Controlled supramolecular oligomerization of C3-symmetrical molecules in water: the impact of hydrophobic shielding.

The supramolecular oligomerization of three water-soluble C(3)-symmetrical discotic molecules is reported. The compounds all possess benzene-1,3,5-tricarboxamide cores and peripheral Gd(III)-DTPA (diethylene triamine pentaacetic acid) moieties, but differ in their linker units and thus in their propensity to undergo secondary interactions in H(2)O. The self-assembly behavior of these molecules was studied in solution using circular dichroism, UV/Vis spectroscopy, nuclear magnetic resonance, and cryogenic transmission electron microscopy. The aggregation concentration of these molecules depends on the number of secondary interactions and on the solvophobic character of the polymerizing moieties. Hydrophobic shielding of the hydrogen-bonding motif in the core of the discotic is of paramount importance for yielding stable, helical aggregates that are designed to be restricted in size through anti-cooperative, electrostatic, repulsive interactions.

Stepwise noncovalent synthesis leading to dendrimer-based assemblies in water.

We provide detailed insight into complex supramolecular assembly processes by fully characterizing a multicomponent model system using dynamic light scattering, cryogenic transmission electron microscopy, atomic force microscopy, and various NMR techniques. First, a preassembly of a host molecule (the fifth-generation urea-adamantyl poly(propylene imine) dendrimer) and 32 guest molecules (a water- and chloroform-soluble ureidoacetic acid guest) was made in chloroform. The association constant in chloroform is concealed by guest self-association and is therefore higher than 10(3) M(-1). Via the neat state the single-host complex was transferred to water, where larger dendrimer-based assemblies were formed. The core of these assemblies, consisting of multiple host molecules (on average three), is kinetically trapped upon dissolution in water, and its size is constant irrespective of the concentration. The guest molecules forming the corona of the assemblies, however, stay dynamic since they are still in rapid exchange on the NMR time scale, as they were in chloroform. A stepwise noncovalent synthesis provides a means to obtain metastable dynamic supramolecular assemblies in water, structures that cannot be formed in one step.

A supramolecular approach to multivalent target-specific MRI contrast agents for angiogenesis.

The synthesis of a cyclic peptide-Gd(III)DTPA molecule equipped with biotin is presented, yielding a well-defined multivalent MRI contrast agent after its coupling to avidin.

Design and synthesis of a bimodal target-specific contrast agent for angiogenesis.

[structure: see text] A bimodal target-specific contrast agent based on a cyclic peptide containing the target-specific NGR sequence, gadolinium(III) diethylenetriaminepentaacetic acid (Gd(III)DTPA), and Oregon Green 488 (OG488) suitable for both MR imaging and optical imaging of angiogenesis is developed. The synthetic strategy for this target-specific contrast agent exploits the use of highly efficient, chemoselective reactions, such as native chemical ligation, and gives a straightforward approach for double labeling of peptides in general.

Self-assembly of soft nanoparticles with tunable patchiness.

Details of the forces between nanoparticles determine the ways in which the nanoparticles can self-assemble into larger structures. The use of directed interactions has led to new concepts in self-assembly such as asymmetric dendrons, Janus particles, patchy colloids and colloidal molecules. Recent models that include attractive regions or 'patches' on the surface of the nanoparticles predict a wealth of intricate modes of assembly. Interactions between such particles are also important in a range of phenomena including protein aggregation and crystallization, re-entrant phase transitions, assembly of nanoemulsions and the organization of nanoparticles into nanowires. Here, we report the synthesis of 6-nm nanoparticles with dynamic hydrophobic patches and show that they can form reversible self-assembled structures in aqueous solution that become topologically more connected upon dilution. The organization is based on guest-host supramolecular chemistry with the nanoparticles composed of a hydrophobic dendrimer host molecule and water-soluble hydrophilic guest molecules. The work demonstrates that subtle changes in hierarchal composition and/or concentration can dramatically change mesoscopic ordering.

Evaluation of Gd(III)DTPA-terminated poly(propylene imine) dendrimers as contrast agents for MR imaging.

