A Model for the Rapid Assessment of Solution Structures for 24-Atom Macrocycles: The Impact of ß-Branched Amino Acids on Conformation.

Experiment and computation are used to develop a model to rapidly predict solution structures of macrocycles sharing the same Murcko framework. These 24-atom triazine macrocycles result from the quantitative dimerization of identical monomers presenting a hydrazine group and an acetal tethered to an amino acid linker. Monomers comprising glycine and the ß-branched amino acids threonine, valine, and isoleucine yield macrocycles G-G, T-T, V-V, and I-I, respectively. Elements common to all members of the framework include the efficiency of macrocyclization (quantitative), the solution- and solid-state structures (folded), the site of protonation (opposite the auxiliary dimethylamine group), the geometry of the hydrazone (E), the C2 symmetry of the subunits (conserved), and the rotamer state adopted. In aggregate, the data reveal metrics predictive of the three-dimensional solution structure that derive from the fingerprint region of the 1D 1H spectrum and a network of rOes from a single resonance. The metrics also afford delineation of more nuanced structural features that allow subpopulations to be identified among the members of the framework. Well-tempered metadynamics provides free energy surfaces and population distributions of these macrocycles. The areas of the free energy surface decrease with increasing steric bulk (G-G > V-V ∼ T-T > I-I). In addition, the surfaces are increasingly isoenergetic with decreasing steric bulk (G-G > V-V ∼ T-T > I-I).

Hydrogen Peroxide Disproportionation Activity Is Sensitive to Pyridine Substitutions on Manganese Catalysts Derived from 12-Membered Tetra-Aza Macrocyclic Ligands.

The abundance of manganese in nature and versatility to access different oxidation states have made manganese complexes attractive as catalysts for oxidation reactions in both biology and industry. Macrocyclic ligands offer the advantage of substantially controlling the reactivity of the manganese center through electronic tuning and steric constraint. Inspired by the manganese catalase enzyme, a biological catalyst for the disproportionation of H2O2 into water and O2, the work herein employs 12-membered tetra-aza macrocyclic ligands to study how the inclusion of and substitution to the pyridine ring on the macrocyclic ligand scaffold impacts the reactivity of the manganese complex as a H2O2 disproportionation catalyst. Synthesis and isolation of the manganese complexes was validated by characterization using UV-vis spectroscopy, SC-XRD, and cyclic voltammetry. Potentiometric titrations were used to study the ligand basicity as well as the thermodynamic equilibrium with Mn(II). Manganese complexes were also produced in situ and characterized using electrochemistry for comparison to the isolated species. Results from these studies and H2O2 reactivity showed a remarkable difference among the ligands studied, revealing instead a distinction in the reactivity regarding the number of pyridine rings within the scaffold. Moreover, electron-donating groups on the 4-position of the pyridine ring enhanced the reactivity of the manganese center for H2O2 disproportionation, demonstrating a handle for control of oxidation reactions using the pyridinophane macrocycle.

Increased Efficiency of a Functional SOD Mimic Achieved with Pyridine Modification on a Pyclen-Based Copper(II) Complex.

A series of Cu(II) complexes with the formula [CuRPyN3]2+ varying in substitution on the pyridine ring were investigated as superoxide dismutase (SOD) mimics to identify the most efficient reaction rates produced by a synthetic, water-soluble copper-based SOD mimic reported to date. The resulting Cu(II) complexes were characterized by X-ray diffraction analysis, UV-visible spectroscopy, cyclic voltammetry, and metal-binding (log ß) affinities. Unique to this approach, the modifications to the pyridine ring of the PyN3 parent system tune the redox potential while exhibiting high binding stabilities without changing the coordination environment of the metal complex within the PyN3 family of ligands. We were able to adjust in parallel the binding stability and the SOD activity without compromising on either through simple modification of the pyridine ring on the ligand system. This goldilocks effect of high metal stabilities and high SOD activity reveals the potential of this system to be explored in therapeutics. These results serve as a guide for factors that can be modified in metal complexes using pyridine substitutions for PyN3, which can be incorporated into a range of applications moving forward.

