Combining the polarizable Drude force field with a continuum electrostatic Poisson–Boltzmann implicit solvation model

In this work, we have combined the polarizable force field based on the classical Drude oscillator with a continuum Poisson–Boltzmann/solvent‐accessible surface area (PB/SASA) model. In practice, the positions of the Drude particles experiencing the solvent reaction field arising from the fixed charges and induced polarization of the solute must be optimized in a self‐consistent manner. Here, we parameterized the model to reproduce experimental solvation free energies of a set of small molecules. The model reproduces well‐experimental solvation free energies of 70 molecules, yielding a root mean square difference of 0.8 kcal/mol versus 2.5 kcal/mol for the CHARMM36 additive force field. The polarization work associated with the solute transfer from the gas‐phase to the polar solvent, a term neglected in the framework of additive force fields, was found to make a large contribution to the total solvation free energy, comparable to the polar solute–solvent solvation contribution. The Drude PB/SASA also reproduces well the electronic polarization from the explicit solvent simulations of a small protein, BPTI. Model validation was based on comparisons with the experimental relative binding free energies of 371 single alanine mutations. With the Drude PB/SASA model the root mean square deviation between the predicted and experimental relative binding free energies is 3.35 kcal/mol, lower than 5.11 kcal/mol computed with the CHARMM36 additive force field. Overall, the results indicate that the main limitation of the Drude PB/SASA model is the inability of the SASA term to accurately capture non‐polar solvation effects. © 2018 Wiley Periodicals, Inc.     

Bonding/antibonding character of “lone pair” molecular orbitals from their energy derivatives; consequences for experimental data

The derivative of molecular orbitals (MO) energies with respect to a bond length (dynamic orbital force [DOF]) is used to estimate the bonding/antibonding character of valence MOs along this bond, with a focus on lone pair MOs, in a series of small molecules: AH (A = F, Cl, Br), AH₂ (A = O, S, Se), AX₃ (A = N, P, As; X = H, F), and H₂CO. The HOMO DOF agrees with the calculated variation of bond length and force constant in the corresponding ground state cation, and of bond length variation by protonation. These results also agree with available experimental data. It is worthy to note that the p‐type HOMOs in AH and AH₂ are found bonding. The lone pair MO is bonding in NH₃, while it is antibonding in PH₃, AsH₃, and AF₃.     

Extension rate of the straight jet in electrospinning of poly(N‐isopropyl acrylamide) solutions in dimethylformamide: Influences of flow rate and applied voltage

In the electrospinning process, the measurement of extension rate of the straight jet is not an easy task. In this study, the diameter profile of the tapering straight jet is determined with a laser light‐scattering technique. Afterwards, the jet extension rate () is derived and used to compare with the solution‐intrinsic rates, for example, the terminal relaxation rate and the Rouse relaxation rate. The extension rate of the straight jet depends on position: it is highest near the cone apex (region I) and decays to a constant value in the major jet (region II) until approaching the jet end (region III), at which the extension rate abruptly drops to nearly zero, that is, _I >_II ≫_III ∼ 0. The jet diameter in region III is independent of solution concentration and applied voltage, but is scaled to the flow rate with an exponent of ∼0.37. The derived exponent is consistent with a simple prediction based on the counterbalance between the stretching electric force and the compressive force induced by the air drag force. Provided that air friction becomes overwhelming at the straight jet end, the long electrified jet is likely to buckle, thereby triggering the instability of jet whipping. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 319–329     

Elasticity‐Dependent Fast Underwater Adhesion Demonstrated by Macroscopic Supramolecular Assembly

Macroscopic supramolecular assembly (MSA) is a recent development in supramolecular chemistry to associate visible building blocks through non‐covalent interactions in a multivalent manner. Although various substrates (e.g. hydrogels, rigid materials) have been used, a general design rule of building blocks in MSA systems and interpretation of the assembly mechanism are lacking and are required. Herein we design three model systems with varied elastic modulus and correlated the MSA probability with the elasticity. Based on the effects of substrate deformability on multivalency, we have proposed an elastic‐modulus‐dependent rule that building blocks below a critical modulus of 2.5 MPa can achieve MSA for the used host/guest system. Moreover, this MSA rule applies well to the design of materials for fast underwater adhesion: Soft substrates (0.5 MPa) can achieve underwater adhesion within 10 s with one order of magnitude higher strength than that of rigid substrates (2.5 MPa).     

Photocontrolled Multidirectional Differentiation of Mesenchymal Stem Cells on an Upconversion Substrate

The effective guidance of mesenchymal stem cell (MSC) differentiation on a substrate by near‐infrared (NIR) light is particularly attractive for tissue engineering and regenerative medicine. However, most of current substrates cannot control multidirectional differentiation of MSCs like natural tissues. Herein, a photocontrolled upconversion‐based substrate was designed and constructed for guiding multidirectional differentiation of MSCs. The substrate enables MSCs to maintain their stem‐cell characteristics due to the anti‐adhesive effect of 4‐(hydroxymethyl)‐3‐nitrobenzoic acid modified poly(ethylene glycol) (P1) attached on the upconversion substrate. Upon NIR irradiation, the P1 is released from the substrate by photocleavage. The detachment of P1 can change cell–matrix interactions dynamically. Moreover, MSCs cultured on the upconversion substrate can be specifically induced to differentiate to adipocytes or osteoblasts by adjusting the NIR laser. Our work provides a new way of using NIR‐based upconversion substrate to modulate the multidirectional differentiation of MSCs.     

