Stable Nanocapsules Formed from Headgroup Polymerization of Natural Phospholipids
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Stable Nanocapsules Formed from Headgroup Polymerization of Natural Phospholipids Glenn E. Lawson and Alok Singh Center for Bio/Molecular Science and Engineering, Naval Research Laboratory 4555 Overlook Ave. Washington, DC 20375 ABSTRACT Formation of lipid based hollow, spherical nanocapsules is reported. N-acylphosphotidylethanolamine (egg) derivatives equipped with divinylbenzoyl functionality in their headgroup region were synthesized to explore their polymerization behavior in vesicles. Polymerizable phospholipids alone produced vesicles as an aqueous dispersion and produced stable nanocapsules upon polymerization at room temperature. These structures survived a redispersed cycle following lyophilization and maintained their structural integrity in non-aqueous solvent. INTRODUCTION Vesicles represent a unique class of technologically relevant structures by virtue of their ever-increasing utility across disciplines. Extensive research activities involving vesicles began after the appearance of a report by Bangham [1] showing electron microscopic evidence to reveal the presence of concentric, bimolecular layers producing vesicular morphologies. Published reports on the formation and properties of vesicles revealed their application potential involving their encapsulation and trans-bilayer transportation capabilities [2,3]. However, vesicles suffer a major drawback of limited stability in working environment. Subsequent research efforts to overcome the stability problem led to development of the following three strategies; a) crosslinking the amphiphiles after the formation of vesicles by polymerization [4-7]; b) formulation of lipid compositions including involvement of lipids containing polyethylene glycol (PEG) in their headgroup [8-10]; and c) development of new, non-lipid materials to form stable vesicles [11, 12]. Despite the superior technical advantage and versatility, the higher cost of polymerized vesicles restricts their overall usage in technology development. From an application point of view, there is need for an inexpensive polymerized vesicle system which is convenient to synthesize on an industrial scale from readily available natural sources. In this report, we present data on the synthesis and selfassembling behavior of a new phospholipid 1, by incorporating a 3,5-divinylbenzoyl functionality in the headgroup region (Figure-1). O O N H
O H P O O OH
O O O
EGG-DVBA (1) Figure 1. Polymerizable phospholipid through the headgroup.
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EXPERIMENTAL Synthesis of Phospholipids and their Intermediates Synthesis of 3,5-divinylbenzoic acid (DVBA). Solid N-bromosuccinimide (2.37 g, 13.3 mmol) was added to a stirred solution of 3,5-dimethylbenzoic acid (1.0 g, 6.6 mmol) in carbon tetrachloride (10 mL). The reaction mixture was refluxed (1-3 h) af
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