Characterization of the sub-transition of hydrated dipalmitoylphosphatidylcholine bilayers: X-ray diffraction study |
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Authors: | Martin J Ruocco G Graham Shipley |
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Institution: | Biophysics Institute, Departments of Medicine and Biochemistry, Boston University School of Medicine, Boston, MA 02118 U.S.A. |
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Abstract: | The structural changes accompanying the recently described sub-transition of hydrated dipalmitoylphosphatidylcholine (Chen, S.C., Sturtevant, J.M. and Gaffney, B.J. (1980) Proc. Natl. Acad. Sci. USA 77, 5060–5063) have been defined using X-ray diffraction methods. Following prolonged storage at ?4°C the usual Lβ′ gel form of hydrated dipalmitoylphosphatidylcholine (DPPC) is converted into a more ordered stable ‘crystal’ form. The bilayer periodicity is 59.1 Å and the most striking feature is the presence of a number of X-ray reflections in the wide angle region. The most prominent of these are a sharp reflection at and a broader reflection at . This diffraction pattern is indicative of more ordered molecular and hydrocarbon chain packing modes in this low temperature ‘crystal’ bilayer form. At the sub-transition (Trmsub = 15–20°C) an increase in the bilayer periodicity occurs () and a strong reflection at approx. with a shoulder at approx. is observed. This diffraction pattern is identical to that of the bilayer gel (Lβ′) form of hydrated DPPC. Thus, the sub-transition corresponds to a bilayer ‘crystal’ → bilayer Lβ′ gel structural rearrangement accompanied by a decrease in the lateral hydrocarbon chain interactions. Differential scanning calorimetry and X-ray diffraction show that on further heating the usual structural changes Lβ′ → Pβ′ and Pβ′ → Lα occur at the pre- and main transitions, at approx. 35°C and 41°C, respectively. |
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Keywords: | Bilayer X-ray diffraction Crystal structure Gel structure Dipalmitoylphosphatidylcholine Sub-transition DSC diferential scanning calorimetry |
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