Manganese (III/IV) µ-Oxo Dimers and Manganese (III) Monomers with Tetraaza Macrocyclic Ligands and Historically Relevant Open-Chain Ligands.

The oxygen-evolving complex (OEC) located in photosystem II (PSII) of green plants is one of the best-known examples of a manganese-containing enzyme in nature, but it is also used in a range of other biological processes. OEC models incorporate two multi-dentate nitrogen-containing ligands coordinated to a bis-µ-oxo Mn(III,IV) core. Open-chain ligands were the initial scaffold used for biomimetic studies, but their macrocyclic counterparts have proven to be particularly appropriate due to their enhanced stability. Dimer and monomer complexes with such ligands have shown to be useful for a wide range of applications, which will be reviewed herein. The purpose of this review is to state with some clarity the different spectroscopic and structural characteristics of the Mn complexes formed with tetraaza macrocyclic ligands both in solution and solid-state that allow the reader to successfully identified the species involved when dealing with similar complexes of Mn.

pH-dependent concatenation of an inorganic complex results in solid state helix formation.

Growth of the library of tetraaza macrocyclic pyridinophane ligands is a result of the potential to treat neurodegenerative diseases by binding unregulated redox active metal-ions, scavenging radicals, and reducing oxidative stress. As part of this work, the copper complex of OH PyN 3 Cu (3,6,9,15-tetraazabicyclo[9.3.1]penta-deca-1(15),11,13-trien-13-ol) was previously identified as a discrete molecule in the solid state when isolated at lower pH values. However, here we report that OH PyN 3 Cu forms a helical structure upon crystallization around pH 6.5. Several properties of the ligand and complex were evaluated to understand the driving forces that led to the concatenation and formation of this solid-state helix. DFT studies along with a comparison of keto/enol tautomerization stability and bond lengths were used to determine the keto-character of the C=O within each subunit. This pH dependent keto-enol tautomerization is responsible for the solid state intermolecular C=O···Cu bonds observed in this metallohelix (Cu1 H ) when produced around pH 6.5. Perchlorate templating that occurs through hydrogen bonding between perchlorate counter ions and each Cu1 H unit is the primary driving factor for the twist that leads to the helix structure. Cu1 H does not exhibit the typical factors that stabilize the formation of helices, such as intra-strand hydrogen bonding or π-stacking. The helix structure further highlights the diversity of inorganic metallohelices and demonstrates the importance of tautomerization and pH that occurs with the pyridinophane ligand used in this study. To our knowledge and although these phenomenon have been observed individually, this is the first example of a pH dependent keto-enol tautomerization in an azamacrocycle being the driving force for the formation of a metallohelix solid state structure and is a particularly unique observation for pyridinophane complexes.

Graphene quantum dot formulation for cancer imaging and redox-based drug delivery.

This work develops a new multifunctional biocompatible anticancer nanoformulation to provide targeted image-guided cancer-selective therapeutics. It consists of three active covalently bound components: (1) biocompatible nitrogen-doped graphene quantum dots (GQDs) as a multifunctional delivery and imaging platform, (2) hyaluronic acid (HA) unit targeted to the CD44 receptors on a variety of cancer cells, and (3) oxidative stress-based cancer-selective ferrocene (Fc) therapeutic. The biocompatible GQD platform synthesized from glucosamine exhibits high-yield intrinsic fluorescence. It is utilized for tracking Fc-GQD-HA formulation in vitro indicating internalization enhancement in HeLa cells targeted by the HA over non-cancer HEK-293 cells not overexpressing CD44 receptor. Fc-GQD-HA, non-toxic at 1 mg/mL to HEK-293 cells, induces cytotoxic response in HeLa enhanced over time, while therapeutic ROS generation by Fc-GQD-HA is ~3 times greater than that of Fc alone. This outlines the targeted delivery, imaging, and cancer-specific treatment capabilities of the new Fc-GQD-HA formulation enabling desired cancer-focused nanotherapeutic approach.

