Nonaqueous capillary electrophoresis and quantum chemical calculations applied to investigation of acid-base and electromigration properties of azahelicenes

Determination of physicochemical parameters of (bio)molecules and (bio)particles by capillary electromigration methods

The review provides an overview of recent developments and applications of capillary electromigration (CE) methods for the determination of important physicochemical parameters of various (bio)molecules and (bio)particles. These parameters include actual and limiting (absolute) ionic mobilities, effective electrophoretic mobilities, effective charges, isoelectric points, electrokinetic potentials, hydrodynamic radii, diffusion coefficients, relative molecular masses, acidity (ionization) constants, binding constants and stoichiometry of (bio)molecular complexes, changes of Gibbs free energy, enthalpy and entropy and rate constants of chemical reactions and interactions, retention factors and partition and distribution coefficients. For the determination of these parameters, the following CE methods are employed: zone electrophoresis in a free solution or in sieving media, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography. In the individual sections, the procedures for the determination of the above parameters by the particular CE methods are described.

Acidity constants and protonation sites of cyclic dinucleotides determined by capillary electrophoresis, quantum chemical calculations, and NMR spectroscopy

Cyclic dinucleotides (CDNs) are important second messengers in bacteria and eukaryotes. Detailed characterization of their physicochemical properties is a prerequisite for understanding their biological functions. Herein, we examine acid-base and electromigration properties of selected CDNs employing capillary electrophoresis (CE), density functional theory (DFT), and nuclear magnetic resonance (NMR) spectroscopy to provide benchmark pKa values, as well as to unambiguously determine the protonation sites. Acidity constants (pKa) of the NH+ moieties of adenine and guanine bases and actual and limiting ionic mobilities of CDNs were determined by nonlinear regression analysis of the pH dependence of their effective electrophoretic mobilities measured by CE in aqueous background electrolytes in a wide pH range (0.98-11.48), at constant temperature (25°C), and constant ionic strength (25 mM). The thermodynamic pKa values were found to be in the range 3.31-4.56 for adenine and 2.28-3.61 for guanine bases, whereas the pKa of enol group of guanine base was in the range 10.21-10.40. Except for systematic shifts of ∼2 pKa, the pKa values calculated by the DFT-D3//COSMO-RS composite protocol that included large-scale conformational sampling and "cross-morphing" were in a relatively good agreement with the pKas determined by CE and predict N1 atom of adenine and N7 atom of guanine as the protonation sites. The protonation of the N1 atom of adenine and N7 atom of guanine in acidic background electrolytes (BGEs) and the dissociation of the enol group of guanine in alkaline BGEs was confirmed also by NMR spectroscopy.

Synthesis of Helical Indolophenanthridines Showing Aggregation-Induced Emission

Helical indolo[2,3-k]- and [3,2-a]phenanthridines were synthesized from amines by amide formation and Morgan-Walls cyclization. The synthetic routes offer the advantage of late-stage derivatization and do not require protecting groups, which makes the compounds directly suitable for further functionalization. The compounds exhibit remarkable acid-dependent bathochromic shifts of the luminescence, solvatochromism, and aggregation-induced emission (AIE) behavior, which make them especially interesting candidates for studies toward optoelectronic applications.

Determination of rhein and physcion in rhubarb by microchip capillary electrophoresis in mixed hydro-organic solvent combined with laser-induced fluorescence detection

Microchip capillary electrophoresis in mixed hydro-organic solvent combined with laser-induced fluorescence detection was developed for the separation and detection of physcion and rhein in rhubarb. In contrast to the conventional capillary electrophoresis method, ammonium acetate-dimethyl sulfoxide was used as the basic buffer system in this method. The effects of background buffer, buffer apparent pH*, buffer concentration, water ratio, sample preparation method, and separation voltage on separation and detection were investigated. Optimized separation and detection conditions were obtained: the buffer consisted of 20 mmol/L of ammonium acetate in hydro-organic solvent composed dimethyl sulfoxide, formamide, and water mixed at 60/20/20 (v/v/v) ratio. The separation voltage was 1.9 kV. Under these conditions, the physcion, rhein, and other components of rhubarb can be completely separated within 150 s. Under the methodological verification, good linearity (R ≥ 0.9995) for physcion and rhein, and low limits of detection (0.085 μg·mL-1 and 0.077 μg·mL-1 , respectively), satisfactory peak area precisions, migration time precisions (1.74%-3.09%), and accuracy (recovery rate 97.8% and 101.4%) were achieved. It is shown that the proposed method is simple, efficient, fast, sensitive, simple instrument, consumes few samples, has low operating cost, and is linear.

Determination of acidity constants of pyridines, imidazoles, and oximes by capillary electrophoresis

The acidity constant in the form of pKa is one of the most important physicochemical quantities. There are prediction tools available for calculating the pKa , but they only deliver precise calculated values for a relatively small set of chemicals. For complex structures with multiple functional groups in particular, the error in the predicted pKa is high due to the application domain of the corresponding models. Thus, we aim to enlarge the dataset of experimentally determined pKa values using capillary electrophoresis. We, therefore, selected various pyridines, imidazoles, and oximes to determine the pKa values using the internal standard approach and the classical method. Especially oximes were not investigated in the past, and predictions for them include larger errors. Thus, our experimentally determined values could contribute to an improved understanding of various functional groups impacting the pKa values and serve as additional datasets to develop improved pKa prediction tools.

Contribution of ionic interactions to stationary phase selectivity in hydrophilic interaction chromatography

We compared the separation selectivities of 19 different hydrophilic interaction chromatography columns. The stationary phases included underivatized silica and hybrid particles, cyano-bonded silica, materials with neutral ligands such as amide, diol, pentahydroxy, and urea, zwitterionic sorbents, and mixed-mode materials with amine functionalities. A set of 77 small molecules was used to evaluate the columns. We visualized the retention behavior of the different columns using retention time correlation plots. The analytes were classified as cations, anions, or neutral based on their estimated charge under the separation conditions. This involved adjusting the dissociation constants of the analytes for the acetonitrile content of the mobile phase and experimentally determining the pH of the mobile phase containing 70% acetonitrile. The retention correlation plots show that the selectivity differences strongly depended on ionic interactions. Comparisons of the neutral stationary phases (e.g., diol vs. amide) showed more similar selectivity than did comparisons of neutral columns versus columns with cation or anion exchange activity (bare silica or amine columns, respectively). The zwitterionic columns did not behave as perfectly neutral. The correlation plots indicated that they exhibited either cation or anion exchange activity, although to a lesser degree than the silica and amine-containing stationary phases.