Carboxyl-terminated Dendrimers Research Articles

PH- and Functionalization-Dependent Host-Guest Interactions Between Fluorescein and Various Poly(amidoamine) Dendrimers.

Dendrimers are branched macromolecules that can be functionalized with a large variety of chemical moieties. Dendrimers can therefore be specifically designed to interact with target molecules. Although tailored dendrimers hold promise for targeted drug delivery and wastewater cleanup, these applications require more detailed and systematic studies on how dendrimer-guest interactions depend on environmental conditions. In light of this need, we studied pH-dependent interactions between fluorescein and poly(amidoamine) dendrimers with three different terminal groups. Crucially, both fluorescein and dendrimers have multiple protonation equilibria, which can enable interactions in different pH windows through various possible mechanisms. Such interactions are studied through UV-vis and fluorescence spectroscopies, which reveal a redshift that occurs upon fluorescein-dendrimer binding. The resulting pH-dependent spectra are complex but can be analyzed quantitatively with an open-source mathematical protocol. Consequently, we show that fluorescein binds across four pH units with amine-terminated dendrimers, across two units with hydroxyl-terminated dendrimers and does not interact attractively with carboxyl-terminated dendrimers. These functionalization-dependent host-guest interactions stabilize fluorescein's dianionic form and are predominantly electrostatically driven, with likely auxiliary hydrogen and CH-π bonding. Notably, these auxiliary mechanisms appear too weak to drive dendrimer-fluorescein interactions on their own. Overall, this work yields valuable insights into dendrimer-fluorescein association and provides a readily reproducible framework for studying host-guest interactions.

Influence of dendrimer surface chemistry and pH on the binding and release pattern of chalcone studied by molecular dynamics simulations.

Poly(amidoamine) (PAMAM) dendrimers are promising nanocarriers that can enhance the solubility of hydrophobic drugs. The surface chemistry of dendrimers is of great relevance as end groups of these nanocarriers can be easily modified to improve the bioavailability and sustained release of the cargo. Therefore, a molecular-level understanding of the host-guest interactions that can give both qualitative and quantitative information is particularly desirable. In this work, fully atomistic molecular dynamics simulations were used to study the association of a bioactive natural product, ie, chalcone, with amine-, acetyl-, and carboxyl-terminated PAMAM dendrimers at physiological and acidic pH environments. Amine- and carboxyl-terminated PAMAM dendrimers have an open microstructure at low pH that is not able to hold the ligand tightly, resulting in an unfavorable encapsulation of the chalcone molecule. In the case of acetyl-terminated dendrimer, chalcone molecule diffuses out of the dendritic cavities a few times during the simulation time and prefers to locate close to the surface of dendrimer. Average center of mass distance values at neutral pH showed that the chalcone molecule bounds firmly in the internal pockets of amine-, acetyl-, and carboxyl-terminated dendrimers and forms stable complexes with these nanovectors. The potential of mean force calculations showed that the release of the ligand from the dendrimers occurs at a controlled rate in the body.

Improving the Gene Transfection in Human Embryonic Stem Cells: Balancing with Cytotoxicity and Pluripotent Maintenance.

Manipulation of genes in human embryonic stem cells (hESCs) is imperative for their highly potential applications; however, the transduction efficiency remains very low. Although existing evidence revealed the type, size, and zeta potential of vector affect gene transfection efficiency in cells, the systematic study in hESCs is scarce. In this study, using poly(amidoamine) (PAMAM) dendrimers ended with amine, hydroxyl, or carboxyl as model, we tested the influences of size and surface group as well as cytotoxicity and endocytosis on hESC gene transfection. We found that in culture medium of mTeSR the particle sizes of G5, G7, G4.5COOH, and G5OH were around 5 nm and G1 had a smaller size of 3.14 nm. G5 and G7 had a slight and significant positive zeta potential, respectively, whereas G1 was slightly negative, and G4.5COOH and G5OH were significantly negative. We demonstrated that only amine-terminated dendrimers accomplished gene transfection in hESCs, which is greater than that from Lipofectamine 2000 transfection. Ten micromolar G5 had the greatest efficiency and was better than 1000 μM G1. Only a low concentration (0.5 and 1 μM) of G7 realized gene delivery. Amine-ended dendrimers, especially with higher generations, were detrimental to the growth and pluripotent maintenance of hESCs. In contrast, similarly sized hydroxyl- and carboxyl-terminated dendrimers exerted much lower cytotoxicity, in which carboxyl-terminated dendrimer maintained pluripotency of hESCs. We also confirmed the endocytosis into and significant exocytosis from hESCs using FITC-labeled G5 dendrimer. These results suggested that careful considerations of size, concentration, and zeta potential, particularly the identity and position of groups, as well as minimized exocytosis in the design of a vector for hESC gene delivery are necessary, which helps to better design an effective vector in hESC gene transduction.

