Competitive ion-exchange reactions of Pb(II) (Pb2+/PbCl+) and Ra(II) (Ra2+) on smectites: Experiments, modeling, and implication for 226Ra(II)/210Pb(II) disequilibrium in the environment

In this study, new experimental data for the adsorption of lead onto a swelling clay mineral with a tetrahedral charge (beidellite) at the ultratrace level (<10^-10 M) are presented. The data were interpreted using an ion-exchange multisite model that considers the sorption of major cations (including H⁺), which always compete with trace elements for sorption onto mineral surfaces in natural environments. The ability of the proposed model to predict experimental K_d values under various conditions of ionic strength (fixed by NaCl solutions) and aqueous cation compositions (including Pb²⁺ and PbCl⁺) was tested. The proposed model was applied to experimental data previously published for other types of swelling clay minerals, and the results were compared with the results obtained using previously published models. The preferential adsorption of chloride ion pairs, as well as the effect of the swelling clay crystal chemistry on lead adsorption, were assessed. Finally, the selective adsorption behavior of ²²⁶Ra compared to ²¹⁰Pb was demonstrated, which has implications for the study of many environmental processes using isotope partitioning.

A preliminary investigation into the use of amino acids as potential ion pairs for diclofenac transdermal delivery

Ion pairing is a potential strategy used to increase the partition and permeation of ionisable drug molecules. This work outlines the process of identifying, selecting and testing potential counter ions for diclofenac (DF). Three screening criteria were considered in the initial selection process. The first, toxicity, was used to eliminate counter ion candidates that could not be used in topical formulations. The second related to the balancing of charges. As DF is a free acid in its unionised state, counter ions should be of a basic character. Finally, molecular size, as represented by molecular mass (Da), was used. Because of the impact on ion pair formation, the counter ion was required to have a lower molecular weight than diclofenac. Basic amino acids L-Arginine, L-Histidine, L-Lysine and their salts were chosen. The selection process concluded with Partition Coefficient (PC) studies. These were used to identify any counter ions able to interact electrostatically with the ionised DF, enabling the 'neutral' ion pair to partition from an aqueous into an organic layer. Permeation studies using porcine skin were performed to test the efficacy of any selected counter ion. These preliminary studies suggest that amino acids may be used as counter ions to increase the partition and permeation of ionisable drugs.

Ab initio study of hydrated cesium iodide dimer (CsI)2-/0(H2O)0-6 and the cation size effect on (MI)2-/0(H2O)0-6 (M = Li, Na, K, Cs)

The structures of microsolvated (CsI)₂^-/0(H₂O)_0-6 clusters were determined using ab initio calculations. Our studies show that one Cs atom at the apex was firstly separated from the pyramid-shaped (CsI)₂^- unit when the water number reaches 3, whereas CsI distances did not increase significantly from n = 0 to 6 for neutrals. Additionally, the atomic charge and reduced density gradient analyses were carried out; the results reveal that the extra electrons are almost entirely localized on terminal Cs atom and the Cs⁺-water interactions dominate in (CsI)₂^-(H₂O)_0-6. The water-water interactions show up at n = 5. The comparison of (CsI)₂^-/0(H₂O)_n with (MI)₂^-/0(H₂O)_n (M = Li, Na, K) shows that neutral (CsI)₂ is the most difficult to be separated, which matches the law of matching water affinity. As for anions, the most difficult separation occurs in the case of small size (LiI)₂^- due to the effect of extra electrons, and (MI)₂^- with larger size cation is more likely to interact with water to form a pyramid structure.

