Ultrafast Transformation of Provitamin D Analogs in Lipid Bilayers

Ultrafast photochemical transformations of 7-Dehydrocholesterol (DHC, Provitamin D₃), DHC-acetate and Ergosterol (Ergo, Provitamin D₂) to previtamin (D₃, D₃-acetate and D₂) occurs upon a ring-opening reaction in the excited state where a cyclohexadiene (CHD) chromophore embedded within the molecules opens to form a hexatriene previtamin D species. The ring opening of isolated CHD happens on a sub-picosecond time scale, therefore it is necessary to use ultrafast techniques such as transient absorption to capture the dynamics of the molecules in the excited state. Here we are studying the excited state dynamics of DHC and its analogs in liposomes as a model for biologically relevant skin membranes. DHC has been studied extensively in isotropic solvents where the molecule is free to isomerize to previtamin D₃, yielding a 0.5ps fast component and 1-2ps long component while in the anisotropic liposomes a third, longer lived, 11ps component is also observed. To understand the contribution in biological membranes we extend our studies to DHC analogs, DHC-Acetate and Ergo in isotropic solvents and in lipid bilayers.