On the molecular mechanism of ion specific Hofmeister series

Investigating Anion Effects on Metal Ion Binding Interactions With Amyloid β Peptide by Ion Mobility Mass Spectrometry

The study of metal ion's role in the biological processes of Alzheimer's disease has spurred investigations into the coordination chemistry of amyloid beta peptide and its fragments. Nano-electrospray ionization mass spectrometry (nESI-MS) has been utilized to examine the stabilization of bound anions on multiprotein complexes without bulk solvent. However, the effects of anions on metal ion binding interactions with amyloid beta peptide have not been explored. This study directly examined metal-peptide complexes using nESI-MS and investigated the effects of various anions on the binding ratio and stability of these complexes from ammonium salt solutions. The results indicate that different anions have distinct effects on the binding ratio and stability of various metal-peptide complexes. Of these, the bicarbonate ion exhibits the highest binding ratios for metal-peptide complexes, while binding ratios for these complexes in phosphate are comparatively low. Our results suggest that acetate, formate, bicarbonate, and phosphate have weak affinities and act as weak stabilizers of the metal-peptide complex structure in the gas phase. Intriguingly, chloride and sulfate act as stabilizers of the metal-peptide complex in the gas phase. The rank order determined from these data is substantially different from the Hofmeister salt series in solution. Although this outcome was anticipated due to the reduced influence of anions and water solvation, our findings correlate well with expected anion binding in solution and emphasize the importance of both hydration layer and anion-metal-peptide binding effects for Hofmeister-type stabilization in solution. This approach proved useful in examining the interactions between metal ions and amyloid beta peptide, which are relevant to Alzheimer's disease, using direct ESI-MS.

Effect of Salt Form on Gelation and Drug Delivery Properties of Diclofenac-Loaded Poloxamer Gels for Delivery to Impaired Skin

PURPOSE

Treating chronic wounds is a significant clinical challenge, and a topical product would be ideal for pain management. Poloxamer 407, a thermosensitive polymer, would allow an analgesic drug to be topically applied to a wound as a liquid that transitions to a gel at physiologic temperature. Using diclofenac as a model analgesic drug, our goal was to determine effects of salt form on poloxamer gelation and drug delivery from poloxamer gels applied to excised skin with impaired barrier function.

METHODS

Gelation properties of 17% and 20% poloxamer gels loaded with 0.4 to 1.7% diclofenac sodium, potassium, epolamine, or diethylamine were evaluated rheologically. Drug release and delivery were quantified using cellulose membranes, porcine skin, and tape-stripped porcine skin.

RESULTS

Poloxamer gelation temperature increased with higher diclofenac concentration, regardless of salt form; the magnitude of increase varied in the following order: sodium>potassium>diethylamine>epolamine. Gelation temperature differences resulting from the various counterions generally matched previously observed trends of ion-specific effects on macromolecule solubility (the Hofmeister series). Despite changes in gelation behavior, we observed minimal corresponding effects on drug release or delivery. There were no significant differences in diclofenac released or delivered through intact porcine skin over 48 h. However, in studies with impaired (tape-stripped) skin, diclofenac delivery was slowest overall with the epolamine salt.

CONCLUSION

Varying the salt form of a model analgesic drug can impact gelation and drug delivery characteristics of poloxamer systems. Further study of the mechanisms of these changes will be important for continued development of topical poloxamer products for clinical wound care.

Ab Initio Molecular Dynamics Study of Aqueous Solutions of Magnesium and Calcium Nitrates: Hydration Shell Structure, Dynamics and Vibrational Echo Spectroscopy

