Effect of molecular weight on the exponential growth and morphology of hyaluronan/chitosan multilayers: a surface plasmon resonance spectroscopy and atomic force microscopy investigation

The layer-by-layer growth of multilayer assemblies of two polysaccharides, the polyanion hyaluronan (HA) and the polycation chitosan (CH), was investigated using atomic force microscopy (AFM) and surface plasmon resonance (SPR) spectroscopy, with primary emphasis on the effect of the polysaccharide molecular weights on the film thickness and surface morphology. The HA/CH multilayers exhibit an exponential increase of the optical film thickness with the number of deposited bilayers. We show that the multilayer thickness at a given stage depends on the size of both CH, the diffusing polyelectrolyte, and HA, the non-diffusing species. Assemblies (12 bilayers) of high molecular weight polysaccharides (HA, 360,000; CH, 160,000) were twice as thick (approximately 900 nm vs approximately 450 nm) as those obtained with low molecular weight polymers (HA, 30,000; CH, 31,000), as assessed by AFM scratch tests. The exponential growth rate is the same for the high and low molecular weight pairs; the larger film thicknesses observed by SPR and by AFM arising from an earlier onset of the steep exponential growth phase in the case of the high molecular weight pair. In all cases, isolated islets form during the deposition of the first CH layer onto the underlying HA. Upon further film growth, individual islets coalesce into larger vermiculate features. The transition from distinct islands to vermiculate structures depends on the molecular weights of the polysaccharides and the lower molecular weight construct presents larger worm-like surface domains than the high molecular weight pair.     

1H and 13C NMR characterization of hemiamidal isoniazid-NAD(H) adducts as possible inhibitors of InhA reductase of Mycobacterium tuberculosis

Isoniazid (INH) is easily oxidized with manganese(III) pyrophosphate, a chemical model of the KatG protein involved in activation of INH inside the bacteria Mycobacterium tuberculosis. Performed in the presence of NAD(+), this oxidation generates a family of isomeric INH-NAD(H) adducts, which have been shown to be effective inhibitors of InhA, an enzyme essential in mycolic acid biosynthesis. In this work, we fully characterized by (1)H and (13)C NMR spectroscopy four main species of INH-NAD(H) adducts that coexist in solution. Two of them are open diastereoisomers consisting of the covalent attachment of the isonicotinoyl radical at position four of the nicotinamide coenzyme. The other two result from a cyclization involving the amide group from the nicotinamide and the carbonyl group from the isonicotinoyl radical to give diastereoisomeric hemiamidals. Although an INH-NAD(H) adduct with a 4S configuration has been characterized within the active site of InhA from Xray crystallography and this bound adduct interpreted as an open form (Rozwarski et al., Science 1998, 279, 98-102), it is legitimate to raise the question about the effective active form(s), open or cyclic, of INH-NAD(H) adduct(s). Is there a single active form or are several forms able to inhibit the InhA activity with different levels of inhibitory potency?     

2-Cyano Δ3piperideine VII1 : the condensation of 2-cyano Δ3piperideine with sodium dimethylmalonate catalyzed by ZnCl2 or zero valent palladium and platinum complexes

ZnCl₂, Pd (PØ₃)₄ and Pt(PØ₃)₄ were found to be effective catalysts for the condensation of sodium dimethyl malonate with 2-cyano Δ³piperideine $\text{1}$ a–d.     

Combined bond-polarization basis sets for accurate determination of dissociation energies. II. Basis set superposition error as a function of the parent basis set

The effect of the parent basis set on the basis set superposition error caused by bond functions is investigated systematically. An important difference between BSSE at the SCF and correlated levels is pointed out. Three new basis sets are defined, denoted 6-311 + G(d,p)B, 6-311 + G(2d,p)B, and 6-311 + G(2df,p)B. BSSE for the first-row hydrides seems to increase uniformly with increasing atomic number of the central atom. Expansion of the valence part of the basis set from 6-31G to 6-311G, as well as adding f functions, has a significant effect on the BSSE. Additional BSSEs incurred by bond functions are less than or equal to 1 kcal/mol for the 6-311 + G(2df,p)B basis set. For the dissociation energies of the first-row hydride species, agreement with experiment within only a few kcal/mol can be obtained even without resorting to isogyric reaction cycles. For high-quality calculations, adding bond functions seems to have definite advantages over expanding the polarization space beyond the [2d1f] level.     

