Reports: DNI750918-DNI7: The Effect of a Polyelectrolyte on the Oppositely Charged Colloidal Suspension

Chongli Yuan, PhD, Purdue University

Stabilization of colloidal suspension has important applications in industry. Polyelectrolytes (PEs) are commonly used additives to enhance the stability of colloidal suspensions. PE can bridge the interactions among similarly charged colloidal particles. The goal of this project is to elucidate the roles of PE in mediating the critical interaction that leads to the stabilization of a colloidal suspension.

During the past year, we primarily focused on a model system consisting of protein colloidal particles with positive surface charges and DNA (polyelectrolyte) with negative surface charges.

We have successfully elucidated how the microstructure of PE determines the interaction between colloidal particle, as well as the conformation and the stability of the formed colloid-PE complexes.

Summary of results

1.      Microstructures of PE determine the binding affinities between PE and colloids

In this study, we modulated the property of PE by inserting a flexible structural unit (CG) and a rigid structural unit (meCG) at different locations along the PE. The effects of these additional structural units on the dynamic conformation and binding affinities to colloidal particles are individually characterized by Fluorescence Correlation Spectroscopy (FCS) and a competitive binding assay.

Our results suggest that the addition of flexible structural units, either as a (CG)n stretch or a (CGX8)n repeats at 10bp intervals, can lead to a reduced hydrodynamic radius and decreased nucleosome-forming ability of DNA. As shown in Fig.1, this effect is more predominant for a DNA stretch (CG)5. Addition of the rigid structural unit meCG, do not significantly alter the conformation of DNA and its binding affinities to the protein colloids. Nevertheless, converting a flexible structural unit to a rigid one, CG to meCG, significantly reduce its binding affinities to colloidal particles. Our results suggest that the microstructure of the PE chain, originated from the distribution of flexible and rigid structural units along DNA, plays a significant role in determining the binding affinities between PE and colloids. The findings of this study have been included in a manuscript accepted for publication at Biopolymers.

2.      Colloid-PE complex assumes a dynamic structure with constant end-breathing motions

We utilized a time-domain fluorescence lifetime measurement to characterize the conformation of colloid-PE complex.  A FRET pair was introduced to the ends of PE. PE chains exhibit a dynamic conformation, with DNA ends constantly breathe away from the colloidal surface. Increased DNA backbone rigidity due to the addition of methyl side chains leads to a more dynamic conformation with DNA ends constantly fraying way from the colloidal surface.  The finding of this study has been included in a published manuscript (Jimenez-Useche, I.,, Biophysical Journal, 103: 2502 (2012)).

3.      The stability of colloid-PE complex is modulated by the microstructure of PE

In this study, we evaluated the stability of the colloid-PE complex by quantifying the change in complex conformation using FRET.  Our results suggest that increases in PE rigidity, by changing CG to meCG, do not always alter the stability of the formed complex as shown in Fig.2.  However, when the modification sites were facing away from the colloidal surface, these sites tend to get overstretched. Consequently, the flexibility change in these particular locations significantly reduces the stability of the complex significantly. Our results suggest that the stability of the colloidal-PE complex is determined by the detailed microstructure of PE and the contacts formed between PE and colloids. The finding of this study has been published in Scientific Reports (Jimenez-Useche, I.,, Scientific Reports, 3: 2121 (2013))

Overall Impact

1.      We have successfully revealed the microscopic features of different PEs in complex with colloidal particles. This knowledge elucidated the molecular mechanism that accounts for the interactions between a polyelectrolyte and an oppositely charged colloidal particle.

2.      One graduate student was supported by this grant. She has received trainings on colloidal science and fluorescence microscopy.

3.      This project provides the PI with the essential resources to establish her lab and recruits her first graduate student.  The findings of this project have led to three peer-reviewed journal publications and one manuscript currently under preparation.