Kinetics evidence for a complex between peroxynitrous acid and titanium(IV)

The reaction between TiO(2+) and ONOOH in 0.9 M H(2)SO(4) provides evidence for direct formation, previously unobserved, of a HOONO-metal complex. The reaction proceeds via formation of an end-on complex (k = 3.0 x 10(2) M(-1) s(-1)) that rearranges to form a side-on complex (k approximately equal to 20 s(-1)). With ONOOH in excess, this rearrangement proceeds more slowly (k approximately equal to 0.1 s(-1)), probably because multiple hydrogen oxoperoxonitrate molecules form end-on complexes with oxotitanium(IV) and hinder rearrangement to the side-on complex. The absorption spectrum of the final product is that of TiO(2)(2+). Presumably, during the rearrangement or later, NO+ is lost.     

Selective alterations of the antibody response to HIV-1

HIV infection leads to progressive alterations of humoral immune functions, including B-cell hyperplasia, hypergammaglobulinemia, elevated autoantibody titers, a poor response to neoantigens and mitogens, polyclonal B-cell activation, monoclonal gammopathies, and a significant deterioration of the antigen-specific humoral response. There is also an important isotypic imbalance of the antibody (Ab) response in the systemic compartment and a profound modification of mucosal immune functions. These abnormalities may contribute to disease progression and development of opportunistic infections, despite the presence of serum-neutralizing anti-HIV Abs. Equally important are the abnormal selection mechanisms of the Ab repertoire that seem to be responsible for B-cell clonal deletions. The V_H3 gene family, which encodes for approx 50% of immunoglobulins expressed by peripheral B-cells from normal adults, is underrepresented in human monoclonal antibodies to HIV-1 and in the peripheral B-cells of AIDS patients. These abnormalities, together with features of germinal center alteration, could be responsible for the clonal elimination of a subset of B-cells, and could contribute to HIV pathogenesis.     

Detection of metal ions using ion-channel sensor based on self-assembled monolayer of thioctic acid

Gold electrodes were chemically modified with thioctic acid monolayer designed to mimic biological ion-channel membranes. The technique was then used in the determination of alkali, alkaline earth, thallium(I), and lanthanum metal cations as analytes. Cyclic voltammograms (CV) of [Fe(CN)6]³⁻ an electroactive marker, were measured in the presence of the various types of analyte cations. In the absence of the analyte cation, electrostatic repulsion between the marker anions and the carboxylate groups of the receptor monolayer hindered the approach of the marker anion to the electrode surface and hence hindered its reduction. The modified electrodes responded well to the metal cations except the alkali metal cations. The sensors could detect the trivalent cation La³⁺ at concentrations as low as 10⁻⁸ M. The response of the sensor to the metal cations increase in the order alkali metal3+ can be discriminated in the ratio 1:100. This makes it possible to determine the trivalent ion in a sample matrix containing monovalent and divalent cations. Thallium(I) ion showed marked deviation in its response as compared to monovalent ions of the alkali metals. The ion-channel sensor based on self-assembled monolayer of thioctic acid therefore offers a potential alternative technique for the selective determination of metal ions.     

Die Acidität von Silanolgruppen. Vorläufige Mitteillung

The intrinsic acidity constant K_int. of surface silanol groups has been determined by coulometric titration of silicagel at 25° in solutions of the constant ionic strength 0,1M consisting primarily of sodium perchlorate. The experimental data are consistent with      

REALISIS: a medicinal chemistry-oriented reagent selection, library design, and profiling platform

REALISIS is a software system for reagent selection, library design, and profiling, developed to fit the workflow of bench chemists and medicinal chemists. Designed to be portable, the software offers a comprehensive graphical user interface and rapid, integrated functionalities required for reagent retrieval and filtering, product enumeration, and library profiling. REALISIS is component-based, consisting of four main modules: reagent searching; reagent filtering; library enumeration; and library profiling. Each module allows the chemist to access specific functionalities and diverse filtering and profiling mechanisms. By implementing the entire process of reagent selection, library design, and profiling and by integrating all the necessary functionalities for this process, REALISIS cuts the time required to design combinatorial and noncombinatorial libraries from several days to a few hours.     

