Structure control within poly(amidoamine) dendrimers: size, shape and regio-chemical mimicry of globular proteins

Assemblies of dendrimers and proteins on carbon and gold electrodes

Dendritic macromolecules of two adjacent (G3.5 and G4) generations have been used to modify gold or carbon electrodes. The structure and stability of deposited films have been explored by quartz crystal microbalance (QCM), Surface Plasma Resonance (SPR) and electrochemistry. Dendrimers have been shown to adsorb spontaneously on electrode materials as compressed macromolecular films. They are able to inhibit (G3.5) or promote (G4) electroactive anionic species such as Fe(CN)(6)(3-/4-) used as a probe system. Mixed protein/dendrimer assemblies have been constructed with proteins differing in charge, nature of the prosthetic groups and sizes such as lysozyme, cytochrome c, polyhemic cytochrome c(3) or glucose oxidase. Generally, the stability of adsorbed films seems to be limited to one dendrimer/protein bilayer. Owing to the satisfactory stability of composite cytochrome c(3)/G3.5 or glucose oxidase/G4 films, biosensing applications are described for metal bioremediation and glucose detection, respectively.

Growth of multi-amine terminated poly(amidoamine) dendrimers on the surface of carbon nanotubes

An in situ repetitive divergent polymerization strategy was employed to grow multi-amine poly(amidoamine) dendritic macromolecules on the surfaces of multiwalled carbon nanotubes (MWNTs), affording novel three-dimensional (3D) molecular nanocomposites. The crude MWNTs were oxidized using H(2)SO(4)/HNO(3) = 3:1 (v/v) and then reacted with thionyl chloride, resulting in MWNTs functionalized with chlorocarbonyl groups (MWNT-COCl). MWNT-COCl, when reacted with an excess of ethylenediamine, produced amine-functionalized MWNT supported initiators (MWNT-NH(2)). Using the MWNT-NH(2) as the growth supporter and methylacrylate/ethylenediamine as building blocks, multi-amine dendritic poly(amidoamine) macromolecules were covalently grafted onto the sidewalls and ends of MWNTs via Michael addition reaction and amidation. Thermal gravimetric analysis (TGA) measurements showed that the weight ratio of the as-grown dendritic polymers on the MWNT surfaces lay in the 10%-50% range. The products were also characterized by Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) analysis. The results indicate that the dendrimers are grafted onto the surface of MWNTs. The as-prepared nanocomposites exhibit excellent dispersibility in water.

Dendritic Arrays of [Re6(μ3-Se)8]2+ Core-Containing Clusters: Exploratory Synthesis and Electrochemical Studies

The reaction between the previously reported site-differentiated cluster solvate [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(MeCN)](SbF(6))(2) (1) with pyridyl-based ditopic ligands 4,4'-trimethylenedipyridine (2), 1,2-bis(4-pyridyl)ethane (3), and (E)-1,2-bis(4-pyridyl)ethene (4) afforded cluster complexes of the general formula [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(L)](SbF(6))(2) (5-7), where L represents one of the pyridyl-based ligands. Reacting these cluster complex-based ligands with the fully solvated cluster complex [Re(6)(mu(3)-Se)(8)(MeCN)(6)](SbF(6))(2) (8) produced dendritic arrays of the general formula {Re(6)(mu(3)-Se)(8)[Re(6)(mu(3)-Se)(8)(PEt(3))(5)(L)](6)}(SbF(6))(14) (9-11), each featuring six circumjacent [Re(6)(mu(3)-Se)(8)(PEt(3))(5)](2+) units bridged to a [Re(6)(mu(3)-Se)(8)](2+) core cluster by the pyridyl-based ligands. Electrochemical studies using a thin-layer electrochemical cell revealed cluster-based redox events in these cluster arrays. For 9 (L = 2), one reversible oxidation event corresponding to the removal of 7 electrons was observed, indicating noninteraction or extremely weak interactions between the clusters. For 10 (L = 3), two poorly resolved oxidation waves were found. For 11 (L = 4), two reversible oxidation events, corresponding respectively to the removal of 1 and 6 electrons, were observed with the 1-electron oxidation event occurring at a potential 150 mV more positive than the 6-electron oxidation. These electrochemical studies suggest intercluster coupling in 11 via through-bond electronic delocalization, which is consistent with electronic spectroscopic studies of this same molecule.

