Paramagnetic 19F chemical shift probes that respond selectively to calcium or citrate levels and signal ester hydrolysis

Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.

Development of Ln(III) Derivatives as 19F Parashift Probes

19F parashift probes with paramagnetically shifted reporter nuclei provide attractive platforms to develop molecular imaging probes. These probes enable ratiometric detection of molecular disease markers using a direct detection technique. Here, we describe a series of trivalent lanthanide (Ln(III)) complexes that are structural analogues of the clinically approved MR contrast agent (CA) ProHance to obtain LnL 19F parashift probes. We evaluated trans-gadolinium paramagnetic lanthanides compared to diamagnetic YL for 19F chemical shift and relaxation rate enhancement. The paramagnetic contribution to chemical shift (δPCS) for paramagnetic LnL exhibited either shifts to lower frequency (δPCS < 0 for TbL, DyL, and HoL) or shifts to higher frequency (δPCS > 0 for ErL, TmL, and YbL) compared to YL 19F spectroscopic signal. Zero-echo time pulse sequences achieved 56-fold sensitivity enhancement for DyL over YL, while developing probe-specific pulse sequences with fast delay times and acquisition times achieved 0.6-fold enhancement in limit of detection for DyL. DyL provides an attractive platform to develop 19F parashift probes for ratiometric detection of enzymatic activity.

19 F VT NMR: Novel Tm3+ and Ce3+ Complexes Provide New Insight into Temperature Measurement Using Molecular Sensors

In the past few decades, magnetic resonance spectroscopy (MRS) and MR imaging (MRI) have developed into a powerful non-invasive tool for medical diagnostic and therapy. Especially 19 F MR shows promising potential because of the properties of the fluorine atom and the negligible background signals in the MR spectra. The detection of temperature in a living organism is quite difficult, and usually external thermometers or fibers are used. Temperature determination via MRS needs temperature-sensitive contrast agents. This article reports first results of solvent and structural influences on the temperature sensitivity of 19 F NMR signals of chosen molecules. By using this chemical shift sensitivity, a local temperature can be determined with a high precision. Based on this preliminary study, we synthesized five metal complexes and compared the results of all variable temperature measurements. It is shown that the highest 19 F MR signal temperature dependence is detectable for a fluorine nucleus in a Tm3+ -complex.

Tuning the pH of Activation of Fluorinated Hydrazone-Based Switches─A Pathway to Versatile 19F Magnetic Resonance Imaging Contrast Agents

Molecular switches have become an area of great interest in recent years. They are explored as high-density data storage and organic diodes in molecular electronics as well as chemosensors due to their ability to undergo a transition between well-defined structures under the action of external stimuli. One of the types of such switches is hydrazones. They work by changing the configuration from E to Z under the influence of pH or light. The change in configuration is accompanied by a change in the absorption band and changes in the nuclear magnetic resonance (NMR) spectrum. In this publication, the structure-property relationship of fluorinated hydrazone switches was established. A linear relationship between the Hammett substituent constants and the pH where the switching occurs was found. Introduction of strong electron-donating groups allowed obtaining a hydrazone switch of pK_a = 6 suitable for application in ¹⁹F MRI as contrast agents.

A ratiometric 19F MR-based method for the quantification of Ca2+ using responsive paramagnetic probes

A custom made lanthanide complex with a high number of fluorine atoms was used to develop a quantitative method to assess calcium ion concentration by means of ¹⁹F chemical shift imaging.

First synthesis of orthogonally 1,7-diprotected cyclens

Six novel orthogonally 1,7-heterodiprotected cyclen derivatives have been prepared through an efficient and chromatography-free procedure.

Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex

The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a "turn-on" europium(III) emission process with a small, water-soluble lanthanide complex triggered by uranyl(VI) ions.

