phospholipids. However, this model has now been superseded, as through the study of lipid polymorphism it is now known that the behaviour of lipids under physiological (and other) conditions is not simple. Main phospholipids Diacylglyceride structures See: Glycerophospholipid Phosphatidic acid (phosphatidate) (PA) Phosphatidylethanolamine (cephalin) (PE) Phosphatidylcholine (lecithin) (PC) Phosphatidylserine (PS) Phosphoinositides: Phosphatidylinositol (PI) Phosphatidylinositol phosphate (PIP) Phosphatidylinositol bisphosphate (PIP2) and Phosphatidylinositol trisphosphate (PIP3) Phosphosphingolipids See Sphingolipid Ceramide phosphorylcholine (Sphingomyelin) (SPH) Ceramide phosphorylethanolamine (Sphingomyelin) (Cer-PE) Ceramide phosphoryllipid Applications Phospholipids have been widely used to prepare liposomal, ethosomal and other nanoformulations of topical, oral and parenteral drugs for differing reasons like improved bio-availability, reduced toxicity and increased permeability across membranes. Liposomes are often composed of phosphatidylcholine-enriched phospholipids and may also contain mixed phospholipid chains with surfactant properties. The ethosomal formulation of ketoconazole using phospholipids is a promising option for transdermal delivery in fungal infections. Advances in phospholipid research lead to exploring these biomolecules and their conformations using lipidomics. Simulations Computational simulations of phospholipids are often performed using molecular dynamics with force fields such as GROMOS, CHARMM, or AMBER. Characterization Phospholipids are optically highly birefringent, i.e. their refractive index is different along their axis as opposed to perpendicular to it. Measurement of birefringence can be achieved using cross polarisers in a microscope to obtain an image of e.g. vesicle walls or using techniques such as dual polarisation interferometry to quantify lipid order or disruption in supported bilayers. Analysis There are no simple methods available for analysis of phospholipids, since the close range of polarity between different phospholipid species makes detection difficult. Oil chemists often use spectroscopy to determine total phosphorus abundance and then calculate approximate mass of phospholipids based on molecular weight of expected fatty acid species. Modern lipid profiling employs more absolute methods of analysis, with NMR spectroscopy, particularly 31P-NMR, while HPLC-ELSD provides relative values. Phospholipid synthesis Phospholipid synthesis occurs in the cytosolic side of ER membrane that is studded with proteins that act in synthesis (GPAT and LPAAT acyl transferases, phosphatase and choline phosphotransferase) and allocation (flippase and floppase). Eventually a vesicle will bud off from the ER containing phospholipids destined for the cytoplasmic cellular membrane on its exterior leaflet and phospholipids destined for the exoplasmic cellular membrane on its inner leaflet. Sources Common sources of industrially produced phospholipids are soya, rapeseed, sunflower, chicken eggs, bovine milk, fish eggs etc. Phospholipids for gene delivery such as distearoylphosphatidylcholine, dioleoyl-3-trimethylammonium propane etc. are