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1. Effect of solution ions on the charge and performance of nanofiltration membranes

   https://doi.org/10.1038/s41545-024-00322-9  

   Roth, Rebecca S.; Birnhack, Liat; Avidar, Mor; Hjelvik, Elizabeth A.; Straub, Anthony P.; Epsztein, Razi (March 2024, npj Clean Water)

   Abstract Considering growing efforts to understand and improve the solute-specific selectivity of nanofiltration (NF) membranes, we explored the ion-specific effects that govern the charge and performance of a loose polyamide NF membrane that is commonly used for solute-solute separations. Specifically, we systematically evaluated the zeta potential of the membrane under different conditions of pH, salinity, and ionic composition, and correlated the obtained data with membrane performance tested under similar conditions. Our results identify the pK_aof both carboxylic and amine groups bonded to the membrane surface and suggest that the highly polarizable chloride anions in the solution adsorb to the polyamide, increasing its negative charge. We also show that monovalent cations of different “stickiness” can neutralize the negative membrane charge to different extents due to their varying tendency to sorb to the polymer matrix or screen the fixed carboxyl groups on the membrane surface. Notably, our correlation between zeta potential measurements and permeability experiments indicates the substantial contribution of solution ions to Donnan exclusion in NF membranes. 

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2. Computational predictions of metal–macrocycle stability constants require accurate treatments of local solvent and pH effects

   https://doi.org/10.1039/D1CP00611H  

   Gentry, Brian M.; Choi, Tae Hoon; Belfield, William S.; Keith, John A. (April 2021, Physical Chemistry Chemical Physics)

   Rational design of molecular chelating agents requires a detailed understanding of physicochemical ligand–metal interactions in solvent phase. Computational quantum chemistry methods should be able to provide this, but computational reports have shown poor accuracy when determining absolute binding constants for many chelating molecules. To understand why, we compare and benchmark static- and dynamics-based computational procedures for a range of monovalent and divalent cations binding to a conventional cryptand molecule: 2.2.2-cryptand ([2.2.2]). The benchmarking comparison shows that dynamics simulations using standard OPLS-AA classical potentials can reasonably predict binding constants for monovalent cations, but these procedures fail for divalent cations. We also consider computationally efficient static procedure using Kohn–Sham density functional theory (DFT) and cluster-continuum modeling that accounts for local microsolvation and pH effects. This approach accurately predicts binding energies for monovalent and divalent cations with an average error of 3.2 kcal mol −1 compared to experiment. This static procedure thus should be useful for future molecular screening efforts, and high absolute errors in the literature may be due to inadequate modeling of local solvent and pH effects. 

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3. Theory and simulations for RNA folding in mixtures of monovalent and divalent cations

   https://doi.org/10.1073/pnas.1911632116  

   Nguyen, Hung T.; Hori, Naoto; Thirumalai, D. (October 2019, Proceedings of the National Academy of Sciences)

   RNA molecules cannot fold in the absence of counterions. Experiments are typically performed in the presence of monovalent and divalent cations. How to treat the impact of a solution containing a mixture of both ion types on RNA folding has remained a challenging problem for decades. By exploiting the large concentration difference between divalent and monovalent ions used in experiments, we develop a theory based on the reference interaction site model (RISM), which allows us to treat divalent cations explicitly while keeping the implicit screening effect due to monovalent ions. Our theory captures both the inner shell and outer shell coordination of divalent cations to phosphate groups, which we demonstrate is crucial for an accurate calculation of RNA folding thermodynamics. The RISM theory for ion–phosphate interactions when combined with simulations based on a transferable coarse-grained model allows us to predict accurately the folding of several RNA molecules in a mixture containing monovalent and divalent ions. The calculated folding free energies and ion-preferential coefficients for RNA molecules (pseudoknots, a fragment of the rRNA, and the aptamer domain of the adenine riboswitch) are in excellent agreement with experiments over a wide range of monovalent and divalent ion concentrations. Because the theory is general, it can be readily used to investigate ion and sequence effects on DNA properties. 

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4. Reversed Cation Selectivity of G ₈ ‐Octamer and G ₁₆ ‐Hexadecamer towards Monovalent and Divalent Cations

   https://doi.org/10.1002/asia.202000016  

   He, Ying; Zhang, Yanbin; Wojtas, Lukasz; Akhmedov, Novruz_G; Pan, Qinhe; Guo, Hao; Shi, Xiaodong (March 2020, Chemistry – An Asian Journal)

   Abstract A reverse‐binding‐selectivity between monovalent and divalent cations was observed for two different self‐assembly G₁₆‐hexadecamer and G₈‐octamer systems. The dissociation constant between G₄‐quadruplex and monomer was calculated via VT‐¹H NMR experiments. Quantitative energy profiles revealed entropy as the key factor for the weaker binding toward Ba²⁺compared with K⁺in the G₈‐octamer system despite stronger ion‐dipole interactions. This study is the first direct comparison of the G₄‐quartet binding affinity between mono and divalent cations and will benefit future applications of G‐quadruplex‐related research. Further competition experiments between the G₈‐octamer and 18‐crown‐6 with K⁺demonstrated the potential of this G₈system as a new potassium receptor. 

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5. Unlocking the potential of polymeric desalination membranes by understanding molecular-level interactions and transport mechanisms

   https://doi.org/10.1039/d2sc04920a  

   Nickerson, Trisha R.; Antonio, Emma N.; McNally, Dylan P.; Toney, Michael F.; Ban, Chunmei; Straub, Anthony P. (January 2023, Chemical Science)

   Polyamide reverse osmosis (PA-RO) membranes achieve remarkably high water permeability and salt rejection, making them a key technology for addressing water shortages through processes including seawater desalination and wastewater reuse. However, current state-of-the-art membranes suffer from challenges related to inadequate selectivity, fouling, and a poor ability of existing models to predict performance. In this Perspective, we assert that a molecular understanding of the mechanisms that govern selectivity and transport of PA-RO and other polymer membranes is crucial to both guide future membrane development efforts and improve the predictive capability of transport models. We summarize the current understanding of ion, water, and polymer interactions in PA-RO membranes, drawing insights from nanofiltration and ion exchange membranes. Building on this knowledge, we explore how these interactions impact the transport properties of membranes, highlighting assumptions of transport models that warrant further investigation to improve predictive capabilities and elucidate underlying transport mechanisms. We then underscore recent advances in in situ characterization techniques that allow for direct measurements of previously difficult-to-obtain information on hydrated polymer membrane properties, hydrated ion properties, and ion–water–membrane interactions as well as powerful computational and electrochemical methods that facilitate systematic studies of transport phenomena. 

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