Surface modifications of bioglass immersed in TRIS-buffered solution. A multitechnical spectroscopic study

Bioglass 45S5 is used in the medical field as a bone regenerative material. In fact, when immersed in body fluid, a layer of hydroxy carbonate apatite (HCA), an analogue to the mineral phase that bones are made of, is deposited on its surface. A mechanism that would explain this process has been hypothesized and includes cation leaching from the glass to the solution and formation of both a silica-rich layer and a Ca/P-rich surface layer, prior to the actual crystallization of HCA. The present paper analyzes the dissolution of 2-mum-size particles of Bioglass in TRIS-buffered solution, focusing on the modifications occurring at the surface of the particles. Results from Transmission FT-IR, Raman, and X-ray Photoelectron Spectroscopy were compared in order to obtain this information. In all cases, precise spectral band assignments were obtained by comparing Bioglass spectra, before and after reaction, with the spectra registered on some selected reference samples. The results confirm the hypothesized mechanism of Bioglass reactivity and yield new insights on the surface modifications of the samples. In particular, the following is shown: the strength of the surface H-bonding system and of water coordination decreases during the reaction; surface carbonates, initially mainly bound to Na, are substituted by an increasing amount of Ca-bound carbonates; and the final calcium phosphate layer obtained is very similar, but not identical, to carbonated hydroxyapatite.     

New mu-oxo octanuclear complexes of 3d elements stabilized by dialkylcarbamato ligands. Synthesis and X-ray crystal and molecular structures

The octanuclear aggregates M(8)(mu(4)-O)(2)(O(2)CN(i)()Pr(2))(12) [M = Mn(II) 1, Co(II) 2, Ni(II) 3] have been prepared in good yields by controlled hydrolysis of the corresponding metal carbamate precursors [M(O(2)CN(i)()Pr(2))(2)](n)(). X-ray analysis has shown compounds 1-3 to be isostructural. The core of 2 contains two distorted [M(4)O] tetrahedra related by an inversion center. The hexanuclear carbamates M(6)(O(2)CNEt(2))(12) in toluene undergo a metal redistribution process with formation of the hexanuclear carbamates M'(x)M' '(6-x)(O(2)CNEt(2))(12), M' = Co, M' ' = Mn, as evidenced by mass-spectrometric data. In the presence of moisture, the mixed octanuclear carbamates Co(x)Mn(6-x)(MnO)(CoO)(O(2)CNEt(2))(12) were promptly formed and detected by DCI/MS measurements. Mass spectral data of Co(8)(mu(4)-O)(2)(O(2)CN(i)Pr(2))(12) are also reported.     

Mirror-image packing in enantiomer discrimination molecular basis for the enantioselectivity of B.cepacia lipase toward 2-methyl-3-phenyl-1-propanol

Synthetic chemists often exploit the high enantioselectivity of lipases to prepare pure enantiomers of primary alcohols, but the molecular basis for this enantioselectivity is unknown. The crystal structures of two phosphonate transition-state analogs bound to Burkholderia cepacia lipase reveal this molecular basis for a typical primary alcohol: 2-methyl-3-phenyl-1-propanol. The enantiomeric alcohol moieties adopt surprisingly similar orientations, with only subtle differences that make it difficult to predict how to alter enantioselectivity. These structures, along with a survey of previous structures of enzyme bound enantiomers, reveal that binding of enantiomers does not involve an exchange of two substituent positions as most researchers assumed. Instead, the enantiomers adopt mirror-image packing, where three of the four substituents at the stereocenter lie in similar positions. The fourth substituent, hydrogen, points in opposite directions.     

Exfoliated graphite nanoplatelets and gold nanoparticles based electrochemical sensor for determination of levodopa

This paper describes a rapid, accurate, and sensitive method for the determination of levodopa in a pharmaceutical sample using a glassy carbon electrode modified with a hybrid nanocomposite constituted of exfoliated graphite nanoplatelets dispersed in a suspension of gold nanoparticles in carboxymethylcelullose (AuNP-CMC-xGnP/GCE). The nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy, UV-Vis spectroscopy, and zeta potential. Electrochemical characterization of the proposed sensor by cyclic voltammetry and electrochemical impedance spectroscopy indicated that the nanocomposite used for the electrode modification facilitated electron transfer. Using square-wave voltammetry (SWV) under optimized conditions (0.50% (m/v) of AuNP-CMC-xGnP, 0.1 mol L^?1 sulfuric acid, frequency 30 Hz, pulse amplitude 50 mV, and scan increment 6.0 mV), the calibration curve showed a linear range for levodopa from 5 to 50 μmol L^?1, with a limit of detection of 0.5 μmol L^?1. The sensor demonstrated good repeatability and electrode-to-electrode repeatability, with relative standard deviations of 2 and 4%, respectively. The proposed method was successfully applied to quantify levodopa in a pharmaceutical sample by SWV, showing good accuracy. Recoveries of 98 to 107% demonstrated that the method is suitable for practical applications. Therefore, the proposed sensor represents a useful tool for rapid and accurate determination of levodopa.     

