PubMed Health⌕ Search

Biomedical subjects

E W Meijer

Publications and source records attributed to E W Meijer.

At least 37 records · Page 2Linked to original sources
PubMed2006

The influence of hydrogen bonding and pi-pi stacking interactions on the self-assembly properties of C3-symmetrical oligo(p-phenylenevinylene) discs.

Three C3-symmetrical discotics containing a 1,3,5-benzenetricarboxamide unit functionalized with pi-conjugated oligo(p-phenylenevinylene)s (OPV)s have been synthesized and fully characterized. For the two amide OPV discs a two-step transition from helical stacks to molecularly dissolved species was observed and surprisingly, the topology of the amide determines the stability and helicity of the fibers in solution and the length of the fibrils at a surface. In case of the bipyridine disc, aggregates were formed that show little chiral ordering while the stacks remain present over a large temperature range. At a surface, completely disordered structures exist probably as a result of competing types of pi-pi stacking interactions that differ in strength and orientation. The results show that the design of functional self-assembled architectures based on hydrogen bonding and pi-pi stacking interactions is an extremely delicate matter and reveal that special demands have to be taken into account to balance the topology, directionality and strength of multiple secondary interactions.

Journal Article↗
PubMed2006

Fractal-like self-assembly of oligo(p-phenylene vinylene) capped gold nanoparticles.

Reversible interparticle self-organization is reported for gold nanoparticles functionalized with an oligo(p-phenylene vinylene) moiety in butanol. The aggregates show a clear melting temperature at 80 degrees C, and atomic force microscopy and transmission electron microscopy indicate a fractal-like organization of the particles.

Journal Article↗
PubMed2006

Pd-catalyzed amidation of 2-chloro- and 2,7-dichloro-1,8-naphthyridines.

[reactions: see text] The catalytic amidation between 2-chloro- and 2,7-dichloro-1,8-naphthyridines and primary amides bearing functional groups is reported. When Pd(OAc)2, xantphos, and K2CO3 are used, it is possible to obtain symmetric as well as nonsymmetric 2,7-diamido-1,8-naphthyridines in 50-90% yield with good functional-group tolerance. Monoamidation of 2,7-dichloro-1,8-naphthyridine using 0.9 equiv of the amide proceeded with good selectivity compared to the formation of the diamide, but as a result of the difficult isolation of the product, isolated yields were poor to moderate (22-42%).

Journal Article↗
PubMed2006

Radical cation formation in characterization of novel C3-symmetric disks and their precursors by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Four C3-symmetrical tris(dipeptide) disks and their precursors were characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). The C3-symmetrical disks were based on a benzene-1,3,5-triscarboxamide core extended by oligopeptides with trialkoxyanilide tails. The results indicate that MALDI TOF MS is a powerful and straightforward analytical technique for characterizing C3-symmetrical disks and their precursors. Clear (pseudo)-molecular ion peaks could readily be identified. It is remarkable that strong radical ion signals were observed for all the compounds, including the anilines that were expected to be protonated prior to laser irradiation using acidic MALDI matrixes. Possible mechanisms for radical ion formation were investigated with the employment of radical scavengers, with various matrixes and with direct laser desorption/ionization (LDI). Most likely the radicals are formed by losing one electron from the aniline nitrogen and stabilized by conjugation through the phenyl ring. It appears that direct photo/thermal ionization of analytes is an important route for the radical ion formation of the compounds with trialkoxy aniline/anilide groups.

Journal Article↗
PubMed2006

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.

Animals↗
PubMed2006

Molecular recognition in poly(epsilon-caprolactone)-based thermoplastic elastomers.

