Bending elasticity of saturated and monounsaturated phospholipid membranes studied by the neutron spin echo technique.

We have used neutron spin echo (NSE) spectroscopy to study the effect of bilayer thickness and monounsaturation (existence of a single double bond on one of the aliphatic chains) on the physical properties of unilamellar vesicles. The bending elasticity of saturated and monounsaturated phospholipid bilayers made of phospholipids with alkyl chain length ranging from 14 to 20 carbons was investigated. The bending elasticity κ(c) of phosphatidylcholines (PCs) in the liquid crystalline (L(α)) phase ranges from 0.38 × 10(-19) J for 1,2-dimyristoyl-sn-glycero-3-phosphocholine to 0.64 × 10(-19) J for 1,2-dieicosenoyl-sn-glycero-3-phosphocholine. It was confirmed that, contrary to the strong effect on the main transition temperature, the monounsaturation has a limited influence on the bending elasticity of lipid bilayers. In addition, when the area modulus K(A) varies little with chain unsaturation or length, the elastic ratios (κ(c)/K(A))(1/2) of saturated and monounsaturated phospholipid bilayers varies linearly with lipid hydrophobic thickness d which agrees well with the theory of ideal fluid membranes.