X-Ray Diffraction Studies of Tubules Formed from a Diacetylenic Phosphocholine Lipid
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X-RAY DIFFRACTION STUDIES OF TUBULES FORMED FROM A DIACETYLENIC PHOSPHOCHOLINE LIPID B. THOMAS*t, C. R. SAFINYA*, R.J. PLANO*, N. A. CLARKt, B.R. RATNA: and R. SHASHIDARt
"*EXXONRESEARCH AND ENGINEERING
CO., ANNANDALE, N.J. 08801 tUNIVERSITY OF COLORADO, BOULDER, CO. 80309 tNAVAL RESEARCH LABORATORY, WASHINGTON, D.C. 20375 ABSTRACT The diacetylenic phosphocholine lipid 1,2-bis(10, 12-tricosadiyonol)-sn-glycerol3-phosphocholine forms, upon cooling below the chain melting temperature, hollow cylindrical structures known as 'tubules'. These tubules are approximately one micron in diameter and may range from tens to hundreds of microns in length. We have carried out x-ray diffraction studies of tubules (1) in 75/25 ethanol/water by volume and (2) in a dessicated state. We present results which show marked differences in the nature of both the in-plane and inter-layer correlations associated with the wrapped multilayer structure of the tubules in the two states. INTRODUCTION The highly unusual morphology exhibited by this phosphocholine is seen in Figure 1. Well-defined ridges along the surface of the tubule are highly suggestive of a formation mechanism that involves winding a lipid sheet along the axis of the tubule with a screw pitch relative to the tubule axis. In considering the formation mechanism, the question of the nature of the "in-plane" order, i.e., correlations of these chiral molecules within the sheet arise: are these correlations solid or fluid in character, do they change during the formation process, and are these correlations the underlying basis of the tubule formation? A thorough understanding of the structure and formation of tubules requires a coordinated study of "macroscopic" probes, such as optical microscopy, and "microscopic" probes, such as x-ray diffraction, to correlate microscopic and macroscopic structures. SAMPLE PREPARATION The starting material, a fine, off-white powder was placed in sufficient USP grade 95% ethanol and water (3:1, v:v) to attain a lipid concentration of 1 milligram lipid per milliliter of solvent. This mixture was heated and magnetically stirred over the course of approximately one hour to 55 °C, yielding a clear, uncolored solution. The solution was cooled to room temperature by turning the power off to the hotplate; the thermal mass of the hotplate plus several liters of solution determined the cooling rate. As the solution cooled it clouded and finally yielded a flocculent, snowy white precipitate consisting of tubules 1 micron in diameter and ranging from 10 to 30 microns in length, the majority being in the vicinity of 20 microns in length. Concentrated tubule samples for x-ray diffraction probes were obtained by centrifuging the mixture at 10,000 x g for 15 to 30 manutes at 5 °C. An aliquot of the lipid pellet was resuspended in cold solvent and observed microscopically to verify the retention of tubule
Mat. Res. Soc. Symp. Proc. Vol. 248. C1992 Materials Research Society
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morphology through the centrifugation process. Tubules at this stage of processin
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