Slow Exchange Kinetics Research Articles

A robust heterodimeric bis-Rh(III)-porphyrin macrocycle for the self-assembly of a kinetically stable [2]-rotaxane.

We report the self-assembly of a robust di-nuclear tetralactam macrocyle based on two symmetric components: a Rh(III) bis-porphyrin and a bis-pyridyl ligand. We probe the binding properties of the tetralactam macrocycle with adipamide derivatives using 1H NMR spectroscopy. On the one hand, we show that the binding of the adipamide having linear alkyl chains that can thread through the intact macrocycle's cavity produces a weakly bound 1 : 1 complex stabilized by four intermolecular hydrogen bonds and featuring a preferred binding geometry of [2]pseudorotaxane topology. On the other hand, we detect the formation of two different complexes in the binding of an analogous adipamide possessing bulky stoppers (dumb-bell axle). The initial addition of the dumb-bell guest induces the formation of a 1 : 1 complex featuring fast exchange kinetics on the 1H chemical shift timescale and exo-cyclic (non-threaded) binding geometry. Notably, in the presence of a large excess of the dumb-bell guest and at suitable concentrations of the macrocycle (>5 mM) we observe the emergence of a second species displaying slow exchange kinetics. This observation allows the undisputed assignment of a [2]rotaxane topology to the second complex. The significant increase in kinetic stability featured by the di-nuclear Rh(III) [2]rotaxane complex contrasts with its reduction in thermodynamic stability (more than one order of magnitude) compared to the previously described di-nuclear Zn(II) counterpart.

Mechanism of photo-ionic stoichiometry changes in SrTiO3

The impact of UV light (λ = 365 nm) on defect chemistry and composition of undoped SrTiO3 single crystals was investigated by means of impedance spectroscopy. In-plane conductivity measurements between 280 and 450 °C in air reveal a drastic increase of the bulk conductivity upon UV illumination. The measured time dependence of this increase is in accordance with oxygen chemical diffusion in SrTiO3. The corresponding photo-induced change in defect concentrations is caused by oxygen being pumped from the gas phase into the oxide under UV irradiation. This affects the entire SrTiO3 crystal rather than only the thin UV absorption zone and leads to very high oxygen chemical potentials with nominal oxygen pressures up to 106 bar. After switching the UV light off, the resulting conductivity increase relaxes extremely slowly due to slow surface exchange kinetics. A mechanistic model is introduced to explain the impact of UV light on the oxygen chemical potential as well as on the oxygen vacancy and hole/electron concentrations in semiconducting oxides, here SrTiO3. This model is based on the formation of oxygen quasi-chemical potentials in the illuminated region. It is discussed how the interplay of four kinetic parameters causes the observed time-dependent stoichiometry and thus conductivity changes.

Picosecond to Second Fluorescence Correlation Spectroscopy for Studying Solute Exchange and Quenching Dynamics in Micellar Media.

Numerous studies have been devoted to understand the reaction kinetics in micelles, where the accessible kinetic time window is often limited by the dynamic range of the employed spectroscopic technique. This is usually accompanied by a selection of probes that comfortably explore time scales where slow solute exchange kinetics is negligible, as compared to the fast excited state reactions. This has led to an undervaluation of the role played by dynamic partitioning of hydrophilic solutes in microheterogeneous media. Here, we employ fluorescence correlation spectroscopy (FCS) and the zwitterionic dye Rhodamine 110 to quantitatively explore the impact of solute exchange on the photoinduced electron transfer between this dye and N,N-dimethylaniline in micellar media. Our study elucidates the coupling and interplay between the kinetics of photophysics, quenching, and solute exchange through a quantitative unified molecular-state quenching-kinetic model that describes the steady-state ensemble and FCS data from subnanosecond photon antibunching to millisecond diffusions.

Optimized Magnesium Force Field Parameters for Biomolecular Simulations with Accurate Solvation, Ion-Binding, and Water-Exchange Properties.

