Because of the inhibitors polyanionic nature of DNA, cationic (and neutral) lipids are typically used for gene delivery, while the use of anionic liposomes has been fairly restricted to the delivery of other therapeutic macromolecules [14]. Liposomes can exhibit a range of sizes and morphologies upon the assembly of pure lipids or lipid mixtures suspended in an aqueous medium [2]. A common morphology which is analogous to the eukaryotic cellular membrane is the unilamellar vesicle. This vesicle is characterized by a single bilayer membrane which encapsulates Inhibitors,research,lifescience,medical an internal aqueous solution, thus separating it from the external (bulk) solution [15].

Both cationic amine head groups and anionic phospholipid head groups can form these single-walled vesicles. Vesicle sizes fall into the nanometer to micrometer range: small unilamellar vesicles are 20–200nm, large unilamellar vesicles are 200nm–1μm, and giant unilamellar vesicles are larger than 1μm [2]. Giant vesicles also include other morphologies such as Inhibitors,research,lifescience,medical multilamellar, which consists of multiple concentric bilayers, oligolamellar, which consists of only two concentric bilayers, and multivesicular, which consists of multiple smaller unilamellar vesicles inside of one giant Inhibitors,research,lifescience,medical one. With

the exception of multilamellar vesicles, these other morphologies are difficult to obtain without highly controlled processes for formation [2]. Giant vesicles also deserve special attention because their sizes are large, Inhibitors,research,lifescience,medical ranging from 1μm to more than 100μm [2]. These large vesicles are studied and well characterized, partially due to the ease of observation via optical microscopy [10]. During the compaction of polynucleotides into liposomal assemblies, a number of structures have been known to appear [5, 6, 16–19]. Each structure

is formed in the most energetically favorable conformation based upon characteristics Inhibitors,research,lifescience,medical of the specific lipids used in the system [13]. A dependent term known as the structure-packing parameter can be used to suggest what shape the amphiphile will take, depending on the ratio of size variables. The packing parameter is defined as P=valc, (1) where v: the volume of the hydrocarbon portion, a: the effective area of the head group, and lc: Tryptophan synthase the length of the lipid tail. This correlation predicts a range of structures according to the following conditions [13, 20] (Figure 2): Figure 2 Structures predicted by the packing parameter P. P<13→spherical  micelle,13≤P<  12→cylindrical  micelle,12≤P<1→flexible  bilayers,vesicles,P=1→planar  bilayers,P>1→inverted  micelles,(hexagonal  (HII)phase). (2) 3. Cationic Lipids A solution of cationic lipids, often formed with neutral helper lipids, can be mixed with DNA to form a positively charged complex termed a lipoplex [21].