1α,25(OH)2-3-Epi-Vitamin D3, a Natural Physiological Metabolite of Vitamin D3: Its Synthesis, Biological Activity and Crystal Structure with Its Receptor

Background: The 1 alpha,25-dihydroxy-3-epi-vitamin-D(3) (1 alpha,25(OH)(2)-3-epi-D(3)), a natural metabolite of the seco-steroid vitamin D(3), exerts its biological activity through binding to its cognate vitamin D nuclear receptor (VDR), a ligand dependent transcription regulator. In vivo action of 1 alpha,25(OH)(2)-3-epi-D(3) is tissue-specific and exhibits lowest calcemic effect compared to that induced by 1 alpha,25(OH)(2)D(3). To further unveil the structural mechanism and structure-activity relationships of 1 alpha,25(OH)(2)-3-epi-D3 and its receptor complex, we characterized some of its in vitro biological properties and solved its crystal structure complexed with human VDR ligand-binding domain (LBD). Methodology/Principal Findings: In the present study, we report the more effective synthesis with fewer steps that provides higher yield of the 3-epimer of the 1 alpha,25(OH)(2)D(3). We solved the crystal structure of its complex with the human VDR-LBD and found that this natural metabolite displays specific adaptation of the ligand-binding pocket, as the 3-epimer maintains the number of hydrogen bonds by an alternative water-mediated interaction to compensate the abolished interaction with Ser278. In addition, the biological activity of the 1 alpha,25(OH)(2)-3-epi-D(3) in primary human keratinocytes and biochemical properties are comparable to 1 alpha,25(OH)(2)D(3). Conclusions/Significance: The physiological role of this pathway as the specific biological action of the 3-epimer remains unclear. However, its high metabolic stability together with its significant biologic activity makes this natural metabolite an interesting ligand for clinical applications. Our new findings contribute to a better understanding at molecular level how natural metabolites of 1 alpha,25(OH)(2)D(3) lead to significant activity in biological systems and we conclude that the C3-epimerization pathway produces an active metabolite with similar biochemical and biological properties to those of the 1 alpha,25(OH)(2)D(3)

Microstructure and thermal stability of electrodeposited nanocrystalline nickel

Nanocrystalline nickel deposits have been prepared using electrodeposition in nickel sulphate based baths. The samples were characterised using microhardness testing, X-ray diffraction (XRD) and transmission electron microscopy (TEM).As-deposited samples showed a narrow grain size distribution with an average grain size of less than 20nm. The greatly increased microhardness of nanocrystalline materials, similar to 500kg/mm(2) for the Ni samples in this study, and their other superior properties have many potential applications. However, the microstructure of nanocrystals is metastable, and grain growth occuring at elevated temperatures can lead to a deterioration of properties.In this work, we have studied grain growth in nanocrystalline Ni annealed at 250degreesC. TEM micrographs revealed that the type of growth is abnormal. 3D atom probe analysis gave no indication of gain boundary segregation in as-deposited materials, and the low levels of segregation found in material annealed for 60 minutes were clearly not sufficient to significantly inhibit grain growth.Upon annealing, an increase in hardness up to about 580kg/mm(2) occurred within the first 5 minutes. Despite fast abnormal grain growth inside the nanocrystalline Ni, the high hardness values were maintained for a surprisingly long time. Only after about 240 minutes when the volume fraction of abnormal grains was above 50%, did the hardness start to decrease significantly. A two-phase model is proposed to relate the hardness data to the observed microstructure