Racemic phosphorothioate as a tool for NMR investigations of protein-DNA complexes

A major driving force for protein-nucleic acid association is electrostatic interactions via ion pairs of the positively charged basic side chains and negatively charged phosphates. For a better understanding of how proteins scan DNA and recognize particular signatures, it is important to gain atomic-level insight into the behavior of basic side chains at the protein-DNA interfaces. NMR spectroscopy is a powerful tool for investigating the structural, dynamic, and kinetic aspects of protein-DNA interactions. However, resonance assignment of basic side-chain cationic moieties at the molecular interfaces remains to be a major challenge. Here, we propose a fast, robust, and inexpensive approach that greatly facilitates resonance assignment of interfacial moieties and also allows for kinetic measurements of protein translocation between two DNA duplexes. This approach utilizes site-specific incorporation of racemic phosphorothioate at the position of a phosphate that interacts with a protein side chain. This modification retains the electric charge of phosphate and therefore is mild, but causes significant chemical shift perturbations for the proximal protein side chains, which facilitates resonance assignment. Due to the racemic nature of the modification, two different chemical shifts are observed for the species with different diastereomers R_P and S_P of the incorporated phosphorothioate group. Kinetic information on the exchange of the protein molecule between R_P and S_P DNA duplexes can be obtained by ¹⁵N_z exchange spectroscopy. We demonstrate the applications of this approach to the Antennapedia homeodomain-DNA complex and the CREB1 basic leucine-zipper (bZIP)-DNA complex.

Choline-amino acid ionic liquids: past and recent achievements about the structure and properties of these really "green" chemicals

The structure of choline-amino acid ionic liquids, atoxic task-specific solvents composed of materials originated from renewable feedstocks, is reviewed in this letter. The varied and strong interactions that these liquids are capable of establishing are largely dependent on their structure and confer them outstanding solvating properties with respect to a large number of different solutes. Among the experimental methods capable of yielding structural insight, the energy-dispersive version of X-Ray diffraction, that uses the Bremsstrahlung radiation of the X-Ray tube, is a technique very well suited to investigate these liquid systems. The diffraction spectra of five choline-amino acid ionic liquids, recently measured, are reported and discussed; in particular, the presence or absence of the medium-range order pre-peak is related to the presence of polar groups within the amino acid side chain that destroys the hydrophobic interactions between aliphatic chains. In the final section, a recent example of choline-amino acid ionic liquids as for ancient paper preservation and two other interesting results are discussed at the end.

Consensus Prediction of Charged Single Alpha-Helices with CSAHserver

Charged single alpha-helices (CSAHs) constitute a rare structural motif. CSAH is characterized by a high density of regularly alternating residues with positively and negatively charged side chains. Such segments exhibit unique structural properties; however, there are only a handful of proteins where its existence is experimentally verified. Therefore, establishing a pipeline that is capable of predicting the presence of CSAH segments with a low false positive rate is of considerable importance. Here we describe a consensus-based approach that relies on two conceptually different CSAH detection methods and a final filter based on the estimated helix-forming capabilities of the segments. This pipeline was shown to be capable of identifying previously uncharacterized CSAH segments that could be verified experimentally. The method is available as a web server at http://csahserver.itk.ppke.hu and also a downloadable standalone program suitable to scan larger sequence collections.

Ab initio and density functional theory (DFT) studies on triflic acid with water and protonated water clusters

The structure, stability and infrared spectral signatures of triflic acid (TA) with water clusters (W_n) and protonated water clusters (TAH⁺W_n, n = 1 - 6) were computed using DFT and MP2 methods. Our calculations show that a minimum of three water molecules are necessary to stabilize the dissociated zwitterionic form of TA. It can be seen from the results that there is no significant movement of protons in smaller (n = 1 and 2) and linear (n = 1 - 6) types of water clusters. Further, the geometries of TAW_n clusters first form a neutral pair (NP) to contact ion pair (CIP), then form a solvent separated ion pair (SSIP) in a water hexamer. These findings reveal that proton transfer may take place through NP to CIP and then CIP to SSIP. The calculated binding energies (BEs) of ion pair clusters is always higher than that of NP clusters (i.e., more stable than the NP). Existing excess proton linear chain clusters transfer a proton to adjacent water molecules via a Grotthuss mechanism, whereas the same isomers in the branched motifs do not conduct protons. Examination of geometrical parameters and infrared frequencies reveals hydronium ion (H₃O⁺ also called Eigen cation) formation in both TAW_n and protonated TAW_n clusters. The stability of Eigen water clusters is three times higher than that of other non-Eigen water clusters. Our study shows clearly that formation of ion pairs in TAW_n and TAH⁺W_n clusters greatly favors proton transfer to neighboring water molecules and also enhances the stability of these complexes.