Ab initio molecular dynamics simulations are performed to study the hydration shell structure, dynamics, and vibrational echo spectroscopy of aqueous Mg(NO₃)₂ and Ca(NO₃)₂ solutions. The hydration shell structure is probed through calculations of various ion-ion and ion-water radial and spatial distribution functions. On the dynamical side, calculations have been made for the hydrogen bond dynamics of hydration shells and also residence dynamics and lifetimes of water in different solvation environments. Subsequently, we looked at the dynamics of frequency fluctuations of OD modes of heavy water in different hydration environments. Specifically, the temporal decay of spectral observables of two-dimensional infrared (2DIR) spectroscopy, three pulse echo peak shift (3PEPS) measurements and also of time correlations of frequency fluctuations are calculated to investigate the dynamics of vibrational spectral diffusion of water in different hydration environments in these solutions. The OD stretch frequencies of water molecules in the vicinity of both divalent cations are found to be red-shifted and also fluctuating at a slower rate than other water molecules present in the solutions. The Mg²⁺ ions are found to be strongly hydrated which can be linked to their lower tendency to form contact ion-pairs and essentially no water exchange between the cationic hydration shells and bulk during the time scale of the current simulations. The stronger hydration of Mg²⁺ ions make their hydration shells structurally and dynamically more rigid and make the dynamics of hydrogen bonds and vibrational spectral diffusion, as revealed through spectral observables of 2DIR and 3PEPS slower than that for the Ca²⁺ ions. The structural and spectral dynamics of water molecules outside the cationic solvation shells in the Mg(NO₃)₂ solution are also found to be relatively slower than that of the Ca(NO₃)₂ solution and pure water which show the effects of stronger electric fields of Mg²⁺ ions extending beyond their first hydration shells. Also, water molecules in the hydration shells of the NO₃^- ions are found to relax at a slower rate in the Mg(NO₃)₂ solution which manifests the effect countercations have on anionic hydration shells for divalent metal nitrate solutions.

Influence of Divalent Metal Ions on the Precipitation of the Plasma Protein Fibrinogen

Fibrinogen nanofibers are very attractive biomaterials to mimic the native blood clot architecture. Previously, we reported the self-assembly of fibrinogen nanofibers in the presence of monovalent salts and have now studied how divalent salts influence fibrinogen precipitation. Although the secondary fibrinogen structure was significantly altered with divalent metal ions, morphological analysis revealed exclusively smooth fibrinogen precipitates. In situ monitoring of the surface roughness facilitated predicting the tendency of various salts to form fibrinogen fibers or smooth films. Analysis of the chemical composition revealed that divalent salts were removed from smooth fibrinogen films upon rinsing while monovalent Na⁺ species were still present in fibrinogen fibers. Therefore, we assume that the decisive factor controlling the morphology of fibrinogen precipitates is direct ion-protein contact, which requires disruption of the ion-surrounding hydration shells. We conclude that in fibrinogen aggregates, this mechanism is effective only for monovalent ions, whereas divalent ions are limited to indirect fibrinogen adsorption.

Assessing the Effect of Hofmeister Anions on the Hydrogen-Bonding Strength of Water via Nitrile Stretching Frequency Shift

The temperature dependence of the peak frequency (ν_max) of the C≡N stretching vibrational spectrum of a hydrogen-bonded C≡N species is known to be a qualitative measure of its hydrogen-bonding strength. Herein, we show that within a two-state framework, this dependence can be analyzed in a more quantitative manner to yield the enthalpy and entropy changes (ΔH_HB and ΔS_HB) for the corresponding hydrogen-bonding interactions. Using this method, we examine the effect of ten common anions on the strength of the hydrogen-bond(s) formed between water and the C≡N group of an unnatural amino acid, p-cyanophenylalanine (Phe_CN). We find that based on the ΔH_HB values, these anions can be arranged in the following order: HPO₄^2- > OAc^- > F^- > SO₄^2- ≈ Cl^- ≈ (H₂O) ≈ ClO₄^- ≈ NO₃^- > Br^- > SCN^- ≈ I^-, which differs from the corresponding Hofmeister series. Because Phe_CN has a relatively small size, the finding that anions having very different charge densities (e.g., SO₄^2- and ClO₄^-) act similarly suggests that this ranking order is likely the result of specific ion effects. Since proteins contain different backbone and side-chain units, our results highlight the need to assess their individual contributions toward the overall Hofmeister effect in order to achieve a microscopic understanding of how ions affect the physical and chemical properties of such macromolecules. In addition, the analytical method described in the present study is applicable for analyzing the spectral evolution of any vibrational spectra composed of two highly overlapping bands.

The Role of Counter-Ions in Peptides-An Overview

Peptides and proteins constitute a large group of molecules that play multiple functions in living organisms. In conjunction with their important role in biological processes and advances in chemical approaches of synthesis, the interest in peptide-based drugs is still growing. As the side chains of amino acids can be basic, acidic, or neutral, the peptide drugs often occur in the form of salts with different counter-ions. This review focuses on the role of counter-ions in peptides. To date, over 60 peptide-based drugs have been approved by the FDA. Based on their area of application, biological activity, and results of preliminary tests they are characterized by different counter-ions. Moreover, the impact of counter-ions on structure, physicochemical properties, and drug formulation is analyzed. Additionally, the application of salts as mobile phase additives in chromatographic analyses and analytical techniques is highlighted.

