd·3]3+, and 8 μs for [Gd·4]3+) to limit the measured relaxivity. Direct 1H NMR observation of the bound water resonance is possible for the corresponding Eu complexes at low temperature in CD3CN, and the rate of water proton exchange is about 50 times faster in the twisted square antiprismatic isomer (m) than in the isomeric square antiprismatic (M) complex. The ratio of these two isomers in solution is sensitive to the steric demand of the amide substituent, with m/M = 2 for [Eu·4]3+, but 0.25 for [Eu·2]3+. The slowness of coordinated water exchange has allowed the rate of prototropic exchange to be studied:  in basic media deprotonation of the bound water molecule or of proximate ligand amide NH protons leads to relaxivity enhancements, whereas in acidic media, hydration around the strongly ion-paired complexes is perturbed, facilitating water exchange. The X-ray crystal structure of ligand 3 reveals a hydrogen-bonded structure with two pairs of ring N-substituents related in a trans arrangement, contrasting with the structure of diprotonated DOTA in which the ligand is predisposed to bind metal ions. In the dysprosium complex [Dy·3·OH2](PF6)3, the metal ion adopts a regular monocapped square antiprismatic coordination geometry, with a water Dy−O bond length of 2.427(3) Å, and a PF6 counterion is strongly hydrogen-bonded to this bound water molecule.

Reversible anion binding in aqueous media at chiral Eu III and Tb III centers has been characterized by 1 H NMR and by changes in the emission intensity and circular polarization following direct or sensitized (365 nm) excitation via an alkylphenanthridinium chromophore. Using a series of heptadentate tri-amide or polycarboxylate ligands, the affinity for CO 3 2-/HCO 3 -, phosphate, lactate, citrate, acetate, and malonate at pH 7.4 was found to decrease as a function of the overall negative charge on the complex: citrate and malonate bound most strongly, and lactate and hydrogen carbonate also formed chelated ternary complexes in which displacement of both of the metal-bound water molecules occurred, which was confirmed by VT 17-O NMR measurements of the corresponding Gd complexes. The binding of carbonate was studied in particular, and 1 H NMR and CPL data were obtained that were consistent with the formation of a complex with a reduced helical twist about the metal center. Monohydrogen phosphate was bound in a monodentate manner, giving a monoaqua adduct. The binding of carbonate to cationic Eu complexes in the presence of a simulated extra-cellular anionic background at pH 7.4 was monitored by variation in the emission intensity, ratio of intensities (615/ 594 nm), and dissymmetry factors as a function of added total carbonate.

The nature of the ternary complexes formed in aqueous media at ambient pH on reversible binding of acetate, lactate, citrate, and selected amino acids and peptides to chiral diaqua europium, gadolinium, or ytterbium cationic complexes has been examined. Crystal structures of the chelated ytterbium acetate and lactate complexes have been defined in which the carboxylate oxygen occupies an "equatorial" site in the nine-coordinate adduct. The zwitterionic adduct of the citrate anion with [EuL1] was similar to the chelated lactate structure, with a 5-ring chelate involving the apical 3-hydroxy group and the alpha-carboxylate. Analysis of Eu and Yb emission CD spectra and lifetimes (H2O and D2O) for each ternary complex, in conjunction with 1H NMR analyses of Eu/Yb systems and 17O NMR and relaxometric studies of the Gd analogues, suggests that carbonate, oxalate, and malonate each form a chelated (q = 0) square-antiprismatic complex in which the dipolar NMR paramagnetic shift (Yb, Eu) and the emission circular polarization (gem for Eu) are primarily determined by the polarizability of the axial ligand. The ternary complexes with hydrogen phosphate, with fluoride, and with Phe, His, and Ser at pH 6 are suggested to be monoaqua systems with Eu/Gd with an apical bound water molecule. However, for the ternary complexes of simple amino acids with [YbL1]3+, the enhanced charge demand favors a chelate structure with the amine N in an apical position. Crystal structures of the Gly and Ser adducts confirm this. In peptides and proteins (e.g. albumin) containing Glu or Asp residues, the more basic side chain carboxylate may chelate to the Ln ion, displacing both waters.