Magnesium ions play an essential role in many vital processes. To correctly describe their interactions in molecular dynamics simulations, an accurate parametrization is crucial. Despite the importance and considerable scientific effort, current force fields based on the commonly used 12–6 Lennard-Jones interaction potential fail to reproduce a variety of experimental solution properties. In particular, no parametrization exists so far that simultaneously reproduces the solvation free energy and the distance to the water oxygens in the first hydration shell. Moreover, current Mg2+ force fields significantly underestimate the rate of water exchange leading to unrealistically slow exchange kinetics. In order to make progress in the development of improved models, we systematically optimize the Mg2+ parameters in combination with the TIP3P water model in a much larger parameter space than previously done. The results show that a long-ranged interaction potential and modified Lorentz–Berthelot combination rules allow us to accurately reproduce multiple experimental properties including the solvation free energy, the distances to the oxygens of the first hydration shell, the hydration number, the activity coefficient derivative in MgCl2 solutions, the self-diffusion coefficient, and the binding affinity to the phosphate oxygen of RNA. Matching this broad range of thermodynamic properties, we present two sets of optimal parameters: MicroMg yields water exchange on the microsecond timescale in agreement with experiments. NanoMg yields water exchange on the nanosecond timescale facilitating the direct observation of ion-binding events. As shown for the example of the add A-riboswitch, the optimized parameters correctly reproduce the structure of specifically bound ions and permit the de novo prediction of Mg2+-binding sites in biomolecular simulations.

Leveraging Exchange Kinetics for the Synthesis of Atomically Precise Porous Catalysts

AbstractMetal‐organic frameworks (MOFs) have attracted significant attention as porous catalyst platforms due to the synthetic modularity of these materials and the diversity of lattice‐confined catalytic active sites that are readily embedded within periodic crystalline frameworks. MOFs offer platforms to heterogenize molecular catalysts, stabilize novel coordination motifs, and leverage confinement effects in catalysis. Crystallinity allows diffraction‐based methods to be employed in the characterization of these catalysts. Access to crystalline MOFs typically requires reversible construction of the metal−ligand (M−L) bonds that connect the SBUs, which provides a mechanism to anneal defects during crystallization. While the required M−L bond reversibility is often promoted by synthesis at elevated temperature, access to crystalline materials based on either transition metals with characteristic slow exchange kinetics or highly basic donor ligands remains a synthetic challenge. Here, we highlight synthetic strategies that leverage M−L exchange kinetics to access MOFs based on kinetically inert ions and extensions of these strategies to the assembly of atomically precise multimetallic materials.

A Modular DNA Scaffold to Study Protein-Protein Interactions at Single-Molecule Resolution

We introduce junctured-DNA (J-DNA) forceps as a generic platform for real-time observation, at the single-molecule level, of biomolecular interactions. The tool is based on a modular double-strand DNA construct to which proteins of interest can be attached using various tagging strategies. When combined with magnetic tweezers, J-DNA allows us to simultaneously monitor individual interactions on multiple forceps in parallel, and over periods of several days. Interactions with a lifetime ranging from 50 ms to 5000 s can be probed at a constant applied force as low as 30 fN, providing solution-like measurements as well as access to the force-dependence of the interaction. This tool thus provides straightforward and robust access to single-molecule force spectroscopy, in particular for the study of weak interactions. We present experiments characterizing the interactions between rapamycin and the FKBP12 and FRB proteins, a system relevant in both medicine and chemical biology. The zero-force lifetime of the ternary complex is 32.5 ± 0.4 s (SEM) at 21.7 oC and varies with temperature as well as magnitude and direction of the probing force, yielding after analysis the position of the transition state in space and energy as well as the direction of the dissociation axis. We demonstrate the broad applicability of the approach by using it to study the strength and regulation of receptor-scaffold interactions between components of inhibitory synapses, namely the glycine receptor (GlyR) and its scaffolding protein, gephyrin. Our results reveal that the association and dissociation of the cytoplasmic β-loop of the GlyR with a trimer of gephyrin displays a mean interaction lifetime on the order of 10 s, in line with the slow exchange kinetics of the receptor at synapses in living cells.

