Coil-globule transition of DNA molecules induced by cationic surfactants: a dynamic light scattering study

The compaction and aggregation of DNA induced by cationic surfactants was studied by dynamic light scattering (DLS). Furthermore, the effect on surfactant-compacted DNA of the addition of nonionic amphiphiles and salt was studied. When using sufficiently low amounts of DNA and cetyltrimethylammonium bromide (CTAB), compacted DNA molecules could be monitored by the appearance of a band characterized by lower hydrodynamic radius and by the decrease in the intensity of the peak corresponding to extended DNA molecules. Notably, we observed a region where compacted molecules coexist with extended ones; these two populations were found to be stable with time. For higher concentrations of CTAB, only compacted molecules were observed and the size of the particles increased with time indicating aggregation. The number of globules present in the coexistence region increased linearly with the surfactant concentrations, as given by the area of the band corresponding to this population, which indicates a double-cooperativity of the binding. The DLS experiments were in good agreement with previous fluorescence microscopy studies, with certain advantages over this technique since there is no need to add fluorescence dyes and antioxidants. Furthermore, it allows the study of molecules which are too small to be visualized by fluorescence microscopy.     

Controlled delivery achieved with bi-layer matrix devices produced by co-injection moulding

The aim of this study was to design new soy protein-based bi-layered co-injection moulded matrix systems aimed to achieve controlled drug delivery. The devices consisted of a drug-free outer layer (skin) and a drug-containing core. The systems overcame the inherent disadvantage of non-linear release associated with diffusion-controlled single-layer matrix devices by providing additional releasing area with time to compensate for the decreasing release rate. As expected, the bi-layer devices presented a significant decrease in drug release rate when compared with a correspondent single layer matrix system. The skin thickness and the degree of crosslinking of the core appeared to be very important tools to tailor the release patterns. Furthermore, due to the amphoteric nature of the soy protein, the developed devices evidenced a pH-dependent behaviour. The mechanisms of drug release were also elucidated at two different pH values: i) pH 5.0, near the isoelectric point of soy (low matrix solubility); and ii) pH 7.4, physiological pH (high matrix solubility). Consequently, changing the release medium from pH 5.0 to pH 7.4 after two hours, led to an abrupt increase in drug release and the devices presented a typical controlled drug delivery profile: slow release/fast release. These evidences may provide for the development of individual systems with different release onsets that in combination may exhibit drug releases at predetermined times in a pre-programmed way. Another possibility is the production of three-layer devices presenting bimodal release profiles (fast release/slow release/fast release) by similar technologies. Scanning electron micrograph of a developed bi-layer device.     

Symmetry Groupoids and Admissible Vector Fields for Coupled Cell Networks

The space of admissible vector fields, consistent with the structureof a network of coupled dynamical systems, can be specifiedin terms of the network's symmetry groupoid. The symmetry groupoidalso determines the robust patterns of synchrony in the network– those that arise because of the network topology. Inparticular, synchronous cells can be identified in a canonicalmanner to yield a quotient network. Admissible vector fieldson the original network induce admissible vector fields on thequotient, and any dynamical state of such an induced vectorfield can be lifted to the original network, yielding an analogousstate in which certain sets of cells are synchronized. In thepaper, necessary and sufficient conditions are specified forall admissible vector fields on the quotient to lift in thismanner. These conditions are combinatorial in nature, and theproof uses invariant theory for the symmetric group. Also thesymmetry groupoid of a quotient is related to that of the originalnetwork, and it is shown that there is a close analogy withthe usual normalizer symmetry that arises in group-equivariantdynamics.     

Copolymerization of ethylene and non-conjugated diene with metallocene/methylaluminoxane system supported on MCM-41 mesoporous material

Ethylene was copolymerized with the non-conjugated diene 5,7-dimethylocta-1,6-diene by metallocene catalysts, homogenous and supported on MCM-41 mesoporous material activated by methylaluminoxane at room temperature. The employed metallocenes were (BzCp)₂ZrCl₂ (1), (C₁₈H₃₇Cp)₂ZrCl₂ (2), (CH₃)₂Si(Ind)₂ZrCl₂ (3) and ?₂C(Cp,Flu)ZrCl₂ (4). Diene incorporation and thermal characteristic of the polymers were determined by ¹³C-NMR and DSC. For the homogeneous systems, only the unbridged metallocene (1) showed a decrease in the activity as the diene content was increased. The ability for diene incorporation in the polymer chain followed the catalyst order (1) < (3) < (2) < (4). Regarding the supported systems, it was observed that all of them showed lower activity than the corresponding homogeneous catalysts. A sharp loss of activity in relation to the homogeneous counterpart was noticed by employing the ansa-metallocenes on MCM-41, especially the complex (4) supported catalyst. The effect of the mesoporous material as support on the behavior of the studied metallocene homogeneous systems in ethylene-diene copolymerization was an increase of polymer molecular weight, maintaining the same level of diene incorporation, except for catalyst (2), when a decrease of the diene incorporation was observed.     

Large <Emphasis Type="Italic">P</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> distributions at RHIC and percolation of strings

We discuss, in the framework of percolation of strings, the general features of the transverse momentum distributions obtained at RHIC.Arrival of the final proofs: 1 April 2005PACS: 25.75.-q,12.38.Mh,24.85. + p     

Enhancement of Europium Luminescence in Tetracycline–Europium Complexes in the Presence of Urea Hydrogen Peroxide

An increase in the europium emission band was observed, for the first time, with addition of urea hydrogen peroxide to the tetracycline–europium (Tc–Eu)solution. We have observed that the wavelength, the band width and the area of ⁵D₀→⁷F₂ europium transition change with the urea hydrogen peroxide concentration. We claim that the tetracycline–europium complexes can be used as probes of urea hydrogen peroxide concentration.     

The solutions of a variational inequality as critical points of a real valued smooth function

Let H be a real Hilbert space, A : H → H a self-adjoint continuous linear operator. Given a variational inequality related to A, we construct a Lipschitz continuously differentiable function F:H→$\text{R}$ so that the critical points of F are the solutions of the variational inequality.     

Quarkonia with a variational method

A variational method, based in independent minimization of energy levels, is applied to quantum mechanical quarkonium systems. A discussion is presented of the method, with emphasis on quark mass dependence of energy levels (Feynman-Hellman theorem), on the behaviour of wave functions at the origin (Martin's theorem) and on the ordering of energy levels. The potential used in the applications is the Coulomb (4-vector) + Linear (4-scalar) potentials, the fine and hyperfine splittings being included by means of the Fermi-Breit hamiltonian. An attempt is made for an overall treatment of the known splittings (radials-wave, hyperfine and fine splittings) for all quarkonium systems, using an approximately flavour independent potential. Thep-wave splittings, as well as the hyperfine splittings, are shown to be particularly sensitive to the nature and mass dependence of the potential. In particular, asymptotic freedom can be more easily tested there. On the other hand, radial excitations provide the place where the set up of short range potential effects should be first detected. Some results concerning the ordinary mesons are presented, and it is specifically pointed out that strong restrictions exist for the masses of the ρ′ and theA ₁. Predictions for the toponium family are also presented.