Organic ionic plastic crystals (OIPCs) have emerged as promising solid-state electrolytes owing to their intrinsic ionic conductivity and plasticity. To further enhance their mechanical stability and transport properties, polymer–OIPC composites have been developed. However, the role of polymer–OIPC interfaces in governing ionic transport at the molecular level remains elusive. Herein, we employ atomistic molecular dynamics simulations to investigate the interfacial effects on structure and ionic transport in polymer–OIPC composites. Polymer–OIPC composites based on diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate ([P1,2,2,4][PF6]) are constructed with two representative polymers, poly(vinylidene fluoride) (PVDF) and poly(ethylene oxide) (PEO). Our results reveal that both PVDF and PEO induce similar interfacial structures, yet the mobility of OIPC ions is enhanced in PEO–OIPC composites. Upon the introduction of Li+ ions as dopants, we find that Li+ ions follow distinct diffusion mechanisms in the two composites: anion-mediated diffusion in PVDF–OIPC and polymer-mediated diffusion in PEO–OIPC. The faster relaxation of Li+–PF6– coordination shells in PVDF–OIPC allows Li+ ions to exhibit normal diffusion along the interfacial plane, whereas Li+ ions in PEO–OIPC remain subdiffusive due to the slower relaxation of Li+–O(PEO) coordination shells. These findings highlight the importance of understanding ion diffusion mechanisms at the molecular level for the rational design of solid-state electrolytes.
Unraveling the fine details of hydrogen-bond structure and conformation within catalyst-substrate complexes is key to advancing organocatalysis. By employing two-dimensional infrared spectroscopy, this work provides evidence for a conformational transition of diphenylthiourea, induced by hydrogen-bond formation to 1,3-diphenyl-2-propenone in solution. Spectral shifts and coupling patterns indicate only one N–H (probed here via N–D) donor participating in hydrogen bonding. The vibrational spectra reveal a striking inhomogeneity in hydrogen-bond strength, and cross-peak analysis is consistent with single-site donor–acceptor geometry, rendering bifurcated interactions unlikely. Comparison of experimental spectral shifts upon H-bonding to harmonic frequencies from density functional theory calculations suggests a transition of diphenylthiourea from the cis–trans to the trans–trans conformation upon H-bond formation. These results highlight the interplay between molecular geometry and vibrational coupling, thereby helping to resolve the molecular structure of reactive intermediates in thiourea catalysis.
Abstract Amelogenin is an intrinsically disordered enamel matrix protein whose pH-dependent self-assembly is central to enamel biomineralization. Although AMELX-associated mutations and engineered variants are known to alter enamel structure, how these sequence changes affect amelogenin solution-state ensembles and assembly pathways remains unclear. Here, we combine small-angle X-ray scattering (SAXS) with monomeric replica exchange molecular dynamics (REMD) simulations to characterize nonphosphorylated murine amelogenin (rM179), Ser16-phosphorylated amelogenin (pS16), the amelogenin point mutants P41T and T21I, and the amelogenin deletion variants H3 (minus H67, H68, and H69) and Delta (minus A166 - D179 at C-terminal) at pH 3, 6, and 8. SAXS data shows that all variants remain disordered at pH 3 but differ in compaction and degree of self-association: H3 samples a more compact ensemble, Delta exhibits expansion or weak self-association, and T21I forms ribbon-like aggregates. REMD provides monomer-level context for these SAXS trends, predicting variant-dependent differences in conformational dimensions and transient structural propensities. At pH 6, pS16 and P41T favor smaller, more dynamic low-order assemblies, whereas T21I and Delta promote larger self-associated species; H3 occupies an intermediate regime. At pH 8, most variants form nanosphere-like assemblies with sequence-dependent size and heterogeneity, while the soluble Delta fraction lacks a well-defined nanosphere population. These results show that phosphorylation, point mutation, histidine deletion, and C-terminal truncation redistribute amelogenin among distinct pH-dependent assembly states, linking sequence variation to enamel-relevant self-assembly behavior.
Abstract Ion association in aqueous electrolyte solutions depends critically on the interplay between ion hydration and electrostatic attraction such that different ion-pair species have been detected in aqueous 2:2 electrolytes with sulfate as the prototypical divalent anion. Dielectric relaxation (DR) spectroscopy has the potential to resolve and quantify the various ion-pair species, yet literature studies are too scarce to systematically address the effect of the nature of the cation on ion association. Here, we report a DR study of aqueous solutions of MnSO4 and FeSO4 at 25 °C, at frequencies 0.2 ≤ ν/GHz ≤ 115 and concentrations up to near-saturation (∼2.7 and ∼0.8 mol·L–1 respectively). The observed spectra were consistent with the presence of five Debye modes: two at higher frequencies (ν > ca. 10 GHz) that correspond to bulk-water processes, and three solute-related modes at lower frequencies. The slowest mode, centered at ν ≈ 0.3 GHz, was consistent with overlapping contributions from the rotation of double-solvent-separated ion pairs (2SIPs) and hydrated triple ions (M2SO42+, T+) as has been found for some other divalent metal sulfate solutions. The other lower frequency modes, centered at ν ≈ 1.5 GHz and ∼8 GHz, were assigned (respectively) to the rotation of solvent-shared (SIPs) and contact (CIPs) ion pairs, with the latter mode possibly also incorporating an unresolved contribution from “slow” bulk-water molecules. Effective total cation solvation numbers, Zt(M2+, aq), derived from the water-related modes, indicated that both Mn2+ and Fe2+ immobilize approximately 11 water molecules on the DR time scale, independent of solute concentration. Combination of the present results with literature data revealed a strong correlation between the types of ion aggregates detected by DR spectroscopy and the hydration enthalpy of the divalent metal cations (excepting Zn2+). Concentrations of the various species derived from the solute-related modes were used to calculate formation constants for each species that were, where comparisons were possible, in reasonable agreement with literature values obtained by other techniques.
Abstract The chemical identity of the oxygen-evolving complex (OEC) in the metastable heterogeneous S3 state that precedes water oxidation remains intensely debated. Multiple ideas regarding the oxidation states of the four constituent Mn ions, as well as the number and protonation states of oxygen ligands are being considered, entailing distinct mechanistic pathways for the subsequent O–O bond formation. Here we describe a previously unreported possibility for the OEC in the S3 state identified by quantum chemical calculations. The model contains a hydroxyl radical that is bridging a terminal Mn center and the redox-inactive Ca2+ ion. In contrast to past ligand-radical suggestions the hydroxyl radical form has a crystallographically consistent intracluster O···O distance of 2.0 Å, while its total proton count is fully aligned with experimental data. Calculations suggest that the hydroxyl radical intermediate has a low-spin or diamagnetic ground state. Importantly, its unusual excited state profile make it a rare, though not definitive, candidate for the near-infrared absorbing component of the S3 state. The hydroxyl radical model enriches our perspectives on S3 state heterogeneity and expands the map of structure–property correlations in biological water oxidation.
