Previtamin D: Z-E photoisomerization via a Hula-twist conical intersection

Area of Science:

• Photochemistry
• Organic Chemistry
• Molecular Dynamics

Background:

• Photoisomerization of previtamin D, a steroid Z-triene, is crucial for understanding steroid chemistry.
• Previous studies proposed different mechanisms, including Hula twist (HT) and double-bond twist (DBT), for tachysterol formation from 7-dehydrocholesterol.
• Conflicting interpretations exist regarding the role of hot-ground-state reactions and the number of previtamin D conformers.

Purpose of the Study:

• To re-evaluate the mechanism of previtamin D photoisomerization.
• To clarify the role of Hula twist (HT) versus double-bond twist (DBT) in tachysterol formation.
• To assess the significance of hot-ground-state reactions and previtamin D conformers.

Main Methods:

• Analysis of published circular dichroism data.
• Consideration of related chemical reactions.
• Theoretical modeling and mechanistic interpretation.

Main Results:

• Hot-ground-state reactions are negligible under the studied conditions.
• Only a single conformer of previtamin D is observed after ring opening.
• All observed tachysterol products are consistent with the Hula twist (HT) mechanism via a conical intersection.

Conclusions:

• The Hula twist (HT) mechanism is strongly supported for previtamin D photoisomerization.
• The NEER (nonequilibration of excited rotamers) principle is consistent with the HT mechanism.
• The HT mechanism offers potential for controlling conformer populations in photochemical reactions.