Different generations of Gd(III)DTPA-terminated poly(propylene imine) dendrimers {G1 [n = 4 Gd(III) ions per molecule], G3 (n = 16) and G5 (n = 64)} and reference Gd(III)DTPA complex [G0 (n = 1)] were characterized in terms of (i) longitudinal (r1) and transverse (r2) relaxivities in mouse blood plasma, (ii) concentration detection limits in vitro and (iii) in vivo contrast-enhanced MR imaging (CE-MRI) in mice at 1.5 T. Serial and dynamic CE-MRI were performed to monitor the distribution of MRI contrast agent in the heart, arteries, renal system, liver, spleen, bladder and tumor periphery. The relaxivities increased non-linearly with molecular weight (for G0 ionic r1 = 8.1 mM(-1) s(-1) and ionic r2 = 8.6 mM(-1) s(-1) to G5 19.3 and 25.0, respectively). The minimal detectable dendrimer concentration was more than two orders of magnitude lower for G5 (8.1 x 10(-8) M) than for G0 (3.1 x 10(-5) M). Sub-millimeter-sized blood vessels were well visualized with serial CE-MRI with each contrast agent. Dynamic CE-MRI showed timely renal clearance for all contrast agents, but a stronger and a prolonged blood signal enhancement for the higher generations of the dendritic contrast agent. Moreover, G0 and G1 showed a rapid tumor wash-in and wash-out, whereas G3 and G5 displayed a more gradual and prolonged tumor wash-in. In conclusion, both G0 and dendritic contrast agents G1, G3 and G5 are well suited for non-tissue-specific MRI of sub-millimeter-sized blood vessels and evaluating tumor microcirculatory characteristics in mice. Higher generations of dendritic contrast agents display lower concentration detection limits, which suggests their future use for molecular imaging.

Multicomponent host-guest chemistry of carboxylic acid and phosphonic acid based guests with dendritic hosts: an NMR study.

A new way to analyze supramolecular dendritic architectures is reported by making use of (13)C NMR and (31)P NMR. Two ethylene glycol guest molecules have been synthesized containing a (13)C labeled carboxylic acid headgroup (2) and a phosphonic acid headgroup (3). The binding of these guests to urea-adamantyl modified poly(propylene imine) dendrimers has been investigated with (13)C NMR and (31)P NMR next to 1D and 2D (1)H NMR techniques. Different amounts of guest 2 have been added to fifth generation dendrimer 1e, and the observed chemical shift values in (13)C NMR were fitted to a model that assumes 1:1 binding between guest and binding site. An association constant of 400 +/- 95 M(-)(1) is obtained for guest 2 with 41 binding sites per dendrimer. When different amounts of phosphonic acid guest 3 are added to dendrimer 1e, two different signals are observed in (31)P NMR. Deconvolution gives the fractions of free and bound guest, resulting in an association constant of (4 +/- 3) x 10(4) M(-)(1) and 61 +/- 1 binding sites. A statistical analysis shows that guest 2 forms a "polydisperse supramolecular aggregate", while guest 3 is able to form a "monodisperse supramolecular aggregate" when the amount of guest is high enough. The NMR results are compared with dynamic light scattering experiments, and a remarkable agreement is found. Phosphonic acid guest 3 is able to exchange with guest 2, which is in agreement with the obtained association constants, and shows that these techniques can be used to analyze multicomponent dendritic aggregates.

Molecular recognition: comparative study of a tunable host-guest system by using a fluorescent model system and collision-induced dissociation mass spectrometry on dendrimers.

Host-guest interactions between the periphery of adamantylurea-functionalized dendrimers (host) and ureido acetic acid derivatives (guest) were shown to be specific, strong and spatially well-defined. The binding becomes stronger when using phosphonic or sulfonic acid derivatives. In the present work we have quantified the binding constants for the host-guest interactions between two different host motifs and six different guest molecules. The host molecules, which resemble the periphery of a poly(propylene imine) dendrimer, have been fitted with an anthracene-based fluorescent probe. The two host motifs differ in terms of the length of the spacer between a tertiary amine and two ureido functionalities. The guest molecules all contain an acidic moiety (either a carboxylic acid, a phosphonic acid, or a sulfonic acid) and three of them also contain an ureido moiety capable of forming multiple hydrogen bonds to the hosts. The binding constants for all 12 host-guest complexes have been determined by using fluorescence titrations by monitoring the increase in fluorescence of the host upon protonation by the addition of the guest. The binding constants could be tuned by changing the design of the acidic part of the guest. The formation of hydrogen bonds gives, in all cases, higher association constants, demonstrating that the host is more than a proton sensor. The host with the longer spacer (propyl) shows higher association constants than the host with the shorter spacer (ethyl). The gain in association constants are higher when the urea function is added to the guests for the host with the longer spacer, indicating a better fit. Collision-induced dissociation mass spectrometry (CID-MS) is used to study the stability of the six motifs using the corresponding third generation dendrimer. A similar trend is found when the six different guests are compared.

