Paramagnetic Agent Research Articles

Localisation of the Antimicrobial Peptide Maculatin 1.1 in Lipid Bilayers using Solid-State NMR

Antimicrobial peptides that target membranes are an attractive alternative to classic antibiotics, since they do not require internalization nor target a specific stereo-structure, thus limiting development of bacterial resistance. Their mode of action involves the disruption of lipid membranes, leading to growth inhibition and ultimately death of the bacteria. However, the molecular details of the killing mechanism and, more particularly, the difference in potency observed between different bacterial strains, remain unclear. Structural information is crucial for defining the molecular mechanism by which these peptides recognize and interact with a particular lipid membrane. Solid-state NMR is a non-invasive tool that allows study of the structural details of lipid-peptide interactions. Explicitly, we have combined 2H, 31P, {19F}13C and {31P}13C REDOR solid-state NMR experiments to obtain the localisation of the antimicrobial peptide maculatin 1.1 (Mac1) in anionic lipid bilayers (PC/PG 3:1). Mac1 showed reduced dynamics of d54-DMPG acyl chain compared to d54-DMPC. 31P NMR experiments also showed more pronounced effect on DMPG than neutral DMPC headgroups. Penetration depth of the peptide was further probed by REDOR experiments where the distance between the 13C labelled peptide and the acyl chain mono-fluorinated DPPC lipid or the phosphorous headgroup was measured. Furthermore, solution NMR using isotropic bicelles with similar lipid composition showed a helical peptide structure for Mac1 and addition of paramagnetic agents confirmed a transmembrane insertion. Overall, Mac1 showed a greater affinity for the anionic DMPG, recruiting the lipid while spaning the bilayer in a transmembrane orientation.

Translational theranostic methodology for diagnostic imaging and the concomitant treatment of malignant solid tumors

Pathologic hyperpermeability exists in the spectrum of disease states, including neuro-ischemic, neuro-inflammatory and neuro-oncological. To-date the characterization of disease pathology with T 1 -weighted quantitative dynamic contrast-enhanced magnetic resonance imaging has relied on the study of modeled microvascular parameters such as diseased tissue transvascular flow rates of small molecule paramagnetic imaging agents with short plasma half-lives, based on which it is difficult to assess the specific nature of the disease state. Another type of T 1 -weighted quantitative dynamic contrast-enhanced magnetic resonance imaging involves the use of paramagnetic heavy metal-chelated dendrimer nanoparticles that possess longer plasma half-lives with pre-determined molar relaxivities to quantitatively image concentration of macromolecular contrast agent accumulation over time in hyperpermeable pathology such as solid tumor tissue with the important advantage of concomitantly treating the pathology, which, in recent years, has been shown to be possible with the use of optimally-sized theranostic dendrimer nanoparticles in the 7 to 10 nanometer size range that are functionalized biocompatibly with a small molecule therapeutic. In this focused review, this translational theranostic methodology for quantitative dynamic contrast-enhanced magnetic resonance imaging and the concomitant treatment of malignant solid tumors with optimally designed theranostic nanoparticles within the 7 to 10 nanometer size range is discussed in context of fine-tuning its suitability for the study and treatment of other disease states with pathologic hyperpermeability and without.

Abstract 215: Tracking transplanted cells with paramagnetic fluorinated nanoemulsions