Probing the molecular weight of poly(amidoamine) dendrimers and derivatives using SDS-PAGE

As a simple and versatile technique, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to analyze the molecular weight (Mw) of poly(amidoamine) (PAMAM) dendrimers and their derivatives. By optimizing the gel concentration and electrophoretic voltage, we were able to probe the Mw of PAMAM dendrimers of different generations terminated with amine, acetyl, hydroxyl, and carboxyl groups. We show that for acetyl-, hydroxyl-, and carboxyl-terminated dendrimers with neutral, slightly positive, and negative surface charges, the protein standards can be directly used for the determination of their Mws. However, for amine-terminated PAMAM dendrimers, one has to subtract the contribution of SDS molecules adsorbed onto the dendrimer surfaces. Our results clearly indicate that besides the utilization of other techniques, SDS-PAGE could be used as an alternative inexpensive, rapid, and reliable method to determine the Mw of PAMAM dendrimers with different surface functionalities.

Enhanced specificity and efficiency of polymerase chain reactions using poly(amidoamine) dendrimers and derivatives

The polymerase chain reaction (PCR) has become a fundamental technique in molecular biology. Improvements to enhance the specificity and efficiency of PCR are always required and remain a great challenge. Here we introduce generation 4 (G4) and 5 (G5) poly(amidoamine) (PAMAM) dendrimers and their derivatives with different terminal groups as a novel class of enhancers to improve the specificity and efficiency of PCR. We show that the dendrimers are effective in optimizing error-prone two-round PCR and non-specific PCR in specificity and efficiency. With the increase of the number and density of terminal amine groups, the optimum concentration of polycationic dendrimers could be decreased to as low as 1.35 nM, which is four orders of magnitude smaller than that of fully acetylated dendrimers and carboxyl-terminated dendrimers. The present study underlines a fact that dendrimers could be used as a powerful additive to enhance the specificity and efficiency of PCR.

Fluorometric assay for detection and quantitation of polyamidoamine dendrimers

Polyamidoamine dendrimers are protein-like nanomaterials that have many potential biological applications. Research studies using dendrimers often require their detection and quantitation. This article describes a simple, inexpensive, and rapid fluorometric assay based on the ability of dendrimers to bind the fluorescent probe, 8-anilino-1-naphthalene-sulfonic acid. This interaction leads to an enhanced fluorescence intensity of the dye that shows a linear relationship with dendrimer concentration. The procedure is applicable to dendrimers that have various surface functional groups and internal cores. Amine- and hydroxyl-terminated dendrimers gave the best calibration curves under alkaline conditions (pH 7.4). Carboxyl-terminated dendrimers required acidic conditions for their measurement. The assay for dendrimers with an amine surface and an ethylene diamine core was accurate between 0 and 100 μM and produced a within-run coefficient of less than 5%. Under the assay conditions used, the fluorescence signal strength was dependent on type of surface group and core length of the dendrimer.

Role of Polyelectrolyte Persistence Length in the Binding of Oppositely Charged Micelles, Dendrimers, and Protein to Chitosan and Poly(dimethyldiallyammonium chloride)

The effect of polyelectrolyte chain flexibility on adsorption to oppositely charged colloidal particles and its parametrization by persistence length have been investigated by comparison of PDADMAC and chitosan, polycations of equal linear charge density but of respectively low and high bare persistence lengths, relatively. Their relative affinity to (i) nonionic/anionic mixed micelles, (ii) carboxyl-terminated dendrimers, and (iii) bovine serum albumin (BSA) was studied by turbidimetric titrations. Potentiometric titrations and dynamic light scattering experiments as well as molecular modeling with SPARTAN were used to characterize and model these systems. The experimental and modeling results lead to the conclusion that while chain stiffness does influence the binding of polyelectrolytes to oppositely charged colloids, the persistence length is not necessarily an appropriate measure of chain flexibility on short length scales.

Control of composition in the multilayer films fabricated from mixed solutions containing two dendrimers

A series of multilayer films were fabricated alternately from poly(4-vinylpyridine) solution and mixed solutions of two carboxyl-terminated dendrimers. UV-Vis absorption spectroscopy was applied to study the composition of the multilayer films. In contrast to mixed solutions of linear polymers, the total amount of dendrimers in multilayer film is independent of the ratio of dendrimers in mixed solution, which is possibly due to the globular conformation of the dendrimer. Furthermore, the ratio of two dendrimers in films is linearly dependent on the ratio of the two dendrimers in solutions, which facilitates control of the composition in multilayer films.

