Helix Formation by Alanine-Based Peptides in Pure Water and Electrolyte Solutions: Insights from Molecular Dynamics Simulations

Specific ion effects on oligopeptide conformations in solution are attracting strong research attention, because of their impact on the protein-folding problem and on several important biological–biotechnological applications. In this work, we have addressed specific effects of electrolytes on the tendency of oligopeptides toward formation and propagation of helical segments. We have used replica-exchange molecular dynamics (REMD) simulations to study the conformations of two short hydrophobic peptides [Ace-(AAQAA)₃-Nme (AQ), and Ace-A₈-Nme (A8)] in pure water and in aqueous solutions of sodium chloride (NaCl) and sodium iodide (NaI) with concentrations of 1 and 3 M. The average helicities of the AQ peptide have been analyzed to yield Lifson–Roig (LR) parameters for helix nucleation and helix propagation. The salt dependence of these parameters suggests that electrolytes tend to stabilize the helical conformations of short peptides by enhancing the <i>helix nucleation</i> parameter. The helical conformations of longer oligopeptides are destabilized in the presence of salts, however, because the <i>helix propagation</i> parameters are reduced by electrolytes. On top of this general trend, we observe a significant specific salt effect in these simulations. The hydrophobic iodide ion in NaI solutions has a high affinity for the peptide backbone, which reflects itself in an enhanced helix nucleation and a reduced helix propagation parameter with respect to pure water or NaCl solutions. The present work thus explains the computational evidence that electrolytes tend to stabilize the compact conformations of short peptides and destabilize them for longer peptides, and it also sheds additional light on the specific salt effects on compact peptide conformations

Bis(hydroxylamino)triazines: High Selectivity and Hydrolytic Stability of Hydroxylamine-Based Ligands for Uranyl Compared to Vanadium(V) and Iron(III)

The development of ligands with high selectivity and affinity for uranium is critical in the extraction of uranium from human body, radioactive waste, and seawater. A scientific challenge is the improvement of the selectivity of chelators for uranium over other heavy metals, including iron and vanadium. Flat ligands with hard donor atoms that satisfy the geometric and electronic requirements of the U^VIO₂²⁺ exhibit high selectivity for the uranyl moiety. The bis­(hydroxylamino)­(triazine) ligand, 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat), a strong binder for hard metal ions (Fe^III, Ti^IV, V^V, and Mo^VI), reacted with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in aqueous solution and resulted in the isolation of the complexes [U^VIO₂(bihyat)­(H₂O)], [U^VIO₂(bihyat)₂]^2–, and {[U^VIO₂(bihyat)­(μ-OH)]}₂^2–. These three species are in equilibrium in aqueous solution, and their abundance varies with the concentration of H₂bihyat and the pH. Reaction of H₂bihyat with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in CH₃CN gave the trinuclear complex [U^VI₃O₆(bihyat)₂(μ-bihyat)₂]^2–, which is the major species in organic solvents. The dynamics between the U^VIO₂²⁺ and the free ligand H₂bihyat in aqueous and dimethyl sulfoxide solutions; the metal binding ability of the H₂bihyat over pyridine-2,6-dicarboxylic acid (H₂dipic) or glutarimidedioxime for U^VIO₂²⁺, and the selectivity of the H₂bihyat to bind U^VIO₂²⁺ in comparison to V^VO₄^3– and Fe^III in either U^VIO₂²⁺/V^VO₄^3– or U^VIO₂²⁺/Fe^III solutions were examined by NMR and UV–vis spectroscopies. The results revealed that H₂bihyat is a superior ligand for U^VIO₂²⁺ with high selectivity compared to Fe^III and V^VO₄^3–, which increases at higher pHs. Thus, this type of ligand might find applications in the extraction of uranium from the sea and its removal from the environment and the human body

Amphiphilic Polymer Conetworks Based on End-Linked “Core-First” Star Block Copolymers: Structure Formation with Long-Range Order

Amphiphilic polymer conetworks are cross-linked polymers that swell both in water and in organic solvents and can phase separate on the nanoscale in the bulk or in selective solvents. To date, however, this phase separation has only been reported with short-range order, characterized by disordered morphologies. We now report the first example of amphiphilic polymer conetworks, based on end-linked “core-first” star block copolymers, that form a lamellar phase with long-range order. These mesoscopically ordered systems can be produced in a simple fashion and exhibit significantly improved mechanical properties