Salt Bridge Rearrangement (SaBRe) Explains the Dissociation Behavior of Noncovalent Complexes

Native electrospray ionization-mass spectrometry, with gas-phase activation and solution compositions that partially release subcomplexes, can elucidate topologies of macromolecular assemblies. That so much complexity can be preserved in gas-phase assemblies is remarkable, although a long-standing conundrum has been the differences between their gas- and solution-phase decompositions. Collision-induced dissociation of multimeric noncovalent complexes typically distributes products asymmetrically (i.e., by ejecting a single subunit bearing a large percentage of the excess charge). That unexpected behavior has been rationalized as one subunit "unfolding" to depart with more charge. We present an alternative explanation based on heterolytic ion-pair scission and rearrangement, a mechanism that inherently partitions charge asymmetrically. Excessive barriers to dissociation are circumvented in this manner, when local charge rearrangements access a lower-barrier surface. An implication of this ion pair consideration is that stability differences between high- and low-charge state ions usually attributed to Coulomb repulsion may, alternatively, be conveyed by attractive forces from ion pairs (salt bridges) stabilizing low-charge state ions. Should the number of ion pairs be roughly inversely related to charge, symmetric dissociations would be favored from highly charged complexes, as observed. Correlations between a gas-phase protein's size and charge reflect the quantity of restraining ion pairs. Collisionally-facilitated salt bridge rearrangement (SaBRe) may explain unusual size "contractions" seen for some activated, low charge state complexes. That some low-charged multimers preferentially cleave covalent bonds or shed small ions to disrupting noncovalent associations is also explained by greater ion pairing in low charge state complexes. Graphical Abstract ᅟ.

NMR studies on the dynamics of hydrogen bonds and ion pairs involving lysine side chains of proteins

Hydrogen bonds and ion pairs involving side chains play vital roles in protein functions such as molecular recognition and catalysis. Despite the wealth of structural information about hydrogen bonds and ion pairs at functionally crucial sites on proteins, the dynamics of these fundamental chemical interactions are not well understood largely due to the lack of suitable experimental tools in the past. NMR spectroscopy is a powerful tool for investigations of protein dynamics, but the vast majority of NMR methods had been applicable only to the backbone or methyl groups. Recently, a substantial progress has been made in the research on the dynamics of hydrogen bonds and ion pairs involving lysine side-chain NH3+ groups. Together with computational/theoretical approaches, the new NMR methods provide unique insights into the dynamics of hydrogen bonds and ion pairs involving lysine side chains. Here, the methodology and its applications are reviewed.

Influence of molecular structure on the photoluminescence of 2-methyl benzimidazolium picrate: a new fluorescent material

This work reports the structural and photo luminescent characterization of the 2-methyl benzimidazolium picrate salt for the first time. This new material exhibits the π-π stacking of aromatic rings involving the benzimidazolium cation and picrate anion, which is additionally supported with a rare n→π interaction as well as with the extended networks of intermolecular hydrogen bonds in the solid state. The FT-IR and UV characterizations further support the solid state structural features of the salt. However, the spectroscopic results reveal that the stacking of ion pairs, which is observed in the solid state, is not maintained in solution. The DSC result indicates that the material is stable under ambient temperature conditions. Interestingly, upon photo excitation at 325 nm in the solid state, the material shows an unusual red emission around 615 nm, which is probably attributed to the supramolecular stacked nature of the ion pairs, along with a usual picrate centered green emission at 530 nm. However, the fluorescence measurement in solution, wherein the stacking of ion pairs are not maintained, shows only a single peak at lower wavelength. These observations highlight the influencing role of the supramolecular stacking interactions on the photoluminescence of the material, due to which, a chemical sensing action may be envisaged.

Computational and experimental approaches to the molecular structure of the HCl adduct of Me3PO

The reaction of anhydrous HCl(g) with trimethyl phosphane oxide yields trimethylhydroxy phosphonium chloride. A crystal structure analysis showed that the prevalent mesomeric structure in the solid state is the phosphonium chloride ion pair. Ab initio calculations in the gas phase cannot reproduce these findings, whereas higher correlated methods (CISD) and solvation models predict the experimental structure correctly.