Counter-ion binding and mobility in the presence of hydrophobic polyions – combining molecular dynamics simulations and NMR

Abstract

Counter-ion binding and mobility in aqueous solutions of partially hydrophobic ionene oligoions is studied here by a combination of all-atomic molecular dynamics (MD) , as well as their mixtures; the latter allowing us to probe counter-ion selectivity of these oligoions. We consolidate both structural and dynamic information, in particular simulated radial distribution functions and average residence times of counter-ions in the vicinity of ionenes and NMR data in the form of counter-ion chemical shift and self-diffusion coefficients. On one hand, previously reported enthalpy of dilution and mixing measurements show a reverse counterion sequence for 12, 12-ionenes with respect to their less hydrophobic 3, 3-and 6, 6-analogues. On the other hand, the current MD and NMR data, reflecting the counter-ion binding tendencies to the ionene chain, give evidence for the same ordering as that observed by MD for 3, 3-ionenes. This is not seen as a contradiction and can be rationalized on the basis of increasing chain hydrophobicity, which has different consequences for enthalpy and ion-binding. The latter is reflecting free energy changes and as such includes both enthalpic and entropic contributions. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Ion-specificity and surface water dynamics in protein solutions

“…It has already been established that different proteins may exhibit distinct ion-specific trends for a given solution property. 13,21 In this section we present a comparison of NMR relaxation measurements for lysozyme (LZM) and BSA proteins. Both of these systems have been studied extensively in the past, lysozyme is overall considered as a more model system.…”

“…A number of studies has investigated the consequences of ion binding on the NMR relaxation of ionic nuclei. These include 35 Cl NMR to study the binding of Cl − ions at the surface of metalloenzymes and in competition with metal ions in solution, 42–45 but also dealing with charge compensation at a protein surface, 46–49 at a charged micellar surface, 50 or next to a polyelectrolyte chain, 21 which is more closely related to the current scenario. Past studies have shown that ion relaxation is sensitive to the internal motion of the protein 49 and that the quadrupolar coupling constant (QCC) v q = e 2 qQ/h , where e is the elementary charge, Q is the nuclear quadrupole, q the electric field gradient and h the Planck constant.…”

“…20 However, depending on the (thermo)dynamic property measured, the ranking within the series can change. 13,21 The ion-protein interaction is dictated by the arrangement and dynamics of the hydration water molecules around the interacting groups. How then is the above-mentioned ranking of ions reflected in the changes of the water behaviour at the protein surface?…”

“…In the case of Cl − and NO 3 − ions, we also present ionic relaxation rates ( 35 Cl and 14 N nuclei) to probe directly ion binding at the protein surface. 42–50 Ion-binding to macromolecules and the link to ion-specific effects has already been studied using techniques other than NMR relaxation, including ionic self-diffusion coefficients, 21,51 scattering methods, 52,53 and molecular simulations. 21,54 While ion-binding tendencies are important, they remain just one of several ingredients for the Hofmeister ranking of ions.…”

Volumetric and calorimetric properties of aqueous ionene solutions

“…The thermodynamic and transport properties, such as osmotic coefficients, heats of dilution, transport numbers, and conductivity, show strong dependence on the charge density of the polyelectrolyte’s backbone as well. Interestingly, the spectroscopic methods, such as dielectric relaxation spectroscopy [18, 19], neutron scattering data and NMR [20–22], and ultra-fast optical Kerr effect [23], suggest that the ionenes studied, in spite their relatively low backbone charge density (linear charge density parameter ξ varying between 1.44 and 0.67; see Figure 1) show surprisingly hydrophilic character, an exception being 12,12–ionenes with ξ = 0.43 [20]. …”