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Title: Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions

Abstract

The structural properties of generation 4 (G4) poly(amidoamine) starburst dendrimers (PAMAM) with an ethylenediamine ne (EDA) central core in D O 2 solutions have been studied by small angle neutron scattering. Upon the addition of DCl , SANS patterns show a pronounced inter-particle 2 correlation peaks due to the strong repulsion introduced by the protonation of the amino groups of the dendrimers. By solving the Ornstein-Zernike integral equation (OZ) with hypernetted chain closure (HNC), the dendrimer-dendrimer er structure factor S(Q) is determined and used to fit the experimental data. Quantitative information such as the effective charge per dendrimer and its conformational change at different conditions can be obtained. The results obtained show clear evidence that significant counterion association occurs, strongly mediating the inter-dendrimer interaction. The influence of interplay between counterions and molecular protonation of dendrimers has strong effect on the dendrimer conformation and effective interaction.

Authors:
 [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Spallation Neutron Source (SNS)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
978214
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Macromolecules; Journal Volume: 40; Journal Issue: 16
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; AQUEOUS SOLUTIONS; CHAINS; CLOSURES; CONFORMATIONAL CHANGES; EFFECTIVE CHARGE; INTEGRAL EQUATIONS; NEUTRONS; SCATTERING; STRUCTURE FACTORS

Citation Formats

Chen, Wei-Ren. Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions. United States: N. p., 2007. Web. doi:10.1021/ma0626564.
Chen, Wei-Ren. Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions. United States. doi:10.1021/ma0626564.
Chen, Wei-Ren. Mon . "Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions". United States. doi:10.1021/ma0626564.
@article{osti_978214,
title = {Small Angle Neutron Scattering Studies of the Counterion Effects on the Molecular Conformation and Structure of Charged G4 PAMAM Dendrimers in Aqueous Solutions},
author = {Chen, Wei-Ren},
abstractNote = {The structural properties of generation 4 (G4) poly(amidoamine) starburst dendrimers (PAMAM) with an ethylenediamine ne (EDA) central core in D O 2 solutions have been studied by small angle neutron scattering. Upon the addition of DCl , SANS patterns show a pronounced inter-particle 2 correlation peaks due to the strong repulsion introduced by the protonation of the amino groups of the dendrimers. By solving the Ornstein-Zernike integral equation (OZ) with hypernetted chain closure (HNC), the dendrimer-dendrimer er structure factor S(Q) is determined and used to fit the experimental data. Quantitative information such as the effective charge per dendrimer and its conformational change at different conditions can be obtained. The results obtained show clear evidence that significant counterion association occurs, strongly mediating the inter-dendrimer interaction. The influence of interplay between counterions and molecular protonation of dendrimers has strong effect on the dendrimer conformation and effective interaction.},
doi = {10.1021/ma0626564},
journal = {Macromolecules},
number = 16,
volume = 40,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Our previous study of the structure change of poly(amidoamine) starburst dendrimers (PAMAM) dendrimer of generation 5 (G5) have demonstrated that although the overall molecular size is practically unaffected by increasing DCl concentration, a configurational transformation, from a diffusive density profile to a more uniform density distribution, is clearly observed. In the current paper, the focus is placed on understanding the effect of counterion identity on the inter-molecular structure and the conformational properties by studying the effect due to DBr using small angle neutron scattering (SANS) and integral equation theory. While the overall molecular size is found to be essentially unaffectedmore » by the change in the pD of solutions, it is surprising that the intra-molecular configurational transformation is not observed when DBr is used. The overall effective charge of a dendrimer is nearly the same for < 1, independent of the type of acids. However, when > 1, the effect of counterion identity becomes significant, the effective charge carried by a charged G5 PAPAM protonated by DBr becomes smaller than that of solutions with DCl. As a consequence, a counterion identity dependence of counterion association is revealed: Under the same level of molecular protonation, the specific counterion association, which is defined as the ratio of bound chloride anions to positively charged amines per molecule, is larger for the G5 PAMAM dendrimer charged by DBr than the one by DCl.« less
  • Small-angle neutron scattering (SANS) experiments were carried out to investigate the structure of aqueous (D2O) G4 PAMAM dendrimer solutions as a function of molecular protonation and dendrimer concentration. Our results indicate unambiguously that, although the radius of gyration RG remains nearly invariant, the dendrimer radial density profile (r) decreases in the dendrimer core with a continuous increase in protonation. This discovery also suggests that RG, which is commonly adopted by numerous simulation and experimental works in describing the global dendrimer size, is not suitable as the index parameter to characterize the dendrimer conformation change. We also found that RG andmore » (r), for dendrimers dissolved in both neutral and acidified solutions, remain nearly constant over the studied concentration range. We further demonstrate that the outcome of the widely used Guinier method is questionable for extracting RG in the concentration range studied. Our results reveal the polymer colloid structural duality as benchmarks for future experimental and theoretical studies and provide a critical step toward understanding drug encapsulation by ionic bonds.« less
  • We investigate a series of poly(amidoamine) starburst dendrimers (PAMAM) of different generations in acidic, aqueous solutions using small-angle neutron scattering (SANS). While the overall molecular size is found to be practically unaffected by a pD change, a strong generational dependence of counterion association is revealed. Upon increasing the dendrimer generation, the effective charge obtained from our SANS experiments only shows a small increase in contrast to the nearly exponential increase predicted by a recent atomic simulation. We also find that with the same degree of molecular protonation the specific counterion association, which is defined as the ratio of bound chloridemore » anions to positively charged amines in solutions, is larger for higher-generation PAMAM dendrimer. The associated counterion density also increases upon increasing generation number.« less
  • In this report, we present a contrast variation small angle neutron scattering (SANS) study of a series of neutral PAMAM dendrimer in aqueous solutions using three different generations (G4-6) at a concentration of about 10 mg/ml. Varying the solvent hydrogen-deuterium ratio, the scattering contributions from the water molecules and the constituent components of PAMAM dendrimer can be determined. Using an analytical model of the scattering cross section I(Q) incorporating the effect of water penetration, we have quantified the intra-molecular space of PAMAM dendrimer by evaluating the number of guest water molecules and we draw a direct comparison to computational predictions.more » As expected, the overall available internal cavity was seen to increase as a function of increasing dendrimer generation. However, the fraction of water accessible volume in the internal cavity of a dendrimer was found to remain invariant for the three generation PAMAM dendrimers studied in this report. We have also estimated the average water density inside a dendrimer, which is found to be higher than that of bulk water.« less
  • By quantitatively analyzing small angle neutron scattering (SANS) results of protonated generation 5 (G5) polyamidoamine (PAMAM) dendrimers dissolved in aqueous solution, we have accurately determined their structural characteristics. The investigation has focused on understanding the effect of counterion valency on the counterion association and its effect to the intra-molecular density profile. It is found that the association of extremely large amount of divalent counterions significantly reduces the effective charge of a dendrimer molecule. Similar to the results of G4, we found that there is still an intra-molecular structure transition in G5 when DCl is used to protonate a dendrimer. However,more » surprisingly, no discernible transition of the density distribution profile is observed for the G5 dendrimer charged by D2SO4. We have attributed the reason to the more effective screening effect of divalent counterions.« less