Reactions between alkaloids and tetraphenylboron and their analytical application : A heterometric study

The reactions between tetraphenylboron (TPB) and acriflavine, rivanol, nitron, cinchonine, quinine, strychnine, papaverine, sparteine, phenanthroline, pyramidon, antipyrine, atropine and oxine were studied heterometrically at pH values 1–9. The reactions were about one third as sensitive as with bismuth nitrate in iodide solution. The sensitivity in the above series decreases from acriflavine to oxine. Precise titrations could always be carried out with TPB. Compounds with two adjacent N atoms gave no precipitation, at pH 5–7. The structures of the compounds and the analytical aspects are discussed and a method of determination is presented.     

On intersection multigraphs of hypergraphs

The sum of the cardinalities of all the edges of a hypergraph is computed in two different ways. This result is used to treat the generalisation of the notion of cyclomatic number for hypergraphs. Among others the following result is obtained: The cyclomatic number of the hypergraph H vanishes if and only if some maximum forest of the weighted intersection graph of H has the property that for every vertex of H the subgraph of the forest induced by those edges containing that vertex is connected.     

Equimatchable claw-free graphs

A graph is equimatchable if all of its maximal matchings have the same size. A graph is claw-free if it does not have a claw as an induced subgraph. In this paper, we provide the first characterization of claw-free equimatchable graphs by identifying the equimatchable claw-free graph families. This characterization implies an efficient recognition algorithm.     

Structural and Functional Consequences of the Weak Binding of Chlorin e6 to Bovine Rhodopsin

The chlorophyll‐derivative chlorin e6 (Ce6) identified in the retinas of deep‐sea ocean fish is proposed to play a functional role in red bioluminescence detection. Fluorescence and ¹H NMR spectroscopy studies with the bovine dim‐light photoreceptor, rhodopsin, indicate that Ce6 weakly binds to it with μm affinity. Absorbance spectra prove that red light sensitivity enhancement is not brought about by a shift in the absorbance maximum of rhodopsin. ¹⁹F NMR experiments with samples where ¹⁹F labels are either placed at the cytoplasmic binding site or incorporated as fluorinated retinal indicate that the cytoplasmic domain is highly perturbed by binding, while little to no changes are detected near the retinal. Binding of Ce6 also inhibits G‐protein activation. Chemical shift changes in ¹H‐¹⁵N NMR spectroscopy of ¹⁵N‐Trp labeled bovine rhodopsin reveal that Ce6 binding perturbs the entire structure. These results provide experimental evidence that Ce6 is an allosteric modulator of rhodopsin.     

Photochemistry of a retinal protonated schiff-base analogue mimicking the opsin shift of bacteriorhodopsin

A retinal Schiff base analogue which artificially mimics the protein-induced red shifting of absorption in bacteriorhodopsin (BR) has been investigated with femtosecond multichannel pump probe spectroscopy. The objective is to determine if the catalysis of retinal internal conversion in the native protein BR, which absorbs at 570 nm, is directly correlated with the protein-induced Stokes shifting of this absorption band otherwise known as the "opsin shift". Results demonstrate that the red shift afforded in the model system does not hasten internal conversion relative to that taking place in a free retinal-protonated Schiff base (RPSB) in methanol solution, and stimulated emission takes place with biexponential kinetics and characteristic timescales of approximately 2 and 10.5 ps. This shows that interactions between the prosthetic group and the protein that lead to the opsin shift in BR are not directly involved in reducing the excited-state lifetime by nearly an order of magnitude. A sub-picosecond phase of spectral evolution, analogues of which are detected in photoexcited retinal proteins and RPSBs in solution, is observed after excitation anywhere within the intense visible absorption band. It consists of a large and discontinuous spectral shift in excited-state absorption and is assigned to electronic relaxation between excited states, a scenario which might also be relevant to those systems as well. Finally, a transient excess bleach component that tunes with the excitation wavelength is detected in the data and tentatively assigned to inhomogeneous broadening in the ground state absorption band. Possible sources of such inhomogeneity and its relevance to native RPSB photochemistry are discussed.     

Broadly tunable dual-wavelength light source for coherent anti-Stokes Raman scattering microscopy

The signal and idler beams from a picosecond, synchronously pumped optical parametric oscillator (OPO) provide the two colors necessary for coherent anti-Stokes Raman scattering (CARS) microscopy. The OPO provides a continuously tunable frequency difference between the two beams over a broad range of Raman shifts (100-3700 cm(-1)) by varying the temperature of a single nonlinear crystal. The near-infrared output (900-1300 nm) allows for deep penetration into thick samples and reduced nonlinear photodamage. Applications of this light source to in vivo cell and ex vivo tissue imaging are demonstrated.     

Acidity and alkylation of 4-phenyl-3,5-dihydroxypyrazole and its derivatives. C versus O and N alkylation

4-Phenyl-3,5-dihydroxypyrazole is a relatively strong acid, with a pKa of 3.70. The effect of substitution, both in the phenyl ring and on the heterocyclic ring, on the acidity was studied. Electron attracting groups on the phenyl group enhance the acidity. Selective replacement by an alkyl group of one or two of the heterocyclic hydrogens lowers the acidity. Meerwein reagent, as well as methyl iodide bring about alkylation on carbon, whereas diazomethane and diethyl sulfate do not. Michael addition proceeds through both carbon and nitrogen.     

-complete sequences of integers

An infinite sequence is -complete if every sufficiently large integer is the sum of such that no one divides the other. We investigate -completeness of sets of the form and with nonnegative.