Ground-state conformational equilibrium of previtamin D and its E-isomer: Effect on the absorption characteristics

The ground-state conformational analysis of previtamin D, its E-isomer, tachysterol, their 10-desmethyl analogues and 1-methyl-1-hydroxy-previtamins have been performed by force-field calculations. Differences in the absorption characteristics of these compounds are discussed in view of the chromophore geometry and abundance (%) of the calculated conformers according to a Boltzmann distribution at 298 K. On the basis of present calculations, the red shift of previtamin D low-temperature UV and CD spectra determined earlier is attributed to the shift of conformational equilibrium in favour of more stable cZc geometries. The effect of complex formation with simplest model cluster of silica surface ((H3SiO)(3)SiOH), on previtamin D and tachysterol conformational equilibria and on UV-absorbance maxima has been evaluated.