Skew-adjacency matrices of graphs

The spectra of the skew-adjacency matrices of a graph are considered as a possible way to distinguish adjacency cospectral graphs. This leads to the following topics: graphs whose skew-adjacency matrices are all cospectral; relations between the matchings polynomial of a graph and the characteristic polynomials of its adjacency and skew-adjacency matrices; skew-spectral radii and an analogue of the Perron–Frobenius theorem; and, the number of skew-adjacency matrices of a graph with distinct spectra.     

Thiazolopyridine analogs of nalidixic acid. 2. Thiazolo[4,5-b]pyridines

A series of 2-substituted 4-ethyl-4,7-dihydro-7-oxothiazolo[4,5-b]pyridine-6-carboxylic acids were synthesized. Antibacterial activity was tested in vitro. None of the new compounds prepared showed any interesting antibacterial activity in vitro against the strains tested.     

Graphs cospectral with distance-regular graphs

We determine all graphs with the spectrum of a distance-regular graph with at most 30 vertices (except possibly for the Taylor graph on 28 vertices)     

Single-particle self-energy and optical conductivity of the simplified Hubbard model

It is shown that the single-particle self-energy of the one and two-dimensional simplified Hubbard model exhibits different behavior characterized by Fermi-liquid, non-Fermi-liquid quasiparticle, or non-quasiparticle excitations, as a function of the strength of the on-site Coulomb repulsionU, temperature, and electron filling. For half-filled lattices, results for the optical conductivity indicate that the d.c. conductivity is zero for all temperatures andU>0.     

Study of the enzymatic degradation of vasostatin I and II and their precursor chromogranin A by dipeptidyl peptidase IV using high-performance liquid chromatography/electrospray mass spectrometry

The interaction of dipeptidyl peptidase IV with structurally related proteins differing in chain length, namely vasostatin I and II and their precursor protein chromogranin A, was examined using high-performance liquid chromatography in combination with electrospray mass spectrometry. Suitable analytical procedures were developed involving the use of reversed-phase high-performance liquid chromatography for purification of the enzymatic degradation products and a peptide mapping procedure for evaluating the enzymatic degradation of the large precursor protein chromogranin A. While vasostatin I was found to be a substrate for dipeptidyl peptidase IV, no N-terminal cleavage of Leu-Pro could be noted for chromogranin A. With respect to vasostatin II, N-terminal degradation was only observed after degradation in the C-terminal domain to proteins containing < or = 78 amino acids. The specificity of the N-terminal release of Leu-Pro was proved by addition of a DPP IV specific inhibitor.     

Quinolone antibacterials. 2. 6-Substituted-7-(2-thiazolyl and thiazolidinyl)quinolones

A series of 6-substituted-1-ethyl-1,4-dihydro-4-oxo-7-(2-thiazolyl- and thiazolidinyl)quinoline-3-carboxylic acids were synthesized. Substitution at the 6-position was H, F or Cl. The Hantzsch method was used for the preparation of the thiazolylquinolones. The thiazolidinylquinolones were synthesized by quaternization of the corresponding thiazolyl analogues, followed by reduction of the obtained thiazolium salts with sodium borohydride in aqueous solution. Antibacterial activity was tested in vitro. The compounds were inactive against Gram-negative bacteria but some of them showed good activity against Gram-positive bacteria and mycobacteria. This activity pattern is rarely found among the quinolone antibacterials.     

Isothiazolo[5,4-b]pyridine analogs of nalidixic acid

The synthesis and antibacterial evaluation of a number of isothiazolo[5,4-b]pyridine isosteres of nalidixic acid are described.     

An inequality for generalized hexagons

We show that a generalized hexagon withs + 1 points on a line andt + 1 lines through a point satisfies s=1 orts ³.     

Capillary electrophoresis for the speciation of arsenic

Capillary zone electrophoresis (CZE) with on-line UV-detection was used for the determination of arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine and arsenocholine. The method is simple and rapid (<10 min) and allows the determination of six different arsenic species without sample pretreatment. Several instrumental parameters were studied to obtain the best performance (pH of buffer, injection mode, injection time, applied voltage). To determine the arsenic compounds, the instrument was used with a negative potential applied to the injection side of the capillary so that the anions can migrate towards the anode because of their own mobility and charge. The capillary wall was coated with an electro-osmotic flow modifier which reversed the electro-osmotic flow and thus increased also the overall migration of the anions towards the anode. The influence of high concentrations of matrix components such as NaCl, KNO₃ and NaNO₃, as well as the presence of acids such as HNO₃ and HCl was studied. CZE was used for the determination of the oxidation state of arsenic in percolate waters and in the leachate of solidified arsenic containing waste. The lowest detectable concentration was about 100 g/l. A comparison with the results obtained with hydride generation coupled to ICP-MS was made.