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M.Kriechbaum(1), M. Steinhart(2), P. Laggner(1), H. Amenitsch(1) and S. Bernstorff(3)

We have performed kinetic studies on pressure-jump induced phase transitions of phospholipids monitored by time-resolved X-ray scattering with millisecond time-resolution in the SAXS (10-200 Å) and WAXS (3-6 Å) region using a high-pressure X-ray cell as described in [1, 2] at the SAXS-beamline at ELETTRA, Trieste, Italy. In these studies we have focused on the kinetics of barotropic phase transitions of the phosphatidyl-ethanolamine (PE) lipids DOPE and SOPE, respectively, by applying pressure-jump amplitudes up to 1.7 kbar (0.17 GPa) in both directions (compression and decompression) at different temperatures. Using an optimized setup (Fig.1), SAXS and WAXS diffraction data with good statistics could be obtained with 5 ms time-resolution in a single-shot experiment (Fig.2). It was shown that the time constants for a completion of a phase transition depend strongly on the magnitude of the depth of the quench into the new phase (Fig.3).

Figure 1. Set-up of the p-jump experiment: Thermocouple (A), high pressure X-ray cell (B), pressure sensors (C). Two ‘pressure-circuits’ are separated by a pneumatic driven valve (D) and are kept at different pressure levels before activating a p-jump, which is accomplished by quickly opening the valve D, resulting in a quick pressure equilibration between both reservoirs within a few milli-seconds. Double-stem valve (E) and a motor(G)-driven pressure pump (F) are used for generating hydrostatic high pressures.
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Figure 2. Time-resolved p-jump experiments of SOPE with p-jump amplitudes as indicated in the plots with a maximum time-resolution of 5 ms at T = 40°C. P-jumps (completed within 10 ms) from the lamellar-fluid to the lamellar-gel phase (left; compression) and vice versa (right; decompression) are shown in the SAXS region (lamellar lattice-spacing d) and the WAXS region (lateral (short-range order) lattice-spacing d within the lipid bilayer). Each image shows 512 frames of a single-shot experiment with decreasing time-resolution (5/50/500ms) from the bottom to the top (time-normalized, unsmoothed intensity-data displayed in a linear gray-scale).
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Figure 3. Pressure-Temperature phase diagram of DOPE in excess water (left), consisting of two lamellar phases, gel and fluid (liquid crystalline), respectively, and an inverted hexagonal phase. The figure to the right shows for three different barotropic phase-transitions the dependency of the transition time of the respective phase undergoing a p-jump induced transition as a function of the applied p-jump amplitude p-pt, where p is the final pressure after the jump and pt the transition pressure in the p-T equilibrium phase diagram. The phase transformation proceeds the faster the higher the jump amplitude (quench into the new phase as being indicative for the driving force) is chosen.
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[1] Pressl, K., Kriechbaum, M., Steinhart, M. & Laggner, P. (1997): High Pressure Cell for Small- and Wide-Angle X-Ray Scattering. Rev.Sci.Instrum. 68, 4588-4592 .
[2] Steinhart, M., Kriechbaum, M., Pressl, K., Amenitsch, H., Laggner, P. & Bernstorff, S. (1999): High- Pressure Instrument for Small- and Wide-Angle X-Ray Scattering. II. Time-Resolved Experiments. Rev.Sci.Instrum. 70, 1540-1545 .

Author: Manfred Kriechbaum (March 30th 2000)