Synthesis of 12-Membered Tetra-aza Macrocyclic Pyridinophanes Bearing Electron-Withdrawing Groups.

The number of substituted pyridine pyridinophanes found in the literature is limited due to challenges associated with 12-membered macrocycle and modified pyridine synthesis. Most notably, the electrophilic character at the 4-position of pyridine in pyridinophanes presents a unique challenge for introducing electrophilic chemical groups. Likewise, of the few reported, most substituted pyridine pyridinophanes in the literature are limited to electron-donating functionalities. Herein, new synthetic strategies for four new macrocycles bearing the electron-withdrawing groups CN, Cl, NO2, and CF3 are introduced. Potentiometric titrations were used to determine the protonation constants of the new pyridinophanes. Further, the influence of such modifications on the chemical behavior is predicted by comparing the potentiometric results to previously reported systems. X-ray diffraction analysis of the 4-Cl substituted species and its Cu(II) complex are also described to demonstrate the metal binding nature of these ligands. DFT analysis is used to support the experimental findings through energy calculations and ESP maps. These new molecules serve as a foundation to access a range of new pyridinophane small molecules and applications in future work.

Manganese Complex of a Rigidified 15-Membered Macrocycle: A Comprehensive Study.

Owing to the increasing importance of manganese(II) complexes in the field of magnetic resonance imaging (MRI), large efforts have been devoted to find an appropriate ligand for Mn(II) ion encapsulation by providing balance between the seemingly contradictory requirements (i.e., thermodynamic stability and kinetic inertness vs low ligand denticity enabling water molecule(s) to be coordinated in its metal center). Among these ligands, a large number of pyridine or pyridol based open-chain and macrocyclic chelators have been investigated so far. As a next step in the development of these chelators, 15-pyN3O2Ph and its transition metal complexes were synthesized and characterized using established methods. The 15-pyN3O2Ph ligand incorporates both pyridine and ortho-phenylene units to decrease ligand flexibility. The thermodynamic properties, protonation and stability constants, were determined using pH-potentiometry; the solid-state structures of two protonation states of the free ligand and its manganese complex were obtained by single crystal X-ray diffractometry. The results show a seven-coordinate metal center with two water molecules in the first coordination sphere. The longitudinal relaxivity of [Mn(15-pyN3O2Ph)]2+ was found to be 5.16 mM-1 s-1 at 0.49 T (298 K). Furthermore, the r2p value of 11.72 mM-1 s-1 (0.49 T), which is doubled at 1.41 T field, suggests that design of this Mn(II) complex does achieve some characteristics required for contrast imaging. In addition, 17O NMR measurements were performed in order to access the microscopic parameters governing this key feature (e.g., water exchange rate). Finally, manganese complexes of ligands with analogous polyaza macrocyclic scaffold have been investigated as low molecular weight Mn(CAT) mimics. Here, we report the H2O2 disproportionation study of [Mn(15-pyN3O2Ph)]2+ to demonstrate the versatility of this ligand scaffold as well.

Enhancement of the Antioxidant Activity and Neurotherapeutic Features through Pyridol Addition to Tetraazamacrocyclic Molecules.

