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           COLLOQUIUM OF THE COMPUTATIONAL MATERIALS SCIENCE CENTER
                              College of Science
                       (CDS Department CSI 898-Sec 001)
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    Lipid Tubules: A Paradigm for Molecularly Engineering Structures
                        Joel M. Schnur
      College of Science, George Mason University, Fairfax, VA
				
The importance of chirality, or molecular handedness, has long been 
recognized in many areas of chemistry and physics. It is well known, 
for example, that the interactions between chiral molecules change 
dramatically when one molecule is replaced by its mirror image.  
This chiral specificity is the basis of a major industry producing 
chiral drugs.  Likewise, in the area of liquid crystals, molecular 
chirality leads to the formation of phases with long-range helical 
modulations, such as the cholesteric phase.  Chirality also changes 
the interactions of liquid crystals with electric fields, leading to 
ferroelectric phases with display applications.

In recent years, researchers have found that chirality plays another 
role - controlling the shape of self-assembled supramolecular aggregates.  
Amphiphilic molecules self-assemble into aggregates in aqueous solution 
because of the competition between hydrophilic and hydrophobic interactions.  
Such aggregates are often bilayers, with the hydrophilic head groups of 
the molecules exposed to water and the hydrophobic tails shielded from 
water.  One would normally expect the lowest-energy state of the bilayers 
to be flat, or to be large spherical vesicles with the minimum curvature 
needed to close off the hydrophobic region from the water.  However, 
experiments have found that bilayers of many chiral molecules instead 
form long, narrow cylinders, known as tubules.  The radius of such 
tubules depends on the material, but they are always high-curvature 
structures, with a radius much smaller than spherical vesicles.  In 
many cases tubules show helical markings that wind around the cylinders, 
and in many cases they seem to form from helical ribbons.  What determines 
the size and shape of tubules and helical ribbons, and how are the size 
and shape related to the chirality of the molecules?  One pertinent 
experimental system is tubules that form from synthetic diacetylenic lipids. 
Diacetylenic lipids are chiral molecules consisting of a hydrophilic head 
linked to hydrophobic chains.  The most commonly studied lipid of this type, 
1, 2-bis (tricosa-10, 12-diynoyl)-sn-glycero-3-phosphocholine (designated 
DC_8,9 PC), makes tubules with diameters of 0.5 micrometers and lengths of 
50-200 micrometers. These structures have been extensively studied for use 
in technological applications, such as electroactive composites and 
controlled-release systems.  I will give an overview of the experimental 
research on this system, discuss some of the theoretical approaches that 
have been formulated to explain the observations, and then discuss some 
of the technological consequences of the research.

                        Monday, September 8, 2008
                                  4:30 pm
                     Room 301, Research I, Fairfax Campus

Refreshments will be served at 4:15 PM.
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Find the schedule at www.cmasc.gmu.edu/seminar/schedule.html
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