Triple hydrogen bonding in a circular arrangement: ab initio, DFT and first-principles MD studies of tris-hydroxyaryl enamines

First-principles Car-Parrinello molecular dynamics, ab initio (MP2) and density functional schemes have been used to explore the tautomeric equilibrium in three tris(amino(R)methylidene)cyclohexane-1,3,5-triones (R?=?hydrogen, methyl or phenyl group). The dynamic nature of the cyclic hydrogen bonding has been studied by the first-principles MD method. The comparison of the results obtained by aforesaid methods has been accomplished on the basis of calculations of structural and spectroscopic characteristics of the compounds. The conformational analysis of the studied compounds has been carried out at the MP2/6-31+G(d,p) and B3LYP/6-31+G(d,p) levels of theory. The influence of steric and electronic effects on the cyclic hydrogen bonding has been analysed. The extent of the proton delocalization has been modified by the substituents according to the sequence: hydrogen?<?phenyl?<?methyl. This fact is verified by the spectroscopic and structural data as well as the energy potential curve. A prevalence of the keto-enamine tautomeric form has been observed in the static ab initio and DFT models, and confirmed by the first-principles MD.     

Development and validation of a reversed-phase HPLC method for determination of alkaloids from Papaver somniferum L. (Papaveraceae)

An HPLC method for the separation of six target alkaloids from Papaver somniferum L. (morphine, codeine, oripavine, thebaine, papaverine, and noscapine) was developed, optimized, and validated. The chromatographic behavior of these alkaloids was investigated using a reversed-phase chromatography at acidic and alkaline pH. The effects of ion-pairing agents, pH value of the mobile phase, concentration of the buffer components, mobile phase organic modifier, and column temperature were studied. Regardless of the large differences in their pKa values, all alkaloids were separated within a close retention window, and good peak shape was achieved for each of the six alkaloids. The proposed method has adequate selectivity, linearity, accuracy, precision, and reproducibility and is applicable for poppy straw.     

Determination of benzoate,sorbate, citrate and orthophosphate ions in beverage samples using two-dimensional isotachophoretic method

A two-dimensional capillary isotachophoretic method (cITP-cITP) using electrolyte system consisting of leading electrolytes (LE1): [10 mM HCl + β-alanine (pH 3.9) + 0.1% hydroxyethylcellulose (HEC)] and (LE2): [10 mM HCl + aminocaproic acid (pH 5.00) + 0.1% HEC], and 5 mM caproic acid as terminating electrolyte (TE) was studied. Two methods of detection, conductometric and UV-Vis, were applied to the determination of selected food preservatives and additives. Practical applicability was demonstrated by simultaneous determination of benzoates, sorbates, citrates and orthophosphates in 12 samples of beverages. The proposed method revealed linearity with R ² between 0.9992 and 0.9999 for the concentration ranges: 10–100 mg/L (orthophosphate and citrate ions), 20–100 mg/L (sorbates) and 40–120 mg/L for benzoates. The detection limits for all studied ions were from 0.85 to 3.1 mg/L whereas the quantification ones were from 2.8 to 10 mg/L. The variation coefficients for five-fold analysis of all ions ranged between 0.4 and 9.1%. Obtained recoveries (from 97 to 104%) confirmed satisfactory accuracy of the proposed cITP-cITP method for the determination of tested food additives.     

Determination of the N-methyl stereochemistry in tropane and granatane derivatives in solution: a computational and NMR spectroscopic study

Tropane (8-methyl-8-azabicyclo[3.2.1]octane) and granatane (9-methyl-9-azabicyclo[3.3.1]nonane) derivatives undergo fast N-methyl inversion. The distribution of axial and equatorial N-methyl invertomers of protonated and free amine forms in aqueous and methanol solutions was studied by room and low temperature NMR spectroscopy on tropinone, tropine, granatan-3-one (pseudopelletierine or pseudopelletrierin) and α-granatan-3-ol. Theoretical (DFT) distributions in both gas phase and solutions are also reported. Applicability of the computational model for the studied system was verified by comparison with invertomer distributions inferred from inverse-gated ¹³C NMR experiments. Among the tested functionals, the BH&HLYP/cc-pVDZ method yields the best agreement with experiment. Moreover, accounting for orbital relaxation upon immersion in the solution was found to be of importance in order to properly reproduce observed distributions.     

