Magnetically modified single and turbostratic stacked graphenes from tris(2,2'-bipyridyl) iron(II) ion-exchanged graphite oxide

Loading of graphite oxide (GO) with tris(2,2'-bipyridyl) iron(II) ions and subsequent calcination affords a novel graphene-based composite with magnetic and electrically conductive properties. The pH of the starting aqueous suspension and the washing procedure play a crucial role in the successful immobilization of the iron precursor, which is mainly governed by ion exchange. The complex is intercalated between the graphene oxide layers, where it adopts a distorted conformation. Rapid heating of this solid results in the deflagration of GO and the formation of ultrafine ( d = 2-14 nm) Fe2O3 particles with maghemite as the dominant phase. The superparamagnetic maghemite crystals are dispersed uniformly in the high-surface-area diamagnetic matrix built up from single or turbostratic stacked graphenes.     

Total synthesis of (+/-)-crinine via the regioselective Stille coupling and Diels-Alder reaction of 3,5-dibromo-2-pyrone

The regioselective synthesis and Diels-Alder cycloaddition of 3-(3,4-methylenedioxyphenyl)-5-bromo-2-pyrone provided a new synthetic route to crinine. The vinyl bromide group can be used as a handle for further derivatization.     

Determination of phenols and chlorophenols as trimethylsilyl derivatives using gas chromatography-mass spectrometry

A rapid and simple method is described for the simultaneous determination of 6 phenols (phenol, o-, m-, p-cresol, catechol and resorcinol) and 19 chlorophenols (all mono-, di-, tri-, and tetrachlorophenol isomers and pentachlorophenol) present in aqueous samples. The method is based on derivatization with trimethylsilyl-N,N-dimethylcarbamate (TMSDMC). In contrast to other derivatization agents, TMSDMC instantaneously reacts with the phenolic compounds at room temperature and no further sample processing is necessary prior to instrumental analysis. The determination of the derivatives was performed by capillary gas chromatography-mass spectrometry (GC-MS). The stability of the most instable trimethylsilyl derivative (pentachlorophenol) was studied using different excess levels of the derivatization reagent. The derivatization method was tested on spiked water samples preconcentrated by solid phase extraction on Isolute ENV+ cartridge. The overall method gave detection limits of 0.01-0.25 microg/L for all compounds and < 0.05 microg/L for 17 of them.     

Synthesis and stereochemistry of 3‐acetyl‐5‐aminospiro[1,3,4‐thiadiazoline‐2,4′‐thioflavans]

Under acetylating conditions racemic thioflavanone thiosemicarbazones cyclize into racemic 3‐acetyl‐spiro[1,3,4‐thiadiazoline‐2,4′‐thioflavans] and a racemic 3‐acetylspiro[1,3,4‐oxadiazoline‐2,4′‐thioflavan] with trans O(1) or S(1) and Ph(2′_eq). Hindered rotation of the endocyclic N(3) acetyl group spirothia‐diazolines caused the formation of isomers separable by HPLC. X‐ray diffraction analyses, ¹H‐, ¹³C‐, and ¹⁵N NMR measurements as well as MOPAC QM calculations were performed to reveal the structures of these isomers.     

Structure and stability of short beta-peptide nanotubes: a non-natural representative of collagen?