Anionic copper complex fragmentations from enkephalins under low-energy collision-induced dissociation in an ion trap mass spectrometer

Peptide metallation with Cu2+ was explored in the negative ESI mode using an ion trap mass spectrometer. Under these conditions, the [(M-3H) + CuII]- species formed were investigated under low-energy collision-induced dissociation conditions. MS2 experiments indicate a very different behavior of CuII metallated complexes compared with [M-H]- species. CuII induces an easy loss of CO2 and specific side-chain cleavages (by radical losses) at the C-terminal residue, as observed previously by prompt 'in source' dissociation experiments. The loss of CO2 yields an unstable carbylide that leads to further dissociations involving the migration of a proton or a hydrogen radical (through the reduction of CuII). Multistage MS3 experiments were carried out to rationalize this behavior. Fragmentation pathways are proposed in order to explain the product ions observed. The side-chain radical loss at the C-terminus was demonstrated to be a consecutive process. Finally, evidence is provided that the specific side-chain cleavages can be used for the differentiation of Leu/Ile and Gln/Lys residues when they are located at the C-terminus. The existence of a zwitterionic form in the case of the anionic YGGFK-CuII complex is proposed.     

Normal mode analysis of oligomeric proteins: Reduction of the memory requirement by consideration of rigid geometry and molecular symmetry

A method is presented to reduce the memory requirement of normal mode analysis applied to systems containing two or more large proteins when these systems exhibit symmetry properties. We use a rigid geometry model (i.e., only the dihedral angles of the polypeptide chain are considered as variables). This model allows a reduction by a factor of 8 on average of the number of variables with a concomitant freezing of the high-frequency modes. The symmetry properties of the system are used to reduce further the number of variables that must be considered in the computation. Application of group theory leads to a factorization of the matrices of interest (the coefficient and the Hessian matrices) into independent blocks along the diagonal. The initial, reducible representation is thus transformed into a number of irreducible representations of smaller dimensions. In the case of the C₂ symmetry group, the method leads to a reduction of the size of the matrices that must be manipulated during the computation (coefficient matrix, Hessian matrix, and eigenvectors matrix) by a factor of 256 compared with the usual normal mode analysis in Cartesian coordinate space. The method is particularly well adapted to the study of the dynamics of oligomeric proteins because these proteins often display symmetry properties (e.g., virus coat proteins, immunoglobulins, hemoglobin, etc.). In favorable cases, in conjunction with X-ray diffuse scattering data, the study of systems showing allosteric properties might be considered. © 1994 by John Wiley & Sons, Inc.     

Development and validation of a high-performance liquid chromatographic method for the determination of clomazone residues in surface water

A method is described for the determination of clomazone residues in surface water by high-performance liquid chromatography with UV detection. The method involves solid-phase extraction with C18 extraction tubes. Clomazone was separated on a C18 column with a mobile phase of methanol-water (65:35, v/v) at pH 4.0 and a flow-rate of 1.0 ml/min. After optimization of the extraction and separation conditions, the method was validated. The method developed can be used for determination of clomazone in surface water, at the limit of 0.1 mcirog/l set by the European Union drinking water directive, with a 400-fold preconcentration.     

Supercritical fluid extraction of flumetralin in tobacco

Summary This work evaluates the level of Flumetralin residues in real tobacco samples. Conventional extraction methods were compared with SFE methods in both static and dynamic modes using CO₂ and CO₂ with modifier. Additionally, in the dynamic SFE mode, different collection approaches, such as collection at room temperature without solvent, collection at room temperature in the presence of solvent (hexane) and collection in an ice bath, without solvent, were studied. SFE showed itself to be a promising extraction technique for pesticide residues in tobacco samples.     

Expeditious synthesis of porphyrin-cobaltacarborane conjugates

Two porphyrin-cobaltacarborane conjugates and were prepared in high yields via a nucleophilic ring-opening reaction of . These novel boron-rich and fluorescent compounds have potential application as boron delivery agents for the boron neutron capture therapy of tumors.     

Competitive adsorption and desorption of a bi-solute mixture: effect of activated carbon type

This study aims to clarify the effects of carbon activation type and physical form on the extent of adsorption capacity and desorption capacity of a bi-solute mixture of phenol and 2-chlorophenol (2-CP). For this purpose, two different PACs; thermally activated Norit SA4 and chemically activated Norit CA1, and their granular countertypes with similar physical characteristics, thermally activated Norit PKDA and chemically activated Norit CAgran, were used. The thermally activated carbons were better adsorbers for phenol and 2-CP compared with chemically activated carbons, but adsorption was more reversible in the latter case. 2-CP was adsorbed preferentially by each type of activated carbon, but adsorption of phenol was strongly suppressed in the presence of 2-CP. The simplified ideal adsorbed solution (SIAS) model underestimated the 2-CP loadings and overestimated the phenol loadings. However, the improved and modified forms of the SIAS model could better predict the competitive adsorption. The type of carbon activation was decisive in the application of these models. For each activated carbon type, phenol was desorbed more readily in the bi-solute case, but desorption of 2-CP was less compared with single-solute. This was attributed to higher energies of 2-CP adsorption.