Fluorinated Paramagnetic Complexes: Sensitive and Responsive Probes for Magnetic Resonance Spectroscopy and Imaging

Fluorine magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) of chemical and physiological processes is becoming more widespread. The strength of this technique comes from the negligible background signal in in vivo¹⁹F MRI and the large chemical shift window of ¹⁹F that enables it to image concomitantly more than one marker. These same advantages have also been successfully exploited in the design of responsive ¹⁹F probes. Part of the recent growth of this technique can be attributed to novel designs of ¹⁹F probes with improved imaging parameters due to the incorporation of paramagnetic metal ions. In this review, we provide a description of the theories and strategies that have been employed successfully to improve the sensitivity of ¹⁹F probes with paramagnetic metal ions. The Bloch-Wangsness-Redfield theory accurately predicts how molecular parameters such as internuclear distance, geometry, rotational correlation times, as well as the nature, oxidation state, and spin state of the metal ion affect the sensitivity of the fluorine-based probes. The principles governing the design of responsive ¹⁹F probes are subsequently described in a "how to" guide format. Examples of such probes and their advantages and disadvantages are highlighted through a synopsis of the literature.

Fe(ii) and Co(ii) N-methylated CYCLEN complexes as paraSHIFT agents with large temperature dependent shifts

Several complexes of Co(ii) or Fe(ii) with 1,4,7,10-tetraazacyclododecane (CYCLEN) appended with 1,7-(6-methyl)2-picolyl groups are studied as ¹H NMR paraSHIFT agents (paramagnetic shift agents) for the registration of temperature. Two of the complexes, [Co(BMPC)]²⁺ and [Fe(BMPC)]²⁺, contain methyl groups only on the methyl picolyl pendents. Two other complexes, [Co(2MPC)]²⁺ and [Fe(2MPC)]²⁺, contain picolyl groups and also methyl groups on the macrocyclic amines. All macrocyclic complexes are in high spin form as shown by solution magnetic moments in the range of 5.0-5.9μ_BM and 5.3-5.8μ_BM for Co(ii) and Fe(ii) complexes, respectively. The ¹H NMR spectra of both of the Fe(ii) complexes and one of the Co(ii) complexes are consistent with a predominant diastereomeric form in deuterium oxide solutions. The highly shifted methyl proton resonances for [Co(2MPC)]²⁺ appear at 164 and -113 ppm for macrocycle and pendent picolyl methyls and show temperature coefficients of -0.58 ppm °C^-1 and 0.49 ppm °C^-1, respectively. Fe(ii) complexes have less shifted methyl proton resonances and smaller temperature coefficients. The ¹H resonances of [Fe(2MPC)]²⁺ appear at 105 ppm and -46 ppm with corresponding temperature coefficients (CT) of -0.29 ppm °C^-1 and 0.22 ppm °C^-1, respectively. The relatively narrow linewidths of [Fe(2MPC)]²⁺, however, produce superior CT/FWHM values of 0.44 and 0.31 °C^-1 for the N-methyl and picolyl proton resonances where FWHM is the full width at half maximum of the ¹H resonance. The crystal structure of [Co(BMPC)]Cl₂ shows a six-coordinate Co(ii) bound to the macrocyclic amines and two pendent picolyl groups. The distorted trigonal prismatic geometry of the complex resembles that of an analogous complex containing four 6-methyl-2-picolyl groups, in which only two picolyl pendents are coordinated.

Pseudocontact shifts from mobile spin labels

This paper presents a detailed analysis of the pseudocontact shift (PCS) field induced by a mobile spin label that is viewed as a probability density distribution with an associated effective magnetic susceptibility anisotropy. It is demonstrated that non-spherically symmetric density can lead to significant deviations from the commonly used point dipole approximation for the PCS. Analytical and numerical solutions are presented for the general partial differential equation that describes the non-point case. It is also demonstrated that it is possible, with some reasonable approximations, to reconstruct paramagnetic centre probability distributions from the experimental PCS data.

