Difference between revisions of "C2AB: A Molecular Glue for Lipid Vesicles with a Negatively Charged Surface"

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[[Image:C2AB3.jpg|500px|thumb|left|(a) shows TIR images before and after the introduction of <math>Ca^{2+}</math>. (b) and (c) show TEM images of the liposomes before and after the addition of <math>Ca^{2+}</math>]]
 
[[Image:C2AB3.jpg|500px|thumb|left|(a) shows TIR images before and after the introduction of <math>Ca^{2+}</math>. (b) and (c) show TEM images of the liposomes before and after the addition of <math>Ca^{2+}</math>]]
  
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Organic field effect transistors (OFETs) have many favorable properties over current field effect transistors (FETs).  The most popular OFET, due to its optimal electronic properties derived from its delocalized pi bond, is pentacene.  These properties however, are degraded by the existence of grain boundaries in polycrystalline pentacene as a result of the reduced mobility in charge carriers.  Therefore, single crystals of pentacene are primarily used.  The most popular way of synthesizing the single crystalline pentacene is currently by physical vapor deposition.
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The academic group conducting this research is experimenting with liposomes as a drug delivery vehicle.  They believe they can solve the problem of targeting the liposome to the correct cells, however there is a perceived problem that the concentration of the drug in each liposome will not have the necessary efficacy.  They hope to be able to fuse the liposomes together at the site of the target to ensure that the drug concentration is high enough to provide a sufficient dosage.
  
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An alternative method of creating pentacene single crystals is by liquid-phase crystallization.  In this process pentacene dissolved in trichlorobenzene is purified via various methods.  Then the pentacene solution is heated and very slowly cooled so that precipitation occurs as a single crystal.  In fact, in this experiment the pentacene was heated to 200C, then cooled at 0.1C per hour.  The phase change from solute to solid single crystal is clearly applicable to the soft matter discussion.  The image on the left shows the single crystal as seen from AFM.  Each step shown is a single molecule step.  The entire image has an area of 4umx4um.
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They are collaborating this work with a group that has been studying the protein C2AB which is used in cells as a calcium sensor in synaptic vesicle exocytosis.  The protein has two active sites, one of which binds 2 <math>Ca^{2+}</math>, the other binding 3 <math>Ca^{2+}</math>.
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The crystalline pentacene was then fabricated onto a thin film of heavily doped silicon oxide to create the OFET.  The OFET electrical properties were measured by standard methods.  The most important of these properties was the field effect mobility which was measured to be <math>0.4 \frac{cm^2}{Vs}</math>.  This number is similar to PVD single crystal pentacene which shows that the fabrication technique was a success.  Liquid-phase crystallization is a practical method in producing pentacene single crystals for OFETs.
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There are some differences between this method and PVD.  For example, there seems to be a large anisotropy in the method used here.  That means that the orientation of the crystal, and the direction by which you measure your voltage across makes a difference.  This created mobilities that were both higher and lower than expected.  Perhaps this can be tuned to create an even better OFET.
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Revision as of 01:35, 3 November 2009

Original entry: William Bonificio, AP 225, Fall 2009

Information

C2AB: A Molecular Glue for Lipid Vesicles with a Negatively Charged Surface. Jiajie Diao, Tae-Young Yoon, Zengliu Su, Yeon-Kyun Shin, Taekjip Ha. Langmuir 2009 25 (13), 7177-7180

Soft matter keywords

Liquid-Phase Crystallization, Pentazene, Organic Field Effect Transistor, Pentacene, Tricholorbenzene.

Summary

The purpose of this study was to test the efficacy of a calcium binding protein, currently found in cells, to fuse liposomes to one another. The binding protein, C2AB, binds calcium at both ends of it. This binding gives it a positive charge on both sides, which in turn gives the C2AB an affinity to negatively charged liposomes. When liposomes, C2AB and <math>Ca^{2+}</math> are combined the liposomes fuse.

Soft matter discussion

Schematic illustration of experiment. Immobilized vesicles 'glued' to mobile vesicles via C2AB.


(a) shows TIR images before and after the introduction of <math>Ca^{2+}</math>. (b) and (c) show TEM images of the liposomes before and after the addition of <math>Ca^{2+}</math>

The academic group conducting this research is experimenting with liposomes as a drug delivery vehicle. They believe they can solve the problem of targeting the liposome to the correct cells, however there is a perceived problem that the concentration of the drug in each liposome will not have the necessary efficacy. They hope to be able to fuse the liposomes together at the site of the target to ensure that the drug concentration is high enough to provide a sufficient dosage.

They are collaborating this work with a group that has been studying the protein C2AB which is used in cells as a calcium sensor in synaptic vesicle exocytosis. The protein has two active sites, one of which binds 2 <math>Ca^{2+}</math>, the other binding 3 <math>Ca^{2+}</math>.

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