Seeking faster, alternative methods for glycolipid biosurfactant characterization and purification.

Biosurfactants have been investigated as potential alternatives for synthetic surfactants in several areas, for example, in environmental and pharmaceutical fields. In that regard, extensive research has been carried out with sophorolipids and rhamnolipids that also present various biological properties with therapeutic significance. These biosurfactants are obtained as complex mixtures of slightly different molecules, and thus when studying these microbial glycolipids, the ability to identify and purify the produced compounds is of extreme importance. This study aimed to develop improved methodologies for the identification, separation, and purification of sophorolipids and rhamnolipids. Therefore, an ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method was modified to ensure faster characterization of both sophorolipids and rhamnolipids, enabling the identification and fragmentation pattern description of 10 and 13 congeners, respectively. The separation and purification of these biosurfactants was achieved with novel reversed-phase solid-phase extraction methods guaranteeing the isolation of different glycolipids, including those considered for their significant biological activity (e.g. antimicrobial, anticancer). It was possible to isolate sophorolipids and rhamnolipids with purity of 94% and 99%, respectively. The methods presented herein can be easily implemented and are expected to make purification of these biosurfactants easier, facilitating the study of their individual properties in further works.

Atomic-Scale Explanation of O2 Activation at the Au-TiO2 Interface.

By a combination of electron paramagnetic resonance spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently in this way than on the bare TiO2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes.

Bicanonical ab Initio Molecular Dynamics for Open Systems.

Performing ab initio molecular dynamics simulations of open systems, where the chemical potential rather than the number of both nuclei and electrons is fixed, still is a challenge. Here, drawing on bicanonical sampling ideas introduced two decades ago by Swope and Andersen [ J. Chem. Phys. 1995 , 102 , 2851 - 2863 ] to calculate chemical potentials of liquids and solids, an ab initio simulation technique is devised, which introduces a fictitious dynamics of two superimposed but otherwise independent periodic systems including full electronic structure, such that either the chemical potential or the average fractional particle number of a specific chemical species can be kept constant. As proof of concept, we demonstrate that solvation free energies can be computed from these bicanonical ab initio simulations upon directly superimposing pure bulk water and the respective aqueous solution being the two limiting systems. The method is useful in many circumstances, for instance for studying heterogeneous catalytic processes taking place on surfaces where the chemical potential of reactants rather than their number is controlled and opens a pathway toward ab initio simulations at constant electrochemical potential.

Efficient calculation of electronic absorption spectra by means of intensity-selected time-dependent density functional tight binding.

During the last two decades density functional based linear response approaches have become the de facto standard for the calculation of optical properties of small- and medium-sized molecules. At the heart of these methods is the solution of an eigenvalue equation in the space of single-orbital transitions, whose quickly increasing number makes such calculations costly if not infeasible for larger molecules. This is especially true for time-dependent density functional tight binding (TD-DFTB), where the evaluation of the matrix elements is inexpensive. For the relatively large systems that can be studied the solution of the eigenvalue equation therefore determines the cost of the calculation. We propose to do an oscillator strength based truncation of the single-orbital transition space to reduce the computational effort of TD-DFTB based absorption spectra calculations. We show that even a sizable truncation does not destroy the principal features of the absorption spectrum, while naturally avoiding the unnecessary calculation of excitations with small oscillator strengths. We argue that the reduced computational cost of intensity-selected TD-DFTB together with its ease of use compared to other methods lowers the barrier of performing optical property calculations of large molecules and can serve to make such calculations possible in a wider array of applications.

Cu/ZnO nanocatalysts in response to environmental conditions: surface morphology, electronic structure, redox state and CO2 activation.

Methanol synthesis is one of the landmarks of heterogeneous catalysis due to the great industrial significance of methanol as a clean liquid fuel and as a raw material for industry. Understanding in atomistic detail the properties of the underlying metal/oxide catalyst materials as a function of temperature and composition of the reactive gas phase is of utmost importance in order to eventually improve the production process. By performing extensive density functional theory based slab calculations in combination with a thermodynamic formalism we establish an atomistic understanding of gas phase-induced changes of surface morphology, redox properties and reactivity of ZnO supported Cu nanocatalysts. Extending our recent insights [Phys. Rev. Lett., 2013, 110, 086108], we explore surface stabilization mechanisms and site-dependent redox states of both catalyst components as well as the pronounced electronic charge transfer processes across the metal-support interface. Moreover, ab initio molecular dynamics simulations unveil the vital role played by dynamical shape fluctuations of the deposited Cu8 cluster. The pronounced structural flexibility of the metal nanoparticle is found to enhance CO2 activation over Cu8 at the elevated temperature conditions of the industrial methanol synthesis process, in addition to activation of CO2via electronic charge transfer from the ZnO support.

Methanol synthesis on ZnO(0001¯). IV. Reaction mechanisms and electronic structure.

