Carboxylate ion pairing with alkali-metal ions for β-Lactoglobulin and its role on aggregation and interfacial adsorption

Frank R. Beierlein, Timothy Clark, Björn Braunschweig, Kathrin Engelhardt, Lena Glas, Wolfgang Peukert

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Abstract

We report a combined experimental and computational study of the whey protein β-lactoglobulin (BLG) in different electrolyte solutions. Vibrational sum-frequency generation (SFG) and ellipsometry were used to investigate the molecular structure of BLG modified air–water interfaces as a function of LiCl, NaCl, and KCl concentrations. Molecular dynamics (MD) simulations and thermodynamic integration provided details of the ion pairing of protein surface residues with alkali-metal cations. Our results at pH 6.2 indicate that BLG at the air–water interface forms mono- and bilayers preferably at low and high ionic strength, respectively. Results from SFG spectroscopy and ellipsometry are consistent with intimate ion pairing of alkali-metal cations with aspartate and glutamate carboxylates, which is shown to be more effective for smaller cations (Li+ and Na+). MD simulations show not only carboxylate–alkali-metal ion pairs but also ion multiplets with the alkali-metal ion in a bridging position between two or more carboxylates. Consequently, alkali-metal cations can bridge carboxylates not only within a monomer but also between monomers, thus providing an important dimerization mechanism between hydrophilic surface patches.
Original languageEnglish
Pages (from-to)5505-5517
JournalThe Journal of Physical Chemistry B
Volume119
Issue number17
Early online date31 Mar 2015
DOIs
Publication statusPublished - 30 Apr 2015

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