Photoisomerization of Pre- and Provitamin D3 in EPA at 77 K: One-Bond-Twist, Not Hula-Twist.

The photoisomerizations of previtamin D3 (Pre) and provitamin D3 (Pro) in EPA at 77 K were monitored using fluorescence spectroscopy. In the glassy EPA medium equilibrated tachysterol (Tachy) exists as a mixture of three conformers, the major of which we assign to s-trans,s-cis- and s-cis,s-cis-conformers (tEc- and cEc-Tachy). By contrast, Pre exists exclusively as the s-cis,s-cis-conformer (cZc-Pre) and undergoes cis → trans photoisomerization to cEc-Tachy. Light-induced ring-opening of Pro gives three Pre conformers, the major of which is tZc-Pre instead of the expected cZc-Pre. Curve resolution based on singular value decomposition yields the fluorescence spectra of the conformers. Structural assignments are based on experimental and theoretical evidence showing that the s-cis,s-cis-conformers of Pre and Tachy absorb UV light to the red of s-trans-s-cis-conformers. The photoisomerizations of tZc- and cZc-Pre in EPA proceed by the one bond twist (OBT) mechanism to give tEc- and cEc-Tachy, respectively.