Myung Duk Jang

Solvatochromic hydrogen bond donor acidity of aqueous binary solvent mixtures for reversed-phase liquid

Solvatochromic hydrogen bond (HB) donor acidity (αm) values for aqueous mixtures of methanol, ethanol, isopropanol, acetonitrile and tetrahydrofuran were measured by electronic absorption spectroscopy employing bis[α-(2-pyridyl)-benzylidine-3,4-dimethylaniline]bis(cyano)iron(II) as an indicator, and compared to available literature values. The HB donor acidity of all the aqueous organic mixtures increases in a nonlinear fashion as water is added to the organic solvent. For mixtures of acetonitrile and tetrahydrofuran, which are not acidic or are only slightly acidic, with water αm increases rapidly as a small amount of water is first added, then shows a plateau region, and finally slowly rises to the acidity value of water. In contrast, the αm for the mixtures of methanol, ethanol and isopropanol, which are quite acidic, increase slowly as the first small amount of water is added and then increase rapidly as the mixtures approaches pure water. Literature values of αm for water-methanol mixtures, which were determined from the DT(30) values of Reichardts betaine dye, showed a minimum in the plot of αmvs. volume fraction of organic component. It was also shown that the solvophobicity parameter (Sp) of methanol- and ethanol-water mixtures can be described by a combination of the dipolarity (πm*) and HB donor acidity (αm) of the mixtures.

Reversed phase liquid chromatographic separation of some mono-substituted phenols with calix[6]arene-ρ-sulfonate-modified eluents

SummaryThe utility of water-soluble calix[6]arene-ρ-sulfonate as a mobile phase additive has been investigated for the reversed phase liquid chromatographic separation of some monosubstituted phenol isomers. Retention factors and separation factors for regioisomers of methoxyphenol, aminophenol, and nitrophenol were measured using methanol — water and acetonitrile —water mobile phases of varying composition containing calix[6]arene-ρ-sulfonate, and the values were compared with those obtained using mobile phases containing no additive. It was observed that addition of calix[6]arene-ρ-sulfonate to both acetonitrile — water and methanol —water caused a reduction in the retention of the phenol isomers but generally increased the separation between them, thereby improving the overall separation efficiency.

Solvatochromic hydrogen bond donor acidity of cyclodextrins and reversed-phase liquid chromatographic retention of small molecules on a β-cyclodextrin-bonded silica stationary phase

Abstract Values of the Kamlet-Taft hydrogen bonding (HB) donor acidity (α) and dipolarity/polarizability (π*) parameters for α-, β- and λ-cyclodextrins (CDs) are reported. They possess HB donor acidities in the range 0.21-0.12 and dipolarity/polarizability parameters of about 0.43. Both values are smaller than those for the aliphatic alcohol analogues. The reversed-phase liquid chromatographic retention behaviour of some small molecules on the β-CD bonded silica was compared with that on an octadecylsilylsilica based on the linear solvation energy relationship. It was found that he factors affecting retention on the two stationary phases are very different in that on the ODS column cavity formation and type A HB interactions determine retention whereas on the β-CD column dipolar and type A HB interaction determine retention. Differences in the retention properties of β-CD and ODS phases are rationalized in terms of types of solute-stationary phase interactions involved in the retention process.

Selectivity of amino-, cyano- and diol-bonded silica in reversed-phase liquid chromatography

Amino-, cyano- and diol-bonded silica stationary phases were characterized by estimating their characteristic interaction constants in reversed-phase liquid chromatography (RPLC) based on linear solvation energy relationships. Five characteristic interaction constants of the stationary phases, the hydrophobicity (v), polarizability (r), dipolarity (s), hydrogen bond (HB) acceptor basicity (a) and HB donor acidity strength (b) were determined by multiple regression analyses of logarithmic retention factors (k) for a set of test solutes measured on them in 10% (v/v) methanol-water vs. the solute properties represented by characteristic molecular volume (Vx), excess polarization (R2), dipolarity/polarizability (pi*), HB donor acidity (alpha) and HB acceptor basicity (beta). Magnitudes of the five constants for the phases in RPLC were compared with those in normal-phase LC to see the differences in chromatographic selectivity in the two LC modes.

UNIFAC model as a heuristic guide for estimating retention in reversed-phase liquid chromatography

The utility of the UNIFAC activity coefficient determination method for understanding the magnitude of solute-solvent interactions in reversed-phase liquid chromatography (RPLC) is discussed. UNIFAC-computed partition coefficients for the transfer of various homologous series of solutes from aqueous mixtures of methanol, acetonitrile and tetrahydrofuran to hexadecane, octane, butane and benzene were used to investigate the effects of the carbon number, the type of functional group of the solutes, the composition of the mobile phases, the chain length of alkyl-bonded stationary phases and temperature on retention in reversed-phase liquid-liquid partition chromatography. Although UNIFAC is not accurate enough to be useful for the quantitative determination of retention in RPLC, it is useful in explaining a wide variety of issues of general importance in RPLC.

UNIFAC model as a heuristic guide for estimating retention in chromatography

Abstract The usefulness of the UNIFAC activity coefficient estimation method for understanding the magnitude of solute-solvent interactions in gas and liquid chromatography has been investigated. To demonstrate the power of UNIFAC it was used to investigate the origin of the Martin equation, to explore the methylene group selectivity and relative retention in reversed-phase liquid-liquid partition chromatography, to examine solute- solvent interactions in eluents in normal-phase liquid chromatography and to examine mixed stationary phase effects in gas-liquid chromatography. Although not accurate enough to be useful for quantitative predictions of retention, UNIFAC is fairly useful in explaining a wide variety of issues of general importance in chromatography such as the prediction of the order of elution of polar versus non-polar solutes and the relative strengths of solvents in all types of chromatography.