Fe- and Ln-DOTAm-F12 Are Effective Paramagnetic Fluorine Contrast Agents for MRI in Water and Blood

A series of fluorinated macrocyclic complexes, M-DOTAm-F12, where M is La^III, Eu^III, Gd^III, Tb^III, Dy^III, Ho^III, Er^III, Tm^III, Yb^III, and Fe^II, was synthesized, and their potential as fluorine magnetic resonance imaging (MRI) contrast agents was evaluated. The high water solubility of these complexes and the presence of a single fluorine NMR signal, two necessary parameters for in vivo MRI, are substantial advantages over currently used organic polyfluorocarbons and other reported paramagnetic ¹⁹F probes. Importantly, the sensitivity of the paramagnetic probes on a per fluorine basis is at least 1 order of magnitude higher than that of diamagnetic organic probes. This increased sensitivity is due to a substantial-up to 100-fold-decrease in the longitudinal relaxation time (T₁) of the fluorine nuclei. The shorter T₁ allows for a greater number of scans to be obtained in an equivalent time frame. The sensitivity of the fluorine probes is proportional to the T₂/T₁ ratio. In water, the optimal metal complexes for imaging applications are those containing Ho^III and Fe^II, and to a lesser extent Tm^III and Yb^III. Whereas T₁ of the lanthanide complexes are little affected by blood, the T₂ are notably shorter in blood than in water. The sensitivity of Ln-DOTAm-F12 complexes is lower in blood than in water, such that the most sensitive complex in water, Ho^III-DOTAm-F12, could not be detected in blood. Tm^III yielded the most sensitive lanthanide fluorine probe in blood. Notably, the relaxation times of the fluorine nuclei of Fe^II-DOTAm-F12 are similar in water and in blood. That complex has the highest T₂/T₁ ratio (0.57) and the lowest limit of detection (300 μM) in blood. The combination of high water solubility, single fluorine signal, and high T₂/T₁ of M-DOTAm-F12 facilitates the acquisition of three-dimensional magnetic resonance images.

Hyperpolarized Multi-Metal 13C-Sensors for Magnetic Resonance Imaging

We introduce hyperpolarizable ¹³C-labeled probes that identify multiple biologically important divalent metals via metal-specific chemical shifts. These features enable NMR measurements of calcium concentrations in human serum in the presence of magnesium. In addition, signal enhancement through dynamic nuclear polarization (DNP) increases the sensitivity of metal detection to afford measuring micromolar concentrations of calcium as well as simultaneous multi-metal detection by chemical shift imaging. The hyperpolarizable ¹³C-MRI sensors presented here enable sensitive NMR measurements and MR imaging of multiple divalent metals in opaque biological samples.

(19)F Magnetic Resonance Imaging Signals from Peptide Amphiphile Nanostructures Are Strongly Affected by Their Shape

Magnetic resonance imaging (MRI) is a noninvasive imaging modality that provides excellent spatial and temporal resolution. The most commonly used MR probes face significant challenges originating from the endogenous (1)H background signal of water. In contrast, fluorine MRI ((19)F MRI) allows quantitative probe imaging with zero background signal. Probes with high fluorine content are required for high sensitivity, suggesting nanoscale supramolecular assemblies containing (19)F probes offer a potentially useful strategy for optimum imaging as a result of improved payload. We report here on supramolecular nanostructures formed by fluorinated peptide amphiphiles containing either glutamic acid or lysine residues in their sequence. We identified molecules that form aggregates in water which transition from cylindrical to ribbon-like shape as pH increased from 4.5 to 8.0. Interestingly, we found that ribbon-like nanostructures had reduced magnetic resonance signal, whereas their cylindrical counterparts exhibited strong signals. We attribute this drastic difference to the greater mobility of fluorinated tails in the hydrophobic compartment of cylindrical nanostructures compared to lower mobility in ribbon-like assemblies. This discovery identifies a strategy to design supramolecular, self-assembling contrast agents for (19)F MRI that can spatially map physiologically relevant changes in pH using changes in morphology.

Nickel(ii) complexes of N-CH2CF3 cyclam derivatives as contrast agents for 19F magnetic resonance imaging

Nickel(ii) complexes of N,N′′-2,2,2-trifluoroethyl cyclam derivatives show significant ¹⁹F NMR relaxation rate enhancement useful for ¹⁹F MRI imaging.

