Carbon and proton magnetic resonance studies on the sites of Mn2+ and Ni2+ ion binding with thiamin and thiamin pyrophosphate and on the solution conformation of the coenzyme

Carbon magnetic resonance was employed to determine the Mn²⁺ and Ni²⁺ binding sites on thiamin and thiamin pyrophosphate and to help elucidate the solution conformation of the latter with Mn²⁺. In comparison with previous proton magnetic resonance studies, the results here presented elucidate direct binding to thiamin even without the presence of the pyrophosphate. In thiamin Mn²⁺ is bound to both the pyrimidine ring and to the oxygen of the β-hydroxyethyl chain, albeit more weakly to the former, Ni²⁺, on the other hand is indicated in binding to the pyrimidine ring only. From observation of water relaxation data, potentiometric determination of the pyrimidinium pK_a with metal ions added and carbon magnetic resonance (competition of thiamin and thiamin pyrophosphate for Mn²⁺) we have very strong evidence to indicate that Mn²⁺ is separated from the pyrimidine ring by a solvent molecule. We were able to estimate association constant between the pyrimidine and Mn²⁺ (50 M^?1) and OH and Mn²⁺ (100 M^?1) in thiamin.In thiamin pyrophosphate both Mn²⁺ and Ni²⁺ much prefer the pyrophosphate as binding sites and at least in the former an inner sphere complex is indicated. Also with Mn²⁺ present the dimethylene pyrophosphate is folded back onto the thiazolium ring according to analysis of proton broadening data and relaxation mechanisms.     

A DFT study of solvation effects on the tautomeric equilibrium and catalytic ylide generation of thiamin models

Thiamin diphosphate (ThDP) is the biologically active form of vitamin B1 and an essential cofactor for a number of enzymes. The effect of solvent polarity on the tautomeric equilibria of ThDP using three model systems of the 4'-aminopyrimidine ring is studied by density functional theory calculations (B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)) in the gas phase and selected solvents (cyclohexane, ether, dichloroethane, and water). Solvation effects are investigated using three different schemes: implicit solvation by a continuum model, explicit solvation by inclusion of three water molecules mimicking the first solvation shell of the enzymatic environment, and by a mixed implicit/explicit solvation model. The 4'-aminopyrimidine tautomer is more stable than the 1',4'-iminopyrimidine tautomer in all solvation schemes employed; however, the trend for the stabilities of the 1',4'-iminopyrimidine tautomer in the solvents depends on the specific ThDP-model. Formation of the catalytic important ylide for ThDP-dependent enzymes by deprotonation of ThDP(C2) is also investigated by localization of transition states for two possible pathways. Only the less stable tautomer, 1',4'-iminopyrimidine ThDP, is able to form the catalytic active ylide. Generation of the ylide through a direct intramolecular proton transfer from ThDP(C2) to the ThDP(N4') nitrogen lone pair is favored by 6 kcal/mol in the gas phase, as compared to a water-mediated ylide generation. However, inclusion of a dielectric medium reduces this difference dramatically. Furthermore, inclusion of two water molecules to model the apoenzymatic environment lowers the activation energies of both direct and water-mediated ylide generation.     

系列双磷维生素B1离子盐化合物的合成、结构及分子构象

将Ni(ClO4)2和NH4PF6分别与硫胺素焦磷酸在甲醇体系中反应, 得到了两个新的离子盐型化合物[TPP·ClO4·H2O](1)和[TPP·PF6·CH3OH](2)(TPP为硫胺素焦磷酸酯). 通过元素分析、红外光谱及X射线衍射等方法对它们进行了表征. 结构分析表明, 它们属于离子型化合物, 而且硫胺素焦磷酸与高氯酸根, 六氟磷酸根形成了大量的氢键网络结构. 结合计算结果进一步分析了化合物的活性及电子结构特征.     

Radiosynthesis of 5-[18F]Fluoro-1,2,3-triazoles through Aqueous Iodine–[18F]Fluorine Exchange Reaction

In this report, a simple and efficient process to achieve fluorine-18-labeled 1,2,3-triazole is reported. The heteroaromatic radiofluorination was successfully achieved through an iodine–fluorine-18 exchange in an aqueous medium requiring only trace amounts of base and no azeotropic drying of fluorine-18. This methodology was optimized on a model reaction and further validated on multiple 1,2,3-triazole substrates with 18–60% radiochemical conversions. Using this strategy—the radiosynthesis of a triazole-based thiamin analogue—a potential positron emission tomography (PET) probe for imaging thiamin-dependent enzymes was synthesized with 10–16% isolated radiochemical yield (RCY) in 40 min (uncorrected, n > 5).