Alzheimer's and other neurodegenerative diseases are chronic conditions affecting millions of individuals worldwide. Oxidative stress is a consistent component described in the development of many neurodegenerative diseases. Therefore, innovative strategies to develop drug candidates that overcome oxidative stress in the brain are needed. To target these challenges, a new, water-soluble 12-membered tetraaza macrocyclic pyridinophane L4 was designed and produced using a building-block approach. Potentiometric data show that the neutral species of L4 provides interesting zwitterionic behavior at physiological pH, akin to amino acids, and a nearly ideal isoelectric point of 7.3. The copper(II) complex of L4 was evaluated by X-ray diffraction and cyclic voltammetry to show the potential modes of antioxidant activity derived, which was also demonstrated by 2,2-diphenyl-1-picrylhydrazyl and coumarin carboxylic acid antioxidant assays. L4 was shown to have dramatically enhanced antioxidant activity and increased biological compatibility compared to parent molecules reported previously. L4 attenuated hydrogen peroxide (H2O2)-induced cell viability loss more efficiently than precursor molecules in the mouse hippocampal HT-22 cell model. L4 also showed potent (fM) level protection against H2O2 cell death in a BV2 microglial cell culture. Western blot studies indicated that L4 enhanced the cellular antioxidant defense capacity via Nrf2 signaling activation as well. Moreover, a low-cost analysis and high metabolic stability in phase I and II models were observed. These encouraging results show how the rational design of lead compounds is a suitable strategy for the development of treatments for neurodegenerative diseases where oxidative stress plays a substantial role.

Increase of Direct C-C Coupling Reaction Yield by Identifying Structural and Electronic Properties of High-Spin Iron Tetra-azamacrocyclic Complexes.

Macrocyclic ligands have been explored extensively as scaffolds for transition metal catalysts for oxygen and hydrogen atom transfer reactions. C-C reactions facilitated using earth abundant metals bound to macrocyclic ligands have not been well-understood but could be a green alternative to replacing the current expensive and toxic precious metal systems most commonly used for these processes. Therefore, the yields from direct Suzuki-Miyaura C-C coupling of phenylboronic acid and pyrrole to produce 2-phenylpyrrole facilitated by eight high-spin iron complexes ([Fe3+L1(Cl)2]+, [Fe3+L4(Cl)2]+, [Fe2+L5(Cl)]+, [Fe2+L6(Cl)2], [Fe3+L7(Cl)2]+, [Fe3+L8(Cl)2]+, [Fe2+L9(Cl)]+, and [Fe2+L10(Cl)]+) were compared to identify the effect of structural and electronic properties on catalytic efficiency. Specifically, catalyst complexes were compared to evaluate the effect of five properties on catalyst reaction yields: (1) the coordination requirements of the catalyst, (2) redox half-potential of each complex, (3) topological constraint/rigidity, (4) N atom modification(s) increasing oxidative stability of the complex, and (5) geometric parameters. The need for two labile cis-coordination sites was confirmed based on a 42% decrease in catalytic reaction yield observed when complexes containing pentadentate ligands were used in place of complexes with tetradentate ligands. A strong correlation between iron(III/II) redox potential and catalytic reaction yields was also observed, with [Fe2+L6(Cl)2] providing the highest yield (81%, -405 mV). A Lorentzian fitting of redox potential versus yields predicts that these catalysts can undergo more fine-tuning to further increase yields. Interestingly, the remaining properties explored did not show a direct, strong relationship to catalytic reaction yields. Altogether, these results show that modifications to the ligand scaffold using fundamental concepts of inorganic coordination chemistry can be used to control the catalytic activity of macrocyclic iron complexes by controlling redox chemistry of the iron center. Furthermore, the data provide direction for the design of improved catalysts for this reaction and strategies to understand the impact of a ligand scaffold on catalytic activity of other reactions.

Crystallographic Characterization and Non-Innocent Redox Activity of the Glycine Modified DOTA Scaffold and Its Impact on EuIII Electrochemistry.