Modifying effects and mechanisms of graphene on dehydrogenation properties of sodium borohydride

Sodium borohydride (NaBH4) is a promising solid-state hydrogen storage material because of its low toxicity, high environmental stability and release of high-purity hydrogen. Nevertheless, the practical application of NaBH4 is still hampered by its high desorption temperature and slow hydrogen exchange kinetics. Using experimental and first-principles calculations approaches, the dehydrogenation properties and modifying mechanisms of NaBH4+10 wt%graphene composite acquired by ball-milling are systematically investigated in this work. The results show that the graphene plays a cooperative catalysis–confinement effect on NaBH4. X-ray diffraction analysis displays that no new phases formed due to the mutual inertia between NaBH4 and graphene during ball-milling. Scanning electron microscopy and transmission electron microscopy observations show that the NaBH4 particles are significantly refined after graphene addition, which effectively restrains the agglomeration of NaBH4 particles. Thermogravimetry testing and mass spectrometry testing indicate that the onset dehydrogenation temperature of the NaBH4+10 wt%graphene composite is decreased by about 114 °C relative to the milled pristine NaBH4. First-principles calculations reveal that the enhanced dehydrogenation properties of NaBH4 after graphene addition should be ascribed to the reduced dehydrogenation enthalpy of NaBH4 and strong binding energy between NaBH4 and graphene as well as the electron transfer from NaBH4 to graphene.

The MyMOMA domain of MYO19 encodes for distinct Miro-dependent and Miro-independent mechanisms of interaction with mitochondrial membranes.

MYO19 interacts with mitochondria through a C-terminal membrane association domain (MyMOMA). Specific mechanisms for localization of MYO19 to mitochondria are poorly understood. Using promiscuous biotinylation data in combination with existing affinity-capture databases, we have identified a number of putative MYO19-interacting proteins. We chose to explore the interaction between MYO19 and the mitochondrial GTPase Miro2 by expressing mchr-Miro2 in combination with GFP-tagged fragments of the MyMOMA domain and assaying for recruitment of MYO19-GFP to mitochondria. Coexpression of MYO19898-970 -GFP with mchr-Miro2 enhanced MYO19898-970 -GFP localization to mitochondria. Mislocalizing Miro2 to filopodial tips or the cytosolic face of the nuclear envelope did not recruit MYO19898-970 -GFP to either location. To address the kinetics of the Miro2/MYO19 interaction, we used FRAP analysis and permeabilization-activated reduction in fluorescence analysis. MyMOMA constructs containing a putative membrane-insertion motif but lacking the Miro2-interacting region displayed slow exchange kinetics. MYO19898-970 -GFP, which does not include the membrane-insertion motif, displayed rapid exchange kinetics, suggesting that MYO19 interacting with Miro2 has higher mobility than MYO19 inserted into the mitochondrial outer membrane. Mutation of well-conserved, charged residues within MYO19 or within the switch I and II regions of Miro2 abolished the enhancement of MYO19898-970 -GFP localization in cells ectopically expressing mchr-Miro2. Additionally, expressing mutant versions of Miro2 thought to represent particular nucleotide states indicated that the enhancement of MYO19898-970 -GFP localization is dependent on Miro2 nucleotide state. Taken together, these data suggest that membrane-inserted MYO19 is part of a larger complex, and that Miro2 plays a role in integration of actin- and microtubule-based mitochondrial activities.