Effect of triethylamine in the mobile phase on the retention properties of conventional polymeric and horizontally polymerized octadecylsilica in RPLC

SummaryThe effect of triethylamine (TEA) in the mobile phase on the RPLC retention behavior of small organic solutes has been studied on a conventional polymeric octadecylsilica (ODS) and on a horizontally polymerized ODS. Retention factors for a set of solutes were measured on the two phases with methanol-water mobile phases containing triethylamine at different concentrations and analyzed by use of linear solvation energy relationships (LSER). Variation of the resulting LSER coefficients—v (hydrophobicity),r (polarizability),s (dipolarity),b (hydrogen-bond (HB) donating acidity), anda (HB accepting strength)—were examined to see how TEA affects the intermolecular interaction properties of the mobile and stationary phases and hence the retention of the solutes. Addition of TEA to the mobile phase changes the interaction properties of both conventionally polymerized and horizontally polymerized ODS; the effect is greater for the conventional phase. The HB donating acidity (b) of conventional polymeric ODS is significantly reduced by addition of TEA. For the mobile phases studied the magnitudes of theb andv coefficients for the horizontally polymerized ODS phase are greater than for the conventional phase. The different interaction properties of the two polymeric phases arise mainly as a result of differential adsorption of TEA, because of the very different amounts of surface silanol groups present on the two phases.

Binding forces contributing to reversed-phase liquid chromatographic retention on a β-cyclodextrin bonded phase

Abstract Reversed-phase liquid chromatographic (RPLC) capacity factors for a number of organic solutes on a β-cyclodextrin (β-CD) bonded phase column in pure water mobile phase ( k w ) were compared with formation constants of inclusion complexes ( k ′ f ) between β-CD and the solutes based on the linear solvation energy relationship (LSER) in order to understand the types and relative strengths of various intermolecular forces between CD and the guest solute affecting the stability of inclusion complexes and hence retention in RPLC. A close fit of capacity factors ( k ′ w ) with complexation constants indicated that inclusion complexation is the major driving force in retention on the β-CD-bonded phase in RPLC. Comparison of LSERs for K f and k ′ w showed that an increasing guest molecular size stabilizes the complex by virtue of increasing dispersive interactions between the hydrophobic interior of CD cavity and the guest and hence increases the RPLC retention, and that increasing guest dipolarity and hydrogen bond (HB) acceptor basicity lead to a decrease in the stability of the complex due to the stronger dipolar and HB interactions with water, and hence decrease the retention.

Doxycycline as a new chiral selector in capillary electrophoresis

Doxycycline (DOX) is one of the tetracycline class of antibiotics and has not been examined for its enantioseparation abilities previously. The purpose of the present work was to evaluate DOX as a chiral selector (CS) using capillary electrophoresis (CE) in nonaqueous mode. Systematic experiments were performed to investigate the effects of concentration of CS, compositions of organic solvents and background electrolytes, applied voltage on chiral separation of a set of acidic chiral compounds. Excellent resolutions of enantiomers of acidic chiral compounds were attained in a background electrolytes composed of 50:50 acetonitrile/methanol+214mM acetic acid+ 63mM triethylamine and 38mM DOX. The results show that DOX is a viable chiral selector in CE for the enantioseparation of the type of chiral compounds investigated.

Enantioselective light switch effect of Δ- and Λ-[Ru(phenanthroline)2 dipyrido[3,2-a:2′, 3′-c]phenazine]2+ bound to G-quadruplex DNA

The interaction of Δ- and Λ-[Ru(phen)2DPPZ]2+ (DPPZ = dipyrido[3,2-a:2′, 3′-c]phenazine, phen = phenanthroline) with a G-quadruplex formed from 5′-G2T2G2TGTG2T2G2–3′(15-mer) was investigated. The well-known enhancement of luminescence intensity (the ‘light-switch’ effect) was observed for the [Ru(phen)2DPPZ]2+ complexes upon formation of an adduct with the G-quadruplex. The emission intensity of the G-quadruplex-bound Λ-isomer was 3-fold larger than that of the Δ-isomer when bound to the G-quadruplex, which is opposite of the result observed in the case of double stranded DNA (dsDNA); the light switch effect is larger for the dsDNA-bound Δ-isomer. In the job plot of the G-quadruplex with Δ- and Λ-[Ru(phen)2DPPZ]2+, a major inflection point for the two isomers was observed at x ≈ .65, which suggests a binding stoichiometry of 2:1 for both enantiomers. When the G base at the 8th position was replaced with 6-methyl isoxanthopterin (6MI), a fluorescent guanine analog, the excited energy of 6-MI transferred to bound Δ- or Λ-[Ru(phen)2DPPZ]2+, which suggests that at least a part of both Ru(II) enantiomers is close to or in contact with the diagonal loop of the G-quadruplex. A luminescence quenching experiment using [Fe(CN)6]4- for the G-quadruplex-bound Ru(II) complex revealed downward bending curves for both enantiomers in the Stern–Volmer plot, which suggests the presence of Ru(II) complexes that are both accessible and inaccessible to the quencher and may be related to the 2:1 binding stoichiometry.