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An ultrafine (6–7 nm) and well dispersed nano-SnO2/carbon nanotube hairball (SnO2/CNTH) composite material with a three-dimensional (3D) hierarchical structure is prepared by spray drying and solvo...

The competitive adsorption of the Na salt of phytic acid (PA) and 4-methyl-4H-1,2,4-triazole-3-thiol (4-MTTL) was investigated by using the surface-enhanced Raman scattering (SERS) technique and electrochemical measurements. On the basis of the recorded SERS spectra, it was found that PA molecules could competitively occupy the silver surface modified with 4-MTTL monolayers and that the competitive adsorption of 4-MTTL at the silver surface with the PA monolayers also happened, but the effect of coadsorption of PA on the structure of 4-MTTL monolayers was smaller than that of 4-MTTL on the PA monolayers. In addition, according to the electrochemical measurements, the only 4-MTTL monolayers or the only PA monolayers at the silver surface exhibited a reasonable stability. The coadsorption of 4-MTTL and PA formed a mixed film at the silver surface immersed in the solution with the coexistence of 4-MTTL and PA, and the film was unstable when a potential was applied.

Surface-enhanced Raman scattering (SERS) spectroscopy was applied to observe the process and structure of N-acetylalanine molecules self-assembled at the silver surface. SERS data show that the N-acetylalanine molecules adsorb at the silver surface through the amino group and the entire carboxyl group in a perpendicular way after a self-reorganization. Results of SERS and X-ray photoelectron spectroscopic experiments indicate that the Ce3+ affects the structure of monolayers because of the binding reaction between the Ce3+ and the carbonyl along with the amino groups in N-acetylalanine. The Ce3+ capped by N-acetylalanine monolayers can be removed by adding EDTA solution and the original structure of monolayers is recovered. However, the impact of K+ on the structure of N-acetylalanine monolayers is slight and can be ignored.

Penicillin-binding protein 2a (PBP2a) is the primary beta-lactam resistance determinant of methicillin-resistant Staphylococcus aureus (MRSA). MecA, the gene coding for PBP2a, was cloned with the membrane-anchoring region at the N-terminus deleted. The truncated protein (PBP2a) was overexpressed in Escherichia coli mostly in the soluble form accounting for approximately 25% of soluble cell protein and was purified to homogeneity. The purified protein was shown to covalently bind beta-lactams in an 1:1 ratio as determined by electrospray mass spectrometry. A novel method based on HPLC-elctrospray mass spectrometry has been developed to quantitatively determine the formation of the covalent adducts or acyl-PBP2a complexes. By using this method, combined with kinetic techniques including quench flow, we have extensively characterized the interactions between PBP2a and three beta-lactams and determined related kinetic parameters for the first time. The apparent first-order rate constants (ka) of PBP2a acylation by benzylpenicillin showed a hyperbolic dependence on the concentration of benzylpenicillin. This is consistent with the mechanism that the binding of the penicillin to PBP2a consists of reversible formation of a Michaelis complex followed by formation of the penicilloyl-PBP2a adduct, and allowed the determination of the individual kinetic parameters for these two steps, the dissociation constant Kd of 13.3 mM and the first-order rate constant k2 of 0.22 s-1. From these values, the second-order rate constant k2/Kd, the value reflecting the overall binding efficiency of a beta-lactam, of 16.5 M-1 s-1 was obtained. The fairly high Kd value indicates that benzylpenicillin fits rather poorly into the protein active site. Similar studies on the interaction between PBP2a and methicillin revealed k2 of 0.0083 s-1 and Kd of 16.9 mM, resulting in an even smaller k2/Kd value of 0.49 M-1 s-1. The rate constants k3 for deacylation of the acyl-PBP2a complexes, the third step in the interactions, were measured to be

The main objective of this work was to use positive-ion fast atom bombardment mass spectrometry (FAB-MS) B/E linked scan spectra to investigate the possibility of differentiating positional isomers of various authentic glycine- and taurine-conjugated bile acids. Sodium salts of 14 conjugated bile acids were individually ionized by FAB-MS and characterized by scanning simultaneously the magnetic field B and the electric sector field E such that B/E remained constant throughout the scan. The dominant fragment ions could be related to cleavage of the aliphatic side chain with charge retention on the conjugated end of the bile acids. However, fragment ions arising from ring cleavages were also observed and could be used to distinguish the positions of substituent hydroxyl groups. For example, ring cleavage of conjugated dihydroxy bile acids at C-7/C-8 and C-9/C-10 permitted the differentiation of chenodeoxycholyltaurine (3 alpha,7 alpha-substitution pattern) from deoxycholyltaurine (3 alpha,12 alpha-substitution pattern) based on the presence of fragment ions at m/z 388 or m/z 404, which were indicative of hydroxyl group substitutions at either the 7- or 12-positions, respectively. It was concluded that B/E linked scans can be used to discriminate positional isomers of conjugated bile acids.