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Ion Pairing and Dielectric Decrement in Glycosaminoglycan Brushes

preprint
revised on 21.02.2021, 18:49 and posted on 22.02.2021, 12:19 by James Sterling, Wenjuan Jiang, Wesley M. Botello-Smith, Yun L. Luo

Cell-surface polysaccharides are essential to many aspects of physiology, serving as a highly-conserved evolutionary feature of life and as an important part of the innate immune system in mammals. Here, as simplified biophysical models of these sugar-coatings, we present results of molecular dynamics simulations of hyaluronic acid and heparin brushes that show important effects of ion-pairing, water dielectric decrease, and co-ion exclusion. As in prior studies of macromolecular crowding under physiologically-relevant salt concentrations, our results show equilibria with electroneutrality attained through screening and pairing of brush anionic charges by monovalent cations at atomistic detail. Most surprising is the reversal of the Donnan potential obtained from both nonpolarizable and Drude polarizable force fields, in contrast to what would be expected based on electrostatic Boltzmann partitioning alone. Water dielectric decrement within the brush domain is also associated with Born hydration-driven cation exclusion from the brush. We observe that the primary partition energy attracting cations to attain brush electroneutrality is the ion-pairing or salt-bridge energy. Potassium and sodium pairing to glycosaminoglycan carboxylates and sulfates show similar abundance of contact-pairing and solvent-separated pairing. We conclude that in these crowded macromolecular brushes, ion-pairing, Born-hydration, and electrostatic potential energies all contribute to attain electroneutrality and should therefore contribute in mean-field models to accurately represent brush electrostatics.

Funding

NIH Grant R01-GM130834

NSF ACI-154862

History

Email Address of Submitting Author

jim_sterling@kgi.edu

Institution

Keck Graduate Institute

Country

USA

ORCID For Submitting Author

0000-0001-6110-0505

Declaration of Conflict of Interest

The authors declare no conflict of interest.

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