Dynamic aggregation of the mid-sized gadolinium complex {Ph4[Gd(DTTA)(H2O)2](-)3}.

A compound binding three Gd(3+) ions, {Ph4[Gd(DTTA)(H2O)2](-) 3} (where H5DTTA is diethylenetriaminetetraacetic acid), has been synthesized around a hydrophobic center made up of four phenyl rings. In aqueous solution the molecules start to self-aggregate at concentrations well below 1 mM as shown by the increase of rotational correlation times and by the decrease of the translational self-diffusion constant. NMR spectra recorded in aqueous solution of the diamagnetic analogue {Ph4[Y(DTTA)(H2O)2](-)3} show that the aggregation is dynamic and due to intermolecular π-stacking interactions between the hydrophobic aromatic centers. From estimations of effective radii, it can be concluded that the aggregates are composed of two to three monomers. The paramagnetic {Ph4[Gd(DTTA)(H2O)2](-)3} exhibits concentration-dependent (1)H NMR relaxivities with high values of approximately 50 mM(-1) s(-1) (30 MHz, 25 °C) at gadolinium concentrations above 20 mM. A combined analysis of (1)H NMR dispersion profiles measured at different concentrations of the compound and (17)O NMR data measured at various temperatures was performed using different theoretical approaches. The fitted parameters showed that the increase in relaxivity with increasing concentration of the compound is due to slower global rotational motion and an increase of the Lipari-Szabo order parameter S(2).

Supramolecular balance: using cooperativity to amplify weak interactions.

Gathering precise knowledge on weak supramolecular interactions is difficult yet is of utmost importance for numerous scientific fields, including catalysis, crystal engineering, ligand binding, and protein folding. We report on a combined theoretical and experimental approach showing that it is possible to vastly improve the sensitivity of current methods to probe weak supramolecular interactions in solution. The concept consists of using a supramolecular platform involving a highly cooperative configurational transition, the perturbation of which (by the modification of the molecular building blocks) can be monitored in a temperature scanning experiment. We tested this concept with a particular bisurea platform, and our first results show that it is possible to detect the presence of interaction differences as low as 60 J/mol, which may be due to steric repulsion between vinyl and alkyl groups or may be the result of solvation effects.

Gold nanoparticles functionalized with gadolinium chelates as high-relaxivity MRI contrast agents.

Water-dispersible gold nanoparticles functionalized with paramagnetic gadolinium have been fully characterized, and the NMRD profiles show very high relaxivities up to 1.5 T. Characterization using TEM images and dynamic light scattering indicate a particle size distribution from 2 to 15 nm. The gold cores of the nanoparticles do not contribute significantly to the overall magnetic moment.

Coupled sensitizer-catalyst dyads: electron-transfer reactions in a perylene-polyoxometalate conjugate.

Ultrafast discharge of a single-electron capacitor: A variety of intramolecular electron-transfer reactions are apparent for polyoxometalates functionalized with covalently attached perylene monoimide chromophores, but these are restricted to single-electron events. (et=electron transfer, cr=charge recombination, csr=charge-shift reaction, PER=perylene, POM=polyoxometalate).A new strategy is introduced that permits covalent attachment of an organic chromophore to a polyoxometalate (POM) cluster. Two examples are reported that differ according to the nature of the anchoring group and the flexibility of the linker. Both POMs are functionalized with perylene monoimide units, which function as photon collectors and form a relatively long-lived charge-transfer state under illumination. They are reduced to a stable pi-radical anion by electrolysis or to a protonated dianion under photolysis in the presence of aqueous triethanolamine. The presence of the POM opens up an intramolecular electron-transfer route by which the charge-transfer state reduces the POM. The rate of this process depends on the molecular conformation and appears to involve through-space interactions. Prior reduction of the POM leads to efficient fluorescence quenching, again due to intramolecular electron transfer. In most cases, it is difficult to resolve the electron-transfer products because of relatively fast reverse charge shift that occurs within a closed conformer. Although the POM can store multiple electrons, it has not proved possible to use these systems as molecular-scale capacitors because of efficient electron transfer from the one-electron-reduced POM to the excited singlet state of the perylene monoimide.