EuDOTA-glycine derivatives have been explored as alternatives to typical gadolinium-containing complexes for MRI agents used in diagnostic imaging. Different imaging modalities can be accessed (T 1 or PARACEST) dependent on the oxidation state of the europium ion. Throughout the past 30 years, there have been significant manipulations and additions made to the DOTA scaffold; yet, characterizations related to electrochemistry and structure determined through XRD analysis have not been fully analyzed. In this work, electrochemical analysis using cyclic voltammetry was carried out on EuDOTA derivatives, including the free ligand DOTAGly4 (4) and the complexes. Effects of glycinate substitution on the DOTA scaffold, specifically, ligand interactions with the glassy carbon electrode were observed. A range of electrochemical investigations were carried out to show that increased glycinate substitution led to increased interaction with the electrode surface, thus implicating a new factor to consider when evaluating the electrochemistry of glycinate substituted ligands. In addition, the solid-state structure of EuDOTAGly4 (Eu4) was determined by X-ray diffraction and a brief analysis is presented compared to known Ln3+ structures found within literature. The Eu4 complex crystalizes in a rare polymer type arrangement via bridging side-arms between adjacent complexes.

Electrochemical investigation of the Eu3+/2+ redox couple in complexes with variable numbers of glycinamide and acetate pendant arms.

The Eu3+/2+ redox couple provides a convenient design platform for responsive pO2 sensors for magnetic resonance imaging (MRI). Specifically the Eu2+ ion provides T1w contrast enhancement under hypoxic conditions in tissues, whereas, under normoxia, the Eu3+ ion can produce contrast from chemical exchange saturation transfer in MRI. The oxidative stability of the Eu3+/2+ redox couple for a series of tetraaza macrocyclic complexes was investigated in this work using cyclic voltammetry. A series of Eu-containing cyclen-based macrocyclic complexes revealed positive shifts in the Eu3+/2+ redox potentials with each replacement of a carboxylate coordinating arm of the ligand scaffold with glycinamide pendant arms. The data obtained reveal that the complex containing four glycinamide coordinating pendant arms has the highest oxidative stability of the series investigated.

Structural, spectral, and electrochemical properties of nickel(II), copper(II), and zinc(II) complexes containing 12-membered pyridine- and pyridol-based tetra-aza macrocycles.

The structural, electronic, and electrochemical properties of a series of novel 12-membered pyridine- and pyridol-based tetra-aza transition-metal (Ni, Cu, Zn) complexes {[M(II)(L1)Cl](ClO4), [M(II)(L2)Cl](ClO4), and [M(II)(L3)Cl](ClO4)} are described (L1 (Pyclen) = 1,4,7,10-tetra-aza-2,6-pyridinophane; L2 = 3,6,9,15-tetraazabicyclo[9.3.1]penta-deca-1(15),11,13-trien-13-ol; L3 = 3,6,9,15-tetra-azabicyclo[9.3.1]penta-deca- 1(15),11,13-trien-12-ol). The subtle variations in the chemical properties of these complexes were investigated using X-ray crystallography, UV-vis and NMR spectroscopy, and cyclic voltammetry. In the solid-state, the Ni(II) complexes adopt a unique bimetallic and cis-octahedral (µ-Cl)2 coordination sphere, and the electronic studies provide further evidence for the existence of a six-coordinate Ni(II) species in solution. The pyridol-based Cu(II) and Zn(II) complexes contain five-coordinate (N4Cl) geometries in the solid-state, in which the four N-donor atoms are not coplanar. Hydroxylation of the pyridine ring was found to increase the amount of π electronic charge density residing throughout the aromatic system of the ligand backbone, increase the strength of the M-Cl and M-N (pyridine) basal x- and y-plane interactions, and decrease the axial M-N bonding interaction. The electrochemical studies demonstrate that (i) the Lewis-acidity of the metal center systematically decreases across the series {[Cu(II)(L3)Cl](ClO4) > [Cu(II)(L1)Cl](ClO4) > [Cu(II)(L2)Cl](ClO4)}, and (ii) the aromatic backbones allow access to both Cu(I) and Cu(III) species in solution. Overall, the experimental findings are consistent with the idea that p-hydroxylation enhances the Lewis-basicity of pyridine-based macrocycle and decreases the Lewis-acidity of the metal-ion, while m-hydroxylation decreases the electron-donating ability of the backbone and increases the metal-ion Lewis-acidity.

