Diffusion of flexible, charged, nanoscopic molecules in solution: size and pH dependence for PAMAM dendrimer

Maiti, Prabal K. ; Bagchi, Biman (2009) Diffusion of flexible, charged, nanoscopic molecules in solution: size and pH dependence for PAMAM dendrimer Journal of Chemical Physics, 131 (21). 214901_1-214901_7. ISSN 0021-9606

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Official URL: http://jcp.aip.org/jcpsa6/v131/i21/p214901_s1

Related URL: http://dx.doi.org/10.1063/1.3266512

Abstract

In order to understand self-diffusion (D) of a charged, flexible, and porous nanoscopic molecule in water, we carry out very long, fully atomistic molecular dynamics simulation of PAMAM dendrimer up to eight generations in explicit salt water under varying pH. We find that while the radius of gyration (Rg) varies as N1/3, the self-diffusion constant (D) scales, surprisingly, as N, with α = 0.39 at high pH and 0.5 at neutral pH, indicating a dramatic breakdown of Stokes-Einstein relation for diffusion of charged nanoscopic molecules. The variation in D as a function of radius of gyration demonstrates the importance of treating water and ions explicitly in the diffusion process of a flexible nanoscopic molecule. In agreement with recent experiments, the self-diffusion constant increases with pH, revealing the importance of dielectric friction in the diffusion process. The shape of a dendrimer is found to fluctuate on a nanosecond time scale. We argue that this flexibility (and also the porosity) of the dendrimer may play an important role in determining the mean square displacement of the dendrimer and the breakdown of the Stokes-Einstein relation between diffusion constant and the radius.

Item Type:Article
Source:Copyright of this article belongs to American Institute of Physics.
ID Code:3963
Deposited On:13 Oct 2010 07:08
Last Modified:07 Jan 2011 04:56

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