Abstract Visualization of distinct cell populations in vivo is one of the most formidable challenges in biomedical sciences. Clinical translation of cutting-edge therapies that involve administration of live immunotherapeutic or stem cells can benefit from understanding the fate of injected cells in vivo. Magnetic resonance imaging (MRI) is emerging as a prime method for non-invasive, longitudinal cell tracking. Fluorine-19 MRI (19F MRI) involves the use of non-radioactive 19F label in the form of biologically inert, cytocompatible perfluorocarbons (PFC). Immediately prior to injection, cells of interest are labeled ex vivo with water-based PFC nanoemulsions. 19F MRI yields background-free images of labeled cells in an anatomical context provided by conventional 1H MRI. Both datasets are acquired on the same scanner in one imaging session. This technology has recently been used to detect immunotherapeutic dendritic cells delivered to colorectal adenocarcinoma patients. We sought to enhance sensitivity and versatility of 19F MRI by using paramagnetic relaxation agents. Covalent modification of clinically relevant PFCs with metal-binding ligands enabled stable and uniform incorporation of gadolinium and iron in the fluorous phase of nanoemulsions. Paramagnetic relaxation enhancement resulted in up to 50-fold reduction in 19F longitudinal relaxation time (T1) compared to state-of-the-art commercial tracers. The ultralow 19F T1 of our paramagnetic tracers enabled in vivo cell tracking with rapid, three-dimensional, zero time-to-echo (ZTE) 19F MRI pulse sequences in mice bearing subcutaneous allografts of 19F-labeled GL261 cells. Moreover, the high local concentration of paramagnetic metals imparted negative contrast in T2*-weighted 1H images. Combination of the new agents with conventional 19F tracers comprises a previously unavailable toolbox of 19F contrast agents, selectively detectable by appropriate acquisition methods. This enables “multicolor” 19F MRI tracking of multiple cell populations, e.g., for monitoring the interaction of host immune cells with injected therapeutic cells or cancer xenografts. Overall, the design of biocompatible, fast-relaxing 19F tracers addresses the issue of modest sensitivity of 19F MRI and reduces the barriers to widespread adoption of this powerful imaging technique. Citation Format: Alexander A. Kislukhin, Hongyan Xu, Stephen R. Adams, Kazim Narsinh, Roger Y. Tsien, Eric T. Ahrens. Tracking transplanted cells with paramagnetic fluorinated nanoemulsions. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 215. doi:10.1158/1538-7445.AM2015-215

Molecular Sensing with Hyperpolarized (129) Xe Using Switchable Chemical Exchange Relaxation Transfer.

An approach for hyperpolarized (129) Xe molecular sensors is explored using paramagnetic relaxation agents that can be deactivated upon chemical or enzymatic reaction with an analyte. Cryptophane encapsulated (129) Xe within the vicinity of the paramagnetic center experiences fast relaxation that, through chemical exchange of xenon atoms between cage and solvent pool, causes accelerated hyperpolarized (129) Xe signal decay in the dissolved phase. In this proof-of-concept work, the relaxivity of Gadolinium(III) -DOTA on (129) Xe in the solvent was increased eightfold through tethering of the paramagnetic molecule to a cryptophane cage. This potent relaxation agent can be 'turned off' specifically for (129) Xe through chemical reactions that spatially separate the Gd(III) centre from the attached cryptophane cage. Unlike (129) Xe chemical shift based sensors, the new concept does not require high spectral resolution and may lead to a new generation of responsive contrast agents for molecular MRI.

Co-encapsulating the fusogenic peptide INF7 and molecular imaging probes in liposomes increases intracellular signal and probe retention.

Liposomes are promising vehicles to deliver diagnostic and therapeutic agents to cells in vivo. After uptake into cells by endocytosis, liposomes are degraded in the endolysosomal system. Consequently, the encapsulated cargo molecules frequently remain sequestered in endosomal compartments; this limits their usefulness in many applications (e.g. gene delivery). To overcome this, various fusogenic peptides have been developed to facilitate delivery of liposomally-encapsulated molecules into the cytosol. One such peptide is the pH-sensitive influenza-derived peptide INF7. Liposomal delivery of imaging agents is an attractive approach for enabling cell imaging and cell tracking in vivo, but can be hampered by inadequate intracellular accumulation and retention of probes caused by exocytosis (and possible degradation) of endosome-entrapped probes. Such signal loss could be minimized by facilitating escape of probe molecules from endolysosomal compartments into the cytosol. We investigated the ability of co-encapsulated INF7 to release liposomally-delivered rhodamine fluorophores into the cytosol after endosomal acidification/maturation. We co-encapsulated INF7 and fluorescent rhodamine derivatives having vastly different transport properties to show that after endocytosis by CV1 cells, the INF7 peptide is activated by acidic endosomal pH and facilitates efficient release of the fluorescent tracers into the cytosol. Furthermore, we show that INF7-facilitated escape from endosomes markedly enhanced retention of tracers that cannot be actively extruded from the cytosol. Minimizing loss of intracellular probes improves cellular imaging by increasing the signal-to-noise ratio of images and lengthening the time window that imaging can be performed. In particular, this will enhance in vivo electron paramagnetic resonance imaging, an emergent magnetic resonance imaging modality requires exogenous paramagnetic imaging agents and is highly promising for cellular and molecular imaging.

Dissolution dynamic nuclear polarization of non-self-glassing agents: spectroscopy and relaxation of hyperpolarized [1-13C]acetate.

