The photosynthesis of vitamin D₃ in mammalian skin results from UV-B irradiation of provitamin D₃ (7-dehydrocholesterol, DHC) at ca. 290 nm. Upon return to the ground state, the hexatriene product, previtamin D₃, undergoes a conformational equilibration between helical gZg and more planar tZg and tZt forms. The helical gZg forms provide a pathway for the formation of vitamin D₃ via a [1,7]-sigmatropic hydrogen shift. Steady state photolysis and UV transient absorption spectroscopy are combined to explore the conformational relaxation of previtamin D₃ formed from DHC in isotropic solution and confined to lipid bilayers chosen to model the biological cell membrane. The results are compared with measurements for two analogues: previtamin D₂ formed from ergosterol (provitamin D₂) and previtamin D₃ acetate formed from DHC acetate. The resulting spectral dynamics are interpreted in the context of simulations of optical excitation energy and oscillator strength as a function of conformation. In solution, the relaxation dynamics and steady state product distributions of the three compounds are nearly identical, favoring tZg forms. When confined to lipid bilayers, the heterogeneity and packing forces alter the conformational distributions and enhance the population of a gZg conformer capable of vitamin D formation.

中文翻译：

---

探索前维生素 D3 和类似物在溶液和脂质双层中的形成和构象松弛

哺乳动物皮肤中维生素 D ₃的光合作用是由维生素 D ₃（7-脱氢胆固醇，DHC）在约 290 纳米。回到基态后，己三烯产物前维生素 D ₃经历螺旋 gZg 与更平面的 tZg 和 tZt 形式之间的构象平衡。螺旋形 gZg 形式通过 [1,7]-σ 氢位移为形成维生素 D ₃提供了途径。结合稳态光解和紫外瞬态吸收光谱探索前维生素 D ₃的构象弛豫由 DHC 在各向同性溶液中形成，并局限于选择用来模拟生物细胞膜的脂质双层。将其结果与比较测量两个类似物：前维生素d ₂从麦角甾醇形成的（前维生素d ₂）和前维生素d ₃由 DHC 醋酸盐形成的醋酸盐。由此产生的光谱动力学在模拟光激发能量和振荡器强度作为构象函数的背景下进行解释。在溶液中，三种化合物的弛豫动力学和稳态产物分布几乎相同，有利于 tZg 形式。当仅限于脂质双层时，异质性和堆积力会改变构象分布并增强能够形成维生素 D 的 gZg 构象异构体的数量。