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This article is part of the supplement: Proceedings of the Eighth Annual MCBIOS Conference. Computational Biology and Bioinformatics for a New Decade

Open Access Proceedings

A modified Stokes-Einstein equation for Aβ aggregation

Srisairam Achuthan1, Bong Jae Chung2, Preetam Ghosh3, Vijayaraghavan Rangachari4 and Ashwin Vaidya5*

Author Affiliations

1 Neuroscience Center of Excellence, Louisiana State University Health Sciences Center, 2020 Gravier Street, Suite D, New Orleans, LA, 70112, USA

2 Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA

3 Department of Computer Science, Virginia Commonwealth University, Richmond, VA-23284, USA

4 Department of Chemistry and Biochemistry, University of Southern Mississippi, 118 College Drive, Box No.5043, Hattiesburg, MS-39406, USA

5 Department of Mathematical Sciences, Montclair State University, Montclair, NJ 07043, USA

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BMC Bioinformatics 2011, 12(Suppl 10):S13  doi:10.1186/1471-2105-12-S10-S13

Published: 18 October 2011

Abstract

Background

In all amyloid diseases, protein aggregates have been implicated fully or partly, in the etiology of the disease. Due to their significance in human pathologies, there have been unprecedented efforts towards physiochemical understanding of aggregation and amyloid formation over the last two decades. An important relation from which hydrodynamic radii of the aggregate is routinely measured is the classic Stokes-Einstein equation. Here, we report a modification in the classical Stokes-Einstein equation using a mixture theory approach, in order to accommodate the changes in viscosity of the solvent due to the changes in solute size and shape, to implement a more realistic model for Aβ aggregation involved in Alzheimer’s disease. Specifically, we have focused on validating this model in protofibrill lateral association reactions along the aggregation pathway, which has been experimentally well characterized.

Results

The modified Stokes-Einstein equation incorporates an effective viscosity for the mixture consisting of the macromolecules and solvent where the lateral association reaction occurs. This effective viscosity is modeled as a function of the volume fractions of the different species of molecules. The novelty of our model is that in addition to the volume fractions, it incorporates previously published reports on the dimensions of the protofibrils and their aggregates to formulate a more appropriate shape rather than mere spheres. The net result is that the diffusion coefficient which is inversely proportional to the viscosity of the system is now dependent on the concentration of the different molecules as well as their proper shapes. Comparison with experiments for variations in diffusion coefficients over time reveals very similar trends.

Conclusions

We argue that the standard Stokes-Einstein’s equation is insufficient to understand the temporal variations in diffusion when trying to understand the aggregation behavior of Aβ42 proteins. Our modifications also involve inclusion of improved shape factors of molecules and more appropriate viscosities. The modification we are reporting is not only useful in Aβ aggregation but also will be important for accurate measurements in all protein aggregation systems.