Interaction between Histidine and Zn(II) Metal Ions over a Wide pH as Revealed by Solid-State NMR Spectroscopy and DFT Calculations

The interactions between histidine and metal species play essential roles in a wide range of important biological processes including enzymes catalysis and signal transduction. In this work, solid-state NMR techniques were employed to determine the interaction between histidine and Zn­(II) from pH 3.5 to 14. 2D homo- and heteronuclear correlation NMR experiments were utilized to extract the ¹H, ¹³C, and ¹⁵N chemical shifts in various histidine–Zn­(II) binding complexes. Several histidine–Zn­(II) binding models were proposed on the basis of experimental results as well as DFT theoretical calculations. No direct interaction could be found between biprotonated histidine and Zn­(II) at acidic pH. At pH 7.5, one zinc ion could be hexa-coordinated with two histidine molecules on C′, N_α and deprotonated N_δ1 sites. As the pH increases to 11–14, both of the N_δ1 and N_ε2 sites could be deprotonated as acceptors to be bound to either Zn­(II) or water. All of these findings give a comprehensive set of benchmark values for NMR parameters and structural geometries in variable histidine–Zn­(II) binding complexes over a wide pH range and might provide insights into the structure–property relationship of histidine–metal complexes in biological metalloproteins

Polarization Switching Induced by Slowing the Dynamic Swinglike Motion in a Flexible Organic Dielectric

Molecular motions with large amplitude in close-packed crystals accompany large distortions of the molecular configuration, which generally generate orientational structural transitions between diverse states and enable the tuning of their bulk physical properties. We present a flexible organic dielectric, di-<i>n</i>-butylammonium chlorodifluoroacetate (<b>1</b>), which exhibits a reversible temperature-induced spontaneous polarization switching at 243 K (<i>T</i>_c). Ferroelectric hysteresis loop measurements and second harmonic generation experiments reveal its excellent polarization switching capacity with spontaneous polarization of 3.9 μC cm^–2. Temperature-dependent solid-state nuclear magnetic resonance measurements clearly elucidate the dynamical mechanism of polarization switching of <b>1</b>. Above <i>T</i>_c, an active swinglike motion in long-chain di-<i>n</i>-butylammonium (DBA) cation is confirmed, resulting in complete obliteration of the dipole moments. When the temperature decreases below <i>T</i>_c, the swinglike motions are frozen and the whole DBA cation becomes considerably more rigid, corresponding to polar order, which greatly contributes to polarization switching. It is believed that this finding opens up a potential strategy for the design of new polar materials as switchable electric devices

Precise Distance Control and Functionality Adjustment of Frustrated Lewis Pairs in Metal–Organic Frameworks

We report the construction of frustrated Lewis pairs (FLPs) in a metal–organic framework (MOF), where both Lewis acid (LA) and Lewis base (LB) are fixed to the backbone. The anchoring of a tritopic organoboron linker as LA and a monotopic linker as LB to separate metal oxide clusters in a tetrahedron geometry allows for the precise control of distance between them. As the type of monotopic LB linker varies, pyridine, phenol, aniline, and benzyl alcohol, a series of 11 FLPs were constructed to give fixed distances of 7.1, 5.5, 5.4, and 4.8 Å, respectively, revealed by 11B–1H solid-state nuclear magnetic resonance spectroscopy. Keeping LA and LB apart by a fixed distance makes it possible to investigate the electrostatic effect by changing the functional groups in the monotopic LB linker, while the LA counterpart remains unaffected. This approach offers new chemical environments of the active site for FLP-induced catalysis

Self-Consistent Implementation of a Solvation Free Energy Framework to Predict the Salt Solubilities of Six Alkali Halides

To assess the salt solubilities of six alkali halides in aqueous systems, we proposed a thermodynamic cycle and an efficient molecular modeling methodology. The Gibbs free energy changes for vaporization, dissociation, and dissolution were calculated using the experimental data of ionic thermodynamic properties obtained from the NBS tables. Additionally, the Marcus’ and Tissandier’s solvation free energy data for Li+, Na+, K+, Cl–, and Br– ions were compared with the conventional solvation free energies by substituting into our self-consistent thermodynamic cycle. Furthermore, Tissandier’s absolute solvation free energy data were used as the training set to refit the Lennard-Jones parameters of OPLS-AA force field for ions. To predict salt solubilities, an assumption of a pseudo-solvent was proposed to characterize the coupling work of a solute with its environment from infinitely diluted to saturated solutions, indicating that the Gibbs energy change of solvation process is a function of ionic strength. Following the self-consistency of the cycle, the newly derived formulas were used to determine the salt solubilities by interpolating the intersection of Gibbs free energy of dissolution and the zero free energy line. The refined ion parameters can also predict the structure and thermodynamic properties of aqueous electrolyte solutions, such as densities, pair correlation functions, hydration numbers, mean activity coefficients, vapor pressures, and the radial dependences of the net charge at 298.15 K and 1 bar. Our method can be used to characterize the solid–liquid equilibria of ions or charged particles in aqueous systems. Furthermore, for highly concentrated strong electrolyte systems, it is essential to introduce accurate water models and polarizable force fields

Self-Assembly of Cetyltrimethylammonium Bromide and Lamellar Zeolite Precursor for the Preparation of Hierarchical MWW Zeolite

Construction of hierarchical zeolite catalysts from lamellar zeolite precursor is challenging and promising for industrial catalysis. Although numerous efforts have been dedicated to control the organization of zeolite nanosheets by postsynthetic approaches or employing complex surfactants in hydrothermal synthesis, there is still no successful case that the hierarchical lamellar zeolite is hydrothermally synthesized by the self-assembly of the commercially available simple surfactant cetyltrimethylammonium bromide (CTAB) and inorganic zeolite precursor. In traditional syntheses, the self-assembly of simple surfactants and the growth of microporous framework are hardly compatible from both thermodynamic and kinetic viewpoints, preferring to cause phase separation. Herein, we approach for the first time the hydrothermal synthesis of a mesostructured multilamellar zeolite ECNU-7P, consisting of an alternative stacking of inorganic MWW zeolite nanosheets and organic CTAB layers with large interlayer spacing (25 Å), by a zeolite seed and CTAB-assisted dissolution–recrystallization route. Correlated 2D ¹H–²⁹Si solid-state NMR, X-ray, electron microscopy, and rotation electron diffraction analyses provide molecular-level insights into the guest–host interactions between organic surfactant and inorganic framework during the self-assembly and structure evolution process. Moreover, the calcined Al-ECNU-7 possessing a hierarchical mesostructure proves to serve as a highly active, selective, and stable solid acid catalyst for triisopropylbenzene cracking as well as acylation of anisole

Host–Guest Interactions in Dealuminated HY Zeolite Probed by ¹³C–²⁷Al Solid-State NMR Spectroscopy

Host–guest interactions in dealuminated HY zeolite have been investigated by advanced ¹³C–²⁷Al solid-state NMR experiments. This analysis allows us to report new insights into the adsorption geometry of acetone and its interaction with acid sites in the zeolite channels