Hysteresis-like Behavior Research Articles

Dynamic light scattering study of heat aggregation of insulin

Dynamic light scattering measurements on heat aggregation of zinc-insulin between temperaturesT=20 °C and 40 °C in 0.1 M NaCl aqueous solutions with pH= 4.01, 7.5 and 9.03, are reported. The experiments were performed on very dilute solutions in the concentration range 20 to 160 μg/cc. It has been observed that the process of heat aggregation was linear with temperature. The concentration dependence of translational diffusion coefficient was found to be negative in consistence with an earlier result. Upon temperature reversal the hysteresis-like behavior was found to be predominantly large at pH=7.5. The process of dissociation of Insulin aggregates with temperature reversal was also found to be linear with temperature decrement. Both the coefficients of heat aggregation and dissociation (upon temperature reversal) were larger at pH=7.5 than at pH=4.01 and 9.03. The low polydispersity in the correlation spectra indicated that both the association and dissociation processes were apparently stepwise in nature.

Origin of Hysteresis-like Behavior in Flow Transition in Simulations of the Czochralski Method with Tetradecane

A hysteresis-like behavior in the flow transition is experimentally discussed in terms of interactions of the natural convection with the forced convection from the viewpoint of crystal growth from a melt.

19F nuclear magnetic resonance as a probe of structural transitions and cooperative interactions in heavy meromyosin.

An 19F NMR probe has been attached to the reactive sulfhydryl SH1 of the globular heads of rabbit skeletal heavy meromyosin. It serves as a sensitive monitor of the conformational state of the heads of heavy meromyosin in a manner similar to that seen for subfragment-1 (Shriver, J.W., and Sykes, B.D. (1982) Biochemistry 21, 3022-3028; Tollemar, U., Cunningham, K., and Shriver, J.W. (1986) Biochim. Biophys. Acta 873, 243-251). The NMR spectra indicate that there are at least two states for the heads in the SH1 region. The energetics of the interconversion of the two states of heavy meromyosin (HMM) differs significantly from that of S-1. In HMM in the absence of divalent cations, there are two reversible paths between the low temperature and high temperature states with a hysteresis-like behavior. One path is consistent with the head groups behaving independently and similar to S-1 alone. The second path indicates a coupling of the globular head region observed in S-1 with a second region forming a distinctly different cooperative unit. Upon addition of Ca(II) the hysteresis effect is lost and only the second cooperative unit is observed. Two explanations are offered for these results: the globular heads in HMM may couple with the S-2 segment, or the two globular heads of HMM may couple to form a larger cooperative unit. The ability to stabilize the larger cooperative unit with a divalent metal ion implicates a role for the LC2 light chain in coupling regions of the myosin molecule.

Piezoelectric properties and ferroelectric hysteresis effects in uniaxially stretched nylon-11 films

Oriented nylon-11 films [initially quenched (δ′ form) or slow cooled (α′ form) from the melt] subjected to a series of static positive and negative poling fields were found to show a hysteresislike behavior of the piezoelectric strain constant d31 and the piezoelectric stress constant e31. This behavior was very similar to that observed in poly(vinylidene fluoride) (PVF2) and attributed to ferroelectric dipole switching, in a previous study. No change in the x-ray diffraction patterns of the δ′-form films was observed after poling. Changes in the x-ray diffraction patterns of the α′-form films after poling were consistent with dipole reorientation in the crystalline regions. In both cases the magnitude of the piezoelectric response of the oriented films was considerably higher than that of the unoriented films. The melting points of the poled films increased significant ly relative to that of the unpoled films with the same thermal history, which may suggest field-induced annealing effects.

Stages of tubulin assembly and disassembly studied by time-resolved synchrotron X-ray scattering

