Conformer-specific photoconversion of 25-hydroxytachysterol to 25-hydroxyprevitamin D3: role in the production of vitamin Ds

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

On photoisomerization of previtamin D - a steroid Z-triene - produced in situ by ring opening of 7-dehydrocholesterol in a cold matrix, it was found in A. M. Müller et al. [Angew. Chem., Int. Ed., 1998, 37, 505-507] that the product (tachysterol) had rotated not only its central double bond but also an adjacent single bond. This is called a Hula twist (HT) due to the alternative description, in which it is just one central CH group that rotates. It was pointed out that the results directly support the calculated molecular structure at a conical intersection, which mediates the Z-E isomerization of polyenes. With a more sophisticated technique, Saltiel et al. (J. Phys. Chem. Lett., 2013, 4, 716-721) confirmed this tachysterol rotamer as the main product but found two additional conformers. They believed to have seen also three previtamin D conformers, suggested to be a result of hot-ground-state reactions from the primary rotamer, and interpreted all tachysterol products to be a result of a double-bond twist (DBT), not a HT. On the basis of published circular dichroism data and consideration of other reactions, it is here shown that under these conditions hot-ground-state reactions are unimportant or even negligible and that there is practically only a single conformer of previtamin D after ring opening. All products can be easily understood on the basis of an HT-type conical intersection, which is thus further supported. Invoking a published pretwist model even rationalizes product ratios. The two twists in HT are concerted. Furthermore HT is fully consistent with the NEER principle (nonequilibration of excited rotamers) and even offers additional possibilities for conformer control.

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

To investigate the role of tachysterol in the regulation of vitamin D photosynthesis, we studied its absorption properties and photodynamics by ab initio methods and non-adiabatic molecular dynamics.

Vitamin D: Photobiological and Ecological Aspects

Vitamin D was discovered as a result of its ability to cure rickets, but recently a wide range of other functions for it in the human body has been suggested. Vitamin D is not a vitamin in the strict sense as it can be synthesised in the human body following exposure of the skin to ultraviolet radiation. Provitamin D (7-dehydrocholesterol) is converted to previtamin D which is further modified by a series of reactions to the active form, 1,25-dihydroxyvitamin D. This Chapter summarises the discovery of vitamin D and reviews the chemistry and photochemistry of its precursors, transformations and metabolites. The production of vitamin D in various human populations is described, and how to assess vitamin D status. The skeletal and non-skeletal effects of vitamin D are discussed, particularly its role in immunomodulation with consequences for protection against a variety of human diseases. The Chapter concludes with evolutionary aspects, the occurrence and role of vitamin D in the plant kingdom, biogeographical considerations, and the nonphotochemical production of vitamin D in certain plants.

Bicycle pedal photoisomerization of 1-phenyl-4-(4-pyridyl)-1,3-butadienes in glassy isopentane at 77 K

In glassy isopentane at 77 K, 1-phenyl-4-(4-pyridyl)-cis-1,cis-3-butadiene (cc-PPyB) and 1-phenyl-4-(4-pyridyl)-cis-1,trans-3-butadiene (ct-PPyB) can undergo simultaneous two-bond photoisomerization. Under the same conditions, 1-phenyl-4-(4-pyridyl)-trans-1,cis-3-butadiene (tc-PPyB) gives tt-PPyB, the ultimate photoproduct in all cases.

Feasibility of ionic liquids as extractants for selective separation of vitamin D₃ and tachysterol₃ by solvent extraction

A selective separation of vitamin D₃ and tachysterol₃ by solvent extraction with 7 organic solvents and 11 ionic liquids (ILs) has been reported. Among organic solvents sulfolane showed optimal extraction performance, giving only a selectivity of 1.44 for tachysterol₃ over vitamin D₃. ILs with unsaturated bonds demonstrated high selectivity probably due to their different π-π interactions with the two compounds. A pyrrolidinium-based ionic liquid, for example, [BMPr][NTf2], provided the highest selectivity up to 1.77. Acceptable selectivity and distribution coefficients were observed by a combination of organic solvents and ILs as extracting agents. In this work, the effects of concentrations, anions, cations, and substituent of ILs were investigated, which may provide a rational strategy for the design of novel ILs for extractive separation of structural analogues. The purification and recovery of vitamin D₃3 via continuous multistage extractions were simulated, indicating that IL-based liquid-liquid extraction might be superior to traditional organic solvents in practical production.

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.

Unravelling the details of vitamin D photosynthesis by non-adiabatic molecular dynamics simulations

We investigate the photodynamics of vitamin D derivatives by a fully analytical implementation of the linear response time-dependent density functional theory surface hopping method (LR-TDDFT-SH). Our study elucidates the dynamics of the processes involved in vitamin D formation at the molecular level and with femtosecond resolution. We explain the major experimental findings and provide new insights that cannot directly be obtained from experiments: firstly, we investigate the dynamics of the photoinduced ring-opening of provitamin D (Pro) and cyclohexadiene (CHD) and the subsequent rotational isomerization. In agreement with recent experiments and CC2 calculations, only the bright S(1) state is involved in the ring-opening reaction. Our calculations confirm the experimentally reported 5 : 1 ratio between the excited state lifetimes of Pro and CHD. The longer lifetimes of Pro are attributed to steric constraints of the steroid skeleton and to temperature effects, both emerging directly from our simulations. For CHD and Pro, we present an explanation of the biexponential decay recently reported by Sension and coworkers [Tang et al., J. Phys. Chem., 2011, 134, 104503]: our calculations suggest that the fast and slow components arise from a reactive and an unreactive reaction pathway, respectively. Secondly, we assess the wavelength dependent photochemistry of previtamin D (Pre). Using replica exchange molecular dynamics we sample the Pre conformers present at thermal equilibrium. Based on this ensemble we explain the conformation dependent absorption and the essential features of Pre photochemistry. Consistent with the experiments, we find ring-closure to occur mostly after excitation of the cZc conformers and at lower energies, whereas Z/E isomerization of the central double bond preferably occurs after excitation at higher energies. For the isomerization we provide the first theoretical evidence of the proposed hula-twist mechanism. Our results show that LR-TDDFT-SH is a highly valuable tool for studying the photochemistry of moderately large systems, even though challenges remain in the vicinity of conical intersections.

Electronic design criteria for O-O bond formation via metal-oxo complexes

Metal-oxos are critical intermediates for the management of oxygen and its activation. The reactivity of the metal-oxo is central to the formation of O-O bonds, which is the essential step for oxygen generation. Two basic strategies for the formation of O-O bonds at metal-oxo active sites are presented. The acid-base (AB) strategy involves the attack of a nucleophilic oxygen species (e.g., hydroxide) on an electrophilic metal-oxo. Here, active-site designs must incorporate the assembly of a hydroxide (or water) proximate to a high-valent metal-oxo of even d electron count. For the radical coupling (RC) strategy, two high-valent metal-oxos of an odd d electron count are needed to drive O-O coupling. This Forum Article focuses on the different electronic structures of terminal metal-oxos that support AB and RC strategies and the design of ligand scaffolds that engender these electronic structures.

Steroids: reactions and partial synthesis

This article reviews the progress in the chemistry of the steroids that was published between January and December 2003. The reactions and partial synthesis of estrogens, androgens, pregnanes, cholic acid derivatives, cholestanes and vitamin D analogues are covered. There are 152 references.