Methanol synthesis from CO and H2 over ZnO, which requires high temperatures and high pressures giving rise to a complex interplay of physical and chemical processes over this heterogeneous catalyst surface, is investigated using ab initio simulations. The redox properties of the surrounding gas phase are known to directly impact on the catalyst properties and thus, set the overall catalytic reactivity of this easily reducible oxide material. In Paper III of our series [J. Kiss, J. Frenzel, N. N. Nair, B. Meyer, and D. Marx, J. Chem. Phys. 134, 064710 (2011)] we have qualitatively shown that for the partially hydroxylated and defective ZnO(0001¯) surface there exists an intricate network of surface chemical reactions. In the present study, we employ advanced molecular dynamics techniques to resolve in detail this reaction network in terms of elementary steps on the defective surface, which is in stepwise equilibrium with the gas phase. The two individual reduction steps were investigated by ab initio metadynamics sampling of free energy landscapes in three-dimensional reaction subspaces. By also sampling adsorption and desorption processes and thus molecular species that are in the gas phase but close to the surface, our approach successfully generated several alternative pathways of methanol synthesis. The obtained results suggest an Eley-Rideal mechanism for both reduction steps, thus involving "near-surface" molecules from the gas phase, to give methanol preferentially over a strongly reduced catalyst surface, while important side reactions are of Langmuir-Hinshelwood type. Catalyst re-reduction by H2 stemming from the gas phase is a crucial process after each reduction step in order to maintain the catalyst's activity toward methanol formation and to close the catalytic cycle in some reaction channels. Furthermore, the role of oxygen vacancies, side reactions, and spectator species is investigated and mechanistic details are discussed based on extensive electronic structure analysis.

Methanol synthesis on ZnO(0001). II. Structure, energetics, and vibrational signature of reaction intermediates.

A rigorous characterization of a wealth of molecular species adsorbed at oxygen defects on ZnO(0001) is given. These defects represent the putative active sites in methanol synthesis from CO and H2. The oxidation state of the ZnO catalyst and thus the preferred charge state and the reactivity of the oxygen vacancies depend on the gas phase temperature and pressure conditions. Considering charge states of oxygen vacancies relevant at the reducing conditions of the industrial process, i.e., F(++)/H2, F(0), F(0)/H2, and F(--), as well as the F(++) center which is abundant at UHV conditions and therefore important to allow for comparison with surface science experiments, we have investigated the structure, energetics, and vibrational frequencies of an exhaustive catalog of reaction intermediates using electronic structure calculations. After having identified the characteristic adsorption modes of CO, formate, formic acid, hydroxymethylene, formyl, formaldehyde, dioxomethylene, hydroxymethyl, hydroxymethoxide, methoxide, as well as methanol itself, the thermodynamic stability of all species with respect to the charge state of the oxygen vacancy and their electronic stabilization is discussed in detail and summarized in an energy level diagram.

Tuning the reactivity of a Cu/ZnO nanocatalyst via gas phase pressure.

By calculation of a thermodynamic phase diagram we provide an atomistic understanding of the morphological changes in ZnO-supported Cu nanocatalysts, which are subject to strong metal-support interactions, in response to the redox properties of the surrounding gas phase, i.e., depending on temperature and pressure. The reactivity, and thus the strong metal-support interactions, of this catalyst is traced back to a redox-state dependent occupation of delocalized ZnO substrate bands and localized Cu cluster states at the Fermi level. It is shown that at the conditions of industrial methanol synthesis complex electronic charge transfer processes across the metal-support interface, driven by morphological and electronic changes, explain the enhanced catalytic reactivity toward CO(2).

Methanol synthesis on ZnO(0001). III. Free energy landscapes, reaction pathways, and mechanistic insights.

The interplay of physical and chemical processes in the heterogeneous catalytic synthesis of methanol on the ZnO(0001) surface with oxygen vacancies is expected to give rise to a complex free energy landscape. A manifold of intermediate species and reaction pathways has been proposed over the years for the reduction of CO on this catalyst at high temperature and pressure conditions as required in the industrial process. In the present study, the underlying complex reaction network from CO to methanol is generated in the first place by using ab initio metadynamics for computational heterogeneous catalysis. After having "synthesized" the previously discussed intermediates in addition to finding novel species, mechanistic insights into this network of surface chemical reactions are obtained based on exploring the global free energy landscape, which is refined by investigating individual reaction pathways. Furthermore, the impact of homolytic adsorption and desorption of hydrogen at the required reducing gas phase conditions is probed by studying such processes using different charge states of the F-center.

On the reticular construction concept of covalent organic frameworks.

The concept of reticular chemistry is investigated to explore the applicability of the formation of Covalent Organic Frameworks (COFs) from their defined individual building blocks. Thus, we have designed, optimized and investigated a set of reported and hypothetical 2D COFs using Density Functional Theory (DFT) and the related Density Functional based tight-binding (DFTB) method. Linear, trigonal and hexagonal building blocks have been selected for designing hexagonal COF layers. High-symmetry AA and AB stackings are considered, as well as low-symmetry serrated and inclined stackings of the layers. The latter ones are only slightly modified compared to the high-symmetry forms, but show higher energetic stability. Experimental XRD patterns found in literature also support stackings with highest formation energies. All stacking forms vary in their interlayer separations and band gaps; however, their electronic densities of states (DOS) are similar and not significantly different from that of a monolayer. The band gaps are found to be in the range of 1.7-4.0 eV. COFs built of building blocks with a greater number of aromatic rings have smaller band gaps.

Imogolite nanotubes: stability, electronic, and mechanical properties.

The aluminosilicate mineral imogolite is composed of single-walled nanotubes with stoichiometry of (HO)(3)Al(2)O(3)SiOH and occurs naturally in soils of volcanic origin. In the present work we study the stability and the electronic and mechanical properties of zigzag and armchair imogolite nanotubes using the density-functional tight-binding method. The (12,0) imogolite tube has the highest stability of all tubes studied here. Uniquely for nanotubes, imogolite has a minimum in the strain energy for the optimum structure. This is in agreement with experimental data, as shown by comparison with the simulated X-ray diffraction spectrum. An analysis of the electronic densities of states shows that all imogolite tubes, independent on their chirality and size, are insulators.