Factors controlling surface oxygen exchange in oxides

Reducing the working temperature of solid oxide fuel cells is critical to their increased commercialization but is inhibited by the slow oxygen exchange kinetics at the cathode, which limits the overall rate of the oxygen reduction reaction. We use ab initio methods to develop a quantitative elementary reaction model of oxygen exchange in a representative cathode material, La0.5Sr0.5CoO3−δ, and predict that under operating conditions the rate-limiting step for oxygen incorporation from O2 gas on the stable, (001)-SrO surface is lateral (surface) diffusion of O-adatoms and oxygen surface vacancies. We predict that a high vacancy concentration on the metastable CoO2 termination enables a vacancy-assisted O2 dissociation that is 102–103 times faster than the rate limiting step on the Sr-rich (La,Sr)O termination. This result implies that dramatically enhanced oxygen exchange performance could potentially be obtained by suppressing the (La,Sr)O termination and stabilizing highly active CoO2 termination.

CoMg olivine: cation partitioning, thermal expansion and structural variation studied by in situ neutron and synchrotron powder diffraction

The crystal structure of CoMg olivine has been investigated by in situ neutron and synchrotron X-ray powder diffraction at temperatures between 23°C and 1050°C. The olivine sample was prepared at 1450°C by solid-state reaction. During quench, the structure acquired a partially anti-ordered Co,Mg site occupancy, X Co M 1 > X Co M 2 , which upon ensuing heating persisted up to 550°C due to slow exchange kinetics. Above 600°C, internal equilibrium was readily obtained. The temperature variation of the Co,Mg site occupancies was modelled by R T ln( K D )= − 12080 ( ±620 ) −160 ( ±0.58 ) T [ J mol − 1 ]. The pronounced negative exchange enthalpy strongly favours the anti-ordered state, as does the positive exchange entropy so that CoMg olivine will remain in this state up to the melting point. The anisotropy of thermal expansion was analysed using an isothermal equation-of-state (EoS) (Kumar, 1995) coupled with the thermal Mie-Gruneisen EoS. This coupling allowed for relating the anisotropic expansivity to an anisotropic thermal pressure. Both properties are linked to the temperature-dependent rotations of the M1 and M2 octahedra in the [001] plane. The rotations increase from Mg via CoMg to Fe olivine. The Kumar and Birch-Murnaghan EoS’s gave virtually indistinguishable results upon modelling thermal expansion. The increase of the mean bond distances and with temperature changes at ~550°C when the redistribution of Co and Mg begins. In terms of a distortion parameter defined as a normalized standard deviation, the bond length distortion of the M2 octahedron is larger than that of M1. Conversely, M1 is more distorted than M2, if the distortion is defined by the variation of the octahedral faces as well as by the variation of the O-M-O angles subtended at the M sites. The former distortion relates to the exchange entropy.

Gd3TCAS2: An Aquated Gd(3+)-Thiacalix[4]arene Sandwich Cluster with Extremely Slow Ligand Substitution Kinetics.

In aqueous solution, Gd3+ and thiacalix[4]arene-p-tetrasulfonate (TCAS) form the complex [Gd3TCAS2]7–, in which a trinuclear Gd3+ core is sandwiched by two TCAS ligands. Acid-catalyzed dissociation reactions, as well as transmetalation and ligand exchange with physiological concentrations of Zn2+ and phosphate, showed [Gd3TCAS2]7– to be extremely inert compared to other Gd complexes. Luminescence lifetime measurements of the Tb analogue Tb3TCAS2 allowed estimation of the mean hydration number q to be 2.4 per Tb ion. The longitudinal relaxivity of [Gd3TCAS2]7– (per Gd3+) was r1 = 5.83 mM–1 s–1 at 20 Hz (37 °C, pH 7.4); however, this relaxivity was limited by an extremely slow water exchange rate that was 5 orders of magnitude slower than the Gd3+ aqua ion. Binding to serum albumin resulted in no relaxivity increase owing to the extremely slow water exchange kinetics. The slow dissociation and water exchange kinetics of [Gd3TCAS2]7– can be attributed to the very rigid coordination geometry.

Enclathration and Confinement of Small Gases by the Intrinsically 0D Porous Molecular Solid, Me,H,SiMe2.

