Chemistry Reference
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dynamic light scattering (Zhang et al. 1997; Miura et al. 1999; Zhang et al. 1999),
capillary electrophoresis (Seyrek et al. 2004), and potentiometric titration (Zhang
et al. 1997). These studies indicate that the dendrimers adopt a spherical structure
having a uniform charge density that is modulated by both pH and salt conditions.
Small-angle neutron scattering (SANS) studies confirm the localization of counter-
ions at the surface of these dendrimers at high pH (Huang et al. 2005).
The conformational behavior of polyelectrolyte dendrimers having charges distrib-
uted throughout the volume of the dendrimers has been somewhat controversial.
Welch and Muthukumar (1998) predicted a reversible conformational transition from
a compact, dense core to a hollow, dense shell structure (up to a 180% increase in
volume) as the ionic strength of the medium was changed from high to low. Their pre-
dictions were based on a series of Monte Carlo simulations on dendritic polyelectro-
lytes, using a mean-field approach to implicitly treat water and counterion molecules.
The structural variation with the ionic strength was attributed to the ability of ions to
screen the coulombic interactions between charged groups, thereby allowing for the
usual dense core structure of neutral dendrimers. At low ionic strength, higher effective
charge produces a structural expansion that minimizes charge repulsion (Fig. 11.11).
Similar conclusions were reported by Lyulin et al. (2004) using Brownian
dynamics simulations, which treated the solvent as an effective viscous continuum.
In contrast to these predictions, Nisato et al. (2000) observed experimentally using
SANS that the size of a generation 8 PAMAM dendrimer was invariant with the
pH and ionic strength. The discordance between experiment and theory arose from
the implicit treatment of water and ions by a mean-field approach. Several molecular
dynamics (MD) attempts to explicitly treat the water and counterions have been
reported that qualitatively predict a small conformational change with pH
(Lee, Athey, et al. 2002; Terao and Nakayama 2004; Maiti et al. 2005; Maiti
and Goddard 2006; Opitz and Wagner 2006; Terao 2006; Giupponi et al. 2007;
Lin et al. 2007) Moreover, the explicit treatment of water molecules in these MD
Figure 11.11 Schematic representation of the structural impact of counterion screening on
the size of the dendrimer.
