Lumisterol to Tachysterol Photoisomerization in EPA Glass at 77 K. A Comparative Study

We present a comparative study of the photoisomerizations of lumisterol (Lumi), previtamin (Pre), and provitamin D3 (Pro) to tachysterol (Tachy) at 77 K in EPA (5:5:2 ether, isopentane, and ethanol by volume) glass. Fluorescence, fluorescence excitation, and UV spectra, measured in the course of these reactions, were analyzed using singular value decomposition with self-modeling (SVD-SM). This represents an extension of previous work that led to the conclusion that in the EPA glass Pre exists as an s-cis,s-cis-conformer (cZc-Pre) which gives, exclusively, an unstable s-cis,s-cis-conformer of Tachy (cEc-Tachy) and Pro gives mainly the tEc-Tachy, that corresponds to a stable s-trans,s-cis-conformer. (Redwood, C.; et al. J. Phys. Chem. Lett. 2013, 4, 716−721.) The surprising result was that the major Pre photoproduct from Pro also has a tZc-Pre conformation instead of the expected cZc-Pre conformation. Accordingly, the Pre to Tachy cis–trans photoisomerization proceeds via a conformer specific one-bond-twist...