The molecular recognition properties of the hydrogen bonding segments in biodegradable thermoplastic elastomers were explored, aiming at the further functionalization of these potentially interesting biomaterials. A poly(epsilon-caprolactone)-based poly(urea) 2 was synthesized and characterized in terms of mechanical properties, processibility and histocompatibility. Comparison of the data with those obtained from the structurally related poly(urethane urea) 1 revealed that the difference in hard segment structure does not significantly affect the potency for application as a biomaterial. Nevertheless, the small differences in hard block composition had a strong effect on the molecular recognition properties of the hydrogen bonding segments. High selectivity was found for poly(urea) 2 in which bisureidobutylene-functionalized azobenzene dye 3 was selectively incorporated while bisureidopentylene-functionalized azobenzene dye 4 was completely released. In contrast, the incorporation of both dyes in poly(urethane urea) 1 led in both cases to their gradual release in time. Thermal analysis of the polymers in combination with variable temperature infrared experiments indicated that the hard blocks in 1 showed a sharp melting point, whereas those in 2 showed a very broad melting trajectory. This suggests a more precise organization of the hydrogen bonding segments in the hard blocks of poly(urea) 2 compared to poly(urethane urea) 1 and explains the results from the molecular recognition experiments. Preliminary results revealed that a bisureidobutylene-functionalized GRGDS peptide showed more supramolecular interaction with the PCL-based poly(urea), containing the bisureidobutylene recognition unit, as compared to HMW PCL, lacking this recognition unit.

3T3 Cells↗
PubMed2006

Modified poly(propylene imine) dendrimers as effective transfection agents for catalytic DNA enzymes (DNAzymes).

The major bottleneck in gene therapy remains the issue of delivery. In this work, various modified poly(propylene imine) (PPI) dendrimers are introduced as gene transfection agents. Commercially available PPI-dendrimers have been modified (i) at the exterior primary amines with acetyl groups or glycol gallate (PEG-like) groups, and (ii) at the interior tertiary amines with methyl iodide (MeI) or MeCl to produce multiple quaternized cationic sites in the core of the dendrimer. The prepared materials have been tested with respect to their binding capabilities to DNA, their toxicity in cell cultures, their in vitro transfection efficiency and their in vivo delivery possibilities. In all cases, a 33-mer oligonucleotide (DNAzyme) was used. Polyacrylamide gel electrophoresis (PAGE) studies have demonstrated strong but reversible binding, where the quarternized and higher generation dendrimer species have shown more potent binding. Typically, for the modified fourth PPI-dendrimers, binding is observed at a concentration of about 4 microM DNA and a dendrimer-DNA charge ratio of around 2:1-1:1. All the tested PPI-dendrimers display a low cellular toxicity, especially when higher serum contents are used in the culture medium. For example, most of the prepared fourth generation PPI-dendrimers are not or hardly toxic up to at least 20 microM in 20% serum. An in vitro characterization has revealed a high dendrimer-mediated intracellular uptake of the DNAzyme: all the tested fourth generation PPI-dendrimers display transfection efficiencies close to or exceeding 80%, even when the concentration of serum in the medium is increased from 10 to 40%. Finally, the potential of using modified PPI-dendrimers for in vivo gene therapy experiments is demonstrated. Injecting a G4-PEG(MeI)-ssDNA complex intravenously into Nude mice has resulted in a high nuclear uptake as confirmed by co-localization studies.

Animals↗
PubMed2005

Energy-transfer efficiency in stacked oligo(p-phenylene vinylene)s: pronounced effects of order.

The supramolecular structure of two types of oligo(p-phenylene vinylene) (OPV) building blocks in dodecane solution is studied. Monofunctional chromophores (MOPV) form well-defined helical assemblies, whereas bifunctional molecules (BOPV) aggregate into so-called frustrated stacks, lacking any higher helical order. This difference in organization has a major influence on the transfer of excitation energy through the assemblies. Energy transfer to supramolecularly incorporated guests (MOPV with lower bandgap) is used to probe the intrinsic differences in exciton mobility in these two types of mixed aggregates. From the observed donor fluorescence quenching, it can be concluded that the helically ordered nature of the MOPV stacks facilitates the transfer of excitation energy, yielding evidence for higher exciton mobility in the well-ordered assemblies than in the frustrated stacks. Finally, the concept of energy transfer in supramolecular assemblies is extended to the solid state by the successful implementation in a light-emitting diode (LED).

Journal Article↗
PubMed2005

Scanning tunneling microscopy and spectroscopy of donor-acceptor-donor triads at the liquid/solid interface.

