(50j) Solvatochromic Property in Lipid Bilayer Interphases Analyzed Based on Time Resolved Emission Spectrum of Laurdan

Self-assembled lipid bilayer membranes are considered to have important roles in various bioprocesses. Raft domains, which are mainly composed of cholesterol and sphingomyelin, form highly ordered membrane domains with various sizes. In order to understand the property and function of raft domains, it is necessary to discuss the heterogeneity of lipid diffusion because distributions of domains could determine that of proteins and lead to control of cell functions. Evaluation of lateral diffusion regarding lipid mobility has been used as a major strategy to discuss the differences in fluidity. Besides, the membrane polarity or hydration property of water molecules such as permeability and solvatochromic relaxation should be discussed. Exclusion of water molecules from the membrane surface can be considered to be a trigger for the interaction with external molecules. Despite the importance on this matter, the behavior of water molecules around lipid membrane surface is still unclear. The fluorescence from Laurdan (time resolved emission spectra) was analyzed to detect how water molecules affect the solvent relaxation in the hydrophilic region of lipid bilayer. Lifetime deconvolution could possibly help to understand the property or localization of the fluorophore. Our results suggest that specific solvent relaxation states can be observed in sphingomyelin-containing bilayers. The hydration around lipid head groups was evaluated with an aim for deeper insight into the formation of heterogeneity in the lipid bilayer membrane.