The structural transitions occurring during the assembly and disassembly of pig brain microtubule protein were investigated by time-resolved X-ray scattering using synchrotron radiation. The reactions were introduced by a slow temperature scan (2 deg.C/min) from 0 °C to 37 °C and back. Several structurally distinct states could be resolved during one cycle of assembly/disassembly. During the temperature rise, one observes four main phases: prenucleation events, microtubule nucleation, growth, and postassembly events. Heating from 0 °C to 22 °C results in a biphasic breakdown of rings and other aggregates, while the apparent mean diameter increases from 38 to 41 nm. Parallel time-resolved electron microscopic observations suggest that the initial solution contains several types of aggregates, mostly double concentric and single rings, but also rod-like particles, clusters of rings and other aggregates. All of these tend to break down with increasing temperature. Double concentric rings seem to dissociate into large and small single rings before both types of rings break down into protofilament fragments and tubulin subunits. From the breakdown products, associations of several protofilament fragments are formed, which are important for initiating microtubule nucleation. Assembly of nuclei begins around 22 °C. Microtubule elongation takes place between 25 and 30 °C. They grow mainly by addition of tubulin subunits but not via rings. During the reverse temperature scan, microtubules shorten by the release of subunits and/or small protofilament fragments from their ends. The degree of disassembly is strongly increased below 22 °C. Below about 10 °C rings are reformed, probably from the fragments, but their final number is much less than initially. Conditions that prevent microtubule nucleation such as GDP or Ca 2+ also stabilize rings, even at 37 °C. Thus, rings are viewed as storage aggregates of tubulin and microtubule associated proteins, whose breakdown is a prerequisite for microtubule formation, and whose reformation is independent of microtubule breakdown. The midpoints of microtubule growth and breakdown differ by about 12 deg.C so that the system shows hysteresis-like behavior. It is dependent on microtubule formation and is not seen when the temperature is cycled below that required for nucleation. Thus, even during a slow temperature scan, microtubule assembly is kinetically limited by nucleation. By contrast, depolymerization proceeds close to equilibrium. The radius of gyration of the tubulin heterodimers is 3.1 nm. The weight average diameter of rings in cold solutions is 38 nm, that of microtubules is 24.5 nm. At radiation dose rates of about 100 rad/s. radiation damage is of minor importance, as judged by the criterion of polymerizability. Total doses of up to 500,000 rad can be applied. Some concepts of analyzing time-resolved X-ray scattering data are presented. They make use of the fact that the scattering intensities vary continuously both with scattering angle and time. Cross-correlation of different regions of the pattern, and comparison of their temperature derivatives, reveals structural transitions not seen by other techniques.

Orientation of crystalline dipoles in poly(vinylidene fluoride) films under electric field

Orientation of molecular dipoles in poly(vinylidene fluoride) (PVF2) films under electric field has been measured by ir spectroscopy using ir transparent indium-tin-oxide electrodes. Significant changes in ir transmission with electric field have been observed at 512 and 446 cm−1, which demonstrate that crystalline dipoles in β-phase PVF2 orient along the electric field. As the field is cycled between positive and negative values, the dipole orientation exhibits a hysteresislike behavior, demonstrating that the polarization of β-phase PVF2 and its switching result directly from the orientation of crystalline dipoles. Time response of dipole orientation shows that the total orientation consists of an instantaneous part and a slowly developing part. Only the latter seems to contribute to the remnant polarization and its switching.

Hysteresis-like behavior of oxygen association-dissociation equilibrium curves of sickle cells determined by a new method.

SummaryAn instrument to determine the oxygen association-dissociation equilibrium of whole blood has been developed. Only one drop of blood sample is required to trace automatically the oxygen-red blood cell equilibrium curve in the direction of deoxygenation and reoxygenation within a total of less than 10 min. The whole blood is suspended in the form of a thin liquid film reenforced by a micromesh. The partial pressure of oxygen is changed continuously with a gas-mixing pump while simultaneously being recorded by polarographic oxygen electrodes. The spectral change of red blood cells is recorded by a dual-beam spec-trophotometer.Using this instrument it is shown that the oxygen equilibrium curve of sickle cells obtained during deoxygenation is shifted to a higher oxygen affinity as compared to those obtained during reoxygenation. This hysteresis-like behavior of equilibrium curves is also reflected in the Hill constant, n, which is greater for deoxygenation than for reoxygenation. On the other hand, if ...

Nuclear Magnetic Resonance Study of Liquid Crystal Transitions

The spin-lattice relaxation time (T1) and the transverse relaxation time (T2) are determined, by the spinecho nuclear magnetic resonance method, for the isotropic-liquid state of the liquid-crystal compounds p-azoxyanisole (PAA), cholesteryl benzoate, cholesteryl n-butyrate, and cholesteryl propionate. The T1 values are 1.2 sec for PAA, and 0.06 to 0.15 sec for the cholesteryl compounds. The T2 values are 0.02 to 0.03 sec for the cholesteryl compounds, and at least 0.05 sec for PAA. The results are consistent with those for other isotropic liquids. As the temperature decreases toward the transition to the anisotropic phase, a pretransitional decrease of T2 is observed for the cholesteryl compounds, and a similar decrease of T1 for PAA. An explanation in terms of Frenkel's theory of heterophase fluctuations is discussed. Thermal expansion curves and interphase surface tension for PAA are also considered, and hysteresis-like behavior of the phase transition.