Stretched--exponential relaxation in arrays of coupled rotators

We consider the non--equilibrium dynamics of a chain of classical rotators coupled at its edges to an external reservoir at zero temperature. We find that the energy is released in a strongly discontinuous fashion, with sudden jumps alternated with long stretches during which dissipation is extremely weak. The jumps mark the disappearance of strongly localized structures, akin to the rotobreather solutions of the Hamiltonian model, which act as insulating boundaries of a hot central core. As a result of this complex kinetics, the ensemble--averaged energy follows a stretched exponential law until a residual pseudo--stationary state is attained, where the hot core has reduced to a single localized object. We give a statistical description of the relaxation pathway and connect it to the properties of return periods of rare events in correlated time series. This approach sheds some light into the microscopic mechanism underlying the slow dynamics of the system. Finally, we show that the stretched exponential law remains unaltered in the presence of isotopic disorder.Comment: 13 Figure

Apoptosis of Inflammatory Cells in Immune Control of the Nervous System: Role of Glia

Normal individuals have T lymphocytes capable of reacting to central nervous system (CNS) antigens such as myelin basic protein (MBP) (Martin et al., [1990]). In view of recent evidence indicating that T cells are much more cross-reactive than previously thought (Mason, [1998]), it is likely that these autoreactive T cells are often primed by exposure to cross-reacting environmental antigens. Indeed it has been shown that viral and bacterial peptides can activate myelin-reactive human T cells (Wucherpfennig and Strominger, [1995]; Hemmer et al., [1997]). Furthermore, normal healthy subjects experience surges of increased frequencies of circulating myelin-reactive T cells that might be driven by cross-reactive environmental antigens (Pender et al., [2000]). Such activated myelin-reactive T cells would be expected to enter the CNS in healthy individuals, because activated T cells of any specificity, including autoreactive T cells, enter the normal CNS parenchyma (Wekerle et al., [1986]; Hickey et al., [1991]). If CNS-reactive T cells survive in the CNS, they have the potential to attack the CNS, either directly or through the recruitment of other inflammatory cells, and thus lead to CNS damage such as demyelination. Therefore, the physiological control of autoreactive T cells in the CNS is likely to have an important role in preventing the development of autoimmune CNS disorders such as multiple sclerosis (MS) (Pender, [1998]). T-cell apoptosis in the CNS has been proposed to be an important mechanism for controlling autoimmune attacks on the CNS (Pender et al., [1992]; Schmied et al., [1993]). Although other mechanisms, such as immune deviation (Wenkel et al., [2000]), may possibly also contribute to the control of the immune response in the CNS, this review will focus on T-cell apoptosis in the CNS and the role of glia in this process

Anomalous relaxation and self-organization in non-equilibrium processes

We study thermal relaxation in ordered arrays of coupled nonlinear elements with external driving. We find, that our model exhibits dynamic self-organization manifested in a universal stretched-exponential form of relaxation. We identify two types of self-organization, cooperative and anti-cooperative, which lead to fast and slow relaxation, respectively. We give a qualitative explanation for the behavior of the stretched exponent in different parameter ranges. We emphasize that this is a system exhibiting stretched-exponential relaxation without explicit disorder or frustration.Comment: submitted to PR

Conversion of holes into reducing species on surface modified small-particle TiO{sub 2}

Complexation of colloidal titanium dioxide nanoparticles (40 {angstrom}) by cysteine as a surface derivative was investigated by electron paramagnetic resonance (EPR) and infra-red (diffusion reflectance infra-red Fourier Transform DRIFT) spectroscopies. It was found that cysteine strongly binds to the colloid surface. The authors have demonstrated with EPR spectroscopy that cysteine modifies the TiO{sub 2} surface with formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine modified TiO{sub 2} at 77K resulted in formation of a sulfur centered radical observed by EPR spectroscopy at 200 K. Upon addition of lead ions, a new complex of cysteine that bridges surface titanium atoms and lead ions was detected by IR spectroscopy. Illumination of lead/cysteine modified TiO{sub 2} did not result in the formation of sulfur centered radical, but symmetrical, lattice defect type EPR signal for trapped holes was observed. However, addition of methanol to this system resulted in the formation of {center_dot}CH{sub 2}OH radical following illumination at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at particle surface (Ti(3){sub surf}) was observed. On further increase of the temperature to 200 K the EPR signal for trapped electrons disappeared as a result of the reduction of Pb{sup 2+} ions, and metallic lead was observed to precipitate. Conversion of photogenerated holes into trapped electrons due to the presence of methanol doubles the yield of trapped electrons that can reduce Pb{sup 2+}. Direct reduction of Pb{sup 2+} ions by {center_dot}CH{sub 2}OH radical on TiO{sub 2} was not detected

Protein Dynamics: From Molecules, to Interactions, to Biology

Proteins have a remarkably rich diversity of dynamical behaviors, and the articles in this issue of the International Journal of Molecular Sciences are a testament to that fact. From the picosecond motions of single sidechains probed by NMR or fluorescence spectroscopy, to aggregation processes at interfaces that take months, all time scales play a role. Proteins are functional molecules, so by their nature they always interact with their environment. This environment includes water, other biomolecules, or larger cellular structures. In a sense, it also includes the protein molecule itself: proteins are large enough to fold and interact with themselves. These interactions have been honed by evolution to produce behaviors completely different from those of random polymers