Study on interaction between poly(amidoamine) dendrimer and CdSe nanocrystal in chloroform

The binding of different categories of molecules to quantum dot has been studied for many years through different spectroscopic techniques to elucidate details of binding mechanism. In this work we present the results of the study of the interactions between CdSe and poly(amidoamine) dendrimer monitored by photoluminescence spectroscopy of CdSe in chloroform. Dendrimers with different terminal groups and different generations were used to bind with CdSe nanocrystal of different size. Significant differences in the values of binding constant Kb(n) and K(SV) were found in these experiments. The binding constant for poly(amidoamine) dendrimer of generation 4.0 is higher as compared to generation 3.5. The interaction of CdSe with poly(amidoamine) dendrimer shows an increase of binding constants with increasing dendrimer generation from 2.0 to 4.0, as well as with decreasing CdSe diameter. From HRTEM and FTIR analysis, we suggest that dendrimer/CdSe interactions are primarily hydrogen-bonding.

Polymeric nanoparticles for gene delivery

Since the evolution of the concept of gene therapy, delivering therapeutic genes to the diseased cells has been a major challenge. Although viral vectors have been shown to be efficient in delivering genes, the issue of their safety is still to be solved. Meanwhile, the field of developing nonviral expression vectors has seen considerable progress. As compared with viruses, these are relatively safe but are confronted with the problem of poor transfection efficiency. With the growing understanding of the biology of gene transfection, and the continued efforts at enhancing the efficiency of nonviral expression vectors, it could soon become a preferred option for human gene therapy. In this review, the potential of polymeric nanoparticles as a gene expression vector is discussed. Furthermore, the importance of understanding the pathophysiology of disease conditions in developing gene expression vectors is discussed in Section 6.

Phosphinic derivative of DTPA conjugated to a G5 PAMAM dendrimer: an 17O and 1H relaxation study of its Gd(III) complex

A DTPA-based chelate containing one phosphinate group was conjugated to a generation 5 polyamidoamine (PAMAM) dendrimer via a benzylthiourea linkage. The Gd(III) complex of this novel conjugate has potential as a contrast agent for magnetic resonance imaging (MRI). The chelates bind Gd3+via three nitrogen atoms, four carboxylates and one phosphinate oxygen, and one water molecule completes the inner coordination sphere. The monomer Gd(III) chelates bearing nitrobenzyl and aminobenzyl groups ([Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-) as well as the dendrimeric Gd(III) complex G5-(Gd(DTTAP))63) were studied by multiple-field, variable temperature 17O and 1H NMR. The rate of water exchange is faster than that of [Gd(DTPA)(H2O)]2- and very similar on the two monomeric complexes (8.9 and 8.3 x 10(6) s-1 for [Gd(DTTAP-bz-NO2)(H2O)]2- and [Gd(DTTAP-bz-NH2)(H2O)]2-, respectively), while it is decreased on the dendrimeric conjugate (5.0 x 10(6) s-1). The Gd(III) complex of the dendrimer conjugate has a relaxivity of 26.8 mM-1 s-1 at 37 degrees C and 0.47 T (corresponding to 1H Larmor frequency of 20 MHz). Given the contribution of the second sphere water molecules to the overall relaxivity, this value is slightly higher than those reported for similar size dendrimers. The experimental 17O and 1H NMR data were fitted to the Solomon-Bloembergen-Morgan equations extended with a contribution from second coordination sphere water molecules. The rotational dynamics of the dendrimeric conjugate was described in terms of global and local motions with the Lipari-Szabo approach.

Cooperative binding and self-assembling behavior of cationic low molecular-weight dendrons with RNA molecules

Tri(ethylene glycol) derived, low molecular-weight dendrons with various amine end groups were synthesized and characterized for their properties of binding and self-assembling with RNA using the Candida ribozyme as a model RNA molecule. These dendritic compounds form stable complexes and well-defined nanoscale particles with RNA molecules via electrostatic interactions and self-assembly process, while leaving the other terminal of the tri(ethylene glycol) chain accessible for targeting. This suggests that dendrimers of this type hold great promise for specific RNA targeting and RNA delivery.

Polyacrylamide gel electrophoresis separation and detection of polyamidoamine dendrimers possessing various cores and terminal groups

Detection and separation of polyamidoamine dendrimers possessing various cores and surface groups was studied by polyacrylamide gel electrophoresis. Although many dyes and staining techniques were able to detect dendrimers on polyacrylamide gels, Coomassie Blue was found to be the most sensitive and convenient. Amine and hydroxyl terminated dendrimers were best separated under acidic conditions, while dendrimers with carboxyl surfaces required alkaline buffers. Some dendrimers were very susceptible to diffusion that could occur during their separation, staining or destaining steps. In the absence of an appropriate fixation step, dendrimers could be resolved by using small pore size gels and low voltage or current. Increasing core lengths did not significantly affect migration of a given dendrimer generation but exhibited improved separation and staining characteristics. Polyacrylamide gel electrophoresis was found to be a rapid, inexpensive, and reliable procedure to characterize many different water-soluble dendritic macromolecules.