Impact of Indole Inclusion in the Design of Multi-Tactical Metal-Binding Tetra-Aza Macrocycles that Target the Molecular Features of Neurodegeneration.

Numerous small molecules have been studied for their ability to counteract oxidative stress, a key contributor to neurodegenerative diseases such as Alzheimer's. Despite these efforts, the pharmacological properties and structure-activity relationships of these compounds remain insufficiently understood, yet they are critical in evaluating a drug molecule's therapeutic potential. A modified tetra-aza macrocycle has demonstrated strong antioxidant activity through various mechanisms; however, its limited permeability presents challenges for advanced formulation studies. To enhance permeability while preserving the beneficial reactivity of the parent molecule, two synthetic modifications involving indole functionality were explored and compared to modifications using methyl groups alone. New synthetic strategies were developed to produce the indole-containing molecules, which were characterized by 1D/2D NMR techniques. Isoelectric points, metal binding, and radical scavenging activity were determined to validate that the reactivity of the parent molecules was retained. The permeability of all molecules explored was improved. Protection against oxidative stress through activation of the Nrf2 pathway was demonstrated for molecules containing indoles in cellular models by measuring ROS levels upon treatment and mRNA levels of HO-1 and Nrf2. In contrast, no protection or Nrf2 activation was observed with the methylation of the O- or N atom. These results suggest that while alkylation improves permeability overall, concomitant antioxidant protection and positive permeability are achieved with the indole congeners alone.

Synthesis and characterization of two piperazine containing macrocycles and their transition metal complexes.

Rigidification of the ligand scaffolds has been a particular mechanism of interest employed to achieve properties suitable for MRI contrast, catalysis, or other applications of metal complexes. Towards the goal of targeting a 15-anePyN5Pip type ligand, a serendipitous isolation of a 30-anePy2N10Pip2 aza-macrocycle was achieved, instead. X-ray diffraction and determination of pKa events were carried out and compared to 17-anePyN5Pip. Furthermore, the X-ray diffraction of the Cu(II) and Zn(II) complexes of 17-anePyN5Pip was achieved and compared to previous reports of other first-row transition metal derivatives of this ligand. Determination of the log ß with both 30-anePy2N10Pip2 and 17-anePyN5Pip with the divalent MnZn metal-ion series was used to demonstrate the impact that the piperazine ring plays compared to other, less rigid macrocycles reported to date.

Pyridine modifications regulate the electronics and reactivity of Fe-pyridinophane complexes.

12-Membered pyridinophanes are the focus of many studies as biological mimics, chelators, and catalytic precursors. Therefore, the desire to tune the reactivity of pyridinophanes to better control the applications of derivative metal complexes has inspired many structure-activity relationship studies. However, the separation of structural versus electronic changes imparted by ligand modification has made these structure-activity relationship studies of transition metal catalysts challenging to define. In this work we show that 4-substitution of the pyridine ring in 12-membered tetra-aza pyridinophanes successfully provides a regulatory handle on the electronic properties of the metal center and, therefore, the catalytic C-C coupling activity of the respective iron complexes. The C-C coupling reaction catalyzed by Fe(L1-L6) provides a range of yields (32-58%) that directly correlate with iron redox potentials (ΔE1/2 = 152 mV) and metal binding constants (Δlog ß = 3.45), while the geometry of the complexes was virtually indistinguishable. These are the first results to definitively show the redox potential and metal binding as independent properties from the coordination chemistry in one ligand series. Adjustments to these chemical properties were then shown to provide a regulatory handle for the C-C coupling reactivity tuned via pyridine substitution in pyridinophanes.

Europium(III) DOTA-tetraamide complexes as redox-active MRI sensors.

PARACEST redox sensors containing the NAD(+)/NADH mimic N-methylquinolinium moiety as a redox-active functional group have been designed and synthesized. The Eu(3+) complex with two quinolinium moieties was nearly completely CEST-silent in the oxidized form but was "turned on" upon reduction with ß-NADH. The CEST effect of the Eu(3+) complex containing only one quinolinium group was much less redox-responsive but showed an unexpected sensitivity to pH in the physiologically relevant pH range.

