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-pre vitamins 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 c onformers according to a Boltzmann distribution at 298 K. On the basis of present calculations, the red shift of previtamin D low-temperatur e UV and CD spectra determined earlier is attributed to the shift of c onformational equilibrium in favour of more stable cZc geometries. The effect of complex formation with simplest model cluster of silica sur face ((H3SiO)(3)SiOH), on previtamin D and tachysterol conformational equilibria and on UV-absorbance maxima has been evaluated.