The chirality of dendrimer-based supramolecular complexes.

Boc-protected L-phenylalanine has been connected to a spacer-armed ureido-acetic acid derivative, which can form multiple supramolecular complexes with urea-adamantyl modified poly(propylene imine) dendrimers in chloroform. Complexes of this guest with several generations of urea-adamantyl dendrimers were prepared. The dendrimer-guest complexes were characterized in detail by (1)H-NMR, (1)H-(1)H-NOESY spectroscopy and mass spectrometry to prove their formation. Optical rotation experiments performed on different generations of dendrimer-guest complexes showed a constant positive value. These observations indicate that, though guest molecules decrease the flexibility at the periphery of the dendrimer upon binding, the amino acid at the terminus of the guest molecule retains its high local conformational freedom. This is in agreement with values found for covalently modified spacer-armed dendrimers.

Dynamic contrast-enhanced MR imaging kinetic parameters and molecular weight of dendritic contrast agents in tumor angiogenesis in mice.

PURPOSE: To evaluate the relationship between dynamic contrast agent-enhanced magnetic resonance (MR) imaging-derived kinetic parameters and contrast agents of equal chemical composition and configuration but with different molecular weights in a tumor angiogenesis model. MATERIALS AND METHODS: This study was approved by the ethical review committee. Maintenance and care of animals was in compliance with guidelines set by the institutional animal care committee. Dynamic contrast-enhanced MR imaging was performed with dendritic contrast agents in 16 mice with tumor xenografts; mice were placed in groups of four for each molecular weight of the contrast agent. The magnitude and spatial distribution of kinetic parameters (transfer coefficient [K(PS)] and plasma fraction [f(PV)]) were compared with molecular weight of the contrast agent by determining the Spearman correlation coefficient (r) and the quantitative relationship between the endothelial K(PS) and molecular weight. RESULTS: Inverse relationships between molecular weight of contrast agent and K(PS) and f(PV) of tumor rim (r = -0.8, P < .001 and r = -0.5, P = .04, respectively) and core (r = -0.7, P = .004 and r = -0.6, P = .01, respectively) were observed. The quantitative relationship between K(PS) and molecular weight (MW) was K(PS) = 0.4/MW(0.44). A decreasing stepwise pattern in f(PV) was noted between contrast agents with low (0.7- and 3.0-kDa) molecular weight and those with high (12- and 51-kDa) molecular weight. CONCLUSION: Macromolecular permeability is best measured with high-molecular-weight contrast agents; endothelial K(PS) values measured with low-molecular-weight contrast agents incorporate tissue perfusion and permeability and demonstrate heterogeneous microcirculatory flow.

Probing the interaction of the biotin-avidin complex with the relaxivity of biotinylated Gd-DTPA.

A biotinylated Gd-DTPA complex is designed to study the biotin-avidin complexation using the longitudinal relaxivity of this new MRI label, which illustrates the use of MRI contrast agents to probe the formation of supramolecular assemblies in water.

Intermolecular 2hJNN coupling in multiply hydrogen-bonded ureidopyrimidinone dimers in solution.

15N-Labeled ureido-4[1H]-pyrimidinones 4a and 5a were synthesized in order to investigate hydrogen bonding in the strongly hydrogen-bonded dimers in solution with intermolecular (2h)J(NN) coupling. Both direct-detection (15)N NMR and one-dimensional (15)N INADEQUATE (for smaller scalar coupling constants) were employed to determine the coupling constants. For dimers of 4 in CDCl(3), a temperature-dependent (2h)J(NN) was observed ranging from 2 Hz at +10 degrees C to 5.1 Hz at -20 degrees C. In dimers of more slowly exchanging bifunctional derivative 5, the coupling constants could be determined at room temperature from an inverse-gated (1)H-decoupled (15)N NMR experiment. Coupling constants in different isomers of the dimer of 5a (4.96, 5.13, 4.37, and 5.27 Hz) were used to test the relationship between (2h)J(NN) values and N-N distances as proposed by Del Bene et al. The N-N distances calculated with the aid of this relationship show excellent agreement with the distances observed in the X-ray crystal structures of 5b, particularly when the nonlinearity of the hydrogen bonds is taken into account.