Probing the Formation and Conformational Relaxation of Previtamin D3 and Analogues in Solution and in Lipid Bilayers.

The photosynthesis of vitamin D3 in mammalian skin results from UV-B irradiation of provitamin D3 (7-dehydrocholesterol, DHC) at ca. 290 nm. Upon return to the ground state, the hexatriene product, previtamin D3, undergoes a conformational equilibration between helical gZg and more planar tZg and tZt forms. The helical gZg forms provide a pathway for the formation of vitamin D3 via a [1,7]-sigmatropic hydrogen shift. Steady state photolysis and UV transient absorption spectroscopy are combined to explore the conformational relaxation of previtamin D3 formed from DHC in isotropic solution and confined to lipid bilayers chosen to model the biological cell membrane. The results are compared with measurements for two analogues: previtamin D2 formed from ergosterol (provitamin D2) and previtamin D3 acetate formed from DHC acetate. The resulting spectral dynamics are interpreted in the context of simulations of optical excitation energy and oscillator strength as a function of conformation. In solution, the relaxation dynamics and steady state product distributions of the three compounds are nearly identical, favoring tZg forms. When confined to lipid bilayers, the heterogeneity and packing forces alter the conformational distributions and enhance the population of a gZg conformer capable of vitamin D formation.

Rhodamine B degradation by nanosized zeolitic imidazolate framework-8 (ZIF-8).

Adsorption and photodegradation of rhodamine B by ZIF-8 nanocrystals were studied using spectroscopic techniques coupled with density functional theory (DFT) cluster calculations. A fast adsorption rate was observed in the dark, but upon exposure to visible light or UV irradiation the adsorption rate noticeably increased. Although several studies previously reported this phenomenon involving bulk ZIF-8 powder, this is the first mechanistic study to our knowledge that demonstrates adsorption and degradation of rhodamine B by nanosized ZIF-8 under various light conditions. The combined study of N2 sorption pore analysis and surface-sensitive X-ray photoelectron spectroscopy (XPS) confirmed the surface adsorption was mainly due to the open metal sites and surface groups of nanoporous ZIF-8. The fluorescence studies suggested ZIF-8 nanocrystals were able to generate hydroxyl radicals in water but only under UV illumination. The work presented here provides an insight into understanding nanoscale metal-organic frameworks (MOFs) in the removal of organic molecules from wastewater.

Azospirillum brasilense Increases CO2 Fixation on Microalgae Scenedesmus obliquus, Chlorella vulgaris, and Chlamydomonas reinhardtii Cultured on High CO2 Concentrations.

Mutualism interactions of microalgae with other microorganisms are widely used in several biotechnological processes since symbiotic interaction improves biotechnological capabilities of the microorganisms involved. The interaction of the bacterium Azospirillum brasilense was assessed with three microalgae genus, Scenedesmus, Chlorella, and Chlamydomonas, during CO2 fixation under high CO2 concentrations. The results in this study have demonstrated that A. brasilense maintained a mutualistic interaction with the three microalgae assessed, supported by the metabolic exchange of indole-3-acetic acid (IAA) and tryptophan (Trp), respectively. Besides, CO2 fixation increased, as well as growth and cell compound accumulation, mainly carbohydrates, in each microalgae evaluated, interacting with the bacterium. Overall, these results propose the mutualism interaction of A. brasilense with microalgae for improving biotechnological processes based on microalgae as CO2 capture and their bio-refinery capacity.

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

To investigate the role of tachysterol in the photophysical/photochemical regulation of vitamin D photosynthesis, we studied its electronic absorption properties and excited state dynamics using time-dependent density functional theory (TDDFT), second-order approximate coupled cluster theory (CC2), and non-adiabatic surface hopping molecular dynamics in the gas phase. In excellent agreement with experiments, the simulated electronic spectrum shows a broad absorption band with a remarkably higher extinction coefficient than the other vitamin D photoisomers provitamin D, lumisterol, and previtamin D. The broad band arises from the spectral overlap of four different ground state rotamers. After photoexcitation, the first excited singlet state (S1) decays with a lifetime of 882 fs. The S1 dynamics is characterized by a strong twisting of the central double bond. In 96% of all trajectories this is followed by unreactive relaxation to the ground state near a conical intersection. The double-bond twisting in the chemically unreactive trajectories induces a strong interconversion between the different rotamers. 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. In both reaction channels, 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. Hence, our study confirms the previously formed hypothesis that cEc rotamers are more prone to previtamin D formation than other isomers. In addition, we also observe the formation of a cyclobutene-toxisterol in the hot ground state in 3 trajectories (0.7%). Due to its large extinction coefficient and mostly unreactive behavior, tachysterol acts mainly as a Sun shield suppressing previtamin D formation. Tachysterol shows stronger toxisterol formation than previtamin D and can thus be seen as the major degradation route of vitamin D. Absorption of low energy ultraviolet light by the cEc rotamer can lead to previtamin D formation. In addition, the cyclobutene-toxisterol, which possibly reacts thermally to previtamin D, is also preferably formed at long wavelengths. These two mechanisms are consistent with the wavelength dependent photochemistry found in experiments. Our study reinforces a recent hypothesis that tachysterol constitutes a source of previtamin D 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.