The intrinsic physicochemical properties of the sample formulation are the key factors for efficient hyperpolarization through dissolution dynamic nuclear polarization (dissolution-DNP). We provide a comprehensive characterization of the DNP process for Na-[1-(13)C]acetate selected as a model for non-self-glassing agents: the solid-state polarization dynamics of different formulations and the effect of the paramagnetic agent (trityl radical) on the pattern of polarization and the relaxation profile were extensively analyzed. We quantified the effects of the glassing agent and Gd(3+)-chelate on DNP performance. The results reported here describe the constraints of the acetate formulation useful for future studies in this field with non-self-glassing enriched molecules.

MRI CEST at 1T with large µeff Ln(3+) complexes T m(3+)-HPDO3A: An efficient MRI pH reporter.

Chemical exchange saturation transfer (CEST) sensitivity relies on the prototropic exchange rate kex between the agent and the "bulk" water protons. To exploit large kex, a large frequency separation (Δω) between the pools of exchanging protons is necessary. For this reason, high magnetic fields are preferred. Herein it is shown that the use of paramagnetic CEST agents based on lanthanide (III) ions with large effective magnetic moments allows the carrying out of CEST experiments at the relatively low field strength of 1 tesla (T). Measurements were performed on a 1T MR-scanner using continuous wave (cw)-presaturation with a spin echo sequence. ParaCEST complexes have been synthetized by mixing the ligand and Ln(III)Cl3 in a stoichiometric ratio at room temperature and pH 7. Different lanthanide chelates were investigated (Tm-, Dy-, Yb-, Eu-HPDO3A, and Eu-DOTAMGly). Ratiometric (Tm-HPDO3A) and selective detection (Eu-DOTAMGly and Tm-HPDO3A) experiments have been proven feasible in vivo. In vitro experiments demonstrated the feasibility of the CEST methodology at 1T for nearly every paraCEST candidate under investigation, except for Eu-HPDO3A. Among the studied compounds, Tm-HPDO3A proved suitable for the application of a ratiometric method for assessing pH both in vitro and in vivo.

Synergy between surface and core entrapped metals in a mixed manganese–gadolinium nanocolloid affords safer MR imaging of sparse biomarkers

High-relaxivity T1-weighted (T1w) MR molecular imaging nanoparticles typically present high surface gadolinium payloads that can elicit significant acute complement activation (CA). The objective of this research was to develop a high T1w contrast nanoparticle with improved safety. We report the development, optimization, and characterization of a gadolinium–manganese hybrid nanocolloid (MnOL-Gd NC; 138±10 (Dav)/nm; PDI: 0.06; zeta: −27±2mV). High r1 particulate relaxivity with minute additions of Gd-DOTA-lipid conjugate to the MnOL nanocolloid surface achieved an unexpected paramagnetic synergism. This hybrid MnOL-Gd NC provided optimal MR TSE signal intensity at 5nM/voxel and lower levels consistent with the level expression anticipated for sparse biomarkers, such as neovascular integrins. MnOL NC produced optimal MR TSE signal intensity at 10nM/voxel concentrations and above. Importantly, MnOL-Gd NC avoided acute CA in vitro and in vivo while retaining minimal transmetallation risk. From the Clinical EditorThe authors developed a gadolinium-manganese hybrid nanocolloid (MnOL-Gd NC) in this study. These were used as a high-relaxivity paramagnetic MR molecular imaging agent in experimental models. It was shown that MnOL-Gd NC could provide high T1w MR contrast for targeted imaging. As the level of gadolinium used was reduced, there was also reduced risk of systemic side effects from complement activation.

Structure and Membrane Topology of the Pore-Forming Peptide Maculatin 1.1

Antimicrobial peptides (AMP) that target membranes are an attractive alternative to classic antibiotics, since they do not require internalization nor target a specific stereo-structure, thus limiting development of bacterial resistance. Their mode of action involves the disruption of lipid membranes; however, the relationship between lipid membrane structure and peptide potency remains unclear. We present the structural investigation of the AMP maculatin 1.1 (Mac1) in DPC micelles and DHPC/DMPC isotropic (q= 0.5) bicelles. Using solution and solid-state NMR with paramagnetic relaxation enhancement agents (PRE) and molecular dynamics (MD), we demonstrate the important role of the membrane structure in modulating the structure and location of Mac1. HSQC of specifically 15N labeled Mac1 in buffer displayed a narrow chemical shift dispersion that is typical of random coil structures. Introduction of micelles and bicelles produced chemical shift dispersions characteristic of helical structures, with differences suggesting that Mac1 adopts a different degree of helicity dependent on the curvature. 3D TROSY-NOESY allowed assignment of the sequential 15N labeled residues, and determination of a 3D helical structure in phospholipid micelles and bicelles, the latter producing the greatest helical stretch. Titration of the PRE agent Gd3+-(DTPA) showed that the central core of Mac1 is protected in bicelles while in micelles only the N-term is exposed to the PRE effect. MD simulations in DPC micelles revealed N-term exposure to the solvent, and they also suggested that Mac1 bent to adapt the curved micelle structure. Experiments will be repeated with 4 and 8 peptides per micelles.

