Nature of the effective interaction between dendrimers

Simulation of Nondilute Dendrimer Systems with the Bond Fluctuation Model

Using the bond fluctuation model, we performed Monte Carlo simulations for solutions of generation 4 and 5 dendrimers with only an intermediate unit between the branching points at different concentrations, ranging from moderately dilute solutions to concentrated systems close to the melt behavior. This model may represent different real types of dendrimer families. We obtained the mean sizes, asphericities, displacement of units, scattering functions, radial distribution functions and structure factors. We compared the results obtained for the last two properties with much faster Monte Carlo simulations of point-like dendrimers using global potentials obtained through the study of binary interactions. The latter procedure provided good reproductions of these properties but failed in the reproduction of the scattering functions in the range of higher concentrations. In this range, the scattering function cannot be described as the product of the structure function and the form factor, because the intensity decreases when the density of the dendrimer units becomes more homogenous.

Modulating Interdendrimer Interactions through Surface Adsorption

Physical confinement of polymers not only affects their structure but also modifies their effective interaction profiles. In this article, we investigate the nature of graphene-adsorbed poly(amidoamine) (PAMAM) dendrimers' interactions using fully atomistic molecular dynamics simulations. Using the umbrella sampling technique, we calculate the potential of mean force (PMF) profiles for the interaction between two graphene-adsorbed PAMAM dendrimers of generations 3 and 4 as a function of their protonation levels. We find that the attractive PMF profile observed for the interaction between two nonprotonated (high pH) PAMAM dendrimers in bulk becomes repulsive upon adsorption. Also, the repulsive interdendrimer interactions known in bulk for the protonated dendrimers become enhanced for the adsorbed case. We further explain these weakened interactions by explicitly showing that the dendrimer-graphene interaction is an order of magnitude larger than the dendrimer-dendrimer bulk interaction. Using the force integration method, we obtain the contributions from various subinteractions present in the system, that is, dendrimer-water, dendrimer-ions, dendrimer-graphene, and dendrimer-dendrimer to the total PMF. From these contributions, we conclude that the reduced dendrimer-dendrimer interactions in the adsorbed case, as compared to those in bulk, lead to the enhanced repulsive effective interdendrimer interactions. Our PMF profiles fit well with the sum of exponential and Gaussian functions, proposed in the bulk interdendrimer interaction study. We hope the current results provide the microscopic origin of how adsorption weakens the interpolymer interactions in general.

pH and generation dependent morphologies of PAMAM dendrimers on a graphene substrate

The adsorption of PAMAM dendrimers at solid/water interfaces has been extensively studied, and is mainly driven by electrostatic and van der Waals interactions between the substrate and the dendrimers. However, the pH dependence of the adsorption driven predominantly by the van der Waals interactions is poorly explored, although it is crucial for investigating the potentiality of these dendrimers in supercapacitors and surface patterning. Motivated by this aspect, we have studied the adsorption behavior of PAMAM dendrimers of generations 2 (G2) to 5 (G5) with pH and salt concentration variation, on a charge neutral graphene substrate, using fully atomistic molecular dynamics simulations. The instantaneous snapshots from our simulations illustrate that the dendrimers deform significantly from their bulk structures. Based on various structural property calculations, we classify the adsorbed dendrimer morphologies into five categories and map them to a phase diagram. Interestingly, the morphologies we report here have striking analogies with those reported in star-polymer adsorption studies. From the fractional contacts and other structural property analyses we find that the deformations are more pronounced at neutral pH as compared to high and low pH. Higher generation dendrimers resist deformation following the deformation trend, G2 > G3 > G4 > G5 at any given pH level. As the adsorption here is mainly driven by van der Waals interactions, we observe no desorption of the dendrimers as the salt molarity is increased, unlike that reported in the electrostatically driven adsorption studies.

