Previtamin D conformations and the wavelength-dependent photoconversions of previtamin D

Abstract

Conformational abundances calculated at different levels of theory have been used to derive spectral and photophysical properties of a previtamin D model compound. The individual conformer spectra of absorption are approximated by Gaussian functions using excited state characteristics calculated with the QCFF/sol method. The calculated total UV spectrum for 3-desoxy-previtamin D model compound is found to be in a good agreement with experiment. The ratio of quantum yields for the cis– trans and ring-closure photochemical reactions of previtamin D is in excellent agreement with experiment in the long-wavelength spectral region, suggesting that lower-energy excitation photochemistry is conformationally controlled. We postulate that the observed sudden change in the relative efficiency of cis– trans vs. ring-closure photoisomerization of previtamin D around 300 nm reflects the opening of new excited state dynamical channel, possibly involving intersystem crossing of the system to excited triplet states.