Efficient synthesis of branched poly(acrylic acid) derivatives via postpolymerization modification

The utility of pentafluorophenyl esters for the selective introduction of functional units and branch points in well-defined poly(acrylic acid) (PAA) derivatives is demonstrated using a combination of controlled radical polymerization and postpolymerization modification. Reversible addition-fragmentation chain transfer enables the synthesis of well-defined copolymers—poly(pentafluorophenyl acrylate-co-tert-butyl acrylate)—with the active ester repeat units serving as attachment points for reaction with primary amines, specifically tris(2-(t-butoxycarbonyl)ethyl)methyl amine (Behera's amine). Deprotection using trifluoroacetic acid removes both the backbone and side chain t-butyl esters to give a series of branched PAA derivatives containing novel tricarboxylic acid side chains that are well suited to complexation and multidentate interactions. Surprisingly, the active ester homopolymer is shown to have the highest reactivity with Behera's amine when compared to copolymers with lower incorporation of pentafluorophenyl esters, suggesting an intriguing interplay of neighboring group effects and steric interactions. The ability to tune the efficiency of postpolymerization modification gives a library of PAA derivatives.     

Structure of Zinc and Nickel Histidine Complexes in Solution Revealed by Molecular Dynamics and Raman Optical Activity

The histidine residue has an exceptional affinity for metals, but solution structure of its complexes are difficult to study. For zinc and nickel complexes, Raman and Raman optical activity (ROA) spectroscopy methods to investigate the link between spectral shapes and the geometry were used. The spectra were recorded and interpreted on the basis of ionic equilibria, molecular dynamics, ab initio molecular dynamics, and density functional theory. For zwitterionic histidine the dominant tautomer was determined by the decomposition of experimental spectra into calculated subspectra. An octahedral structure was found to prevail for the ZnHis₂ complex in solution, in contrast to a tetrahedral arrangement in the crystal phase. The solution geometry of NiHis₂ is more similar to the octahedral structure found by X-ray. The Raman and ROA structural determinations of metal complexes are dependent on extensive computations, but reveal unique information about the studied systems.     

Partially Hydrogenated Graphene Materials Exhibit High Electrocatalytic Activities Related to Unintentional Doping with Metallic Impurities

Partially hydrogenated graphene materials, synthesized by the chemical reduction/hydrogenation of two different graphene oxides using zinc powder in acidic environment or aluminum powder in alkaline environment, exhibit high electrocatalytic activities, as well as electrochemical sensing properties. The starting graphene oxides and the resultant hydrogenated graphenes were characterized in detail. Their electrocatalytic activity was examined in the oxygen reduction reaction, whereas sensing properties towards explosives were tested by using picric acid as a redox probe. Findings indicate that the high electrocatalytic performance originates not only from the hydrogenation of graphene, but also from unintentional contamination of graphene with manganese and other metals during synthesis. A careful evaluation of the obtained data and a detailed chemical analysis are necessary to identify the origin of this anomalous electrocatalytic activity.     

Band structures of symmetrical graphene superlattice with cells of three regions

We study the electronic band structures of massless Dirac fermions in symmetrical graphene superlattice with cells of three regions. opening gaps and additional Dirac points. Finally, we inspect the potential effect on minibands, the anisotropy of group velocity and the energy bands contours near Dirac points. We also discuss the evolution of gap edges and cutoff region near the vertical Dirac points.     

A spectroscopic method to estimate the binding potency of amphiphile assemblies

A fast and convenient spectroscopic methodology to determine the water uptake capacity of amphiphile assemblies studied in multilayer films is presented. This method was developed to provide a reliable but relatively simple tool for estimating the binding potency of such complex systems. The water-binding potency represents a general propensity of higher-order systems to bind or embed relevant ligands, such as various non-lipid effectors in the case of artificial lipid membranes. In this sense, the binding potency might contribute to a specific functional role of certain lipids. The essence of the new method is that the calibration of data measured by infrared (IR) spectroscopy against those directly obtained by Karl–Fischer titration (KFT) enables one to replace the expensive chemical–analytical technique by a more comfortable and efficient IR-spectroscopic protocol. This approach combines the easy handling, versatility, and availability of IR spectroscopy with the high accuracy of KFT. The usefulness of the procedure is demonstrated on an example set of six amphiphiles with a common chain length of 18 carbon atoms. Despite this similarity, the binding potency data differ tremendously in a way which can be correlated with the systematic variations introduced into the amphiphile structure. Going further beyond the methodical aspect, the scientific relevance of the data is comprehensively discussed especially in terms of the structural factors that govern the binding potency of amphiphiles. That is favored mainly by fluidity and disfavored mainly by inter-amphiphile binding networks. For phosphatidylcholine, our data are strongly in favor of a particular hydration model that involves primary water binding to phosphate as well as the formation of water semi-clathrates hosting the trimethylammonium moiety. Interestingly, stearylamine and diolein assemblies did not take up any water at all. This unexpected hydrophobicity is due to the unusual structures formed in these latter cases: rigid ammonium amide with a strong hydrogen-bonding/salt bridge network in stearylamine, and patches of inverted micelles in diolein, as revealed by molecular dynamics simulations.     

Laser Desorption Ionization of As<Subscript>2</Subscript>Ch<Subscript>3</Subscript> (Ch = S,Se, and Te) Chalcogenides Using Quadrupole Ion Trap Time-of-Flight Mass Spectrometry: A Comparative Study

Laser desorption ionization using time-of-flight mass spectrometer afforded with quadrupole ion trap was used to study As₂Ch₃ (Ch = S, Se, and Te) bulk chalcogenide materials. The main goal of the study is the identification of species present in the plasma originating from the interaction of laser pulses with solid state material. The generated clusters in both positive and negative ion mode are identified as 10 unary (S _p ^+/– and As _m ^+/– ) and 34 binary (As _m S _p ^+/– ) species for As₂S₃ glass, 2 unary (Se _q ^+/– ) and 26 binary (As _m Se _q ^+/– ) species for As₂Se₃ glass, 7 unary (Te _r ^+/– ) and 23 binary (As _m Te _r ^+/– ) species for As₂Te₃ material. The fragmentation of chalcogenide materials was diminished using some polymers and in this way 45 new, higher mass clusters have been detected. This novel approach opens a new possibility for laser desorption ionization mass spectrometry analysis of chalcogenides as well as other materials.

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Tight β-turns in peptides. DFT-based study of infrared absorption and vibrational circular dichroism for various conformers including solvent effects

Vibrational circular dichroism (VCD) has had a large impact on configurational studies of organic molecules largely due to the theoretical advances made by Philip Stephens and co-workers. For peptides, the structural issue is not one of the configuration, but of conformation, and the flexibility of the oligomeric structure raises major computational challenges. Turns are a vital aspect of peptide and protein conformation that allow such structures to fold into a compact unit. However, unlike helices and sheets, they are not extended repeating structures, but each residue has a different local conformation. Also, when turns are part of larger peptides their termini are connected to completely different structural elements. We have done extended comparative density functional theory (DFT) computations to characterize the expected spectral contributions of selected turn structures to the amide IR and VCD spectra of peptides. The isolated vacuum results for tri-amide turns (Ac-X-Y-NH₂) of a few different sequences are compared with calculations involving correction for solvation effects. In particular, we looked at the sequence variation in spectra and structure between Ala-Ala, Aib-Gly and D-Pro-Gly for the turn-specific X–Y residues. The nature of some turn-associated, amide originating, spectral transitions are developed and tested.