The Need and Initial Practice of Parallel Imaging and Compressed Sensing in Hyperpolarized 13C MRI in vivo.

The Need and Initial Practice of Parallel Imaging and Compressed Sensing in Hyperpolarized 13C MRI in vivo.

Characterization of a lanthanide complex encapsulated with MRI contrast agents into liposomes for biosensor imaging of redundant deviation in shifts (BIRDS).

Purposely designed magnetic resonance imaging (MRI) probes encapsulated in liposomes, which alter contrast by their paramagnetic effect on longitudinal (T₁) and transverse (T₂) relaxation times of tissue water, hold promise for molecular imaging. However, a challenge with liposomal MRI probes that are solely dependent on enhancement of water relaxation is lack of specific molecular readouts, especially in strong paramagnetic environments, thereby reducing the potential for monitoring disease treatment (e.g., cancer) beyond the generated MRI contrast. Previously, it has been shown that molecular imaging with magnetic resonance is also possible by detecting the signal of non-exchangeable protons emanating from paramagnetic lanthanide complexes themselves [e.g., TmDOTP⁵⁻, which is a Tm³⁺ -containing biosensor based on a macrocyclic chelate 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate), DOTP⁵⁻] with a method called biosensor imaging of redundant deviation in shifts (BIRDS). Here, we show that BIRDS is useful for molecular imaging with probes like TmDOTP⁵⁻ even when they are encapsulated inside liposomes with ultrastrong T₁and T₂contrast agents (e.g., Magnevist and Molday ION, respectively). We demonstrate that molecular readouts such as pH and temperature determined from probes like TmDOTP⁵⁻ are resilient, because the sensitivity of the chemical shifts to the probe's environment is not compromised by the presence of other paramagnetic agents contained within the same nanocarrier milieu. Because high liposomal encapsulation efficiency allows for robust MRI contrast and signal amplification for BIRDS, nanoengineered liposomal probes containing both monomers, TmDOTP⁵⁻ and paramagnetic contrast agents, could allow high spatial resolution imaging of disease diagnosis (with MRI) and status monitoring (with BIRDS).

Insights on the relaxation of liposomes encapsulating paramagnetic Ln-based complexes.

To describe and quantify the different relaxation mechanisms operating in suspensions of liposomes that encapsulate paramagnetic lanthanide(III) complexes. The transverse relaxation rate of lanthanide-loaded liposomes receives contribution from the exchange between intraliposomal and bulk water protons, and from magnetic susceptibility effects. Phospholipids vesicles encapsulating different Ln(III)-HPDO3A complexes (Ln = Eu, Gd, or Dy) were prepared using the conventional thin film rehydration method. Relaxation times (T1 , T2 , and T2*) were measured at 14 Tesla (T) and 25 °C. The effect of compartmentalization of the paramagnetic agent inside the liposomal cavity was evaluated by means of an IRON-modified MRI sequence. NMR measurements demonstrated that Curie spin relaxation is the dominant contribution (> 90%) to the observed transverse relaxation rate of paramagnetic liposomes. This was further confirmed by MRI that showed the ability of the liposome entrapped lanthanide complexes to generate IRON-MRI positive contrast in a size dependent manner. The Curie spin relaxation mechanism is by far the principal mechanism involved in the T2 shortening of the water protons in suspension of paramagnetic liposomes at 14T. The access to IRON contrast extends the potential of such nanosystems as MRI contrast agents.

Determination of organogenic elements in the composition of functional compounds and materials

The presented analysis methods (automatic and gravimetric) are used to determine the main composition of various organic functional materials such as paramagnetic chelating agents for the synthesis of molecular magnetics, polyfluoroaromatic compounds for the preparation of special latices, new antivirus and analgesic agents, photoinitiator compounds for cationic polymerization, fluorophores for organic light emitting diodes, and so on. Conditions for the quantitative formation of final products of the complete decomposition of the compound (CO2, H2O, N2, SO2) are shown: the application of special oxide catalytic compositions and also the use of selectively adsorbing reagents for the removal of compounds hindering the determination of organogenic elements. Their determination error does not exceed ±0.3 % abs.