Synthesis of chromophores and polyimides with a green chemistry approach for second‐order nonlinear optical applications

This article presents the synthesis of nonlinear optical responsive chromophores by adopting a green chemistry approach by coupling N‐methyl‐N‐(2‐hydroxyethyl)‐4‐amino benzaldehyde with barbituric acid, 1,3‐indanedione, and 1,3‐diethyl‐2‐thiobarbituric acid as the acceptors through stilbene linkage. We performed the synthesis in less than 10 minutes at room temperature with water as a solvent without catalyst. Two different side‐chain polyimides were synthesized from poly(hydroxy‐imide)s with chromophores by Mitsunobu reaction. The chromophores were characterized by Fourier transform infrared, ¹H NMR, ¹³C NMR, and elemental analysis. However, the polyimides were characterized by Fourier transform infrared and ¹H NMR. The inherent viscosities (η_inh) of polyimides were determined by Ubbelohde viscometer, which ranged between 0.1793 and 0.1890 dL/g. The molecular weights of the polyimides were determined using gel permeation chromatography and were in range of 23 000 to 26 000. Polyimides demonstrated an excellent solubility in polar aprotic solvents, indicating good processability. Thermal behavior of these polyimides was studied by differential scanning calorimetry and thermogravimetric analysis. The T_g's were in the range of 185°C to 255°C. The change in the molecular orientation in the polymer films after electrical poling was ascertained using ultraviolet‐visible spectrophotometer and atomic force microscopy. The thicknesses and refractive indices of the thin films were determined by an ellipsometer. The second harmonic generation coefficients of the corona‐poled polymer films at T_opt's, determined by the Maker fringe technique, ranged between 59.33 and 77.82 pm/V. High thermal endurance observed for the polyimides is attributed to the extensive hydrogen bonds in the matrix. The developed polyimides showed no decay in second harmonic generation signals below 110°C, indicating the acceptance for nonlinear optical devices.     

The Narrowest Band Gap Ever Observed in Molecular Ferroelectrics: Hexane‐1,6‐diammonium Pentaiodobismuth(III)

Narrow band gaps and excellent ferroelectricity are intrinsically paradoxical in ferroelectrics as the leakage current caused by an increase in the number of thermally excited carriers will lead to a deterioration of ferroelectricity. A new molecular ferroelectric, hexane‐1,6‐diammonium pentaiodobismuth (HDA‐BiI₅), was now developed through band gap engineering of organic–inorganic hybrid materials. It features an intrinsic band gap of 1.89 eV, and thus represents the first molecular ferroelectric with a band gap of less than 2.0 eV. Simultaneously, low‐temperature solution processing was successfully applied to fabricate high‐quality ferroelectric thin films based on HDA‐BiI₅, for which high‐precision controllable domain flips were realized. Owing to its narrow band gap and excellent ferroelectricity, HDA‐BiI₅ can be considered as a milestone in the exploitation of molecular ferroelectrics, with promising applications in high‐density data storage and photovoltaic conversion.     

A sobering assessment of small‐molecule force field methods for low energy conformer predictions

We have carried out a large scale computational investigation to assess the utility of common small‐molecule force fields for computational screening of low energy conformers of typical organic molecules. Using statistical analyses on the energies and relative rankings of up to 250 diverse conformers of 700 different molecular structures, we find that energies from widely used classical force fields (MMFF94, UFF, and GAFF) show unconditionally poor energy and rank correlation with semiempirical (PM7) and Kohn–Sham density functional theory (DFT) energies calculated at PM7 and DFT optimized geometries. In contrast, semiempirical PM7 calculations show significantly better correlation with DFT calculations and generally better geometries. With these results, we make recommendations to more reliably carry out conformer screening.     

Perturbed structures generated using coordinates derived from compliance constants in internal vibrations for QTAIM dual functional analysis: Intrinsic dynamic nature of interactions

Perturbed structures for QTAIM dual functional analysis (QTAIM‐DFA) are proposed to generate using the coordinates corresponding to the compliance force constants in internal vibrations (CIV). In QTAIM‐DFA, total electron energy densities H_b( r _c) are plotted versus H_b( r _c) – V_b( r _c)/2 at bond critical points (BCPs) of interactions in question, where V_b( r _c) are potential energy densities at BCPs. Each plot of an interaction based on the data from both perturbed structures and fully optimized one takes the form (θ_p, κ_p), where θ_p corresponds to the tangent line of the plot and κ_p is the curvature. The θ_p values evaluated with CIV are equal to those obtained by partial optimizations with the interaction distance in question being fixed suitably, within the calculation errors. Very high applicability of CIV is demonstrated to generate the perturbed structures for QTAIM‐DFA. Dynamic nature of interactions based on (θ_p, κ_p) with CIV is called “intrinsic dynamic nature of interactions.”     

Influence of Coriolis force on thermal convection and impurity segregation during crystal growth under microgravity

In contrast with the generally accepted viewpoint, it is shown that the Coriolis force caused by rotation of an orbital station can appreciably affect natural convection and impurity distribution during the growth of crystals from a melt in orbital flight conditions. 2D and 3D steady and oscillatory convection in a rectangular enclosure is considered. The resonance phenomenon arising due to the interaction of the Coriolis force and harmonic oscillations of the gravity force is demonstrated. It is shown that for moderate values of the Ekman number the Coriolis force suppresses convection in one direction and amplifies it in the other, which in turn results in deformation of the impurity distribution over the cross-section of the crystal.