Two-Dimensional Infrared Spectroscopy of Aqueous Solutions of Metal Nitrates: Slowdown of Spectral Diffusion in the Presence of Divalent Cations

The hydrogen-bonded network of water can be affected both structurally and dynamically by the presence of ions. In the present study, we have considered three aqueous solutions of metal nitrates to investigate the effects of divalent cations (Mg²⁺ and Ca²⁺), compared to that of monovalent Na⁺ ions, on hydrogen-bond fluctuations and vibrational spectral diffusion through calculations of linear and two-dimensional infrared spectra of these solutions at room temperature. We have employed the methods of molecular dynamics simulations using effective polarizable models of ions combined with quantum mechanical calculations of transition variables and statistical mechanical calculations of spectral response functions of vibrational spectroscopy. Divalent cations are found to have much stronger and longer-ranged effects on the structure and dynamics of the hydrogen-bonded network than that induced by the monovalent sodium ions. The blue shifts in the calculated linear spectra are found to follow the Hofmeister trend for the cations. The 2D-IR spectral lineshape and intensity corresponding to three-pulse echo peak shift (3PEPS) experiments are calculated. The timescales of these nonlinear spectral responses and also frequency-time correlations show significant slowing down of spectral diffusion for solutions containing divalent Mg²⁺ and Ca²⁺ ions compared to the corresponding dynamics of the solution containing Na⁺ ions. Unlike NaNO₃ solution, the relaxation of frequency and dipole orientational fluctuations of anion-bound water in Mg(NO₃)₂ and Ca(NO₃)₂ solutions are found to be somewhat slower than bulk water, which can be attributed to the presence of divalent cations whose effects go beyond their first solvation shells. This is also seen in the dynamics of bulk water in these solutions which is found to be notably slower for the solutions containing divalent cations than that in the NaNO₃ solution. Unlike Mg²⁺ and Ca²⁺ ions, no specific cationic effect is observed for the Na⁺ ions.

Heterogeneous Occupancy and Vibrational Dynamics of Spatially Patterned Water Molecules

We performed first-principles molecular dynamics simulations of relatively dilute aqueous solutions of sulfate and thiosulfate dianions to analyze the structure, dynamics, and vibrational spectral properties of water molecules around the solute, especially the spatially patterned solvent molecules in the first solvation layer and the extended layers. This study also involves the investigation of dynamics of dangling OH groups in these layers and their role in patterning the water molecules around the dianions. Structural evaluation of the systems is carried out by radial distribution functions, number integrals, and spatial distribution functions. The lifetime of dangling OH groups inside the solvation shell is compared more to that of the bulk. By constructing the O-H groups in three ensembles (S1, S2, and S3) around the anion, we show that the frequency distribution of OH modes in the S1 ensemble show red-shifting for both sulfate and thiosulfate. The O-H groups in the S2 ensemble of the sulfate-water system show red-shifting by 10 cm^-1, while in the case of thiosulfate-water, these O-H groups show blue-shifting by 8 cm^-1. The water molecules in S1 and S2 subensembles have slower dynamics compared to those in the bulk (S3). The dynamics of various kinds of hydrogen bonds were characterized by hydrogen bond population correlation functions. The spectral diffusion of solvation shell O-H modes was performed through a frequency-time correlation function. We find a significant amount of orientational retardation of water molecules in the S1 layer and moderate retardation in the S2 layer as compared to that in the bulk, S3 layer. All these findings, the red shift of the OH stretching frequency in S1 and S2 layers, slowing down of the orientational dynamics of OH vectors in S1 and S2 layers, and less diffusivity of water in S1 and S2 layers, show the long-range kosmotropic effect of multivalent sulfate and thiosulfate oxyanions. Due to the long-range effect, heterogeneous occupancy of water molecules is observed, and the water molecules are found to arrange in a patterned manner in the vicinity of anions with varied local density.

Hofmeister effect of salt mixtures on thermo-responsive poly(propylene oxide)

The Hofmeister effect of salt mixtures is strongly dependent on composition of the mixture as well as absolute and relative concentration of the salts.

A Simple Theory for the Hofmeister Series

In cells, biological molecules function in an aqueous solution. Electrolytes and other small molecules play important roles in keeping the osmotic pressure of the cellular environment as well as the structure formation and function of biomolecules. The observed empirical rules such as Hofmeister series are still waiting for molecular interpretations. In this Perspective, we will discuss a simple and self-consistent theory that takes into account the cooperative effects of cations and anions in affecting water/air surface tension, water activity, and the solubility of model compounds including polypeptides. Molecular dynamics simulations used to test these theoretical models will also be discussed.