Host-guest chemistry in the gas phase: selected fragmentations of CB[6]-peptide complexes at lysine residues and its utility to probe the structures of small proteins

The gas phase host-guest chemistry between cucurbit[6]uril (CB[6]) and peptide is investigated using electrospray ionization mass spectrometry (ESI-MS). CB[6] exhibits a high preference to interacting with a Lys residue in a peptide forming a CB[6]-peptide complex. Collisionally activated CB[6] complexes of peptides yield a common highly selective fragment product at m/z 549.2, corresponding to the doubly charged CB[6] complex of 5-iminiopentylammonium (5IPA). The process involves the formation of an internal iminium ion, which results from further fragments to an a-type ion from a y-type ion, and the resulting 5IPA ion threads through CB[6]. Numerous peptides are investigated to test the generality of the observed unique host-guest chemistry of CB[6]. Its potential utility in probing protein structures is demonstrated using CB[6] complexes of ubiquitin. Low-energy collision induced dissociation yields CB[6] complex fragments, and further MS(n) spectra reveal details of the CB[6] binding sites, which allow us to deduce the protein structure in the solution phase. The mechanisms and energetics of the observed reactions are evaluated using density functional theory calculations.     

Optical Spectra of Synthetic Spinels in the System MgAl2O4–MgCr2O4

Unpolarized optical spectra were measured in the wavelength range 322–1666 nm by the diffuse reflection technique from spinel powders synthesized in the system MgAl₂O₄–MgCr₂O₄. The spectra were interpreted by the crystal-field theory on the basis of trigonally distorted spinel octahedra with D_3d symmetry. For chromium-rich solid solutions, including the MgCr₂O₄ end-member, results after peak fittings showed octahedral D_3d local symmetry around Cr³⁺ ions, identical to the crystallographic site symmetry. For chromium-poor solid solutions, however, octahedral C_3v local symmetry was suggested around Cr³⁺ ions, different from the D_3d crystallographically expected.     

Peroxide‐catalyzed swell grafting of maleic anhydride onto polypropylene

A new grafting method was developed to incorporate maleic anhydride directly onto solid‐state polypropylene powders. Maleic anhydride grafts altered the nonpolar characteristics of polypropylene so that much better mixing was achieved in blends and composites of polypropylene with many other polymers and fillers. Maleic anhydride was grafted onto polypropylene by the peroxide‐catalyzed swell grafting method, with a maximum extent of grafting of 4.60%. Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, tensile testing, and impact testing were used to characterize the isotactic polypropylene (iPP), maleic anhydride grafted polypropylene (MAH‐g‐iPP), and (isotactic polypropylene)/(calcium carbonate) composites (iPP/CaCO₃). The crystallinity and heat of fusion of the MAH‐g‐iPP decreased as the extent of grafting increased. The mechanical properties of the CaCO₃ filled polypropylene were improved by adding MAH‐g‐iPP as a compatibilizing agent. The dispersion of the fillers in the polymer matrix and the adhesion between the CaCO₃ particles and the polymer matrix were improved by adding the compatibilizer.     

Fractal analysis of pressure fluctuations in a three phase bubble column reactor operating at low pressure

Pressure fluctuations in a three phase bubble column reactor operating at relatively low pressure (92 KPa) have been analyzed by adopting the spectral and fractal analyses to get the engineering informations for the on-line control and fault diagnosis of the reactors. The mean value, standard deviation, skewness and kurtosis of the pressure fluctuations have been obtained. The local fractal dimension has been determined from the Pox diagram obtained by means of the rescaled range analysis of the pressure fluctuations based on the fractional Brownian motion. The local fractal dimension of pressure fluctuations has increased and thus the pressure fluctuation signals have become less persistent and irregular, with increases in the gas flow rate reaction temperature, particle size and solid content in the slurry phase. The local fractal dimension has been well correlated in terms of the operating variables.     

Preparation of nanosize Pt clusters using ion exchange of Pt(NH3) 4 2+ inside mesoporous channel of MCM-41

Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 35 was obtained by hydrothermal synthesis using a gel mixture with a molar composition of 6 SiO₂0.1 Al₂O₃1 hexadecyltrimethylammonium chloride 0.25 dodecyltrimethylammonium bromide 0.25 tetrapropylammonium bromide0.15 (NH₄)₂O1.5 Na₂O300 H₂O. The MCM-41 sample was calcined in O₂ flow at 813 K and subsequently ion exchanged with Ca²⁺. A small Pt cluster has been supported on the MCM-41 sample following a procedure using ion exchange of Pt(NH₃) ₄ ²⁺ . The Pt(NH₃) ₄ ²⁺ ion supported on MCM-41 has been activated in O₂ flow at 593 K and subsequently reduced with Fh flow at 573 K, in the same way used for the preparation of a Pt cluster entrapped inside the supercage of zeolite NaY. The resulting Pt cluster supported on the MCM-41 shows hydrogen chemisorption oftotal two H atoms per Pt at 296 K (based on the total amount of Pt) and high catalytic activity for hydrogenolysis of ethane. The chemical shift in¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the Pt cluster is located inside the mesoporous molecular sieve.     

Morphological core/shell structure and dispersion stability of water-dispersible copolyester graft polymerized with acrylic acid and ethyl acrylate

Anionic water-dispersible copolyester was prepared by the polycondensation of dimethyl isophathalate (DMI)/5-sodium sulfo dimethyl isophathalate (DMS) with ethylene glycol (EG)/ diethylene glycol (DEG) to synthesize water-dispersible copolyester-g-AA/EAs by chemical method. In the case of graft polymerization of AA/EA onto DMI/DMS/EG/DEG copolyester does not. This is a result of the core/shell structure that was ascertained by the graft yield and the zeta potential of copolyester-g-AA/EAs. It appears that grafted AA/EA exists at the surface of copolyester-g-AA/EA particles, and the backbone polymer, i.e., DMI/DMS/EG/DEG copolyester, exists in the inner phase of the copolyester, -g-AA/EA particles. The complete solubility of copolyester-g-AA/EAs in 1N NaOH aqueous solution seems to come mainly from the electrostatic repulsion between  COOs that are present at the surface of copolyester-g-AA/EA particles due to AA grafting and  OHs that are present at the dispersion medium due to NaOH decomposition and which cause the hydrolytic breakage and consequently the coagulation of copolyesters in alkaline medium. © 1996 John Wiley & Sons, Inc.