Anions as efficient chain stoppers for hydrogen-bonded supramolecular polymers.

The chain length of hydrogen-bonded supramolecular polymers and thus their rheological properties can be controlled by the presence of so-called chain stoppers: these monofunctional monomers are able to interact with the monomers and break the polymer chains. In this letter, we show that the use of anions, strong hydrogen bond competitors, instead of precisely designed complementary units is a very simple approach to tuning the rheology of a bisurea-based hydrogen-bonded supramolecular polymer. All of the anions tested were able to break the supramolecular chains, resulting in a dramatic drop in the viscosity of the solutions and were found to be more efficient than a previously described organic stopper. A careful study of the rheological properties of bisurea solutions in the presence of H2PO4,N(C4H9)4 showed that the presence of this ion does not modify the nature of the bisurea supramolecular assembly. For a molar fraction of stopper of only 10(-5), the viscosity of bisurea solutions decreases by a factor of 10 as a result of the formation of shorter supramolecular assemblies.

Convenient synthesis and properties of polypropyleneimine dendrimer-functionalized polymer nanoparticles.

A simple synthesis of polymer core-dendrimer shell nanoparticles (NPs) in the 15-20-nm-diameter range is presented. Amine-terminated polypropyleneimine (PPI) dendrimers DAB-dendri-(NH(2))(4) and DAB-dendri-(NH(2))(16) (DAB4 and DAB16) are covalently attached to the surface of primary polystyrene-based NPs bearing reactive chlorobenzyl groups produced by microemulsion polymerization in the presence of a cationic surfactant. The grafting readily proceeds under mild conditions and leads to translucent aqueous suspensions of core-shell-type NPs with a high density of peripheral amine groups that have been characterized relative to their size and chemical composition. The dendritic shell acts as a protective ionizable outer layer and provides an improvement of the colloidal stability in neutral and acidic media. The metal-binding capacity of the PPI dendrimers is retained, and spectrophotometric titrations show that the dendrimer-grafted NPs can trap a large number of Cu(2+) ions (more than 900 Cu per NP-DAB16). These properties make them potentially valuable templates for the elaboration of hybrid nanomaterials. The reactivity of the external amine groups is used to link covalently azobenzene chromophores (disperse Red 1 residues) through aza-Michael addition in aqueous suspension. This simple method gives access to colored NPs with high dye contents in the outer layer (up to 1000-1500 dye molecules per NP), which indicates that dendrimer-functionalized NPs are valuable building blocks for the construction of multifunctional nanomaterials.

Physicochemical properties of the high-field MRI-relevant [Gd(DTTA-Me)(H2O)2]- complex.

To study the physicochemical properties of the DTTA chelating moiety (H4DTTA = diethylenetriaminetetraacetic acid = N,N'-[iminobis(ethane-2,1-diyl)]bis[N-(carboxymethyl)glycine]), used in several compounds proposed as magnetic resonance imaging (MRI) contrast agents, the methylated derivative H4DTTA-Me (N,N'-[(methylimino)bis(ethane-2,1-diyl)]bis[N-(carboxymethyl)glycine]) was synthesized. Protonation constants of the ligand were determined in an aqueous solution by potentimetry and (1)H NMR pH titration and compared to various DTTA derivatives. Stability constants were measured for the chelates formed with Gd(3+) (log K(GdL) = 18.60 +/- 0.10) and Zn(2+) (log K(ZnL) = 17.69 +/- 0.10). A novel approach of determining the relative conditional stability constant of two paramagnetic complexes in a direct way by (1)H NMR relaxometry is presented and was used for the Gd(3+) complexes [Gd(DTTA-Me)(H2O)2](-) (L1) and [Gd(DTPA-BMA)(H2O)] (L2) [K(L1/L2)*(at pH 8.3, 25 degrees C) = 6.4 +/- 0.3]. The transmetalation reaction of the Gd(3+) complex with Zn(2+) in a phosphate buffer solution (pH 7.0) was measured to be twice as fast for [Gd(DTTA-Me)(H2O)2](-) in comparison to that for [Gd(DTPA-BMA)(H2O)]. This can be rationalized by the higher affinity of Zn(2+) toward DTTA-Me(4-) if compared to DTPA-BMA(3-). The formation of a ternary complex with L-lactate, which is common for DO3A-based heptadentate complexes, has not been observed for [Gd(DTTA-Me)(H2O)2](-) as monitored by (1)H NMR relaxometric titrations. From the results, it was concluded that the heptadentate DTTA-Me(4-) behaves similarly to the commercial octadentate DTPA-BMA(3-) with respect to stability. The use of [Gd(DTTA-Me)(H2O)2](-) as an MRI contrast agent in vitro and in animal studies is conceivable, mainly at high magnetic fields, where an increase of the inner-sphere-coordination water actually seems to be the most certain way to increase the relaxivity.

