Vitamin D/ja: Difference between revisions

=== Photochemistry ===

[[File:Vitamin D biosynthesis in fungi and animals.svg|thumb|upright=1.35|The photochemistry of vitamin D biosynthesis in animal and fungi]]

[[File:Calcitriol-Biosynthese 2.svg|thumb|upright=1.35|Thermal isomerization of [[previtamin D3|previtamin{{nbsp}}D₃]] to vitamin D₃]]

The transformation that converts 7-dehydrocholesterol to vitamin D₃ occurs in two steps. First, 7-dehydrocholesterol is [[photolysis|photolyzed]] by ultraviolet light in a 6-electron [[conrotatory]] ring-opening [[electrocyclic reaction]]; the product is [[previtamin D3|previtamin{{nbsp}}D₃]]. Second, previtamin{{nbsp}}D₃ spontaneously [[isomer]]izes to vitamin{{nbsp}}D₃ ([[cholecalciferol]]) in an [[antarafacial]] [[Sigmatropic shift#.5B1,7.5D Shifts|sigmatropic [1,7] hydride shift]]. At room temperature, the transformation of previtamin{{nbsp}}D₃ to vitamin D₃ in an organic solvent takes about 12 days to complete. The conversion of previtamin{{nbsp}}D₃ to vitamin D₃ in the skin is about 10 times faster than in an organic solvent.