Specific and nonspecific interactions in ultraweak protein-protein associations revealed by solvent paramagnetic relaxation enhancements.

Weak and transient protein–protein interactions underlie numerous biological processes. However, the location of the interaction sites of the specific complexes and the effect of transient, nonspecific protein–protein interactions often remain elusive. We have investigated the weak self-association of human growth hormone (hGH, KD = 0.90 ± 0.03 mM) at neutral pH by the paramagnetic relaxation enhancement (PRE) of the amide protons induced by the soluble paramagnetic relaxation agent, gadodiamide (Gd(DTPA-BMA)). Primarily, it was found that the PREs are in agreement with the general Hwang-Freed model for relaxation by translational diffusion (J. Chem. Phys.1975, 63, 4017–4025), only if crowding effects on the diffusion in the protein solution are taken into account. Second, by measuring the PREs of the amide protons at increasing hGH concentrations and a constant concentration of the relaxation agent, it is shown that a distinction can be made between residues that are affected only by transient, nonspecific protein–protein interactions and residues that are involved in specific protein–protein associations. Thus, the PREs of the former residues increase linearly with the hGH concentration in the entire concentration range because of a reduction of the diffusion caused by the transient, nonspecific protein–protein interactions, while the PREs of the latter residues increase only at the lower hGH concentrations but decrease at the higher concentrations because of specific protein–protein associations that impede the access of gadodiamide to the residues of the interaction surface. Finally, it is found that the ultraweak aggregation of hGH involves several interaction sites that are located in patches covering a large part of the protein surface.

EXPERIMENTAL EVALUATION OF MN(II)-TRANS-1,2-DIAMINOCYCLEHEXANE-N,N,N',N'TETRAACETATE AS PARAMAGNETIC CONTRAST AGENT FOR ENHANCEMENT OF CEREBRAL MENINGEOMAS IN DOGS

Aim of research. We have evaluated the abilities of new original paramagnetic contrast agent Mn-DCTA (0,5 mol solution of Manganese(II) complex with trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetate, registered trade mark Cyclomang) for contrast-enhanced imaging of brain meningeomas in dogs.Material and methods. Twelve animals were included, all with brain tumors reveald during out-patient veterinary examinations. In ten of twelve the diagnosis was verified later by pathomorphologic study. The cerebral MRI has been carred out as set of axial, sagittal and coronal slices as thin as three to five mm, covering all the volume of brain.The scanning parameters in T1-weighted spin-ech mode were as follows: TR = 400–500 ms, TE = 15–20 ms, the dose of injected paramagnetic was standardised as 1 mmol per 10 kg of body weight. For quantitative analysis the index of enhancement was calculated.Results. In all cases the sure enhancement with clear visualization of cerebral tumor was obtained due to highly intensive uptake of Mn-DCTA to the tumor tissue. When evaluated quantitatively the uptake of Mn-DCTA to the tumor gave the index of enhancement in T1-weighted spin-echo mode as high as IE = 1,72 ± 0,18 for the central parts of tumor and IE = 2,08 ± 0,23 for the peripheral ones, where as in control animals it was far below these values. Intravenous injection of Mn-DCTA to dogs with cerebral tumors did not induce any detectable pathologic or even physiologic effects.Conclusion. Henceforth we conclude the Mn-DCTA provides highly available methodologically simple imaging of cerebral meningeomas and can be thought out as promising paramagnetic agent for clinical magnetic resonance imaging in humans.

Technical review, types of imaging, part 4--magnetic resonance imaging.

Technical review, types of imaging, part 4--magnetic resonance imaging.

A new ex vivo method to evaluate the performance of candidate MRI contrast agents: a proof-of-concept study.