Investigations on the interaction of dichloroaluminum carboxylates with Lewis bases and water: an efficient road toward oxo- and hydroxoaluminum carboxylate complexes

A series of dichloroaluminum carboxylates [Cl(2)Al(O(2)CR)](2) (were R = Ph (1a), (t)Bu (1b), CHCH(2) (1c) and C(11)H(23) (1d)) were prepared and extended investigations on their structure and reactivity toward various Lewis bases and H(2)O performed. Compounds [Cl(2)Al(O(2)CR)](2) and their adducts with Lewis bases show a large structural variety, featuring both molecular and ionic forms with different coordination numbers of the metal center and various coordination modes of the carboxylate ligand. Upon addition of a Lewis base of moderate strength the molecular form [Cl(2)Al(O(2)CR)](2) equilibrates with new ionic forms. In the presences of 4-methylpyridine the six-coordinate Lewis acid-base adducts [Cl(2)Al(λ(2)-O(2)CR)(py-Me)(2)] [R = Ph (3a), (t)Bu (3b)] with a chelating carboxylate ligand were formed. The reactions of 1a, 1b, and 1d with 0.33 equiv of H(2)O in THF-toluene solution lead to oxo carboxylates [(Al(3)O)(O(2)CR)(6)(THF)(3)] [AlCl(4)] [where R = Ph (4a(THF)), (t)Bu (4b(THF)), and C(11)H(23) (4d(THF))] in high yield. The similar reaction of 1c in tetrahydrofuran (THF) afforded the chloro(hydroxo)aluminum acrylate [(ClAl)(2)(OH)(O(2)CC(2)H(3))(2) (THF)(4)][AlCl(4)] (5), while the hydrolysis of 1b in MeCN lead to the hydroxoaluminum carboxylate [Al(2)(OH)(O(2)C(t)Bu)(2)(MeCN)(6)][AlCl(4))(3)] (6). All compounds were characterized by elemental analysis, (1)H, (27)Al NMR, and IR spectroscopy, and the molecular structure of 1a, 3a, 3b, 4a(THF), 4b(THF), 4b(py-Me'), 5, and 6 were determined by single-crystal X-ray diffraction. The study provides a platform for testing transformations of secondary building units in Al-Metal-Organic Frameworks toward H(2)O and neutral donor ligands.     

Chain Geometry Determined by the Affine Group

Chain geometry associated with an affine group and with a linear group is studied. In particular, closely related to the respective chain geometries affine partial linear spaces and generalizations of sliced spaces are defined. The automorphisms of thus obtained structures are determined.     

Application of microextraction by packed sorbent to isolation of psychotropic drugs from human serum

A method of microextraction by packed sorbent (MEPS) followed by liquid chromatography with diode array detection has been developed and optimized for the extraction of six tricyclic antidepressants (amitriptyline, nortriptyline, imipramine, desipramine, doxepin, nordoxepin) from human serum. The optimal parameters of MEPS extraction (type of sorbent, volume of sample, composition, and volume of washing and elution solutions) for these drugs in spiked samples were defined. The developed MEPS procedure was validated and then successfully applied to the analysis of serum reference material. The limit of detection (0.02–0.05 μg/mL), intraday (2.7–8.8%) and interday (4.4–11.6%) precision (RSD), and the accuracy of the assay (94.5–108.8%) at three concentration levels—0.2, 0.5, and 0.8 μg/mL—were estimated. The accuracy of the method was evaluated by the analysis of certified reference material. Moreover, the validated procedure was compared with the solid-phase extraction technique. Finally, microextraction by packed sorbent was assessed as a suitable tool in forensic and clinical methods for serum sample preparations.     

Stochastic switching in systems with rare and hidden attractors

Complex biochemical networks are commonly characterised by the coexistence of multiple stable attractors. This endows living systems with plasticity in responses under changing external conditions, thereby enhancing their probability for survival. However, the type of such attractors as well as their positioning can hinder the likelihood to randomly visit these areas in phase space, thereby effectively decreasing the level of multistability in the system. Using a model based on the Hodgkin–Huxley formalism with bistability between a silent state, which is a rare attractor, and oscillatory bursting attractor, we demonstrate that the noise-induced switching between these two stable attractors depends on the structure of the phase space and the disposition of the coexisting attractors to each other.