Membrane equilibration period

The structure and stability of several other channels have also been addressed in computer simulations. In a recent study of the HIV-Vpu transmembrane domain in a water-octane system [84], the calculations were started in a pentameric, coiled-coil bundle that had been suggested as the equilibrium structure, based on simulations of the bundle with water-caps and restraining forces [125]. After initial equilibration periods of 0.5-1.5 ns, the bundle evolved into a conical structure that resembled the K+ channel [117]. This structural rearrangement had a significant effect on the channel region while the initial coiled-coil contained a continuous column of water, most of the water was expelled from the conical structure breaking a continuous water path across the channel. Similar slow relaxation times were seen in simulations of tetrameric bundles of LS3 channels in a water-octane membrane-mimetic system [82], These simulations were started... 

The casting procedure consisted of drawing an aqueous solution of PVA to a thin layer, and after an evaporation period, immersing in the complexing bath. The complexing bath used in our study was basically a saturated CuSO solution, with or without a series of possible additives. After a period of equilibration of over 24 hours, the membranes were dried and subjected to dry heat treatment,in order to stabilize the asymmetric structure obtained. The preparation conditions of several membranes prepared by this method are shown in Table V. 

Three pairs of membranes, each with two different porosities, were installed in the RO cells. The membranes used were PA (92 and 972), CAc (852 and 912), and PBI (892 and 992). Each cell had an effective membrane diameter of 4.1 cm (area of 13.4 cm2). The operating pressure for all runs was 260 10 psig, and the flow rate was adjusted to 410 10 mL/min. The system and membranes were washed by operating with an ethanol/water mixture (1 9 v/v twice) for a 6-8-h period to get rid of any trace organic impurities in the system. The system was then cleaned twice with purified water and equilibrated with purified water (3 X 10 h). During the run, the temperature of the feed solution increased from 20-22 °C to 26-29 °C. 

Equilibrium Sorption Procedure. The sorption (binding) of 8-galactosidase by the various modified and unreacted collagen membranes was measured by the method of equilibrium sorption (18, 13). In this procedure, collagen membrane was impregnated with the purified enzyme (8-galactosidase, J2. coli K12) as a function of enzyme bath concentrations. Enzyme solutions containing 0.023 0.012, 0.0074, 0.0047) 0.0031 and 0.0021 pmole/ml of enzyme in phosphate buffer (0.02H) at pH 7-0 were employed as the sorption baths. All sorption studies were carried out at 4°C for a twenty four hour period. Transient state sorption data established that the system had equilibrated within the twenty four hour period. 

Osmotic equilibrium is not reached quickly after the solvent and solution first contact the membrane. Periods of a few hours or more may be required for the pressure difference to stabilize, and this equilibration process must be repeated for each concentration of the polymer in the solvent. Various ingenious procedures have been suggested to shorten the experimental time. Much of the interest in this problem has waned, however, with the advent of high-speed automatic osmometers. 

When a person is exposed to a volatile organic solvent through inhalation, the solvent vapor diffuses very rapidly torough the alveolar membranes, fire connective tissues and the capillary endothelium and into fire red blood cells or plasma. With respiratory gases the whole process takes less than 0.3 seconds. This results in almost instantaneous equilibration between the concentration in alveolar air and in blood and, flierefore, the ratio of the solvent concentration in pulmonary blood to that in alveolar air should be approximately equal to the partition coefficient. As the exposure continues, the solvent concentration in the arterial blood exceeds that in the mixed venous blood. The partial pressures in alveolar air, arterial blood, venous blood and body tissues reach equilibrium at steady state. When the exposure stops, any unmetabolized solvent vapors are removed from the systemic circulation through pulmonary clearance. During that period the concentration in fire arterial blood is lower than in the mixed venous blood and the solvent concentration in alveolar air will depend on the pulmonary ventilation, the blood flow, the solubifity in blood and the concentration in the... 

Fig. 5. Effect of HCOj -free medium on inactivation and phosphorylation of acetyl-CoA carboxylase. Enzyme preparation was placed in the main compartment of a Warburg flask, capped with a serum stopper, containing a small piece of Whatman filter paper (2x3 cm) plus 0.1 ml of Hyamine 10-X in the central well. Vessels were evacuated and refilled with COj-free Nj several times. This procedure was repeated several times during a 3-hour period at room temperature to completely remove dissolved HCOs firom the medium. Enzjmie preparation was equilibrated at 37°C for 10 minutes prior to the addition of [y- PlATP (specific activity, 28 iiCi/iimole at 0.7 miW final concentration) which was also treated in a similar way to remove COi. AT indicated times, P incorporation was terminated by addition of a cold solution containing ATP and EDTA at 5 times the concentrations of [y- PlATP and MgCl2, respectively. The labeled en me was purified by DE AE-cellulose chromatography as described before, and immunoprecipitation of the enzyme was carried out by incubation with rabbit antibody to the carboxylase for 15 minutes and with the membrane preparation fromS. aureus (18) for another 15 minutes. Enzyme-antibody precipitates were collected and immediately washed by centrifugation. Enzyme activity-control enzyme -ATP, O +ATP, . COj-free enzyme —ATP, A -H ATP, . Radioactivily-control enzyme, C02-free enzyme, . From Lent et al. (71).

If the sensor has to be operated without front turbulence or if the oxygen consumption of the probe cannot be tolerated as, e.g., in some respiration experiments, the sensor may be operated in pulsed mode (Langdon, 1984). In this mode the polarization voltage is pulsed after a period of delay. The oxygen concentration in front of the membrane is allowed to equilibrate and the total oxygen consumption is reduced. The plateau current is determined from the current-time registration during the pulse. 

When the oxygen sensor is immersed in a flowing or stirred solution of the analyte, oxygen diffuses through the membrane into the thin layer of electrolyte immediately adjacent to the disk cathode, where it diffuses to the electrode and is immediately reduced lo water. In contrast with a normal hydrodynamic electrode, two diffusion processes are involved — one through the membrane and the other through the solution between the membrane and the electrode surface. For a steady-state condition to be reached in a reasonable period (10 to 20 s), the thickness of the membrane and the electrolyte film must be 20 pm or less. Under these conditions, it is the rale of equilibration of the transfer of oxygen across the membrane that determines the steadv-stalc current that is reached. 

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