Polyunsaturated fatty acids (PUFAs), commonly known as omega-3 and omega-6 fatty acids, have been shown to prevent chronic illnesses when consumed; PUFA-containing lipids are critical to organism development and membrane function. Currently, research on the structure of PUFA phospholipids is in its nascent stages, leaving much room for further characterization of this group of lipids. Through a combination of small angle neutron scattering (SANS), small angle X-ray scattering (SAXS), and molecular dynamics (MD) simulations, the structures of five PUFA phospholipids were determined, namely: 1,2-diarachidonyl-phosphatidycholine (DAPC), 1,2-didocosahexaenoyl-sn-glycero-3-phosphocholine (DDPC), 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC), 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (SAPC), and 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (SDPC). Structural parameters including area per lipid, total bilayer thickness (DHH), hydrocarbon thickness (2DC), and the area compressibility modulus (KA) were analyzed from scattering density profiles (SDPs) determined from fits to the scattering data, as well as unconstrained and area-constrained MD simulations using the C36 CHARMM force field. Areas per lipid determined from the SDPs and (unconstrained) MD simulations at 20oC and 30oC (except for DDPC) were found to be in good agreement with one another. However, when analyzing DHH and 2DC, the resulting experimental SDP values were approximately 10-15% lower than the values obtained from MD simulations. Area-constrained simulations allowed for determination of the area compressibility modulus and the area per lipid across simulated and scattering data. Joint analysis of SANS and SAXS data along with MD simulations provides an accurate protocol for bilayer structure of PUFA phospholipids and the use of these different, but complementary, techniques offers insights into the discrepancies that are observed within this unique class of biologically relevant lipids.

PUFA phospholipid bilayer structure determination using neutron and X-ray scattering data

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