In Bleaney's theory of magnetic anisotropy, the second-order crystal field coefficient, B o 2 , is predicted to determine the dipolar NMR shift of paramagnetic lanthanide complexes in solution. This parameter has been measured directly, by analysing the europium emission spectra for a series of eight-and nine-coordinate axially symmetric complexes based on cyclen including aza-carboxylate ligands (e.g. DOTA), phosphonates (DOTP), phosphinates and several carboxamides (e.g. DOTAM). For both Yb and Eu complexes with a common coordination number and geometry (square antiprism (SAP) or twisted square antiprism (TSAP)), the dipolar NMR shift correlates very well with this parameter, which also determines the sign and magnitude of a major CD band in the near-IR CD spectra of a series of enantiopure Yb complexes. Measurements of the free energy change associated with axial ligand exchange in a cationic europium tetraamide complex, [Eu(DOTAMPh)](CF 3 SO 3 ) 3 supported by a simple electrostatic perturbation model, have been interpreted in terms of a predominant donor atom polarisation model which affords a simple assessment of Ln ion donor atom preference and ranks the axial second-order ligand field coefficient.

Targeted radiopharmaceuticals offer the possibility of improved tumor imaging and radiotherapy, with reduced side effects. A variety of monoclonal antibodies and antibody fragments have previously been successfully radiolabeled and used in diagnostic imaging and targeted radiotherapy of cancer. Many such antibodies have been shown to recognize the well-characterized MUC1 tumor marker and have recently been in clinical trials. Furthermore, a number of chelators have been synthesized and are currently used as radiopharmaceuticals for imaging and therapy. We now report the synthesis of a novel, cyclen-based ligand with a sulfur-containing arm that offers increased stability of the ligand-metal complex. We have coupled this ligand with previously selected aptamers to the MUC1 tumor marker to generate a novel targeted radiopharmaceutical with improved properties. We have tested the complex against known, commercially available chelators such as MAG3 in model breast cancer systems. To improve the pharmacokinetic properties of the aptamer-based targeted radiopharmaceutical, we have generated multi-aptamer complexes around a central chelator. Such multi-aptamer complexes have increased retention of the complex in circulation, without affecting the lack of immunogenicity of the complex or altering its superior tumor penetration properties.

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Photodynamic therapy (PDT) is an increasingly popular anticancer treatment that uses photosensitizer, light, and tissue oxygen to generate cytotoxic reactive oxygen species (ROS) within illuminated cells. Acting to counteract ROS-mediated damage are various cellular antioxidant pathways. In this study, we combined PDT with specific antioxidant inhibitors to potentiate PDT cytotoxicity in MCF-7 cancer cells. We used disulphonated aluminium phthalocyanine photosensitizer plus various combinations of the antioxidant inhibitors: diethyl-dithiocarbamate (DDC, a Cu/Zn-SOD inhibitor), 2-Methoxyestradiol (2-ME, a Mn-SOD inhibitor), L-buthionine sulfoximine (BSO, a glutathione synthesis inhibitor) and 3-amino-1,2,4-Triazole (3-AT, a catalase inhibitor). BSO, singly or in combination with other antioxidant inhibitors, significantly potentiated PDT cytotoxicity, corresponding with increased ROS levels and apoptosis. The greatest potentiation of cell death over PDT alone was seen when cells were pre-incubated for 24 hours with 300 μM BSO plus 10 mM 3-AT (1.62-fold potentiation) or 300 μM BSO plus 1 μM 2-ME (1.52-fold), or with a combination of all four inhibitors (300 μM BSO, 10 mM 3-AT, 1 μM 2-ME, 10 μM DDC: 1.4-fold).Because many of these inhibitors have already been clinically tested, this work facilitates future in vivo studies.

The development of emissive lanthanide complexes as structural or reactive probes to signal changes in their local chiral or ionic environment has been inhibited by the lack of understanding of correlating structural and electronic spectral information. The definition of relatively rigid enantiopure macrocyclic lanthanide complexes, whose inter- and intramolecular exchange dynamics have been defined, offers scope for remedying this situation. Chiral axially symmetric lanthanide complexes in solution give rise to large emission dissymmetry values (g(em)) in CPL spectra. The sign and magnitude of g(em) are determined by the degree of twist about the principal axis, which is predicted to be a maximum at +/-22.5 degrees, and by the site symmetry and local ligand field. In particular, the polarisability of the ligand donor atoms, especially for any axial donor, is very important. Examples of each case are discussed for structurally related cationic Eu(III) complexes.