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

Abstract

Nonaqueous capillary electrophoresis (NACE) using methanol (MeOH) as a solvent of the BGEs and quantum mechanical density functional theory (DFT) have been applied to determine the thermodynamic acidity (ionization) constants (pK a ) of mono-and diaza[5]helicenes, mono-and diaza[6]helicenes, and their dibenzo derivatives in MeOH and water. First, the mixed acidity constants, pK mix a,MeOH , of ionogenic pyridinium groups of azahelicenes and their derivatives in MeOH were obtained by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by NACE in a large series of methanolic BGEs within a wide conventional pH range (pH MeOH 1.6-12.0) and at ambient temperature (21-26°C) in a home-made CE device. Prior to mixed acidity constant calculation, the effective mobilities were corrected to reference temperature (25°C) and constant ionic strength (25 mM). Then, the mixed acidity constants were recalculated to the thermodynamic acidity constants pK a,MeOH by the Debye-Hückel theory of nonideality of electrolyte solutions. Finally, from the methanolic thermodynamic pK a,MeOH values, the aqueous thermodynamic pK a,H 2 O constants were estimated using the empirical relations between methanolic and aqueous acidity constants derived for structurally related pyridine derivatives. Depending on the number and position of the nitrogen atoms in their molecules, the analyzed azahelicenes were found to be weak to moderate bases with methanolic pK a,MeOH in the range 2.01-8.75 and with aqueous pK a,H 2 O in the range 1.67-8.28. The thermodynamic pK a,MeOH obtained by the DFT calculations were in a good agreement with those determined experimentally by NACE.

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

“…[54, 55], Espinoza et al. [56, 57], and others [58–61]. Since the HILIC mobile phases include high concentrations of ACN, the calculated analyte charges may be significantly different than those in an aqueous environment.…”

“…The limitation of the above-described approach is that mobile phase pH and pK a values of analytes shift in presence of organic solvents; this can alter the apparent analytes' charge. This phenomenon has been studied by Wiczling et al [52,53], Bosch et al [54,55], Espinoza et al [56,57], and others [58][59][60][61]. Since the HILIC mobile phases include high concentrations of ACN, the calculated analyte charges may be significantly different than those in an aqueous environment.…”

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

“…Some work focused on the determination of pK a values at the physiologically relevant temperature of 37 • C [7] and approaches to tackle sparingly soluble chemicals by the application of a methanol-water extrapolation [8]. Šolínová et al even applied nonaqueous CE using methanol as solvent for the determination of pK a s of azahelicenes [9]. Furthermore, pressure assisted CE was used to determine pK a values [10].…”

“…Šolínová et al. even applied nonaqueous CE using methanol as solvent for the determination of p K a s of azahelicenes [9]. Furthermore, pressure assisted CE was used to determine p K a values [10].…”

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

“…Delgado-Povedano et al [19] performed NACE coupled with time-of-flight mass spectrometry for determination of emerging mycotoxins. Solinova et al [20] performed NACE coupled with quantum chemical calculations for investigation of acid-base and electromigration properties of azahelicenes.…”