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Proc Natl Acad Sci U S A. 2009 Aug 11;106(32):13296-300. doi: 10.1073/pnas.0902904106. Epub 2009 Jul 28.

Cation specific binding with protein surface charges.

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Max-Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.


Biological organization depends on a sensitive balance of noncovalent interactions, in particular also those involving interactions between ions. Ion-pairing is qualitatively described by the law of "matching water affinities." This law predicts that cations and anions (with equal valence) form stable contact ion pairs if their sizes match. We show that this simple physical model fails to describe the interaction of cations with (molecular) anions of weak carboxylic acids, which are present on the surfaces of many intra- and extracellular proteins. We performed molecular simulations with quantitatively accurate models and observed that the order K(+) < Na(+) < Li(+) of increasing binding affinity with carboxylate ions is caused by a stronger preference for forming weak solvent-shared ion pairs. The relative insignificance of contact pair interactions with protein surfaces indicates that thermodynamic stability and interactions between proteins in alkali salt solutions is governed by interactions mediated through hydration water molecules.

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