The stable, guest-free crystal form of the simple molecular cavitand, Me,H,SiMe2, is shown to be intrinsically porous, possessing discrete, zero-dimensional (0D) pores/microcavities of about 28 Å(3). The incollapsible 0D pores of Me,H,SiMe2 have been exploited for the enclathration and room temperature (and higher) confinement of a wide range of small gases. Over 20 isostructural x(gas/guest)@Me,H,SiMe2 (x ≤ 1) clathrates (guest = H2O, N2, Ar, CH4, Kr, Xe, C2H4, C2H6, CH3F, CO2, H2S, CH3Cl, CH3OCH3, CH3Br, CH3SH, CH3CH2Cl, CH2Cl2, CH3I, CH3OH, BrCH2Cl, CH3CH2OH, CH3CN, CH3NO2, I2), and a propyne clathrate (CH3CCH@Me,H,SiMe2·2CHCl3), have been prepared and characterized, and their single crystal structures determined. Gas enclathration is found to be highly selective for gases that can be accommodated by the predefined, though slightly flexible 0D pore. The structure determinations provide valuable insight, at subangstrom resolution, into the factors that govern inclusion selectivity, gas accommodation, and the kinetic stability of the clathrates, which has been probed by thermal gravimetric analysis. The activation (emptying) of several clathrates (guest = H2O, N2, CO2, Kr, CH3F) is shown to occur in a single-crystal-to-single-crystal (SC → SC) fashion, often requiring elevated temperatures. Akin to open pore materials, water vapor and CO2 gas are shown to be taken up by single crystals of empty Me,H,SiMe2 at room temperature, but sorption rates are slow, occurring over weeks to months. Thus, Me,H,SiMe2 exhibits very low, but measurable, gas permeability, despite there being no obvious dynamic mechanism to facilitate gas uptake. The unusually slow exchange kinetics has allowed the rates of gas (water vapor and CO2) sorption to be quantified by single crystal X-ray diffraction. The data are well fit to a simple three-dimensional diffusion model.

ChemInform Abstract: Recent Advances in the Use of Sodium Borohydride as a Solid State Hydrogen Store

AbstractReview: 80 refs.

(Invited) Molecular Understanding of Oxygen Exchange in Solid Oxide Fuel Cell Cathodes

Materials with the ability to rapidly exchange oxygen with the environment play a critical role in many existing and developing technologies, e.g., solid oxide fuel cells (SOFCs), gas separation membranes, oxygen sensors, chemical looping devices, and memristors. In particular, reducing the working temperature of SOFCs, which is critical to their increased commercialization, is significantly inhibited by the slow oxygen exchange kinetics at the cathode. The oxygen exchange is an essential step in the overall oxygen reduction reaction (ORR), where O2 (gas) combines with 4e- to incorporate as 2O2- into the electrolyte, facilitated by the cathode. Oxygen exchange rates are typically quantified by the oxygen exchange coefficient, K*, which represents the response of the rate of oxygen exchange to a driving force. In our work we are developing techniques to predict K* and provide a molecular scale understanding of processes involved in oxygen surface exchange, with the long-term goal of designing more active oxygen exchange materials. Our work has focused on perovskite materials. A few years ago we identified the distance of the bulk oxygen 2p-band center to the Fermi level as an easily calculated descriptor for oxygen exchange in electronic conducing perovskites.[1] These results suggested that overall oxyphilicity plays a critical role in determining K* and provides a tool for high-throughput ab initio screening of promising materials. We will report on recent predictions of active oxygen exchange systems using these screening approaches. In more recent work we have initiated an effort to model oxygen exchange at the molecular level for the (La,Sr)CoO3-d (LSC) system. Using ab inito energetics and the elementary kinetics formalism developed by Adler, et al.[2] We have modeled the possible reaction steps on the LSC (001) surface. This modeling has yielded valuable insights in the rate limiting steps of oxygen exchange through a perovskite surface, the role of surface termination and Sr segregation on K*,[3, 4] and the absolute limits of surface exchange rates. Acknowledgement We gratefully acknowledge financial support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0001284, and computing support from NSF National Center for Supercomputing Applications (NCSA - DMR060007).