X-ray structure of physiological copper(II)-bis(L-histidinato) complex

The isolation and the X-ray crystal structure of physiological copper(II)-L-histidine complex are reported. The neutral five-coordinate complex shows distorted square pyramidal geometry with bidentate and tridentate L-histidine ligands. The basic character of the pendent imidazole group and H-bonding interactions of bidentate L-histidine ligand are important for copper transport. The unique structural features help explain the origin of its thermodynamic stability and kinetic reactivity in human blood along with the ternary copper(II)-amino acid complexes. The role of L-histidine in interaction with copper(II)-albumin, in cellular uptake of copper, and in treatment of Menkes disease can be studied using these results.     

Photochemistry of iminium salts : Electron transfer mechanisms for singlet quenching and photoaddition of n-electron donating alcohols and ethers

Several aspects of past and current studies in the area of iminium salt photochemistry are discussed. Investigations of olefin-iminium salt photoaddition and photocyclization reactions are reviewed and conclusions about electron-transfer pathways for fluorescence quenching and reaction are discussed. The results of recent studies of alcohol and ether photoaddition to 2-phenyl-1-pyrrolinium salts are presented. These C-C bond forming processes occur in moderate yields to produce β-amino alcohol or ether products. In addition, alcohols and ethers serve as efficient quenchers of pyrrolinium salt fluorescence. Rate constants for quenching appear to be dependent upon both the oxidation potential of the alcohols and ethers and the availability of C-H bond α to oxygen. This data along with deuterium isotope effects on quenching combine to suggest a common mechanism for both fluorescence quenching and photoaddition. The nature of this mechanism is tested using the comparative quenching effeciencies of the tertiary alcohols t-butyl alcohol and 1,2,2-trimethyl-1-cyclopropanol. The latter alcohol having a weak C-C bond adjacent to the hydroxyl function quenches the fluorescence of 2-phenyl-1-pyrrolium salts at a rate two orders of magnitude greater than for t-butyl alcohol. The observations made are interpreted in terms of a sequential electron-proton transfer mechanism for quenching and photoaddition. Lastly, the relationship of iminium salt photochemical studies to other investigations of electron-transfer photochemistry is discussed.     

Two dimensional reversed-phase-reversed-phase separations isomeric separations incorporating C18 and carbon clad zirconia stationary phases

Informational theory and a geometric approach to factor analysis were employed to evaluate the degree of orthogonality of a two-dimensional reversed-phase-reversed-phase chromatographic system. The system incorporated a C18 column as one dimension and a carbon clad zirconia column as the second dimension. In order to study the resolving power of this system, the separation of a sample matrix containing an artificial mix of 32 isomers (structural and diastereoisomers) was evaluated. Using this system, between 25 and 28 of the 32 isomers could be separated, depending on the mobile phase combinations--with resolution that could not possibly be achieved in a single one dimensional separation. The results from this study indicate that in order to fully evaluate the resolving power of a 2D system multiple methods of analysis are most appropriate. This becomes increasingly important when the sample contains components that are very closely related and the retention of solutes is clustered in one quadrant of the 2D space. Ultimately, the usefulness of the 2D separation is determined by the goals of analyst.     

Rearrangement of phenylethenes on reaction with iodine-xenon difluoride

Phenyl-substituted ethenes react with iodine and xenon difluoride to provide difluorinated products. Iodofluoro intermediates react with xenon difluoride to produce transient iodine difluoride species that lose IF and F(-) and produce carbocations.     

A general strategy for the diastereoselective synthesis of 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignans

A strategy for the stereoselective synthesis of all the possible diastereoisomers of the 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane (furofuran) lignans from a single dihydrofuran precursor is described. The key steps involve a diastereocontrolled templated cationic cyclization followed by stereoselective reduction of the resulting methyl glycoside.