Title: The role of tachysterol in vitamin D photosynthesis - A non-adiabatic molecular dynamics study,
Abstract: To investigate the role of tachysterol in the photophysical/chemical
regulation of vitamin D photosynthesis, we studied its electronic absorption
properties and excited state dynamics using time-dependent density functional
theory (TDDFT), coupled cluster theory (CC2), and non-adiabatic molecular
dynamics. In excellent agreement with experiments, the simulated electronic
spectrum shows a broad absorption band covering the spectra of the other
vitamin D photoisomers. The broad band stems from the spectral overlap of four
different ground state rotamers. After photoexcitation, the first excited
singlet state (S1) decays within 882 fs. The S1 dynamics is characterized by a
strong twisting of the central double bond. 96% of all trajectories relax
without chemical transformation to the ground state. In 2.3 % of the
trajectories we observed [1,5]-sigmatropic hydrogen shift forming the partly
deconjugated toxisterol D1. 1.4 % previtamin D formation is observed via
hula-twist double bond isomerization. We find a strong dependence between
photoreactivity and dihedral angle conformation: hydrogen shift only occurs in
cEc and cEt rotamers and double bond isomerization occurs mainly in cEc
rotamers. Our study confirms the hypothesis that cEc rotamers are more prone to
previtamin D formation than other isomers. We also observe the formation of a
cyclobutene-toxisterol in the hot ground state (0.7 %). Due to its strong
absorption and unreactive behavior, tachysterol acts mainly as a sun shield
suppressing previtamin D formation. Tachysterol shows stronger toxisterol
formation than previtamin D. Absorption of low energy UV light by the cEc
rotamer can lead to previtamin D formation. Our study reinforces a recent
hypothesis that tachysterol can act as a previtamin D source when only low
energy ultraviolet light is available, as it is the case in winter or in the
morning and evening hours of the day.