Recent Advances in the Use of Sodium Borohydride as a Solid State Hydrogen Store

The development of new practical hydrogen storage materials with high volumetric and gravimetric hydrogen densities is necessary to implement fuel cell technology for both mobile and stationary applications. NaBH4, owing to its low cost and high hydrogen density (10.6 wt%), has received extensive attention as a promising hydrogen storage medium. However, its practical use is hampered by its high thermodynamic stability and slow hydrogen exchange kinetics. Recent developments have been made in promoting H2 release and tuning the thermodynamics of the thermal decomposition of solid NaBH4. These conceptual advances offer a positive outlook for using NaBH4-based materials as viable hydrogen storage carriers for mobile applications. This review summarizes contemporary progress in this field with a focus on the fundamental dehydrogenation and rehydrogenation pathways and properties and on material design strategies towards improved kinetics and thermodynamics such as catalytic doping, nano-engineering, additive destabilization and chemical modification.

A time-resolved study on the interaction of oppositely charged bicelles--implications on the charged lipid exchange kinetics.

Time-resolved small-angle X-ray scattering was applied to study charged lipid exchange between oppositely charged disc-shaped bicelles. The exchange of charged lipids gradually reduces the surface charge density and weakens the electrostatic attraction between the oppositely charged bicelles which form alternately stacked aggregates upon mixing. Initially, at a high surface charge density with almost no free water layer between the stacked bicelles, fast exchange kinetics dominate the exchange process. At a later stage with a lower surface charge density and a larger water gap between the stacked bicelles, slow exchange kinetics take over. The fast exchange kinetics are correlated with the close contact of the bicelles when there is almost no free water layer between the tightly bound bicelles with a charged lipid exchange time constant as short as 20-40 min. When the water gap becomes large enough to have a free water layer between the stacked bicelles, the fast lipid exchange kinetics are taken over by slow lipid exchange kinetics with time constants around 200-300 min, which are comparable to the typical time constant of lipid exchange between vesicles in aqueous solution. These two kinds of exchange mode fit well with the lipid exchange models of transient hemifusion for the fast mode and monomer exchange for the slow mode.

Study of the binding interaction between fluorinated matrix metalloproteinase inhibitors and Human Serum Albumin

Fluorinated, arylsulfone-based inhibitors of Matrix Metalloproteinases (MMP) have been used, in the [18F]-radiolabelled version, as radiotracers targeted to MMP-2/9 for Positron Emission Tomography (PET). Although they showed acceptable tumour uptake, specificity was rather low. To get further insights into the reason of low specificity, the binding interaction of these compounds with Human Serum Albumin (HSA) has been investigated. 19F NMR spectroscopy showed that all compounds considered partition between multiple HSA binding sites, being characterized by either slow-exchange kinetics (with Ka in the order of 105 M−1) and fast-exchange kinetics (with Ka in the order of 104 M-1). For 2-(2-(4′-(2-fluoroethoxy)biphenyl-4-ylsulfonyl)phenyl)acetic acid (1a) and 2-(2-(4′-(2-fluoroacetamido)biphenyl-4-ylsulfonyl)phenyl)acetic acid (1c), these slow and fast-exchanging binding sites could be mapped to Sudlow's site I and II, respectively. It is shown that high affinity albumin binding constitutes a theoretical limitation for the specificity achievable by MMP-inhibitors as MMP-targeted PET tracers in cancer imaging, because albumin accumulating aspecifically in tumours lowers the binding potential of radiotracers.

Evaluation of Hydrogen-Bond Acceptors for Redox-Switchable Resorcin[4]arene Cavitands

Various H-bond acceptor groups were evaluated for their propensity to induce conformational switching between the kite and vase forms of diquinone-diquinoxaline resorcin[4]arene cavitands upon redox interconversion. The H-bond acceptors were placed on the quinoxaline walls with the purpose of stabilizing the vase form only in the reduced hydroquinone state of the cavitand by forming H-bonds with the hydroquinone OH groups. Design guidelines for successful acceptors were derived. The carboxamide acceptor was shown to be the best candidate. Based on this moiety, a redox-switchable triptycene-based basket that can completely sterically encapsulate a guest in its closed vase conformation was prepared. The basket binds small molecule guests with association constants of up to 10(4) M(-1) in mesitylene-d12 and exhibits slow guest exchange kinetics with a half-life for guest release in the order of 10(4) s.