Dendrimer assisted dispersion of carbon nanotubes: a molecular dynamics study

Various unique physical, chemical, mechanical and electronic properties of carbon nanotubes (CNTs) make them very useful materials for diverse potential application in many fields. Experimentally synthesized CNTs are generally found in bundle geometry with a mixture of different chiralities and present a unique challenge to separate them. In this paper we have proposed the PAMAM dendrimer to be an ideal candidate for this separation. To estimate the efficiency of the dendrimer for the dispersion of CNTs from the bundle geometry, we have calculated potential of mean forces (PMF). Our PMF study of two dendrimer-wrapped CNTs shows lesser binding affinity compared to the two bare CNTs. PMF study shows that the binding affinity decreases for non-protonated dendrimer, and for the protonated case the interaction is fully repulsive in nature. For both the non-protonated as well as protonated cases, the PMF increases gradually with increasing dendrimer generations from 2 to 4 compared to the bare PMF. We have performed PMF calculations with (6,5) and (6,6) chirality to study the chirality dependence of PMF. Our study shows that the PMFs between two (6,5) and two (6,6) CNTs respectively are ∼-29 kcal mol^-1 and ∼-27 kcal mol^-1. Calculated PMF for protonated dendrimer-wrapped chiral CNTs is more compared to the protonated dendrimer-wrapped armchair CNTs for all the generations studied. However, for non-protonated dendrimer-wrapped CNTs, such chirality dependence is not very prominent. Our study suggests that the dispersion efficiency of the protonated dendrimer is more compared to the non-protonated dendrimer and can be used as an effective dispersing agent for the dispersion of CNTs from the bundle geometry.

pH controlled gating of toxic protein pores by dendrimers

Designing effective nanoscale blockers for membrane inserted pores formed by pore forming toxins, which are expressed by several virulent bacterial strains, on a target cell membrane is a challenging and active area of research. Here we demonstrate that PAMAM dendrimers can act as effective pH controlled gating devices once the pore has been formed. We have used fully atomistic molecular dynamics (MD) simulations to characterize the cytolysin A (ClyA) protein pores modified with fifth generation (G5) PAMAM dendrimers. Our results show that the PAMAM dendrimer, in either its protonated (P) or non-protonated (NP) states can spontaneously enter the protein lumen. Protonated dendrimers interact strongly with the negatively charged protein pore lumen. As a consequence, P dendrimers assume a more expanded configuration efficiently blocking the pore when compared with the more compact configuration adopted by the neutral NP dendrimers creating a greater void space for the passage of water and ions. To quantify the effective blockage of the protein pore, we have calculated the pore conductance as well as the residence times by applying a weak force on the ions/water. Ionic currents are reduced by 91% for the P dendrimers and 31% for the NP dendrimers. The preferential binding of Cl(-) counter ions to the P dendrimer creates a zone of high Cl(-) concentration in the vicinity of the internalized dendrimer and a high concentration of K(+) ions in the transmembrane region of the pore lumen. In addition to steric effects, this induced charge segregation for the P dendrimer effectively blocks ionic transport through the pore. Our investigation shows that the bio-compatible PAMAM dendrimers can potentially be used to develop therapeutic protocols based on the pH sensitive gating of pores formed by pore forming toxins to mitigate bacterial infections.

Dynamical Interactions of 5-Fluorouracil Drug with Dendritic Peptide Vectors: The Impact of Dendrimer Generation, Charge, Counterions, and Structured Water

Molecular dynamics simulations are utilized to investigate the interactions between the skin cancer drug 5-fluorouracil (5FU) and peptide-based dendritic carrier systems. We find that these drug-carrier interactions do not conform to the traditional picture of long-time retention of the drug within a hydrophobic core of the dendrimer carrier. Rather, 5FU, which is moderately soluble in its own right, experiences weak, transient chattering interactions all over the dendrimer, mediated through multiple short-lived hydrogen bonding and close contact events. We find that charge on the periphery of the dendrimer actually has a negative effect on the frequency of drug-carrier interactions due to a counterion screening effect that has not previously been observed. However, charge is nevertheless an important feature since neutral dendrimers are shown to have a significant mutual attraction that can lead to clustering or agglomeration. This clustering is prevented due to charge repulsion for the titrated dendrimers, such that they remain independent in solution.