Enhancing magnetic resonance imaging with contrast agents for ultra-high field strengths

Contrast agents are diagnostic tools that often complement magnetic resonance imaging. At ultra-high field strengths (≥7 T), magnetic resonance imaging is capable of generating desirable high signal-to-noise ratios, but clinically available contrast agents are less effective at ultra-high field strengths relative to lower fields. This gap in effectiveness demands the development of contrast agents for ultra-high field strengths. In this minireview, we summarize contrast agents reported during the last three years that focused on ultra-high field strengths.

Single (19)F probe for simultaneous detection of multiple metal ions using miCEST MRI

The local presence and concentration of metal ions in biological systems has been extensively studied ex vivo using fluorescent dyes. However, the detection of multiple metal ions in vivo remains a major challenge. We present a magnetic resonance imaging (MRI)-based method for noninvasive detection of specific ions that may be coexisting, using the tetrafluorinated derivative of the BAPTA (TF-BAPTA) chelate as a ¹⁹F chelate analogue of existing optical dyes. Taking advantage of the difference in the ion-specific ¹⁹F nuclear magnetic resonance (NMR) chemical shift offset (Δω) values between the ion-bound and free TF-BAPTA, we exploited the dynamic exchange between ion-bound and free TF-BAPTA to obtain MRI contrast with multi-ion chemical exchange saturation transfer (miCEST). We demonstrate that TF-BAPTA as a prototype single ¹⁹F probe can be used to separately visualize mixed Zn²⁺ and Fe²⁺ ions in a specific and simultaneous fashion, without interference from potential competitive ions.

Rapid synthesis of 1,7-bis(t-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (DO2A-t-Bu ester)

A three-step route was used to synthesize 1,7-bis(t-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (DO2A-t-Bu ester) from 1,4,7,10-tetraazacyclododecane (cyclen). The overall time of reaction was reduced from a combined ~56 h to 2.3 h with an overall yield comparable to previously reported methods.

¹⁹F NMR indicator displacement assay using a synthetic receptor with appended paramagnetic relaxation agent

An admixture of zinc(II)-bis(dipicolylamine) receptor with covalently attached paramagnetic relaxation agent and fluorine-labeled phosphate indicator enables (19)F NMR detection of phosphorylated analytes with amplified switched-on signal intensity.

An adhesive (19)F MRI chemical probe allows signal off-to-on-type molecular sensing in a biological environment

We report a new strategy for designing a signal off-to-on-type (19)F MRI chemical probe that operates in biological environments. The present strategy is based on the control of adherence of a (19)F MRI chemical probe to certain blood proteins, accompanied by a change in transverse relaxation time of (19)F nuclei.

Dual-frequency calcium-responsive MRI agents

Responsive or smart magnetic resonance imaging (MRI) contrast agents are molecular sensors that alter the MRI signal upon changes in a particular parameter in their microenvironment. Consequently, they could be exploited for visualization of various biochemical events that take place at molecular and cellular levels. In this study, a set of dual-frequency calcium-responsive MRI agents are reported. These are paramagnetic, fluorine-containing complexes that produce remarkably high MRI signal changes at the (1)H and (19)F frequencies at varying Ca(2+) concentrations. The nature of the processes triggered by Ca(2+) was revealed, allowing a better understanding of these complex systems and their further improvement. The findings indicate that these double-frequency tracers hold great promise for development of novel functional MRI methods.

19F magnetic resonance probes for live-cell detection of peroxynitrite using an oxidative decarbonylation reaction

We have developed ¹⁹F NMR probes for non-invasive monitoring of ONOO⁻ produced by A549 cells using novel oxidative decarbonylation chemistry.

Fluorine NMR reporter for phosphate anions

A fluorine-labelled zinc(II)-dipicolylamine coordination complex reports the presence of phosphate anions in aqueous solution, especially pyrophosphate and ADP, and is used to monitor the enzymatic hydrolysis of ATP.