Development of a computer algorithm for the detection of phase singularities and initial application to analyze simulations of atrial fibrillation

Atrial fibrillation (AF) is a common cardiac arrhythmia, but its mechanisms are incompletely understood. The identification of phase singularities (PSs) has been used to define spiral waves involved in maintaining the arrhythmia, as well as daughter wavelets. In the past, PSs have often been identified manually. Automated PS detection algorithms have been described previously, but when we attempted to apply a previously developed algorithm we experienced problems with false positives that made the results difficult to use directly. We therefore developed a tool for PS identification that uses multiple strategies incorporating both image analysis and mathematical convolution for automated detection with optimized sensitivity and specificity, followed by manual verification. The tool was then applied to analyze PS behavior in simulations of AF maintained in the presence of spatially distributed acetylcholine effects in cell grids of varying size. These analyses indicated that in almost all cases, a single PS lasted throughout the simulation, corresponding to the central-core tip of a single spiral wave that maintained AF. The sustained PS always localized to an area of low acetylcholine concentration. When the grid became very small and no area of low acetylcholine concentration was surrounded by zones of higher concentration, AF could not be sustained. The behavior of PSs and the mechanisms of AF were qualitatively constant over an 11.1-fold range of atrial grid size, suggesting that the classical emphasis on tissue size as a primary determinant of fibrillatory behavior may be overstated. (c) 2002 American Institute of Physics.     

Effects of Adsorbed Polyaniline on Redox Processes on As(2)O(3) Surfaces

Polyaniline deposited on As(2)O(3) surface resulted in a new material, which was characterized by infrared spectoscopy, thermogravimetry, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and cyclic voltammetry. The mass percentage of polymer deposited on oxide surface is approximately 13%. The scanning electron microscopy images as well as the X-ray diffraction patterns provided conclusive evidence that the oxide surface is coated by the polymer. The cyclic voltammograms of the polyaniline adsorbed on As(2)O(3) surface showed that the adsorbate exerts remarkable effects on redox processes on this oxide. The pure oxide exhibited two oxidation/reduction peaks at 0.25/-0.06 and 0.47/-0.25 V attributed tentatively to the processes As(2)O(3)(s)+6H(+)+6e(-)=2As(s)+3H(2)O and As(s)+3H(+)+3e(-)=AsH(3)(g), respectively. The polyaniline-coated sample exhibited a better-defined voltammogram in which the first oxidation peak of the oxide had its intensity increased about four times. Copyright 2000 Academic Press.     

Eckart axis conditions, Gauss' principle of least constraint, and the optimal superposition of molecular structures

The relation of the Eckart axis conditions for polyatomic vibrating molecules to the problem of optimal superposition of molecular structures has been pointed out recently [J. Chem. Phys. 122, 224105 (2005)]. Here, it is shown that both problems are intimately related to Gauss' principle of least constraint, for which a concise derivation is presented. In the context of this article, Gauss' principle leads to a rotational superposition problem of the unconstrained atomic displacements and the corresponding displacements due to a molecular rigid-body motion. The Eckart axis conditions appear here as necessary conditions for a minimum of the constraint function. The importance of Eckart's problem for extracting the internal motions of macromolecules from simulated molecular dynamics trajectories is pointed out, and it is shown how the case of coarse-grained sampled trajectories can be treated.     

Influence of pressure on the slow and fast fractional relaxation dynamics in lysozyme: a simulation study

The article reports on a molecular dynamics simulation study of the influence of moderate, nondenaturing pressure on the slow and fast internal relaxation dynamics of lysozyme. The model parameters of the fractional Ornstein-Uhlenbeck process are used to quantify the changes. We find that the nonexponential character for diffusive motions on time scales above 10 ps is enhanced and that the diffusion processes are slowed down. The diffusive motions on the subpicosecond time scale appear, in contrast, accelerated, whereas the nonexponential character is not altered by pressure. We attribute these findings to the different natures of slow and fast relaxation processes, which are characterized by structural rearrangements and collisions, respectively. The analyses are facilitated by the use of spatially resolved relaxation rate spectra.