Microtubule structure and its stabilisation

Microtubules are designed to be dynamically unstable. GTP hydrolysis converts an initially stable polymeric structure into an unstable one in which strain at the interfaces between longitudinal neighbours in the helical lattice of subunits is balanced by lateral interactions. However, stability can be modulated by a variety of factors, including associated proteins and a variety of drug molecules. Stabilising drugs such as Taxol and the assembly-promoting repeat motifs of tau protein occupy a special pocket in beta-tubulin. Microtubule destabilizing drugs such as colchicine alter the longitudinal interfaces of the subunits so that they cannot assemble into a microtubule lattice. These mechanisms are discussed in terms of the atomic structure of the protein.     

Local optical properties, electron densities, and london dispersion energies of atomically structured grain boundaries

Quantitative analysis of spatially resolved valence electron energy-loss spectra shows strong physical property contrasts for Sigma5 and near Sigma13 grain boundaries in Fe-doped SrTiO3, resulting in London dispersion interaction energies of 14 to 50 mJ/m(2) between the adjacent grains. The determination of local physical properties of grain boundary cores and the appreciable contribution of long-range London dispersion to interface energies provides new information on formation and control of interfaces in materials.     

Selectivity of quadruplex DNA stationary phases toward amino acids in homodipeptides and alanyl dipeptides

Series of dipeptides, including homodipeptides and alanyl dipeptides, were separated using quadruplex (G-quartet) DNA stationary phases in open-tubular capillary electrochromatography (OTCEC). The stationary phases were constructed by covalently attaching the DNA oligonucleotides to the inner capillary surface. Three different G-quartet forming oligonucleotides were investigated: the two-plane G-quartet forming thrombin-binding aptamer, the four-plane analogue of the thrombin-binding aptamer, and a two-plane oligonucleotide identical to the thrombin-binding aptamer except for the replacement of the guanine by thymine in the central bridging loop of the G-quartet structure. Results were compared with results obtained using capillary electrophoresis on a bare capillary and OTCEC using an oligonucleotide with the same base composition as the thrombin-binding aptamer but in a different sequence that does not allow G-quartet formation as the stationary phase.     

Photochemical pinacol rearrangements of unsymmetrical diols

The photochemical pinacol reaction of a series of nonsymmetrical 9-fluorenyl-substituted vic-diols was investigated and compared with their acid-catalyzed thermal reaction. Unlike the thermal reaction, the radiation-induced processes involve only fluorenyl cations, as is reflected in differences of product distribution between the two reactions. From the product studies, substituent migratory aptitudes are reversed in the photochemical process, suggesting that kinetic control takes place under neutral conditions unlike the acid-catalyzed thermal reactions. The presence of fluorenyl cation intermediates and their lifetimes were established by laser flash spectroscopy studies.     

The decomposition of diazo-compounds induced by nucleophiles. The decomposition of 9-diazofluorene in the presence of hydroxide or alkoxide ions

9-Diazofluorene, on treatment with stoichiometric or substoichiometric amounts of quaternary ammonium hydroxide or methoxide or of potassium t-butoxide in solution in aqueous or alcoholic dimethyl sulfoxide or acetonitrile at 30 degrees C, decomposes with evolution of nitrogen to yield fluorenone azine [bis(fluorenylidene)hydrazine] in almost quantitative yield. Studies are reported of the identity of the minor by-products, together with an examination of the kinetics of the reactions and additional spectroscopic experiments. The general rate equation is v = k[FIN2]3/2[Nu-]1/2, where Nu- represents the nucleophile. It is concluded that the oxyanions, most probably after nucleophilic attack on 9-diazofluorene under these basic conditions to generate a new anionic species, are capable of transferring an electron to the diazo-compound. This initiates decomposition of further diazo-molecules in a process of electron-transfer chain catalysis that bears some similarities to, but has also some differences from, that encountered using cathodic initiation. In support of this interpretation it is found that reactions can be accelerated by the introduction of carbon acids (fluorene, 9-phenylfluorene) into the reaction mixtures. The nature of the initiation, propagation and termination steps of the chain mechanism are discussed.     

Retention Time of Unretained Compound Calculation and Determination of Retention Indices of Some Monosubstituted Benzenes Using a Multiparametric Method in Binary, Ternary and Quaternary Solvent RP-LC Systems

In reversed phase liquid chromatography (RP-LC), the validity of a multiparametric (MP) non-linear least-squares regression iterative method has been evaluated for 14 different aqueous mobile phases modified with one, two or three organic solvents [acetonitrile (ACN), methanol (MeOH), or tetrahydrofuran (THF)] for calculating the retention time of unretained compound t _M and the regression parameter (slope b), based on the use of alkan-2-ones and alkyl aryl ketones homologous series. The determination of t _M and b has been studied for eight binary (ACN?CH₂O or MeOH?CH₂O), 3 ternary (ACN?CMeOH?CH₂O) and 3 quaternary (ACN?CMeOH?CTHF?CH₂O) mobile phase systems on an Omnispher C₁₈ column. The multiparametric calculated t _M and b values were compared with those obtained by Guardino??s, and Grobler??s methods. The MP retention indices (RI) of ten monosubstituted benzenes with different functionality (hydroxyl, carbonyl, nitro, etc.) based on the alkan-2-ones retention index standards have been determined and compared for the different mobile phase compositions studied. The influence of organic modifier type, the nature of mobile phase system and water content on the variation of retention parameter studied in this work were discussed.     

An adaptive finite volume box scheme for solving a class of nonlinear parabolic equations

In this work we are interested in a fully nonconforming (Crouzeix–Raviart) finite element scheme for addressing a nonlinear parabolic equation. In addition to showing the equivalence with a finite volume box scheme we derive a posteriori error indicators in time and space, uncoupled as much as possible.