Ultrafast Conformer Relaxation 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. Conformer relaxation back to the ground state of isolated CHD happens on the time scale of picoseconds, therefore it is necessary to use ultrafast techniques such as transient absorption to capture the relaxation. Here we are studying conformer relaxation of DHC and its analogs in liposomes as a simple model for biologically relevant skin membranes. DHC has been studied extensively in isotropic solvents where the molecule is free to relax down to the ground state to form a stable previtamin D₃ species. However in the cell membrane the molecule is hypothesized to be locked in an unstable previtamin D₃ conformation that thermally relaxes down to form Vitamin D_3. To understand the conformer relaxation that takes place in biological membranes we extend our studies to DHC analogs, DHC-Acetate and Ergo in isotropic solvents and in simple lipid bilayers.