Analysis of poly(amidoamine)-succinamic acid dendrimers by slab-gel electrophoresis and capillary zone electrophoresis

Ethylenediamine (EDA)-core poly(amidoamine) (PAMAM) succinamic acid dendrimers (Ex.SAH, where x refers to the generation) were synthesized and analyzed by polyacrylamide gel electrophoresis (PAGE), size-exclusion chromatography (SEC), potentiometric acid-base titration, and capillary zone electrophoresis (CZE). Various generations (E1.SAH-E7.SAH) PAMAMs and a succinamic acid terminated core-shell tecto(dendrimer) (E5(E3.SAH)(n)) were first analyzed by PAGE. PAGE results show that the relative mobilities of generation 2 to generation 7 dendrimers decreased with the increasing number of generations. The molecular mass of a generation 5 core generation 3 shell tecto(dendrimer) (denoted as E5(E3.SAH)(n)) was determined to be between the Mw of E6.SAH and E7.SAH. CZE analysis allowed the evaluation of electrophoretic properties of given-generation dendrimers. The electrophoretic mobilities of individual generations PAMAM polyanions are similar, indicating that the separation mainly depends on their approximately identical charge/mass ratio. The E5(E3.SAH)(n) tectodendrimer had a lower electrophoretic mobility, which was consistent with its lower charge/mass ratio. The combination of PAGE and CZE analysis provides an alternative and effective way to characterize this group of PAMAM-succinamic acid dendrimers.

Capillary Electrophoresis of Poly(amidoamine) Dendrimers: From Simple Derivatives to Complex Multifunctional Medical Nanodevices

Multifunctional poly(amidoamine) (PAMAM) dendrimer-based nanodevices provide novel nanoplatforms for targeting, imaging, and treatment of cancers in vitro and in vivo. Generational, skeletal, and substitutional dispersities are always present in dendrimer-based medical nanodevices. Molecular distribution plays a central role for one to evaluate the quality of PAMAM materials for medical applications. Capillary electrophoresis (CE) has been extensively used as a characterization technique to analyze a range of PAMAM dendrimers, from simple PAMAM derivatives to complex multifunctional PAMAM nanodevices. This review reports the recent advances in the analysis and characterization of a variety of PAMAM dendrimer-based nanoparticles ranging from polycationic and polyanionic PAMAM derivatives to PAMAMs of different generations and defined substitutions, and to complex multifunctional PAMAM nanodevices containing targeting ligands, dyes, and drugs. Understanding the structural complexity of dendrimer nanodevices is crucial for their use as multifunctional imaging, targeting, and cancer therapeutic devices, as well as for their use in biosensing, diagnostics, and control of biological systems.

Rotational Dynamics Account for pH-Dependent Relaxivities of PAMAM Dendrimeric, Gd-Based Potential MRI Contrast Agents

The EPTPA5) chelate, which ensures fast water exchange in GdIII complexes, has been coupled to three different generations (5, 7, and 9) of polyamidoamine (PAMAM) dendrimers through benzylthiourea linkages (H5EPTPA = ethylenepropylenetriamine-N,N,N',N'',N''-pentaacetic acid). The proton relaxivities measured at pH 7.4 for the dendrimer complexes G5-(GdEPTPA)111, G7-(GdEPTPA)253 and G9-(GdEPTPA)1157 decrease with increasing temperature, indicating that, for the first time for dendrimers, slow water exchange does not limit relaxivity. At a given field and temperature, the relaxivity increases from G5 to G7, and then slightly decreases for G9 (r1 = 20.5, 28.3 and 27.9 mM(-1) s(-1), respectively, at 37 degrees C, 30 MHz). The relaxivities show a strong and reversible pH dependency for all three dendrimer complexes. This originates from the pH-dependent rotational dynamics of the dendrimer skeleton, which was evidenced by a combined variable-temperature and multiple-field 17O NMR and 1H relaxivity study performed at pH 6.0 and 9.9 on G5-(GdEPTPA)111. The longitudinal 17O and 1H relaxation rates of the dendrimeric complex are strongly pH-dependent, whereas they are not for the [Gd(EPTPA)(H2O)]2- monomer chelate. The longitudinal 17O and 1H relaxation rates have been analysed by the Lipari-Szabo spectral density functions and correlation times have been calculated for the global motion of the entire macromolecule (tau(gO)) and the local motion of the GdIII chelates on the surface (tau(lO)), correlated by means of an order parameter S2. The dendrimer complex G5-(GdEPTPA)111 has a considerably higher tau(gO) under acidic than under basic conditions (tau(298)gO = 4040 ps and 2950 ps, respectively), while local motions are less influenced by pH (tau(298)lO = 150 and 125 ps). The order parameter, characterizing the rigidity of the macromolecule, is also higher at pH 6.0 than at pH 9.9 (S2 = 0.43 vs 0.36, respectively). The pH dependence of the global correlation time can be related to the protonation of the tertiary amine groups in the PAMAM skeleton, which leads to an expanded and more rigid dendrimeric structure at lower pH. The increase of tau(gO) with decreasing pH is responsible for the pH dependent proton relaxivities. The water exchange rate on G5-(GdEPTPA)111(k(298)ex = 150 x 10(6) s(-1)) shows no significant pH dependency and is similar to the one measured for the monomer [Gd(EPTPA)(H2O)]2-. The proton relaxivity of G5-(GdEPTPA)111 is mainly limited by the important flexibility of the dendrimer structure, and to a small extent, by a faster than optimal water exchange rate.

