Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene: a comprehensive density functional investigation based on the generic [(C(4)H(6))(2)Ni(0)PH(3)] catalyst

We present a comprehensive theoretical investigation of the mechanism for cyclodimerization of butadiene by the generic [bis(butadiene)Ni(0)PH(3)] catalyst employing a gradient-corrected DFT method. We have explored all critical elementary steps of the whole catalytic cycle, namely, oxidative coupling of two butadienes, reductive elimination under ring closure, and allylic isomerization. Oxidative coupling of two butadienes in the [bis(butadiene)Ni(0)L] complex and reductive elimination in the [(bis(eta(3))-octadienediyl)Ni(II)L] species take place under different stereocontrol, which makes isomerization indispensable. Commencing from a preestablished equilibrium between several configurations of the [(octadienediyl)Ni(II)L] complex, the major cyclodimer products, namely, VCH, cis-1,2-DVCB, and cis,cis-COD, are formed along competing reaction paths via reductive elimination, which is found to be the overall rate-determining step. Careful exploration of different possible conceivable routes revealed that bis(eta(1)) species are not involved as critical intermediates either in reductive elimination or in isomerization along the most feasible pathway. The regulation of the selectivity of the cyclodimer formation based on both thermodynamic and kinetic considerations is outlined.     

Generation and intermolecular reactions of 3-indolylacyl radicals

The generation of 3-indolylacyl radicals from the corresponding phenyl selenoesters and the scope of their participation in intermolecular addition reactions to carbon-carbon double bonds under both reductive and nonreductive conditions have been studied.     

Hydrophosphination of boron–boron multiple bonds

Five compounds containing boron–boron multiple bonds are shown to undergo hydrophosphination reactions with diphenylphosphine in the absence of a catalyst. With diborenes, the products obtained are highly dependent on the substitution pattern at the boron atoms, with both 1,1- and 1,2-hydrophosphinations observed. With a symmetrical diboryne, 1,2-hydrophosphination yields a hydro(phosphino)diborene. The different mechanistic pathways for the hydrophosphination of diborenes are rationalised with the aid of density functional theory calculations.

Compounds containing boron–boron double and triple bonds are shown to undergo uncatalysed hydrophosphination reactions with diphenylphosphine.     

Analysis of binary mixtures of flufenamic, meclofenamic and mefenamic acids by derivative synchronous fluorescence spectrometry

Second-derivative synchronous fluorescence spectrometry was used to develop a simple, rapid and sensitive spectrofluorimetric method for the determination of binary mixtures of the nonsteroidal antiinflammatory drugs flufenamic (FFA), meclofenamic (MCFA) and mefenamic (MFA) acids in serum and pharmaceutical formulations. The method is based on the intrinsic fluorescence of these compounds in chloroform. A Deltalambda=105 nm was used for the resolution of FFA-MFA and MFA-MCFA mixtures, whereas the FFA-MCFA mixture was determined at Deltalambda=40 nm. Serum samples are treated with trichloroacetic acid to remove the proteins, and the analytes are extracted in chloroform prior to determination. Pharmaceutical preparations were analysed without prior separation steps.     

Application of photochemical inhibition in flow injection systems: Determination of epinephrine and L-dopa

An automated procedure for the photochemical determination of epinephrine and L-dopa has been developed. It is based on the strong inhibition by these catecholamines on the photochemical reaction between phloxin and ethylenediaminetetraacetic. The proposed flow-injection method allows the fluorimetric determination of epinephrine in the range 1.9-26.4 mug/ml and of L-dopa in the range of 1.5-12.7 mug/ml, with a sampling frequency of 35 samples/hr. The method was applied successfully to the determination of body catecholamines in pharmaceutical preparations.     

Interactions of cationic porphyrins with double-stranded oligodeoxynucleotides: a study by electrospray ionisation mass spectrometry

Electrospray ionisation mass spectrometry (ESI-MS), electrospray ionisation tandem mass spectrometry (ESI-MS/MS) and Ultraviolet-visible (UV-vis) spectroscopy were used to investigate the non-covalent interactions between small oligonucleotide duplexes with the GC motif and a group of cationic meso(N-methylpyridynium-4-yl)porphyrins (four free bases with one to four positive charges, and the zinc complex of the tetracationic free base).The results obtained point to outside binding of the porphyrins, with the binding strength increasing with the number of positive charges. Fragmentations involving losses from both chains were observed for the porphyrins with N-methylpyridinium-4-yl groups in opposite meso positions.     

Domain-specific incorporation of noninvasive optical probes into recombinant proteins

An integrated approach is described that allows the domain-specific incorporation of optical probes into large recombinant proteins. The strategy is the combination of two existing techniques, expressed protein ligation (EPL) and in vivo amino acid replacement of tryptophans with tryptophan (Trp) analogues. The Src homology 3 (SH3) domain from the c-Crk-I adaptor protein has been labeled with a Trp analogue, 7-azatryptophan (7AW), using Escherichia coli Trp auxotrophs. Structural, biochemical, and thermodynamic studies show that incorporation of 7AW does not significantly perturb the structure or function of the isolated domain. Ligation of the 7AW-labeled SH3 domain to the c-Crk-I Src homology 2 (SH2) domain, via EPL, generated the multidomain protein, c-Crk-I, with a domain-specific label. Studies of this labeled protein show that the biochemical and thermodynamic properties of the SH3 domain do not change within the context of a larger multidomain protein. The technology described here is likely to be a useful tool in enhancing our understanding of the behavior of modular domains in their natural context, within multidomain proteins.