A macrocyclic molecule with multiple antioxidative activities protects the lens from oxidative damage.

Growing evidence links oxidative stress to the development of a cataract and other diseases of the eye. Treatments for lens-derived diseases are still elusive outside of the standard surgical interventions, which still carry risks today. Therefore, a potential drug molecule OHPy2N2 was explored for the ability to target multiple components of oxidative stress in the lens to prevent cataract formation. Several pathways were identified. Here we show that the OHPy2N2 molecule activates innate catalytic mechanisms in primary lens epithelial cells to prevent damage induced by oxidative stress. This protection was linked to the upregulation of Nuclear factor erythroid-2-related factor 2 and downstream antioxidant enzyme for glutathione-dependent glutaredoxins, based on Western Blot methods. The anti-ferroptotic potential was established by showing that OHPy2N2 increases levels of glutathione peroxidase, decreases lipid peroxidation, and readily binds iron (II) and (III). The bioenergetics pathway, which has been shown to be negatively impacted in many diseases involving oxidative stress, was also enhanced as evidence by increased levels of Adenosine triphosphate product when the lens epithelial cells were co-incubated with OHPy2N2. Lastly, OHPy2N2 was also found to prevent oxidative stress-induced lens opacity in an ex vivo organ culture model. Overall, these results show that there are multiple pathways that the OHPy2N2 has the ability to impact to promote natural mechanisms within cells to protect against chronic oxidative stress in the eye.

T2 exchange agents: a new class of paramagnetic MRI contrast agent that shortens water T2 by chemical exchange rather than relaxation.

Exchange of water molecules between the frequency-shifted inner-sphere of a paramagnetic lanthanide ion and aqueous solvent can shorten the T(2) of bulk water protons. The magnitude of the line-broadening T(2) exchange (T(2exch)) is determined by the lanthanide concentration, the chemical shift of the exchanging water molecule, and the rate of water exchange between the two pools. A large T(2exch) contribution to the water linewidth was initially observed in experiments involving Eu(3+)-based paramagnetic chemical exchange saturation transfer agents in vivo at 9.4 T. Further in vitro and in vivo experiments using six different Eu(3+) complexes having water exchange rates ranging from zero (no exchange) to 5 × 10(6) s(-1) (fast exchange) were performed. The results showed that the exchange relaxivity (r(2exch)) is small for complexes having either very fast or very slow exchange, but reaches a well-defined maximum for complexes with intermediate water exchange rates. These experimental results were verified by Bloch simulations for two site exchange. This new class of T(2exch) agent could prove useful in the design of responsive MRI contrast agents for molecular imaging of biological processes.

Europium(III) DOTA-derivatives having ketone donor pendant arms display dramatically slower water exchange.

A series of new 1,4,7,10-tetraazacyclododecane-derivatives having a combination of amide and ketone donor groups as side-arms were prepared, and their complexes with europium(III) studied in detail by high resolution NMR spectroscopy. The chemical shift of the Eu(3+)-bound water resonance, the chemical exchange saturation transfer (CEST) characteristics of the complexes, and the bound water residence lifetimes (τ(m)) were found to vary dramatically with the chemical structure of the side-arms. Substitution of ketone oxygen donor atoms for amide oxygen donor atoms resulted in an increase in residence water lifetimes (τ(m)) and a decrease in chemical shift of the Eu(3+)-bound water molecule (Δω). These experimental results along with density functional theory (DFT) calculations demonstrate that introduction of weakly donating oxygen atoms in these complexes results in a much weaker ligand field, more positive charge on the Eu(3+) ion, and an increased water residence lifetime as expected for a dissociative mechanism. These results provide new insights into the design of paramagnetic CEST agents with even slower water exchange kinetics that will make them more efficient for in vivo imaging applications.