The temperature-dependent effects on the ultraviolet photoluminescence enhancement, blueshift, thermal quenching of the two-dimensional Zn nanosheets, 2D-hierarchical ZnO nanostructures, and 2D-hierarchical ZnO/Zn nanostructures are presented.

No abstract

A method for purifying and preparing rebaudioside A (RA) from steviol glycosides at preparative scale was developed with resin based on hydrophilic interaction liquid chromatography (HILIC). In pure water and acetone-water system, the adsorption capacity and selectivity of five anion resins for RA, rebaudioside C (RC) and stevioside (ST) were examined and discussed. Strongly basic IRA458 with the quaternary amine group was chosen as the resin used for separating and preparing RA. The hydroxide form of IRA458 (IRA458-OH) showed the best results in terms of the adsorption behaviors for RA, RC and ST. The retentions of RA, RC and ST on IRA458-OH resin at high concentration of acetone solution followed HILIC mode, in which retention is probably based on surface adsorption of the resin. Under optimized chromatographic conditions, the pooled purity and yield of RA were up to 98.18 and 98.62%, the relative standard deviations (n = 3) for these two parameters were 1.2 and 5.7%, respectively. The present method has the characteristics of simple, low cost, high purity and high yield. The study will be a promising tool for RA industrialized production and also provides a possible mode for purifying and preparing polar components from their analogs.

Self-assembled L,L-diphenylalanine (FF) nanostructures offer an attractive platform for photonics and nonlinear optics. The nonlinear optical (NLO) coefficients of FF nanotubes depend on the diameter of the tube [S. Khanra et al. Phys. Chem. Chem. Phys. 19(4), 3084–3093 (2017)]. To further enhance the NLO properties of FF, we search for structural modifications. Here, we report on the synthesis of fluorinated FF dipeptides by replacing one ortho-hydrogen atom in each of the phenyl groups of FF by a fluorine atom. Density-functional theoretical calculations yield insights into minimum energy conformers of fluorinated FF (Fl-FF). Fl-FF self-assembles akin to FF into micron-length tubes. The effects of fluorination are evaluated on the piezoelectric response and nonlinear optical properties. The piezoelectric d15 coefficient of Fl-FF is found to be more than 10 times higher than that of FF nanotubes, and the intensity of second harmonic generation (SHG) polarimetry from individual Fl-FF nanotubes is more than 20 times that of individual FF nanotubes. Furthermore, we obtain SHG images to compare the intensities of FF and Fl-FF tubes. This work demonstrates the potential of fluorine substitution in other self-assembled biomimetic peptides for enhancing nonlinear optical response and piezoelectricity.

We have been interested in the development of rubisco-based biomimetic systems for reversible CO 2 capture from air. Our design of the chemical CO 2 capture and release (CCR) system is informed by the understanding of the binding of the activator CO 2 ( A CO 2 ) in rubisco (ribulose-1,5-bisphosphate carboxylase/ oxygenase). The active site consists of the tetrapeptide sequence Lys-Asp-Asp-Glu (or KDDE) and the Lys sidechain amine is responsible for the CO 2 capture reaction. We are studying the structural chemistry and the thermodynamics of CO 2 capture based on the tetrapeptide CH 3 COÀ KDDEÀ NH 2 ("KDDE") in aqueous solution to develop rubisco mimetic CCR systems. Here, we report the results of 1 H NMR and 13 C NMR analyses of CO 2 capture by butylamine and by KDDE. The carbamylation of butylamine was studied to develop the NMR method and with the protocol established, we were able to quantify the oligopeptide carbamylation at much lower concentration. We performed a pH profile in the multi equilibrium system and measured amine species and carbamic acid/ carbamate species by the integration of 1 H NMR signals as a function of pH in the range 8 � pH � 11. The determination of ΔG 1 (R) for the reaction RÀ NH 2 + CO 2 !RÀ NHÀ COOH requires the solution of a multi-equilibrium equation system, which accounts for the dissociation constants K 2 and K 3 controlling carbonate and bicarbonate concentrations, the acid dissociation constant K 4 of the conjugated acid of the amine, and the acid dissociation constant K 5 of the alkylcarbamic acid. We show how the multi-equilibrium equation system can be solved with the measurements of the daughter/parent ratio X, the knowledge of the pH values, and the initial concentrations [HCO 3 À ] 0 and [R-NH 2 ] 0 . For the reaction energies of the carbamylations of butylamine and KDDE, our best values are ΔG 1 (Bu) = À 1.57 kcal/mol and ΔG 1 (KDDE) = À 1.17 kcal/mol. Both CO 2 capture reactions are modestly exergonic and thereby ensure reversibility in an energy-efficient manner. These results validate the hypothesis that KDDE-type oligopeptides may serve as reversible CCR systems in aqueous solution and guide designs for their improvement.

