Topological Switching Research Articles

Hypoangular: A Gene Potentially Involved in Specifying Positional Information in a Ciliate, Tetrahymena Thermophila

In Tetrahymena, two unique cell-surface structures, the oral apparatus and the cytoproct, are formed at opposite ends of one ciliary row, the reference meridian, which is propagated longitudinally during clonal growth. A third set of unique structures, the contractile vacuole pore(s) (CVP), is located at a nearly constant proportion of the cell circumference to the cell's right of the reference meridian. Three allelic recessive temperature-sensitive mutations, collectively named hypoangular (hpo), alter both the geometry of propagation of the reference meridian and the location of the CVPs. In mutant cells, the reference meridian typically undergoes a steady rightward shift in successive cell generations ("cortical slippage"); concomitantly, CVP sets come to lie closer to the reference meridian. Although CVP location is still proportional to the cell circumference, the constant of proportionality (the "CVP angle") is reduced. Another effect is an alteration in the widths of morphogenetic domains within the cortex. As the temperature is raised (made more restrictive), these effects are accentuated and the CVP angle becomes reduced further. At the extreme, the CVP angle collapses to zero and less, i.e., there is a topological switch such that CVPs come to lie to the left of the reference meridian, and the direction of cortical slippage reverses from rightward to leftward. These observations are hard to reconcile with existing formal models of pattern specification in this system and suggest that the hpo locus might specify a key component of the intracellular positional system.

Topological Switching of the COOH-Terminal Domain of Peptidylglycine α-Amidating Monooxygenase by Alternative RNA Splicing

Proteins encompassing the two catalytic domains (monooxygenase and lyase) and the COOH-terminal domain of rat peptidyiglycine α-amidating monooxygenase (rPAM) 3 were purified from recombinant Escherichia coli overexpressing each domain and used to raise domain-specific polyclonal antibodies. Four alternatively spliced forms of PAM RNA (PAM-1, -2, -3, and -4) were transcribed in vitro and used to synthesize PAM proteins in a cell-free translation system. The orientation of the proteins in microsomal membrane vesicles was analyzed using trypsin protection assays and immunoprecipitation with the domain-specific antibodies. Only one of the two potential N-glycosylation sites (Asn 765-Phe-Ser) in PAM-1 was efficiently utilized by microsomal membranes. PAM-1 and PAM-2 were shown to be type Ia membrane proteins with their two catalytic domains residing within microsomal vesicles and their COOH-terminal domains exposed to the cytosol. In contrast, PAM-3 and PAM-4 were shown to be soluble proteins contained entirely within vesicles. Thus, the COOH-terminal domain underwent topological switching between the cytosolic (PAM-1 and -2) and luminal (PAM-3) compartments as a function of alternative splicing of exons B a/B b. Computer analyses of the PAM protein sequence correlated the exons encoding PAM-1 with a model for the structural and functional domains of the PAM protein. The dual topologies of the PAM proteins confer an important means of functional regulation to this secretory granule associated neuropeptide processing enzyme.

Some remarks on the theory of SQUID structures. I. Topology of SQUID structures—A unified picture

A topological analysis of different rf SQUID structures is performed. A simple unified picture of these devices is obtained by considering the topological properties of the magnetic force lines around superconducting (or resistive) SQUID structures containing one weak link. Due to the periodic collapse of the superconducting order parameter in the weak link the “normal” space around these structures can change its connectivity. A number of rf SQUID structures are analyzed from the point of view of simple topological homotopy theory. The rf SQUID is then viewed as a device which realize a topological switching between two different classes of simple closed loops of the magnetic force lines. This switching (through the weak superconductivity region) is modulated by the low-frequency closed loops of the magnetic field and results in the well-known periodic response of the rf SQUID. The surfaces surrounding the rf excitation coil of the rf SQUID are analyzed from the point of view of homology theory. The screening currents flowing on these surfaces are represented as one-dimensional closed loops (cycles). It is shown that the SQUID can work only if the surface of the structure has at least one class of so-called nonbounding cycles, which are all “cut” by the weak link. A number of known SQUID structures are analyzed from the point of view of homotopy and homology theory and some new SQUID structures are discussed.

The Leibniz project

A language for quantum physics is derived from set theory by replacing the classical predicate algebra (Boolean) by a certain quantum predicate algebra (rational projective), time space and the Hamilton-Schroedinger dynamics by a Feynman-like graph dynamics, and the Dirac spin operators by topological switching operators on the graph. The development is described from the basic level of elementary monadic processes to the level of the free Dirac equation.

High-speed photography of topological switching and anisotropic saturation velocities in a misoriented garnet film

The topological switching of a bubble array into one of the opposite polarity by a bias field pulse is studied in a garnet film by high-speed photography. The key requirement for regular switching is found to be an anisotropic wall velocity which causes the expanding bubbles to make contact with only four out of six of the nearest neighbours. The same anisotropy of almost 2:1 in the saturation velocity is also observed in isolated bubbles pulsed to expand. The velocity anisotropy and the static bubble ellipticity are related to a 2.2 degrees misorientation of the film's (111)-axis from the surface normal, which gives rise to an in-plane magnetic anisotropy.

The topological switching of magnetic bubble domain arrays

The process of topological switching of magnetic bubble arrays, in which arrays of bubbles of one direction of magnetisation may be stably and repetitively switched to arrays of the opposite polarity, is described. A qualitative analysis of the various steps in the switching process is discussed, which points to the conclusion that an in-plane dynamic anisotropy must exist in garnet film samples which exhibit stable topological switching. Quantitative analysis is assisted by the observation of the variations in transmitted light intensity when the sample is illuminated by a Helium-Neon laser during switching.

The dynamics of magnetic bubble domain arrays

Abstract The dynamics of a close-packed hexagonal array of magnetic bubble domains is considered, and some experiments using epitaxial layers of europium yttrium gallium iron garnet are described which result in measurements of the low-field domain wall mobility and the domain wall saturation velocity in this materials. A new effect in bubble domain arrays termed ‘topological domain switching’ is reported and the saturation velocity is also measured using this effect.