Polymeric and dendrimeric pyridoxal enzyme mimics

Pyridoxal was covalently attached to polyethylenimine polymers, but the resulting materials were found to degrade rapidly. In comparison, the dendrimeric pyridoxals, which possess only one pyridoxal unit at the core of every dendrimer molecule were found to be relatively stable compounds. A total of 12 poly(amidoamine) type dendrimers were synthesized. They range from G1 to G6 with either NMe(2) or NHAc termini. The NMe(2)-terminated pyridoxal dendrimers racemize alpha-amino acids 50-100 times faster than does simple pyridoxal, while the NHAc-terminated pyridoxal dendrimers racemize alpha-amino acids only 3-5 times faster than does simple pyridoxal. Both the NMe(2)- and NHAc-terminated pyridoxal dendrimers decarboxylate 2-amino-2-phenyl-propionic acid 1-3 times faster than simple pyridoxal. The interior polarity in the pyridoxal dendrimers is similar to that of 85:15 water-DMF solution. Furthermore, we successfully incorporated eight lauryl groups to the G5 pyridoxal dendrimer at known positions. The laurylated dendrimer exhibits lower racemization and decarboxylation rates than do the unlaurylated ones, in contrast to the positive rate effects of laurylation in polyethylenimine-pyridoxamines in our previous transamination studies.

Dendrimer-immobilized culture surface as a tool to evaluate formation of cellular cytoskeleton of anchorage-dependent cells

Anchorage-dependent cultivation of human epithelial and keratinocyte cells was carried out on surfaces modified with synthesized dendrimers. Notable elongation of the epithelial cells was recognized on the culture surface immobilized with a dendrimer having D-glucose as a functional ligand, but not when a dendrimer having L-glucose was used or when the dendrimer was ligand-free. This morphological change was attributable to a temporary grasping of the cells at the D-glucose moiety via a glucose transporter-mediated mechanism present in the cell membrane. Following visualization of the actin filaments of the cells, it was considered that the cellular elongation on the D-glucose-bound dendrimer surface reflected the degree of formation of the cellular cytoskeleton. The cellular roundness was calculated by means of image analysis of the individual cells and employed as a parameter to evaluate the formation of the cellular cytoskeleton. In the culture of keratinocytes on the D-glucose-bound dendrimer surface, it was demonstrated that the decrease in the ratio of elongated cells (i.e., cells with a low roundness value) was correlated with the deterioration in the growth potential associated with cellular senescence.

Metal-Core−Organic Shell Dendrimers as Unimolecular Micelles

The synthesis and characterization of nanoparticle-cored dendrimers (NCDs), consisting of a metal core capped by arylpolyethers terminated with ester or carboxylate groups, are reported. These NCDs, comprising nanometer-sized gold clusters at the core and organic dendrons radially connected to the gold core by gold-sulfur bonds, were analyzed by TEM, TGA, UV, IR, and NMR spectroscopies. The density of the branching units connected to the core decreased from 1.90/nm(2) for a first-generation NCD (Au-G1(CO(2)Me)) to 0.80/nm(2) for a fourth-generation NCD (Au-G4(CO(2)Me)). Although the ester-terminated NCDs were stable and resisted aggregation, they were easily hydrolyzed to the corresponding water-soluble sodium salts. Aqueous solutions of (Au-Gn(CO(2)Na)) exhibited micellar properties. Since these NCDs possess a relatively unpassivated metal core and an organic aryl ether shell with micellar and dendritic properties, they are expected to have important potential applications in catalysis.

Dendrimeric Pyridoxamine Enzyme Mimics

PAMAM dendrimers from generations 1-6 were synthesized with pyridoxamine in their core. They transaminated pyruvic and phenylpyruvic acids in water to alanine and phenylalanine, respectively, with Michaelis-Menten kinetics and high effectiveness compared with simple pyridoxamine. The largest dendrimers-similar in size to some globular proteins-were comparable in effectiveness to a previous polyethylenimine (PEI)-pyridoxamine catalyst, and to a protein-pyridoxamine catalyst, but not as effective as a previous PEI-pyridoxamine carrying lauryl hydrophobic groups. The new catalysts showed both general acid/base catalysis by their amino groups and hydrophobic binding of the phenylpyruvate substrate.