Small-Angle Neutron Scattering Studies of Charged Carboxyl-Terminated Dendrimers in Solutions

Small-angle neutron scattering was used to characterize the solution behavior of charged carboxylic acid terminated "cascade" dendrimers (Z-Cascade/methane [4]/3-oxo-6-oxa-2-azaheptylidyne/3-oxo-2-azaheptylidyne/propanoic acids) of third (G3) and fifth (G5) generations as a function of dendrimer concentration, pH, and ionic strength. An increase in dendrimer concentration leads to a single broad peak in the scattering profile arising from interdendrimer interaction. The dissociation of terminal carboxylate groups also gives rise to an interdendrimer interaction peak, which could be suppressed by the addition of excess salt. The results of contrast matching measurements indicate the accumulation of an excess concentration of tetramethylammonium counterions around the surface of these highly charged particles, and the thickness of these counterions lies somewhere between 4 and 6 A. This conclusion is consistent with our previous potentiometric titration (Zhang, H.; et al. J. Phys. Chem. B 1997, 101, 3494) and capillary electrophoresis (Huang, Q. R.; et al. J. Phys. Chem. B 2000, 104, 898) data.

Effect of Electric Field on the Mobility of Carboxyl-Terminated Dendrimers

The effect of an applied electric field on the mobilities of carboxyl-terminated dendrimers has been measured by capillary electrophoresis at applied voltages V ranging from 5 to 30 kV. The mobility increased with the field strength at moderate ionic strength, but varied inversely with V at low ionic strength. These results did not arise from Joule heating or from electroosmotic flow effects. It is suggested that the remarkably high velocities of these dendrimers enables them to outstrip their ion atmospheres at high V, in a manner related to the increase in conductance at high field strength first described by Wien in 1927. At low ionic strength, the increase in the size of the ion atmosphere leads to increased frictional drag with increasing field strength.

Counterion Binding on Charged Spheres: Effect of pH and Ionic Strength on the Mobility of Carboxyl-Terminated Dendrimers

Capillary electrophoresis was used to study the effect of pH on the mobility of carboxyl-terminated dendrimers ranging from generation two to five. Even though the net charge for G5 (−972) is much higher than that of G2 (−36), its electrophoretic mobility is lower. Titration studies reveal that the effective surface charge density of G5 is lower than the geometric surface charge density, which we attribute to counterion binding. The results suggest a critical condition for nonspecific counterion binding to spheres, analogous but not identical to the well-known Manning condition for counterion condensation on polyelectrolytes.

Complex Formation by Electrostatic Interaction between Carboxyl-Terminated Dendrimers and Oppositely Charged Polyelectrolytes

Complex formation between a carboxyl-terminated cascade polymer (generation 3) and several cationic polyelectrolytes of varying linear charge density was studied as a function of ionic strength, by turbidimetric titration and dynamic light scattering. Tetramethylammonium chloride was used to adjust the ionic strength in order to avoid sodium counterion binding to dendrimer carboxyl groups. Complex formation occurred abruptly at a critical pH, as signaled by a sudden change in either the turbidity or the apparent Stokes radius from dynamic light scattering. The pHc of incipient complex formation was converted to the critical surface charge density σc. Under conditions of low or moderate ionic strength (I), it was confirmed that σc is roughly proportional to κ/ξ, where κ ∼ I1/2 is the Debye−Huckel parameter and ξ is the linear charge density of the polyelectrolyte.

Size-Exclusion Chromatography of Carboxyl-terminated Dendrimers as a Model for Permeation of Charged Particles into Like-Charged Cavities

In order to examine the permeation of small charged colloids into cavities of like charge, size-exclusion chromatography was carried out with carboxyl-terminated dendritic polymers on a porous glass stationary phase. Chromatographic partition coefficientsKSECwere measured for solute diameters ranging from 2 to 8 nm, at neutral pH, and ionic strengths from 0.01 to 0.09M,and were reported relative to those of noninteracting solutes of equal size (pullulan and Ficoll). The experimental results were compared to values calculated using the treatment of F. G. Smith III and W. M. Deen for charged spheres in cylindrical pores of like charge (J. Colloid Interface Sci.78,44 (1980);91,571 (1983)). The measured degrees of permeation were typically 20–100% larger than the calculated values, the discrepancies being greatest for the larger solutes at high ionic strength. It is shown that the tendency of the calculations to overestimate the repulsion are likely to arise from linearization of the Poisson–Boltzmann equation.