Bridging the Gap between the Direct and Hydrocarbon Pool Mechanisms of the Methanol‐to‐Hydrocarbons Process

After a prolonged effort over many years, the route for the formation of a direct carbon?carbon (C?C) bond during the methanol‐to‐hydrocarbon (MTH) process has very recently been unveiled. However, the relevance of the “direct mechanism”‐derived molecules (that is, methyl acetate) during MTH, and subsequent transformation routes to the conventional hydrocarbon pool (HCP) species, are yet to be established. This important piece of the MTH chemistry puzzle is not only essential from a fundamental perspective, but is also important to maximize catalytic performance. The MTH process was probed over a commercially relevant H‐SAPO‐34 catalyst, using a combination of advanced solid‐state NMR spectroscopy and operando UV/Vis diffuse reflectance spectroscopy coupled to an on‐line mass spectrometer. Spectroscopic evidence is provided for the formation of (olefinic and aromatic) HCP species, which are indeed derived exclusively from the direct C?C bond‐containing acetyl group of methyl acetate. New mechanistic insights have been obtained from the MTH process, including the identification of hydrocarbon‐based co‐catalytic organic reaction centers.     

Structural motifs of DNA complexes in the gas phase

DNA duplexes are known to be quite stable in the condensed phase but recent mass spectrometry results have shown that DNA complexes are also stable (at least for a limited time) in the gas phase. However, very little is known about the overall shape of the complexes in a solvent-free environment and what factors influence that shape. In this article, we present recent ion mobility and molecular modeling results that address some issues concerning the gas-phase conformations of DNA duplexes. Examples include the effect of metal ions on Watson–Crick base pairing, investigating the onset of helicity in duplexes as a function of strand length, comparison of the stability of C·G and A·T base pairs, and examining the formation of quadruplex structures.     

The experimental electron density distribution in the complex of (E)-1,2-bis(4-pyridyl)ethylene with 1,4-diiodotetrafluorobenzene at 90 K

The experimental electron density of the donor-acceptor complex of (E)-1,2-bis(4-pyridyl)ethylene (bpe) with 1,4-diiodotetrafluorobenzene (F(4)DIB) at 90 K has been determined with the aspherical atom formalism and analyzed by means of the topological theory of molecular structure. The bpe and F(4)DIB molecules are connected by intermolecular I.N bonds into infinite 1D chains. F.H bonds link these chains together to form the crystal assembly. The topological analysis reveals that the Cbond;I bond is of the "closed shell" type. Its bond-critical properties run parallel to those found in metal-metal and metal-ligand bonds of organometallic compounds. The integrated net charges show that the I.N halogen bond has an essentially electrostatic nature. F.F, F.C, and C.C intermolecular interactions, for which a bond path was found, contribute to reinforce the crystal structure.     

Photoinduced N-demethylation of rufloxacin and its methyl ester under aerobic conditions

Irradiation of rufloxacin (RF) under aerobic conditions gives rise to N-demethylation of the piperazinyl ring, which is enhanced in aerated D2O. Two primary processes seem to be involved in RF N-demethylation: photoionization from 1RF and singlet oxygen generation from 3RF. Both processes may lead to the same key intermediates, namely, RF*+ and superoxide radical anion; coupling of these intermediates explains N-demethylation of RF via an iminium cation. Formation of the hydrated electron by a monophotonic process (with a quantum yield of 0.09) is detected along with 3RF (with a intersystem-crossing quantum yield phiISC = 0.36) by laser flash photolysis. Studies performed on RF methyl ester give qualitatively similar results.     

Atomic force microscopy evaluation of aqueous interfaces of immobilized hyaluronan

Hyaluronan (HA) was immobilized on aminated glass surfaces in three different ways: by simple ionic interaction and by covalent linking at low density and at full density. In agreement with previous reports, in vitro experiments show that the outcome of fibroblast adhesion tests is markedly affected by the details of the coupling procedure, suggesting that different interfacial forces are operating at the aqueous/HA interface in the three cases investigated. The interfacial properties of the HA-coated surfaces were probed by force-distance curves obtained with the atomic force microscope (AFM). This approach readily shows significant differences among the tested samples, which are directly related to the coupling strategy and to results of cell adhesion tests. In particular, the range of interaction between the tip and the surface is much lower when HA is covalently linked than when it is ionically coupled, suggesting a more compact surface structure in the former case. Increasing HA surface density minimizes the interaction force between the surface and the AFM tip, likely reflecting more complete shielding by the HA chains of the underlying substrate. In summary, these measurements clearly show the different nature of the aqueous interfaces tested, and underline the role of this analytical approach in the development and control of finely tuned biomaterial surfaces.