Dysprosium Complexes and Their Micelles as Potential Bimodal Agents for Magnetic Resonance and Optical Imaging

Six diethylene triamine pentaacetic acid (DTPA) bisamide derivatives functionalized with p-toluidine (DTPA-BTolA), 6-aminocoumarin (DTPA-BCoumA), 1-naphthalene methylamine (DTPA-BNaphA), 4-ethynylaniline (DTPA-BEthA), p-dodecylaniline (DTPA-BC12PheA) and p-tetradecyl-aniline (DTPA-BC14PheA) were coordinated to dysprosium(III) and the magnetic and optical properties of the complexes were examined in detail. The complexes consisting of amphiphilic ligands (DTPA-BC12PheA and DTPA-BC14PheA) were further assembled into mixed micelles. Upon excitation into the ligand levels, the complexes display characteristic Dy(III) emission with quantum yields of 0.3-0.5% despite the presence of one water molecule in the first coordination sphere. A deeper insight into the energy-transfer processes has been obtained by studying the photophysical properties of the corresponding Gd(III) complexes. Since the luminescence quenching effect is decreased by the intervention of non-ionic surfactant, quantum yields up to 1% are obtained for the micelles. The transverse relaxivity r2 per Dy(III) ion at 500 MHz and 310 K reaches a maximum value of 27.4 s(-1) mM(-1) for Dy-DTPA-BEthA and 36.0 s(-1) mM(-1) for the Dy-DTPA-BC12PheA assemblies compared with a value of 0.8 s(-1) mM(-1) for Dy-DTPA. The efficient T2 relaxation, especially at high magnetic field strengths, is sustained by the high magnetic moment of the dysprosium ion, the coordination of water molecules with slow water exchange kinetics and long rotational correlation times. These findings open the way to the further development of bimodal optical and magnetic resonance imaging probes starting from single lanthanide compounds.

The stereochemistry of amide side chains containing carboxyl groups influences water exchange rates in EuDOTA-tetraamide complexes

Many Eu(III) complexes formed with DOTA-tetraamide ligands (where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have sufficiently slow water exchange kinetics to meet the slow-to-intermediate condition required to serve as chemical exchange saturation transfer (CEST) contrast agents for MRI. This class of MRI contrast agents offers an attractive platform for creating biological sensors because water exchange is exquisitely sensitive to subtle ligand stereochemistry and electronic effects. Introduction of carboxyl groups or carboxyl ethyl ester groups on the amide substituents has been shown to slow water exchange in these complexes, but less is known about the orientation or position of these side-chain groups relative to the inner-sphere Eu(III)-bound water molecule. In this study, a series of Eu(III) complexes having one or more carboxyl groups or carboxyl esters at the δ-position of the pendant amide side chains were prepared. Initial attempts to prepare optically pure EuDOTA-[(S)-Asp]4 resulted in a chemically pure ligand consisting of a mixture of stereochemical isomers. This was traced to racemization of (S)-aspartate diethyl ester during the synthetic procedure. Nevertheless, NMR studies of the Eu(III) complexes of this mixture revealed that each isomer had a different water exchange rate, differing by a factor of 2 or more. A second controlled synthesis and CEST study of EuDOTA-[(S)-Asp]4 and cis-EuDOTA-[(S)-Asp]2[(R)-Asp]2 confirmed that the water exchange rates in these diastereomeric complexes are controlled by the axial versus equatorial orientation of the carboxyl groups on the amide side chains. These observations provide new insights toward the development of even more slowly water exchanging systems which will be necessary for practical in vivo applications.