Heterogeneous Molecular Systems Research Articles

Ab Initio Molecular Dynamics Study of Methanol-Water Mixtures under External Electric Fields

Intense electric fields applied on H-bonded systems are able to induce molecular dissociations, proton transfers, and complex chemical reactions. Nevertheless, the effects induced in heterogeneous molecular systems such as methanol-water mixtures are still elusive. Here we report on a series of state-of-the-art ab initio molecular dynamics simulations of liquid methanol-water mixtures at different molar ratios exposed to static electric fields. If, on the one hand, the presence of water increases the proton conductivity of methanol-water mixtures, on the other, it hinders the typical enhancement of the chemical reactivity induced by electric fields. In particular, a sudden increase of the protonic conductivity is recorded when the amount of water exceeds that of methanol in the mixtures, suggesting that important structural changes of the H-bond network occur. By contrast, the field-induced multifaceted chemistry leading to the synthesis of e.g., hydrogen, dimethyl ether, formaldehyde, and methane observed in neat methanol, in 75:25, and equimolar methanol-water mixtures, completely disappears in samples containing an excess of water and in pure water. The presence of water strongly inhibits the chemical reactivity of methanol.

Photoelectrochemical Properties of SnO2 Photoanodes Sensitized by Cationic Perylene-Di-Imide Aggregates for Aqueous HBr Splitting.

Perylene-sensitized mesoporous SnO2 films were used as electrodes for photoelectrochemical HBr splitting in aqueous solution. Upon AM 1.5 G illumination a 3-4 fold increase of the saturated photocurrent was observed when decreasing the pH of the aqueous solution from pH 3 to pH 0 (j max = 0.05 ± 0.01 mAcm-2 at pH 3 and 0.17 ± 0.02 mAcm-2 at pH 0, respectively). A detailed spectroscopic and electrochemical analysis of the hybrid material was carried out in order to address the impact of interfacial energetics on charge separation dynamics. UV/Vis spectroelectrochemical measurements showed that the energy of semiconductor states in such systems can be adjusted independently from the molecular levels by varying proton concentration. Photoelectrochemical measurements and ns-μs transient absorption spectroscopy reveal that pH-related changes of the interfacial energetics have only a minor impact on the charge injection rate. An increase of the proton concentration improves charge collection mainly by retarding recombination, which in the case of Br- oxidation is in critical competition with perylene regeneration. Control of the back recombination appears to be a key feature in heterogeneous molecular systems tasked to drive energetically demanding redox reactions.

A fast and accurate computational method for the linear-combination-based isotropic periodic sum

An isotropic periodic sum (IPS) is a powerful technique to reasonably calculate intermolecular interactions for wide range of molecular systems under periodic boundary conditions. A linear-combination-based IPS (LIPS) has been developed to attain computational accuracy close to an exact lattice sum, such as the Ewald sum. The algorithm of the original LIPS method has a high computational cost because it needs long-range interaction calculations in real space. This becomes a performance bottleneck for long-time molecular simulations. In this work, the combination of an LIPS and fast Fourier transform (FFT) was developed, and evaluated on homogeneous and heterogeneous molecular systems. This combinational approach of LIPS/FFT attained computational efficiency close to that of a smooth particle mesh Ewald while maintaining the same high accuracy as the original LIPS. We concluded that LIPS/FFT has great potential to extend the capability of IPS techniques for the fast and accurate computation of many types of molecular systems.

Cellular Solid-State NMR Applied to Bacterial and Human cells

Solid-state NMR (ssNMR) is a method that can be applied to gain atomic-level insight into heterogeneous molecular systems. Over the last 15 years, ssNMR has seen strong methodological and instrumental developments that have allowed for the characterization of complex molecules including membrane proteins, amyloid fibrils or protein biopolymers with remarkable structural accuracy and comprehensiveness. More recently, ssNMR has profited from revolutionary enhancements in sensitivity, mainly due to the advent of Dynamic Nuclear Polarization (DNP) and it has seen significant progress in the field of ssNMR 1H detection. In our contribution we show how to make combined use of such methods to probe biomolecules in bacterial and human cells. Applications include protein translocation and insertion machines in bacteria and extend to membrane associated as well as soluble protein complexes in human cells. We also describe how to combine such ssNMR-based studies with modalities such as cryo-electron tomography (CET) and Fluorescence microscopy to obtain structural insight into cell organization from the atomic to sub-micrometer scale.

ChemInform Abstract: Heterogeneous Molecular Systems for Photocatalytic CO2 Reduction with Water Oxidation

AbstractReview: [292 refs.

Precise measurement of 1H 90° pulse in solid-state NMR spectroscopy for complex and heterogeneous molecular systems

The 90 degrees pulse calibration is essential in NMR spectroscopy to prevent artefacts in the liquid state or to enhance cross-polarization efficiency in the solid state. We verified pulse-angle (PA) errors due to circuit impedances in solid-state NMR and suggested a possible solution to prevent the inconvenience of PA errors. The classic pulse sequences used to calibrate (1)H 90 degrees pulse lengths by direct detection of protons or by cross-polarization were modified in order to replace single (1)H pulses with (1)H pulse trains. Pulse trains were found to decrease the effect of PA imperfections in the calibration of basic pulses (i.e. 90 degrees and 180 degrees ) for a number of organic substrates. The modified sequences are especially important to rapidly obtain pulse calibration of complex and heterogeneous molecular systems such as humic substances, which usually require a long time when using single (1)H pulses.

How does the Electrostatic Force Cut-Off Generate Non-uniform Temperature Distributions in Proteins?

Abstract Molecular dynamics simulations (MDS) with electrostatic force cut-off on heterogeneous molecular systems make temperature separation between solute and solvent [1 –4]. In MDS where the temperature separation occurs, we found that there is a non-uniform temperature distribution in protein. The pattern of temperature distribution was shown to be peculiar to the protein, suggesting a possible relationship between dynamical and thermal properties of protein. By principal component analysis, the non-uniform temperature distribution was ascribed to high temperature of large fluctuation modes in protein. Further analysis by thermal diffusion equation between principal modes showed that in protein large fluctuation modes inherently have smaller value of heat capacity than small fluctuation modes have and are easily heated up by truncation noise due to abrupt cut-off of electrostatic forces.