By means of scanning tunneling microscopy (STM), the self-assembly of two organic donor-acceptor-donor triads (donor=oligo(p-phenylene vinylene) (OPV); acceptor=perylene diimide (PDI)) and their mixtures has been investigated at the liquid/solid interface. Both triads differ in the nature of the substituents and, therefore, in the redox properties of the central perylene diimide unit (H or Cl). Thanks to the submolecular resolution, the distinct electronic properties of the units, within a triad and between the two triads, are reflected by the relative STM contrast in the bias-dependent imaging experiments. Moreover, scanning tunneling spectroscopy reveals an inverse rectifying behavior of the OPV and H-substituted PDI units, which is discussed in the framework of quasi-resonant tunneling. A striking difference is observed for the Cl-substituted triad.

Journal Article↗
PubMed2005

Dendrimer-based transient supramolecular networks.

Association of a 16-fold excess of a monodisperse telechelic oligo(THF) (Mw = 1270 g/mol) containing two end groups that selectively bind to the 32 binding sites of a fifth generation dendritic host (Mw = 18,511 g/mol and radius R(h) = 2.4 nm) results in the formation of reversible and dynamic supramolecular complexes. The structure of these complexes in solution depends strongly on the concentration. At low concentration, the two end groups of one guest are proposed to complex to the same host, and flowerlike structures are formed with a radius of R(h) = 3.7 nm. At higher concentrations, both end groups of one guest are complexed to different hosts, forming a bridge between them. This gives rise to the formation of larger associates, and eventually to a transient supramolecular network. Dynamic light scattering unequivocally showed that three distinct relaxation processes, associated with the proposed structures, are present in this system. In addition to the dynamics related to the flowerlike (fast) and the transient network structures (slow), an intermediate dynamic process is attributed to the cooperative motion of a few (approximately 6) connected flowerlike structures. Rheological data elucidate the nature of the intermittent network responsible for the slowest process. A monofunctional guest, not capable of forming a network structure, was used as a reference, and starlike supramolecular structures are formed at all concentrations, indeed.

Acetates↗
PubMed2005

Charge transport in self-organized pi-stacks of p-phenylene vinylene oligomers.

We have studied the mobility of charge carriers along self-organizing pi-stacks of hydrogen-bonded phenylene vinylene oligomers in solution, by time-resolved microwave conductivity measurements. The value deduced for the mobility along the stacks is 3 x 10(-3) and 9 x 10(-3) cm2/(V s) for holes and electrons, respectively. Additionally, we have calculated the mobility along the pi-stacks using a hopping model based on parameters from density functional theory. The mobility values obtained from these calculations are in good agreement with the experimental values if it is assumed that there are relatively large twist angles between neighboring molecules in the stack. It is shown that a significantly higher mobility can be attained if the twist angle between neighboring oligomers is reduced.

Journal Article↗
PubMed2005

Sequential energy and electron transfer in aggregates of tetrakis[oligo(p-phenylene vinylene)] porphyrins and C60 in water.

Chiral aggregation of oligo(p-phenylene vinylene)-functionalized Zn and free-base porphyrins is observed in water. The formation of mixed assemblies containing both porphyrins results in sequential energy transfer from OPV via zinc porphyrin to free-base porphyrin. Furthermore, the incorporation of C60 as electron acceptor yields a charge separated state by ultimate electron transfer.

Journal Article↗
PubMed2005

Crystalline-crystalline block copolymers of regioregular poly(3-hexylthiophene) and polyethylene by ring-opening metathesis polymerization.

Block copolymers of regioregular poly(3-hexylthiophene) (P3HT) and polyethylene (PE) were synthesized through the chain transfer of olefin-terminated P3HT in the presence of cyclooctene via ring-opening metathesis polymerization (ROMP). Subsequent hydrogenation of the poly(cyclooctene) block yielded high molecular weight, crystalline-crystalline P3HT-PE block copolymers, which are thermally stable and resistant to solvents under ambient conditions. These copolymers were characterized by 1H NMR, DSC, and WAXS and represent the first materials of a class of crystalline-crystalline semiconducting-insulating block copolymers.

Calorimetry, Differential Scanning↗