Multiple interfacial hydration of dihydro-sphingomyelin bilayer reported by the Laurdan fluorescence

Abstract

The hydration properties of the lipid bilayer interface are important for determining membrane characteristics. The hydration properties of different lipid bilayer species were evaluated using the solvent sensitive fluorescence probe, 6-lauroyl-2-dimethylamino naphthalene (Laurdan). Sphingolipids, D-erythro-N-palmitoyl-sphingosylphosphorylcholine (PSM) and D-erythro-N-palmitoyl-dihydrosphingomyelin (DHPSM) showed specific, interfacial hydration properties stemming from their intra- and intermolecular hydrogen bonds. As control, the bilayers of glycerophospholipids, such as 1-palmitoyl-2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-oleoyl-2-oleoyl-sn-glycero-3-phosphocholine (DOPC), were also evaluated. The fluorescence properties of Laurdan in sphingolipids indicated multiple excited states according to the results obtained from the emission spectra, fluorescence anisotropy, and the center of mass spectra during the decay time. Deconvolution of the Laurdan emission spectra into four components enabled us to identify the variety of hydration and the configurational states derived from intermolecular hydrogen bonding in sphingolipids. Particularly, the Laurdan in DHPSM revealed more hydrated properties compared to the case in PSM, even though DHPSM has a higher Tm than PSM. Since DHPSM forms hydrogen bonds with water molecules (in 2NH configurational functional groups) and the different flexibility among the head groups compared with PSM, which could modulate space to retain a high amount of water molecules. The careful analysis of Laurdan such as the deconvolution of emission spectra into four components performed in this study gives the important view for understanding the membrane hydration property.