Since secondary structure elements are known to play a key role in stabilizing the 3D-fold of proteins for the design of non-natural proteins composed of beta-amino acid residues, the construction of suitable secondary structural elements is mandatory. Folding analogues of alpha-helices and beta-strands of beta-polypeptides were already described (Chem. Biodiversity 2004, 1, 1111 (1)). Here, we present several collagen-like folds composed exclusively of beta-Ala(s). Unlike their natural counterpart, these tubular nanostructures can be composed of more than three polypeptide chains aligned parallel and/or antiparallel. By using ab initio and DFT calculations we have optimized a large number of versatile collagen-like antiparallel nanostructures. In these tubular systems, oligopeptide strands are interconnected by i --> (i) type H-bonds, except for the "closing" set. This latter is called "the H-bond zipper" and is either (i) --> i, ( i + 1) --> i, or ( i + 2) --> i type. Antiparallel, tubular foldamers composed of l number of strands, each of k number of beta-amino acid residues (e.g., apbeta-T(l) i+l ) k , ap(beta-T(l) i+1 ) k , or ap(beta-T(l) i+2 ) k ), are unexpectedly stable supramolecular complexes. Independent of k and l, the local backbone fold of the amino acid residues is usually spiral, abbreviated as "S(P)" or "S*(P)". Nevertheless, in contrast to parallel, in antiparallel nanotubes the backbone fold can occasionally twist out from S(P) or S*(P) type into an alternative local structure. However, the more the local geometry of the strands resembles to S(P) or S*(P), the higher the stability is. Besides the backbone twisting, the overall stability is determined by the type and the geometrical properties of the constituent H-bonds. Interestingly, higher number of total H-bonds can provide a lower overall stability, when H-bond parameters are inferior. In general, the increase of both the number of strands and their length stabilize the supramolecular complex. Now that, for beta-peptides, collagen-like overall folds with their stability were determined, their POG- or PPG-like sequence specificity has to be revealed.     

Inverse microemulsion polymerization of sterically stabilized polyampholyte microgels

Polyampholyte microgels consisting of various compositions of poly(methacrylic acid) and poly(2-(dimethylamino)ethyl methacrylate) (PMAA-PDMA) cross-linked with allyl methacrylate (AM) were synthesized via the inverse microemulsion polymerization (IMEP) technique. To improve colloidal stability at the isoelectric point (IEP), steric stabilization via the grafting of poly(ethylene glycol) methyl ether methacrylate (PEGMA) on the surface of the microgel was performed. Potentiometric and conductometric titration showed good agreement between the targeted and experimental compositions of the microgel systems. The microgel swelled at low and high pH and possessed a compact structure near the IEP, and the diameter were in good agreement with data from the transmission electron microscopic (TEM) analyses. With increasing pH, the mobility decreased from +2 m(2)s(-1)V (1) at pH 2 to -2 m(2)s(-1)V (1) at pH 10. An empirical relationship describing the PMAA composition and IEP was proposed, where the IEP decreased with increasing PMAA content. The microgel exhibited thermal-responsive properties at high pH, which is dictated by the lower critical solution temperature of PDMA.     

Biochemically controlled bioelectrocatalytic interface

A switchable bioelectrocatalytic system for glucose oxidation controlled by external biochemical signals exemplifies interfacing between bioelectronic and biochemical ensembles.     

Complete structural characterization of metallacyclic complexes in solution-phase using simultaneously X-ray diffraction and molecular dynamics simulation

Wide-angle X-ray diffraction and molecular dynamics simulation has been used to perform complete structural characterization of nitromethane solution of a 16-membered gold(I) ring. The joint application of these two methods was an adequate tool to describe not only the structure of the complex but also the solvation properties of the complex in nitromethane and the effect of the solvation on the bulk structure. It has been found that a relatively diffuse slightly distorted solvation shell is formed around the complex, following the shape of the molecule. Nitromethane molecules in the solvation sphere are distributed randomly; no special orientation can be detected. The interaction energy of the complex with nitromethane molecules is attractive. In bulk, besides the antiparallel orientation of the nitromethane molecules, T-shape orientation and long-range order in antidipole orientation can also be detected.     

A hydrodynamic approach to the measurement of the permeability of small molecules across artificial membranes

An in situ analytical approach to the measurement of supported liquid membrane permeability is reported. The method consists of a spectrophotometric method to measure transport through a membrane-supported lipid solution, using a rotating-diffusion cell configuration to overcome limits arising from transport through the aqueous solution boundary layer in stationary systems. Rotation frequencies are almost two orders of magnitude higher than those employed previously for rotating-diffusion studies of membrane transport. The method is illustrated with the transport of warfarin [1-(4'-hydroxy-3'-coumarinyl)-1-phenyl-3-butanone]. The use of the rotating-diffusion approach permits accurate calculation of the aqueous phase boundary layer thickness, which has hitherto been treated as an adjustable parameter in studies of membrane permeability. Further, it is shown that the analyte diffusion coefficient can be determined readily using liquid-liquid electrochemistry.