N,N,N′,N′-tetramethylethylene diamine as a co-catalyst in the anionic polylymerization of methyl methacrylate

Anionic polymerization of methyl methacrylate in toluene solution has been studied. The polymerizations were initiated by fluorenyllithium as well as by butyl-, benzyl-, and phenylmagnesium halides, and carried out in the presence of polar additives. Especially the organomagnesium initiators gave generally rise to highly isotactic polymers (mm = 0.90-0.95) in the absence of solvating additives. In the presence of the bidentate ligand N,N,N′,N′-tetramethylethylene diamine (TMEDA) at molar ratios of 1–2 relative to magnesium, highly syndiotactic polymers were obtained at ?78°C (rr = 0.8–0.9), while at higher temperatures stereoblock polymers or stereocomplexes were formed. It is proposed that at low temperatures the magnesium cations are strongly solvated under the influence of TMEDA, with the formation of monomeric ion pairs. The large solvated magnesium cations will not have any directional influence on the monomer in the propagation step, with syndiotactic propagation as a result.     

Simulations of lipid transfer between a supported lipid bilayer and adsorbing vesicles

Recent experiments demonstrate transfer of lipid molecules between a charged, supported lipid membrane (SLB) and vesicles of opposite charge when the latter adsorb on the SLB. A simple phenomenological bead model has been developed to simulate this process. Beads were defined to be of three types, ‘n’, ‘p’, and ‘0’, representing POPS (negatively charged), POEPC (positively charged), and POPC (neutral but zwitterionic) lipids, respectively. Phenomenological bead–bead interaction potentials and lipid transfer rate constants were used to account for the overall interaction and transfer kinetics. Using different bead mixtures in both the adsorbing vesicle and in the SLB (representing differently composed/charged vesicles and SLBs as in the reported experiments), we clarify under which circumstances a vesicle adsorbs to the SLB, and whether it, after lipid transfer and changed composition of the SLB and vesicle, desorbs back to the bulk again or not. With this model we can reproduce and provide a conceptual picture for the experimental findings.     

Mathematical and physical aspects of controlling the exact solutions of the 3D Gross-Pitaevskii equation

The possibility of the decomposition of the three-dimensional (3D) Gross-Pitaevskii equation (GPE) into a pair of coupled Schrödinger-type equations, is investigated. It is shown that, under suitable mathematical conditions, it is possible to construct the exact controlled solutions of the 3D GPE from the solutions of a linear 2D Schrödinger equation coupled with a 1D nonlinear Schrödinger equation (the transverse and longitudinal components of the GPE, respectively). The coupling between these two equations is the functional of the transverse and the longitudinal profiles. The applied method of nonlinear decomposition, called the controlling potential method (CPM), yields the full 3D solution in the form of the product of the solutions of the transverse and longitudinal components of the GPE. It is shown that the CPM constitutes a variational principle and sets up a condition on the controlling potential well. Its physical interpretation is given in terms of the minimization of the (energy) effects introduced by the control. The method is applied to the case of a parabolic external potential to construct analytically an exact BEC state in the form of a bright soliton, for which the quantitative comparison between the external and controlling potentials is presented.     

Programmed cross flow asymmetrical flow field-flow fractionation for the size separation of pullulans and hydroxypropyl cellulose

Different functions for the programming of the cross flow in asymmetrical flow field-flow fractionation were studied with the aim to find the flow conditions most suitable for the molar mass distribution analysis of high molecular weight polysaccharides. A mixture of four differently sized pullulans covering the molar mass range 5.8 x 10(3)-1.6 x 10(6) g mol(-1) were used as a model sample. Two types of programs were studied, linear and exponential decays, both with and without initial periods of a constant cross flow. For comparison, nonprogrammed runs, i.e. using constant cross flow, were studied. It was found that exponentially decaying cross flow gave the most uniform molar mass selectivity across the fractogram. The programmed cross flow was applied to the molar mass distribution analysis of a technical quality of hydroxypropyl cellulose.     

Spectrophotometric determination and computational evaluation of the rates of hydrolysis of 9-amino-substituted acridines

Aminoacridines have a long history in the drug and dye industries and display a wide range of biological and physical properties. Despite the historical relevance of 9-aminoacridines, there have been few studies investigating their stability. 9-Aminoacridines are known to hydrolyze at the C9-N15 bond, yielding acridones. In this study, the pH-dependent hydrolysis rates of a series of 9-substituted aminoacridines are investigated. In addition, ground-state physical properties of the compounds are determined using ab initio quantum mechanics calculations to gain insight into the forces that drive hydrolysis. An analysis of the bond orders, bond dissociation energies, and conformational energies show that the rate of hydrolysis depends on two main factors: delocalization across the C9-N15 bond and steric effects. The computational results are applied to explain the change in experimental rates of hydrolysis going from primary to secondary and to tertiary substituted 9-aminoacridines. In the case of tertiary substituted amines, the calculations indicate the C9-N15 bond is forced into a more gauche-like conformation, greatly diminishing delocalization (as shown by reductions in bond orders and bond energy), which leads to rapid hydrolysis. A model of intramolecular hydrogen bonding is also presented, which explains the increased rate of hydrolysis observed for highly substituted compounds under acidic conditions.     

New directions of miniaturization within the proteomics research area

An overview of the current trends within protein expression profiling is given where multidimensional separation of both gel- and liquid-phase techniques linked to mass spectrometry is viewed as a major route in the global proteome mapping. A clear trend in these biochemical developments is the effort to sequence and identify low-abundant protein expressions where assay miniaturization and integrated sample processing plays a central role. Two areas of miniaturization within the proteomics field are addressed: (i) sample cleanup and enrichment, and (ii) silicon microstructure developments for protein chip microarrays.     

Near- to mid-infrared picosecond optical parametric oscillator based on periodically poled RbTiOAsO(4)

We describe a Ti:sapphire-pumped picosecond optical parametric oscillator based on periodically poled RbTiOAsO(4) that is broadly tunable in the near to mid infrared. A 4.5-mm single-grating crystal at room temperature in combination with pump wavelength tuning provided access to a continuous-tuning range from 3.35 to 5microm , and a pump power threshold of 90 mW was measured. Average mid-infrared output powers in excess of 100 mW and total output powers of 400 mW in ~1-ps pulses were obtained at 33% extraction efficiency.