On Two-Graphs of Ovoids inO2n+ 1(q)

Moorhouse and Gunawardena [1] have shown that if the avoids in finite orthogonal spaces of typeO_{2n + 1}(q),qodd, exist then their two-graphs are regular. In this note, we present a new geometrical proof for the regularity of those two-graphs.     

On weak monotonicity of the powers of nonnegative matrices

The purpose of this paper is to study conditions under which the powers of a square matrix A are weak monotone. Necessary and sufficient conditions under which the product of any two weak monotone matrices is weak monotone are also obtained. Several examples are given to illustrate the necessity of the conditions stated.     

Atomic homodyne detection of weak atomic transitions

We have developed a two-color, two-pathway coherent control technique to detect and measure weak optical transitions in atoms by coherently beating the transition amplitude for the weak transition with that of a much stronger transition. We demonstrate the technique in atomic cesium, exciting the 6s(2)S(1/2) --> 8s(2)S(1/2) transition via a strong two-photon transition and a weak controllable Stark-induced transition. We discuss the enhancement in the signal-to-noise ratio for this measurement technique over that of direct detection of the weak transition rate, and project future refinements that may further improve its sensitivity and application to the measurement of other weak atomic interactions.     

Electron transfer versus proton transfer in gas-phase ion/ion reactions of polyprotonated peptides

The ion/ion reactions of several dozen reagent anions with triply protonated cations of the model peptide KGAILKGAILR have been examined to evaluate predictions of a Landau-Zener-based model for the likelihood for electron transfer. Evidence for electron transfer was provided by the appearance of fragment ions unique to electron transfer or electron capture dissociation. Proton transfer and electron transfer are competitive processes for any combination of anionic and cationic reactants. For reagent anions in reactions with protonated peptides, proton transfer is usually significantly more exothermic than electron transfer. If charge transfer occurs at relatively long distances, electron transfer should, therefore, be favored on kinetic grounds because the reactant and product channels cross at greater distances, provided conditions are favorable for electron transfer at the crossing point. The results are consistent with a model based on Landau-Zener theory that indicates both thermodynamic and geometric criteria apply for electron transfer involving polyatomic anions. Both the model and the data suggest that electron affinities associated with the anionic reagents greater than about 60-70 kcal/mol minimize the likelihood that electron transfer will be observed. Provided the electron affinity is not too high, the Franck-Condon factors associated with the anion and its corresponding neutral must not be too low. When one or the other of these criteria is not met, proton transfer tends to occur essentially exclusively. Experiments involving ion/ion attachment products also suggest that a significant barrier exists to the isomerization between chemical complexes that, if formed, lead to either proton transfer or electron transfer.     

Electrochemical nucleation: Part II. The electrodeposition of silver on vitreous carbon.

A study has been made of the electrochemical nucleation of silver on vitreous carbon from aqueous solutions of silver nitrate.The nucleation is shown to be progressive and mass transferred controlled. The rate of nucleation is somewhat better described bythe atomistic theory than by classical theories. The addition of EDTA reduces the rate of nucleation as the result, it is suggested,of the adsorption of EDTA by the graphite surface. The sensitivity of the rate of the nucleation to the condition of the graphitesurface is also shown by the effects of changing the salvation acidity.     

Use of the titanium(IV) chelate of salicylic acid for a selective absorptiometric determination of titanium

Titanium(IV) forms a 1:1 chelate with salicylic acid in 40% aqueous ethanolic solution. The chelate is extractable into chloroform. A selective extraction method for the estimation of titanium in the presence of a variety of commonly interfering species is described. The addition of pyridine is found to increase the sensitivity and selectivity and makes it unnecessary to use alcohol in the extraction. The extracted species is a pyridine adduct of the titanium-salicylic acid chelate.     

An Improved Method for Calculating Zeta-Potentials from Measurements of the Electrokinetic Sonic Amplitude

The surface electric properties of the commercially available silica, Monospher 1000 (Fa. Merck), have been studied by conductivity and ESA (electrokinetic sonic amplitude) experiments. It could be shown that accounting for the contribution of the stagnant layer to surface conductivity is indispensable in the interpretation of electrokinetic data at low ionic strength. A general method has been put forward which allows to take into account the total, experimentally accessible surface conductivity in the evaluation of ESA data of moderately concentrated suspensions. This includes additional conductivity measurements which serve for the independent estimation of the total relative surface conductivity. The resulting zeta-potentials are clearly higher than those obtained after neglecting the contribution of the stagnant layer to surface conductivity. In addition, the ionic mobilities of potassium and magnesium in the hydrodynamically stagnant layer have been investigated in some detail. It has been found that the ionic mobility of potassium is of the same order of magnitude as in the bulk solution while the mobility of magnesium is significantly reduced. Copyright 2000 Academic Press.     

Effects of cation charge-site identity and position on electron-transfer dissociation of polypeptide cations

The effect of cation charge site on gas-phase ion/ion reactions between multiply protonated model peptides and singly charged anions has been examined. Insights are drawn from the quantitative examination of the product partitioning into competing channels, such as proton transfer (PT) versus electron transfer (ET), electron transfer followed by dissociation (ETD) versus electron transfer without dissociation (ET, no D), and fragmentation of backbone bonds versus fragmentation of side chains. Peptide cations containing protonated lysine, arginine, and histidine showed similar degrees of electron transfer, which were much higher than the peptide having fixed-charge sites, that is, trimethyl ammonium groups. Among the four types of cation charge sites, protonated histidine showed the highest degree of ET, no D, while no apparent intact electron-transfer products were observed for peptides with protonated lysine or arginine. All cation types showed side chain losses with arginine yielding the greatest fraction and lysine the smallest. The above trends were observed for each electron-transfer reagent. However, proton transfer was consistently higher with 1,3-dinitrobeznene anions, as was the fraction of side-chain losses. The partitioning of products among the various electron-transfer channels provides evidence for several of the mechanisms that have been proposed to account for electron-transfer dissociation and electron-capture dissociation. The simplest picture to account for all of the observations recognizes that several mechanisms can contribute to the observed products. Furthermore, the identity of the anionic reagent and the positions of the charge sites can affect the relative contributions of the competing mechanisms.