Ring Opening in the Dehydrocholesterol−Previtamin D System Studied by Ultrafast Spectroscopy

Abstract

The rate of the electrocyclic ring opening of 7-dehydrocholesterol to previtamin D is investigated by transient absorption spectroscopy with a time resolution better than 300 fs. The dehydrocholesterol, which is a derivative of 1,3-cyclohexadiene, is excited around 267 nm. The primary product, the s-cis,Z,s-cis conformer of the triene previtamin D, appears with a time constant of 5.2 ps. This result is consistent with literature data. We found that it is temperature independent. So there is no activation energy for this electrocyclic ring opening. A barrierless process, on the other hand, is expected to proceed much faster (in about 10 -13 s), unless there is an entropy of activation. The results suggest that the molecule reaches within a few femtoseconds the lowest excited state (2A 1 ) of the product, from where it goes on through an entropic bottleneck. The primary product isomerizes thermally to the stable s-cis,Z,s-trans conformer of previtamin D within 125 ps in ethanol at room temperature. This time is much longer than the reported corresponding time in the cyclohexadiene/ hexatriene system. We found that it depends on the solvent viscosity and on the temperature. The activation energy was determined to be 15.5 ( 1.0 kJ/mol.

Low-Temperature Photochemistry of Previtamin D: A Hula-Twist Isomerization of a Triene

“…Furthermore it can be concluded that P is more stable than P 0 ; this is consistent with the observed isomerization P 0 3P [3] and with some results [4,5] of the force-field calculations.…”

“…[3] The fact that we obtained a different product than Havinga et al [1] implies that the starting compounds were different, too. Since the reaction we observed corresponds to P 0 3T, the process they observed…”

“…Kinetic measurements on the previtamin D system indicate that thermal reactions are frozen out in the cold matrix. [3] Not the activation energy (16 kJ mol…”

GPU-Accelerated State-Averaged Complete Active Space Self-Consistent Field Interfaced with Ab Initio Multiple Spawning Unravels the Photodynamics of Provitamin D3

“…Aside from its obvious biological importance, Pro is a model system for understanding how to optically control polyene chromophores, which has potential applications in nanoscience. 52 This reaction has been studied both experimentally [53][54][55][56][57][58][59][60][61][62] and theoretically. [63][64] Critical points on Pro potential energy surfaces are represented in Figure 1 (SA-3-CAS(6,4)/6-31G, see SI for details and validation of the electronic structure employed).…”

The role of tachysterol in vitamin D photosynthesis – a non-adiabatic molecular dynamics study

“…Pre itself plays the central role, as it reacts to four distinct main Vita photo isomers (DPI) Pro, Lumi, and Tachy and toxisterols (Toxi). While the provitamin D ringopening and its analogous reaction in cyclohexadiene has been studied extensively, both theoretically [17][18][19][20] and experimentally, 18,[21][22][23][24][25][26] little research has been done to assess the influence of tachysterol on photochemical vitamin D production. In particular, its excited state dynamics has neither been investigated by experiments nor by simulations.…”