BackgroundMagnetic resonance imaging (MRI) plays an important role in tumor detection/diagnosis. The use of exogenous contrast agents (CAs) helps to improve the discrimination between lesion and neighbouring tissue, but most of the currently available CAs are non-specific. Assessing the performance of new, selective CAs requires exhaustive assays and large amounts of material. Accordingly, in a preliminary screening of new CAs, it is important to choose candidate compounds with good potential for in vivo efficiency. This screening method should reproduce as close as possible the in vivo environment. In this sense, a fast and reliable method to select the best candidate CAs for in vivo studies would minimize time and investment cost, and would benefit the development of better CAs.ResultsThe post-mortem ex vivo relative contrast enhancement (RCE) was evaluated as a method to screen different types of CAs, including paramagnetic and superparamagnetic agents. In detail, sugar/gadolinium-loaded gold nanoparticles (Gd-GNPs) and iron nanoparticles (SPIONs) were tested. Our results indicate that the post-mortem ex vivo RCE of evaluated CAs, did not correlate well with their respective in vitro relaxivities. The results obtained with different Gd-GNPs suggest that the linker length of the sugar conjugate could modulate the interactions with cellular receptors and therefore the relaxivity value. A paramagnetic CA (GNP (E_2)), which performed best among a series of Gd-GNPs, was evaluated both ex vivo and in vivo. The ex vivo RCE was slightly worst than gadoterate meglumine (201.9 ± 9.3% versus 237 ± 14%, respectively), while the in vivo RCE, measured at the time-to-maximum enhancement for both compounds, pointed to GNP E_2 being a better CA in vivo than gadoterate meglumine. This is suggested to be related to the nanoparticule characteristics of the evaluated GNP.ConclusionWe have developed a simple, cost-effective relatively high-throughput method for selecting CAs for in vivo experiments. This method requires approximately 800 times less quantity of material than the amount used for in vivo administrations.

In vivo MRI visualization of release from liposomes triggered by local application of pulsed low-intensity non-focused ultrasound

The work aimed at developing a MRI-guided protocol for the visualization of the release of material entrapped in liposomes stimulated by the local application of pulsed low-intensity non-focused ultrasound (pLINFU). The task was achieved by formulating liposomes filled up with the clinically approved paramagnetic agent gadoteridol, because the release of the agent from the nanovesicles is accompanied by a significant MRI signal enhancement. The protocol was validated in vivo on mice-bearing subcutaneous syngeneic B16 melanoma and i.v. injected with the paramagnetic liposomes. Upon exposing tumor to pLINFU (3MHz, insonation time 2min, duty cycle 50%) few minutes after liposomes injection, a signal enhancement of ca. 35% was detected. The effective release of the agent was confirmed by the strong enhancement measured in kidneys calyx and bladder due to the rapid renal excretion of the agent released in the tumor. From the Clinical EditorIn this paper, a pulsed low-intensity non-focused ultrasound-based technique was used to release a paramagnetic MRI contrast agent from liposomes, demonstrating the feasibility of this triggered release system in a mouse melanoma model for future research applications.

Contrast agents for MRI: 30+ years and where are we going?

Thirty years ago, Schering filed the first patent application for a contrast agent for magnetic resonance imaging (MRI) covering the forefather of the gadolinium contrast agents and still the most widely used gadolinium probe: gadolinium(III) diethylenetriaminepentaacetate (Magnevist). To date, 11 contrast agents have been approved by the US Food and Drug Administration for intravenous use. Coordination chemists have done a great deal to move the field forward. Our understanding of lanthanide chemistry now makes possible the design of complexes with long rotational correlation times, fast or slow water-exchange rates, high thermodynamic stabilities, and kinetic inertness, leading to sensitive and nontoxic contrast agents. Chemists did not stop there. The last few decades has seen the development of novel classes of probes that yield contrast through different mechanisms, such as paramagnetic chemical exchange saturation transfer agents. Thirty years since the first patent, chemists are still leading the way. The development of high-sensitivity contrast agents for high magnetic fields, safe probes for patients with kidney disorders, and multimodal, targeted, and responsive agents demonstrates that the field of contrast agents for MRI still has much to offer.

Synthetic transporters for sulfate: a new method for the direct detection of lipid bilayer sulfate transport

The transmembrane transport of anions by small synthetic molecules is a growing field in supramolecular chemistry and has focussed mainly on the transmembrane transport of chloride. On the other hand, the transport of the highly hydrophilic sulfate anion across lipid bilayers is much less developed, even though the inability to transport sulfate across cellular membranes has been linked to a variety of genetic diseases. Tris-thioureas possess high sulfate affinities and have been shown to be excellent chloride and bicarbonate transporters. Herein we report the sulfate transport abilities of a series of tris-ureas and tris-thioureas based on a tris(2-aminoethyl)amine or cyclopeptide scaffold. We have developed a new technique based on 33S NMR that can be used to monitor sulfate transport, using 33S-labelled sulfate and paramagnetic agents such as Mn2+ and Fe3+ to discriminate between intra- and extravesicular sulfate. Reasonable sulfate transport abilities were found for the reported tris-ureas and tris-thioureas, providing a starting point for the development of more powerful synthetic sulfate transporters that can be used in the treatment of certain channelopathies or as a model for biological sulfate transporters.