Fluorescent labeling of a bisurea-based supramolecular polymer.

Bisurea-based supramolecular polymer 2-ethylhexyl-3-[3-(3-(2-ethylhexyl)ureido)-4-methyl-phenyl]urea (EHUT) has been shown previously to self-assemble through hydrogen bonding into high-molecular-weight structures. The present publication reports the study of the thermodynamics of these tubular structures by time-resolved fluorescence spectroscopy, with the help of a tetrazine labeled monomer. Results of calorimetry and time-resolved fluorescence spectroscopy show that the as-modified monomer EHUTz does not interfere with the formation of the supramolecular assembly. When incorporated, these labeled monomers exhibit a longer fluorescence lifetime due to the electron-rich tolyl group buried in the structure. Dilution experiments allowed us to measure their partition coefficient, and to compare it with the critical aggregation concentration of EHUT. Measurements at higher dye loads, where interactions between neighboring tetrazines occur, show that EHUTz is uniformly dissolved in the supramolecular polymer. Tetrazine dye is a good reporter of events occurring in solution, such as disruption of the assembly upon heating. Our results confirm the pseudophase diagram for EHUT solution in toluene obtained previously with other techniques such as infrared spectroscopy and calorimetry.

A ruthenium-based metallostar: synthesis, sensitized luminescence and (1)H relaxivity.

Ru-based metallostars Na(4){Ru[Ln(2)bpy-DTTA(2)(H(2)O)(4)](3)} (Ln = Y, Gd, and Eu) have been self-assembled in aqueous solution and their relaxivity and optical properties unravelled. The synthesis and the purification of the new, highly stable heptametallic entities have been optimized for the diamagnetic Y(3+) complex and followed by (1)H NMR. The europium(iii) ruthenium-based metallostar {Ru[Eu(2)bpy-DTTA(2)(H(2)O)(4)](3)}(4-) displays sensitized (5)D(0)-->(7)F(J) luminescence upon excitation of the tris(2,2'-bipyridyl)ruthenium(ii) unit in the ultraviolet around 293 nm, as well as in the visible around 450 nm ((1)MLCT state). NMRD profiles at two temperatures (25 degrees C and 37 degrees C) were performed on {Ru[Gd(2)bpy-DTTA(2)(H(2)O)(4)](3)}(4-). NMRD profiles of the ruthenium-based {Ru[Gd(2)bpy-DTTA(2)(H(2)O)(4)](3)}(4-) and the iron-based {Fe[Gd(2)bpy-DTTA(2)(H(2)O)(4)](3)}(4-) metallostars were fitted with SBM theory coupled to the model-free Lipary-Szabo method for internal motion as well as with the modified Florence approach. Comparison of both fitting methods shows that the Florence approach is able to fit NMRD profiles up to 100 MHz, fails however at higher frequencies because it does not account for internal motion. Overall, the results detailed point to the heptametallic self-assembled edifices being potential relaxivity and luminescence bimodal bioprobes for use in animal models.