The simultaneous determination of two binding parameters for metal ions on an immobilized metal affinity chromatography column was performed by frontal chromatography. In this study, the binding parameters of Cu(2+) to l-glutamic acid were measured, the metal ion-binding characteristics of the complex ligand were evaluated. The linear correlation coefficients were all greater than 99%, and the relative standard deviations of two binding parameters were 0.58 and 0.059%, respectively. The experiments proved that the frontal chromatography method was accurate, reproducible, and could be used to determine the metal-binding parameters of the affinity column. The effects of buffer pH, type, and concentration on binding parameters were explored by uniform design experiment. Regression, matching and residual analyses of the models were performed. Meanwhile, the optimum-binding conditions of Cu(2+) on the l-glutamic acid-silica column were obtained. Under these binding conditions, observations and regression values of two parameters were similar, and the observation values were the best. The results demonstrated that high intensity metal affinity column could be effectively prepared by measuring and evaluating binding parameters using frontal chromatography combined with a uniform design experiment. The present work provided a new mode for evaluating and preparing immobilized metal affinity column with good metal-binding behaviors.

The central question of the paper concerns the CEISR effect in the case of symmetrical p-disubstituted acetophenone azines. We use intermolecular interaction inventory analyses to firmly establish the inequivalence or equivalence of the two arenes.

Rubisco is the enzyme responsible for CO 2 fixation in nature, and it is activated by CO 2 addition to the amine group of its lysine 201 side chain. We are designing rubisco-based biomimetic systems for reversible CO 2 capture from ambient air. The oligopeptide biomimetic capture systems are employed in aqueous solution. To provide a solid foundation for the experimental solution-phase studies of the CO 2 capture reaction, we report here the results of computational studies of the thermodynamics of CO 2 capture by small alkylamines in aqueous solution. We studied CO 2 addition to methyl-, ethyl-, propyl-, and butylamine with the consideration of the full conformational space for the amine and the corresponding carbamic acids and with the application of an accurate solvation model for the potential energy surface analyses. The reaction energies of the carbamylation reactions were determined based on just the most stable structures (MSS) and based on the ensemble energies computed with the Boltzmann distribution (BD), and it is found that ΔG BD ≈ ΔG MSS . The effect of the proper accounting for the molecular translational entropies in solution with the Wertz approach are much more significant, and the free energy of the capture reactions Δ W G BD is more negative by 2.9 kcal/mol. Further accounting for volume effects in solution results in our best estimates for the reaction energies of the carbamylation reactions of Δ W A BD = −5.4 kcal/mol. The overall difference is ΔG BD − Δ W A BD = 2.4 kcal/mol for butylamine carbamylation. The full conformational space analyses inform about the conformational isomerizations of carbamic acids, and we determined the relevant rotational profiles and their transition-state structures. Our detailed studies emphasize that, more generally, solution-phase reaction energies should be evaluated with the Helmholtz free energy and can be affected substantially by solution effects on translational entropies.